diff --git a/Cargo.lock b/Cargo.lock
index 34dec06..524654e 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -138,6 +138,8 @@ dependencies = [
 name = "claude-connector"
 version = "0.1.0"
 dependencies = [
+ "anyhow",
+ "cultcache-rs",
  "libc",
  "model-connector",
  "rmp-serde",
diff --git a/crates/claude-connector/Cargo.toml b/crates/claude-connector/Cargo.toml
index a35bb4a..7f9b042 100644
--- a/crates/claude-connector/Cargo.toml
+++ b/crates/claude-connector/Cargo.toml
@@ -7,7 +7,13 @@ license.workspace = true
 repository.workspace = true
 publish = false
 
+[[bin]]
+name = "claude-connector"
+path = "src/main.rs"
+
 [dependencies]
+anyhow = "1"
+cultcache-rs = { git = "https://github.com/GameCult/CultLib.git", rev = "a4eae9056152335662f1115f97bf35fb00ab6d23", package = "cultcache-rs" }
 model-connector = { path = "../model-connector", features = ["daemon"] }
 rmp-serde = "1"
 serde = { version = "1", features = ["derive"] }
diff --git a/crates/claude-connector/src/backend.rs b/crates/claude-connector/src/backend.rs
index ad3fb8c..c5a8f81 100644
--- a/crates/claude-connector/src/backend.rs
+++ b/crates/claude-connector/src/backend.rs
@@ -1,5 +1,6 @@
 use std::fs::File;
 use std::io;
+use std::path::PathBuf;
 use std::time::Instant;
 
 use model_connector::{
@@ -11,12 +12,54 @@ use zeroize::Zeroizing;
 
 use crate::config::{ClaudeCodeConfig, PROVIDER};
 use crate::handle::{ClaudeCodeHandle, SessionStore, plan, prefix_digest, stdin_bytes};
-use crate::launch::{PromptFile, TOKEN_VAR, argv, child_env};
+use crate::launch::{PromptFile, argv, child_env};
 use crate::process::{ProcessPort, RunningChild, StdoutRead};
 use crate::stream::{Failure, Finished, Step, StreamState};
 
-/// How the backend reads its own environment.
-type EnvLookup = Box<dyn Fn(&str) -> Option<String> + Send + Sync>;
+/// Where the child's credential comes from, read afresh at every use. The
+/// connector's own environment is never a source (ruling
+/// claude-connector-token-credential).
+pub trait TokenSource: Send + Sync {
+    /// The token, or `None` when there is none or it is empty.
+    fn read(&self) -> Option<Zeroizing<String>>;
+}
+
+/// A token in a file named by a systemd credential: the file
+/// `$CREDENTIALS_DIRECTORY/<name>`, read on every call so a rotated credential
+/// is picked up by the next spawn.
+pub struct CredentialTokenSource {
+    directory: Option<PathBuf>,
+    name: String,
+}
+
+impl CredentialTokenSource {
+    /// The credential `name` under the directory systemd names in
+    /// `CREDENTIALS_DIRECTORY`; with no such variable the source has no token.
+    /// The variable names a directory, never a token.
+    pub fn from_environment(name: impl Into<String>) -> Self {
+        Self {
+            directory: std::env::var_os("CREDENTIALS_DIRECTORY").map(PathBuf::from),
+            name: name.into(),
+        }
+    }
+
+    pub fn in_directory(directory: impl Into<PathBuf>, name: impl Into<String>) -> Self {
+        Self {
+            directory: Some(directory.into()),
+            name: name.into(),
+        }
+    }
+}
+
+impl TokenSource for CredentialTokenSource {
+    fn read(&self) -> Option<Zeroizing<String>> {
+        let directory = self.directory.as_ref()?;
+        let mut token = Zeroizing::new(std::fs::read_to_string(directory.join(&self.name)).ok()?);
+        let trimmed = token.trim_end_matches(['\r', '\n']).len();
+        token.truncate(trimmed);
+        (!token.is_empty()).then_some(token)
+    }
+}
 
 /// Where the tail of a child's stderr goes. The backend never puts it in a
 /// result; the connector binary wires this to its log.
@@ -31,7 +74,7 @@ pub struct ClaudeCodeBackend {
     port: Box<dyn ProcessPort>,
     sessions: Box<dyn SessionStore>,
     sink: Box<dyn StderrSink>,
-    env_lookup: EnvLookup,
+    tokens: Box<dyn TokenSource>,
 }
 
 impl ClaudeCodeBackend {
@@ -42,7 +85,10 @@ impl ClaudeCodeBackend {
         let sessions = Box::new(crate::handle::FsSessionStore::new(
             config.config_dir.clone(),
         ));
-        Self::with_ports(config, port, sessions, sink)
+        let tokens = Box::new(CredentialTokenSource::from_environment(
+            config.token_credential.clone(),
+        ));
+        Self::with_ports(config, port, sessions, sink, tokens)
     }
 
     pub fn with_ports(
@@ -50,29 +96,21 @@ impl ClaudeCodeBackend {
         port: Box<dyn ProcessPort>,
         sessions: Box<dyn SessionStore>,
         sink: Box<dyn StderrSink>,
+        tokens: Box<dyn TokenSource>,
     ) -> Self {
         Self {
             config,
             port,
             sessions,
             sink,
-            env_lookup: Box::new(|name| std::env::var(name).ok()),
+            tokens,
         }
     }
 
-    fn token(&self) -> Option<Zeroizing<String>> {
-        (self.env_lookup)(TOKEN_VAR)
-            .filter(|value| !value.is_empty())
-            .map(Zeroizing::new)
-    }
-
+    /// The one pin check: `ready` and every spawn use it, and nothing caches
+    /// its answer.
     fn binary_matches_pin(&self) -> bool {
-        let Ok(mut file) = File::open(&self.config.claude_binary) else {
-            return false;
-        };
-        let mut hasher = Sha256::new();
-        io::copy(&mut file, &mut hasher).is_ok()
-            && <[u8; 32]>::from(hasher.finalize()) == self.config.claude_binary_sha256
+        binary_matches_pin(&self.config)
     }
 
     fn run(&self, request: &ProviderRequest) -> (Vec<TransportEvent>, TransportOutcome) {
@@ -86,15 +124,21 @@ impl ClaudeCodeBackend {
                 );
             }
         };
-        let Some(token) = self.token() else {
+        let Some(token) = self.tokens.read() else {
             return failed(
                 FailureKind::Authentication,
                 "claude-code: the connector has no credential to give the child".to_string(),
                 Some(zero_usage()),
             );
         };
-        let Ok(prompt) = PromptFile::create(&request.instructions) else {
-            return failure(Failure::unbilled(
+        if !self.binary_matches_pin() {
+            return failure(Failure::never_reached(
+                FailureKind::Provider,
+                "claude-code: the claude binary does not match its pin",
+            ));
+        }
+        let Ok(prompt) = PromptFile::create(&self.config.prompt_dir, &request.instructions) else {
+            return failure(Failure::never_reached(
                 FailureKind::Provider,
                 "claude-code: the system prompt could not be staged",
             ));
@@ -110,7 +154,7 @@ impl ClaudeCodeBackend {
             stdin_bytes(plan.blocks()),
         );
         let Ok(mut child) = spawned else {
-            return failure(Failure::unbilled(
+            return failure(Failure::never_reached(
                 FailureKind::Provider,
                 "claude-code: the child could not be spawned",
             ));
@@ -175,11 +219,8 @@ impl ClaudeCodeBackend {
 }
 
 impl ProviderBackend for ClaudeCodeBackend {
-    fn ready(&self) -> bool {
-        self.config.work_dir.is_dir()
-            && self.config.config_dir.is_dir()
-            && self.token().is_some()
-            && self.binary_matches_pin()
+    fn ready(&self) -> Result<(), &'static str> {
+        readiness(&self.config, self.tokens.as_ref())
     }
 
     fn provider(&self) -> &str {
@@ -206,19 +247,19 @@ fn read_stream(
                 }
             }
             StdoutRead::Eof => {
-                return Err(Failure::unbilled(
+                return Err(Failure::cost_unknown(
                     FailureKind::Provider,
                     "claude-code: the stream ended without a result",
                 ));
             }
             StdoutRead::Timeout => {
-                return Err(Failure::unbilled(
+                return Err(Failure::cost_unknown(
                     FailureKind::Timeout,
                     "claude-code: the deadline passed before a result",
                 ));
             }
             StdoutRead::Breach => {
-                return Err(Failure::unbilled(
+                return Err(Failure::cost_unknown(
                     FailureKind::Provider,
                     "claude-code: the output passed its size cap",
                 ));
@@ -230,6 +271,39 @@ fn read_stream(
     finished
 }
 
+fn binary_matches_pin(config: &ClaudeCodeConfig) -> bool {
+    let Ok(mut file) = File::open(&config.claude_binary) else {
+        return false;
+    };
+    let mut hasher = Sha256::new();
+    io::copy(&mut file, &mut hasher).is_ok()
+        && <[u8; 32]>::from(hasher.finalize()) == config.claude_binary_sha256
+}
+
+/// Whether the backend can take work, naming the first config field that
+/// cannot be used. `config check` and `serve` both ask this.
+pub(crate) fn readiness(
+    config: &ClaudeCodeConfig,
+    tokens: &dyn TokenSource,
+) -> Result<(), &'static str> {
+    if !config.work_dir.is_dir() {
+        return Err("work_dir");
+    }
+    if !config.config_dir.is_dir() {
+        return Err("config_dir");
+    }
+    if !config.prompt_dir.is_dir() {
+        return Err("prompt_dir");
+    }
+    if tokens.read().is_none() {
+        return Err("token_credential");
+    }
+    if !binary_matches_pin(config) {
+        return Err("claude_binary_sha256");
+    }
+    Ok(())
+}
+
 fn zero_usage() -> Usage {
     Usage {
         input_tokens: 0,
diff --git a/crates/claude-connector/src/backend/tests.rs b/crates/claude-connector/src/backend/tests.rs
index 309e2d1..21a8c72 100644
--- a/crates/claude-connector/src/backend/tests.rs
+++ b/crates/claude-connector/src/backend/tests.rs
@@ -13,7 +13,7 @@ use model_connector::{
 use super::*;
 use crate::config::ClaudeCodeLimits;
 use crate::handle::{FsSessionStore, SessionStore};
-use crate::launch::ChildEnv;
+use crate::launch::{ChildEnv, TOKEN_VAR};
 
 const CANARY_TOKEN: &str = "canary-oauth-token-value";
 const MODEL: &str = "claude-haiku-5-5";
@@ -243,17 +243,47 @@ impl StderrSink for Sink {
     }
 }
 
-fn config(instance: &str, limits: ClaudeCodeLimits) -> ClaudeCodeConfig {
+/// The bytes of the stand-in `claude` the scripted rigs pin.
+const PINNED: &[u8] = b"the pinned claude bytes";
+
+fn pin_of(bytes: &[u8]) -> [u8; 32] {
+    Sha256::digest(bytes).into()
+}
+
+/// A directory holding a pinned `claude` file and an empty `prompts` directory.
+fn sandbox() -> TempDir {
+    let dir = TempDir::new();
+    std::fs::write(dir.path().join("claude"), PINNED).unwrap();
+    std::fs::create_dir_all(dir.path().join("prompts")).unwrap();
+    dir
+}
+
+fn config(dir: &Path, instance: &str, limits: ClaudeCodeLimits) -> ClaudeCodeConfig {
     ClaudeCodeConfig {
-        claude_binary: PathBuf::from("/opt/claude"),
-        claude_binary_sha256: [9; 32],
+        claude_binary: dir.join("claude"),
+        claude_binary_sha256: pin_of(PINNED),
         work_dir: PathBuf::from("/work"),
         config_dir: PathBuf::from("/cfg"),
+        prompt_dir: dir.join("prompts"),
+        token_credential: "claude-token".to_string(),
         instance: instance.to_string(),
         limits,
     }
 }
 
+/// A token source with a fixed answer.
+struct FixedToken(Option<&'static str>);
+
+impl TokenSource for FixedToken {
+    fn read(&self) -> Option<Zeroizing<String>> {
+        self.0.map(|token| Zeroizing::new(token.to_string()))
+    }
+}
+
+fn canary_token() -> Box<dyn TokenSource> {
+    Box::new(FixedToken(Some(CANARY_TOKEN)))
+}
+
 fn limits() -> ClaudeCodeLimits {
     ClaudeCodeLimits {
         deadline: Duration::from_secs(30),
@@ -263,12 +293,8 @@ fn limits() -> ClaudeCodeLimits {
     }
 }
 
-fn with_canary_token(mut backend: ClaudeCodeBackend) -> ClaudeCodeBackend {
-    backend.env_lookup = Box::new(|name| (name == TOKEN_VAR).then(|| CANARY_TOKEN.to_string()));
-    backend
-}
-
 struct Rig {
+    dir: TempDir,
     backend: ClaudeCodeBackend,
     spawns: Arc<AtomicUsize>,
     seen: Arc<Mutex<Seen>>,
@@ -285,13 +311,16 @@ fn rig_for(instance: &str, scripts: Vec<Script>, sessions: &[&str], stderr: &[u8
         seen: seen.clone(),
         stderr: stderr.to_vec(),
     };
-    let backend = with_canary_token(ClaudeCodeBackend::with_ports(
-        config(instance, limits()),
+    let dir = sandbox();
+    let backend = ClaudeCodeBackend::with_ports(
+        config(dir.path(), instance, limits()),
         Box::new(port),
         Box::new(Sessions(sessions.iter().map(|s| s.to_string()).collect())),
         Box::new(sink.clone()),
-    ));
+        canary_token(),
+    );
     Rig {
+        dir,
         backend,
         spawns,
         seen,
@@ -443,7 +472,7 @@ fn argv_is_the_fixed_hygiene_set() {
         .expect("the prompt is passed as a file")
         .to_string();
     let expected: Vec<String> = [
-        "/opt/claude",
+        rig.dir.path().join("claude").to_str().unwrap(),
         "-p",
         "--output-format",
         "stream-json",
@@ -569,7 +598,7 @@ fn the_token_never_reaches_a_message_an_event_or_a_debug_form() {
     );
 
     let env = child_env(
-        &config(INSTANCE, limits()),
+        &config(rig.dir.path(), INSTANCE, limits()),
         &request(vec![user("x")]),
         Zeroizing::new(CANARY_TOKEN.to_string()),
     );
@@ -580,59 +609,163 @@ fn the_token_never_reaches_a_message_an_event_or_a_debug_form() {
 
 // ---- ready ----
 
+/// A backend over a scripted port that never has a script: `ready` runs nothing.
+fn ready_backend(config: ClaudeCodeConfig, tokens: Box<dyn TokenSource>) -> ClaudeCodeBackend {
+    ClaudeCodeBackend::with_ports(
+        config,
+        Box::new(ScriptedPort {
+            scripts: Mutex::new(VecDeque::new()),
+            spawns: Arc::new(AtomicUsize::new(0)),
+            seen: Arc::new(Mutex::new(Seen::default())),
+            stderr: Vec::new(),
+        }),
+        Box::new(Sessions(vec![])),
+        Box::new(Sink::default()),
+        tokens,
+    )
+}
+
 #[test]
 fn ready_checks_the_pinned_binary_digest() {
-    let dir = TempDir::new();
+    let dir = sandbox();
     let (work, cfg) = (dir.path().join("work"), dir.path().join("cfg"));
     std::fs::create_dir_all(&work).unwrap();
     std::fs::create_dir_all(&cfg).unwrap();
     let binary = dir.path().join("claude");
-    std::fs::write(&binary, b"the pinned bytes").unwrap();
-    let pin: [u8; 32] = Sha256::digest(b"the pinned bytes").into();
 
-    let ready = |sha: [u8; 32], binary: &Path, work: &Path, token: bool| {
-        let mut config = config(INSTANCE, limits());
+    let ready_field = |sha: [u8; 32], binary: &Path, work: &Path, prompts: &Path, token: bool| {
+        let mut config = config(dir.path(), INSTANCE, limits());
         config.claude_binary = binary.to_path_buf();
         config.claude_binary_sha256 = sha;
         config.work_dir = work.to_path_buf();
         config.config_dir = cfg.clone();
-        let rig = rig(vec![]);
-        let mut backend = ClaudeCodeBackend::with_ports(
-            config,
-            Box::new(ScriptedPort {
-                scripts: Mutex::new(VecDeque::new()),
-                spawns: rig.spawns.clone(),
-                seen: rig.seen.clone(),
-                stderr: Vec::new(),
-            }),
-            Box::new(Sessions(vec![])),
-            Box::new(Sink::default()),
-        );
-        backend.env_lookup =
-            Box::new(move |name| (token && name == TOKEN_VAR).then(|| "t".to_string()));
-        let answer = backend.ready();
-        assert_eq!(rig.spawns(), 0, "ready runs nothing");
-        answer
+        config.prompt_dir = prompts.to_path_buf();
+        let tokens = Box::new(FixedToken(token.then_some("t")));
+        ready_backend(config, tokens).ready()
     };
+    let ready = |sha: [u8; 32], binary: &Path, work: &Path, prompts: &Path, token: bool| {
+        ready_field(sha, binary, work, prompts, token).is_ok()
+    };
+    let prompts = dir.path().join("prompts");
+    let pin = pin_of(PINNED);
+    assert_eq!(
+        ready_field([0; 32], &binary, &work, &prompts, true),
+        Err("claude_binary_sha256")
+    );
+    assert_eq!(
+        ready_field(pin, &binary, &dir.path().join("absent"), &prompts, true),
+        Err("work_dir")
+    );
+    assert_eq!(
+        ready_field(pin, &binary, &work, &dir.path().join("absent"), true),
+        Err("prompt_dir")
+    );
+    assert_eq!(
+        ready_field(pin, &binary, &work, &prompts, false),
+        Err("token_credential")
+    );
 
-    assert!(ready(pin, &binary, &work, true));
-    assert!(!ready([0; 32], &binary, &work, true), "other pin");
+    assert!(ready(pin, &binary, &work, &prompts, true));
+    assert!(!ready([0; 32], &binary, &work, &prompts, true), "other pin");
     assert!(
-        !ready(pin, &dir.path().join("absent"), &work, true),
+        !ready(pin, &dir.path().join("absent"), &work, &prompts, true),
         "no binary"
     );
     assert!(
-        !ready(pin, &binary, &dir.path().join("absent"), true),
+        !ready(pin, &binary, &dir.path().join("absent"), &prompts, true),
         "no work dir"
     );
-    assert!(!ready(pin, &binary, &work, false), "no token");
+    assert!(
+        !ready(pin, &binary, &work, &dir.path().join("absent"), true),
+        "no prompt dir"
+    );
+    assert!(!ready(pin, &binary, &work, &prompts, false), "no token");
     std::fs::write(&binary, b"other bytes").unwrap();
     assert!(
-        !ready(pin, &binary, &work, true),
+        !ready(pin, &binary, &work, &prompts, true),
         "bytes differ from the pin"
     );
 }
 
+#[test]
+fn token_comes_from_the_credential_file_not_the_environment() {
+    static CANARY_PARENT: std::sync::Once = std::sync::Once::new();
+    CANARY_PARENT.call_once(|| {
+        // SAFETY: set once to a value no code path reads; the test proves none does.
+        unsafe { std::env::set_var(TOKEN_VAR, "canary-environment-token") };
+    });
+    let dir = sandbox();
+    let credentials = dir.path().join("credentials");
+    for sub in ["credentials", "work", "cfg"] {
+        std::fs::create_dir_all(dir.path().join(sub)).unwrap();
+    }
+    let seen = Arc::new(Mutex::new(Seen::default()));
+    let mut config = config(dir.path(), INSTANCE, limits());
+    config.work_dir = dir.path().join("work");
+    config.config_dir = dir.path().join("cfg");
+    let backend = ClaudeCodeBackend::with_ports(
+        config,
+        Box::new(ScriptedPort {
+            scripts: Mutex::new(vec![ok_script(SESSION, "x")].into()),
+            spawns: Arc::new(AtomicUsize::new(0)),
+            seen: seen.clone(),
+            stderr: Vec::new(),
+        }),
+        Box::new(Sessions(vec![])),
+        Box::new(Sink::default()),
+        Box::new(CredentialTokenSource::in_directory(
+            &credentials,
+            "claude-token",
+        )),
+    );
+    let mut door = Door::new();
+
+    // The variable is set and the file is absent: nothing to give the child.
+    assert_eq!(backend.ready(), Err("token_credential"));
+    let (claim, _) = door.begin(request(vec![user("hi")]));
+    let (kind, message, usage) = failure_of(&backend.execute(claim.call()));
+    assert_eq!(kind, FailureKind::Authentication);
+    assert_eq!(usage, Some(usage_zero()));
+    assert!(!message.contains("canary-environment-token"));
+    assert!(seen.lock().unwrap().argv.is_empty(), "no child was spawned");
+
+    // The file is read afresh at the call, its trailing newline trimmed, and
+    // the child's environment carries the file's value.
+    std::fs::write(
+        credentials.join("claude-token"),
+        format!("{CANARY_TOKEN}\n"),
+    )
+    .unwrap();
+    assert_eq!(backend.ready(), Ok(()));
+    let mut door = Door::new();
+    let (claim, _) = door.begin(request(vec![user("hi")]));
+    let result = backend.execute(claim.call());
+    assert!(matches!(
+        outcome(&result),
+        TransportOutcome::Completed { .. }
+    ));
+    let env = seen.lock().unwrap().env.clone();
+    assert!(env.contains(&(TOKEN_VAR.to_string(), CANARY_TOKEN.to_string())));
+
+    // An empty credential is no token.
+    std::fs::write(credentials.join("claude-token"), "\n").unwrap();
+    assert_eq!(backend.ready(), Err("token_credential"));
+}
+
+#[test]
+fn an_unreadable_credential_is_no_token() {
+    // A credential that is a directory, or a directory that is absent.
+    let dir = sandbox();
+    std::fs::create_dir_all(dir.path().join("claude-token")).unwrap();
+    let source = CredentialTokenSource::in_directory(dir.path(), "claude-token");
+    assert!(source.read().is_none());
+    assert!(
+        CredentialTokenSource::in_directory(dir.path().join("absent"), "x")
+            .read()
+            .is_none()
+    );
+}
+
 // ---- planning ----
 
 #[test]
@@ -1224,7 +1357,7 @@ fn one_spawn_per_execute() {
 #[test]
 fn a_missing_token_fails_before_spawn() {
     let mut rig = rig(vec![ok_script(SESSION, "x")]);
-    rig.backend.env_lookup = Box::new(|_| None);
+    rig.backend.tokens = Box::new(FixedToken(None));
     let (kind, _, usage) = failure_of(&rig.run(request(vec![user("hi")])));
     assert_eq!(kind, FailureKind::Authentication);
     assert_eq!(usage, Some(usage_zero()));
@@ -1245,18 +1378,29 @@ fn a_port_that_cannot_spawn_is_a_provider_failure() {
             Err(io::Error::from(io::ErrorKind::NotFound))
         }
     }
-    let backend = with_canary_token(ClaudeCodeBackend::with_ports(
-        config(INSTANCE, limits()),
+    let dir = sandbox();
+    let backend = ClaudeCodeBackend::with_ports(
+        config(dir.path(), INSTANCE, limits()),
         Box::new(Refusing),
         Box::new(Sessions(vec![])),
         Box::new(Sink::default()),
-    ));
+        canary_token(),
+    );
     let mut door = Door::new();
     let (claim, _) = door.begin(request(vec![user("hi")]));
-    let (kind, message, usage) = failure_of(&backend.execute(claim.call()));
+    let result = backend.execute(claim.call());
+    let (kind, message, usage) = failure_of(&result);
     assert_eq!(kind, FailureKind::Provider);
-    assert_eq!(usage, None);
-    assert!(!message.contains("/opt/claude"), "no path in a message");
+    assert_eq!(
+        usage,
+        Some(usage_zero()),
+        "a child that never ran bills nothing"
+    );
+    assert_eq!(parts(&result).1.cost_micro_usd, 0);
+    assert!(
+        !message.contains(dir.path().to_str().unwrap()),
+        "no path in a message"
+    );
 }
 
 #[test]
@@ -1384,28 +1528,60 @@ fn usage_the_result_does_not_report_is_billed_the_reservation() {
 }
 
 #[test]
-fn a_refusal_at_the_door_is_billed_nothing() {
-    // Ruling spend-bills-on-identity-not-validity: a bound result is charged
-    // its true cost, and the provider bills nothing for a 401 or 403.
-    // The shape is a real claude 2.1.312 capture with a bogus token.
+fn door_set_is_exactly_401_403_429() {
+    // Ruling claude-code-plan-quota-fuse-2 with spend-bills-on-identity-not-
+    // validity: a refusal at the door is charged nothing, for exactly the
+    // statuses the provider cannot have billed (401, 403, 429) and only when
+    // the result says its usage is explicitly empty. Every other shape is
+    // billed the reservation. The 401 shape is a real claude 2.1.312 capture
+    // with a bogus token.
     let retry = r#"{"type":"system","subtype":"api_retry","error":"authentication_failed"}"#;
     let synthetic = r#"{"type":"assistant","api_error_status":401,"error":"authentication_failed","message":{"id":"e48001b7-cf4f-49b1-9810-db8fd7f73c73","model":"<synthetic>","stop_reason":"stop_sequence","content":[{"type":"text","text":"Failed to authenticate. API Error: 401 OAuth access token is invalid."}]}}"#;
-    for status in [401, 403] {
-        let result_line = format!(
-            r#"{{"type":"result","subtype":"success","is_error":true,"api_error_status":{status},"stop_reason":"stop_sequence","result":"Failed to authenticate.","modelUsage":{{}},"model_usage_since_last_result":{{}},"terminal_reason":"api_error"}}"#
-        );
-        let result = rig(vec![Script::lines(vec![
-            init_line(MODEL, SESSION),
-            retry.to_string(),
-            retry.to_string(),
-            synthetic.to_string(),
-            result_line,
-        ])])
-        .run(request(vec![user("hi")]));
-        let (kind, _, usage) = failure_of(&result);
-        assert_eq!(kind, FailureKind::Authentication, "{status}");
-        assert_eq!(usage, Some(usage_zero()), "{status}");
-        assert_eq!(parts(&result).1.cost_micro_usd, 0, "{status}");
+    let reservation = reservation();
+    let statuses: [Option<u16>; 9] = [
+        Some(400),
+        Some(401),
+        Some(402),
+        Some(403),
+        Some(404),
+        Some(429),
+        Some(500),
+        Some(529),
+        None,
+    ];
+    let shapes = [
+        (Some("{}"), "empty"),
+        (None, "absent"),
+        (Some("null"), "null"),
+    ];
+    for status in statuses {
+        for (since_last, shape) in shapes {
+            let status_field = status
+                .map(|status| format!(r#""api_error_status":{status},"#))
+                .unwrap_or_default();
+            let usage_field = since_last
+                .map(|usage| format!(r#","model_usage_since_last_result":{usage}"#))
+                .unwrap_or_default();
+            let result_line = format!(
+                r#"{{"type":"result","subtype":"success","is_error":true,{status_field}"stop_reason":"stop_sequence","result":"Failed.","modelUsage":{CUMULATIVE}{usage_field},"terminal_reason":"api_error"}}"#
+            );
+            let result = rig(vec![Script::lines(vec![
+                init_line(MODEL, SESSION),
+                retry.to_string(),
+                synthetic.to_string(),
+                result_line,
+            ])])
+            .run(request(vec![user("hi")]));
+            let (_, _, usage) = failure_of(&result);
+            let case = format!("{status:?} {shape}");
+            if matches!(status, Some(401 | 403 | 429)) && shape == "empty" {
+                assert_eq!(usage, Some(usage_zero()), "{case}");
+                assert_eq!(parts(&result).1.cost_micro_usd, 0, "{case}");
+            } else {
+                assert_eq!(usage, None, "{case}");
+                assert_eq!(parts(&result).1.cost_micro_usd, reservation, "{case}");
+            }
+        }
     }
 }
 
@@ -1458,6 +1634,57 @@ fn the_prompt_file_is_removed_when_the_call_fails() {
     }
 }
 
+#[test]
+fn prompt_file_lives_in_prompt_dir_exclusively() {
+    let rig = rig(vec![ok_script(SESSION, "x")]);
+    rig.run(request(vec![user("hi")]));
+    let (path, _) = rig.seen().prompt_file.expect("a prompt file");
+    assert_eq!(
+        path.parent(),
+        Some(rig.dir.path().join("prompts").as_path())
+    );
+
+    // A prompt directory that cannot take the file fails staging: nothing
+    // reached the provider, so it bills nothing, and no child runs.
+    let rig = rig_for(INSTANCE, vec![ok_script(SESSION, "x")], &[SESSION], b"");
+    std::fs::remove_dir_all(rig.dir.path().join("prompts")).unwrap();
+    let result = rig.run(request(vec![user("hi")]));
+    let (kind, _, usage) = failure_of(&result);
+    assert_eq!((kind, usage), (FailureKind::Provider, Some(usage_zero())));
+    assert_eq!(parts(&result).1.cost_micro_usd, 0);
+    assert_eq!(rig.spawns(), 0);
+}
+
+#[test]
+fn pin_is_checked_before_every_spawn() {
+    let rig = rig(vec![ok_script(SESSION, "x"), ok_script(SESSION, "y")]);
+    let binary = rig.dir.path().join("claude");
+    assert!(rig.backend.binary_matches_pin());
+    rig.run(request(vec![user("hi")]));
+    assert_eq!(rig.spawns(), 1);
+
+    // The binary is swapped after readiness: the next call never spawns.
+    std::fs::write(&binary, b"swapped bytes").unwrap();
+    let result = rig.run(request(vec![user("hi")]));
+    let (kind, message, usage) = failure_of(&result);
+    assert_eq!((kind, usage), (FailureKind::Provider, Some(usage_zero())));
+    assert_eq!(
+        parts(&result).1.cost_micro_usd,
+        0,
+        "billed nothing, not the reservation"
+    );
+    assert!(!message.contains(rig.dir.path().to_str().unwrap()));
+    assert_eq!(rig.spawns(), 1, "no child for a binary off its pin");
+    assert!(!rig.backend.binary_matches_pin());
+    let staged = std::fs::read_dir(rig.dir.path().join("prompts")).unwrap();
+    assert_eq!(staged.count(), 0, "no prompt was staged for it");
+
+    // Restoring the pinned bytes is enough: nothing remembers the verdict.
+    std::fs::write(&binary, PINNED).unwrap();
+    rig.run(request(vec![user("hi")]));
+    assert_eq!(rig.spawns(), 2);
+}
+
 #[test]
 fn the_provider_tag_is_claude_code() {
     assert_eq!(rig(vec![]).backend.provider(), "claude-code");
@@ -1540,12 +1767,21 @@ mod real {
         let binary = dir.path().join("claude");
         std::fs::write(&binary, format!("#!/bin/sh\n{body}\n")).unwrap();
         std::fs::set_permissions(&binary, std::fs::Permissions::from_mode(0o755)).unwrap();
-        let mut config = config(INSTANCE, limits);
+        let prompts = dir.path().join("prompts");
+        std::fs::create_dir_all(&prompts).unwrap();
+        let mut config = config(dir.path(), INSTANCE, limits);
+        config.claude_binary_sha256 = pin_of(&std::fs::read(&binary).unwrap());
         config.claude_binary = binary;
         config.work_dir = work;
         config.config_dir = cfg;
         let sink = Sink::default();
-        let backend = with_canary_token(ClaudeCodeBackend::new(config, Box::new(sink.clone())));
+        let backend = ClaudeCodeBackend::with_ports(
+            config.clone(),
+            Box::new(crate::process::StdProcessPort::new(config.limits)),
+            Box::new(FsSessionStore::new(config.config_dir.clone())),
+            Box::new(sink.clone()),
+            canary_token(),
+        );
         Fake {
             _guard: guard,
             dir,
@@ -1655,7 +1891,7 @@ mod real {
             leader.display()
         );
         let fake = fake(&body, limits());
-        let config = config(INSTANCE, limits());
+        let config = config(fake.dir.path(), INSTANCE, limits());
         let env = child_env(
             &config,
             &request(vec![user("x")]),
@@ -1801,6 +2037,11 @@ mod real {
             "600"
         );
         let path = std::fs::read_to_string(probe.path().join("path")).unwrap();
+        assert_eq!(
+            Path::new(path.trim()).parent(),
+            Some(fake.dir.path().join("prompts").as_path()),
+            "staged in the configured prompt directory"
+        );
         assert!(!Path::new(path.trim()).exists(), "removed after the call");
     }
 
@@ -1837,6 +2078,25 @@ mod real {
         assert!(sunk.trim_end().ends_with("END-OF-STDERR"), "{}", sunk.len());
     }
 
+    #[test]
+    fn stderr_join_survives_a_megabyte_after_result() {
+        // Soul s2's probe: the result line first, then a megabyte of stderr
+        // before the child exits. The call must wait for the stderr reader to
+        // finish, or the tail handed to the sink is cut short.
+        let body = format!(
+            "{}\nhead -c 1000000 /dev/zero | tr '\\0' x >&2\necho END-OF-STDERR >&2",
+            printf_lines(&good_lines())
+        );
+        let fake = fake(&body, limits());
+        let result = fake.run(request(vec![user("hi")]));
+        assert!(matches!(
+            outcome(&result),
+            TransportOutcome::Completed { .. }
+        ));
+        let sunk = String::from_utf8(fake.sink.0.lock().unwrap().clone()).unwrap();
+        assert!(sunk.trim_end().ends_with("END-OF-STDERR"), "{}", sunk.len());
+    }
+
     #[test]
     fn a_result_is_enough_even_if_the_child_keeps_stdout_open() {
         let body = format!("{}\nexec sleep 300", printf_lines(&good_lines()));
diff --git a/crates/claude-connector/src/config.rs b/crates/claude-connector/src/config.rs
index 1dd0e8a..3bd0e1f 100644
--- a/crates/claude-connector/src/config.rs
+++ b/crates/claude-connector/src/config.rs
@@ -19,6 +19,12 @@ pub struct ClaudeCodeConfig {
     /// Passed to the child as `CLAUDE_CONFIG_DIR` and `HOME`; it holds the
     /// CLI's session store.
     pub config_dir: PathBuf,
+    /// A directory the connector owns (mode 0700), where each call's system
+    /// prompt is staged for the child.
+    pub prompt_dir: PathBuf,
+    /// The name of the systemd credential that holds the child's token, under
+    /// `$CREDENTIALS_DIRECTORY`.
+    pub token_credential: String,
     /// The connector instance id written into every handle.
     pub instance: String,
     pub limits: ClaudeCodeLimits,
diff --git a/crates/claude-connector/src/daemon.rs b/crates/claude-connector/src/daemon.rs
new file mode 100644
index 0000000..87450a8
--- /dev/null
+++ b/crates/claude-connector/src/daemon.rs
@@ -0,0 +1,571 @@
+//! The claude-connector's configuration document and its serve verb. One typed
+//! CultCache record owns every bound and path the backend and the serve loop
+//! run with; nothing in the connector's environment, flags or JSON can change
+//! them. Validation errors name a field and never echo a value.
+
+use std::collections::{BTreeMap, BTreeSet};
+use std::net::{SocketAddr, TcpListener};
+use std::path::{Path, PathBuf};
+use std::sync::Arc;
+use std::time::Duration;
+
+use cultcache_rs::{CultCache, DatabaseEntry, SingleFileMessagePackBackingStore};
+use model_connector::{
+    CallerAdmission, CallerConfig, MIN_PAYLOAD_BYTES, ModelPrice, ServeError, ServeLimits,
+    ServiceError, load_admissions, serve,
+};
+use thiserror::Error;
+
+use crate::backend::readiness;
+use crate::{
+    ClaudeCodeBackend, ClaudeCodeConfig, ClaudeCodeLimits, CredentialTokenSource, StderrSink,
+};
+
+const CONFIG_KEY: &str = "runtime";
+const MODEL_PREFIX: &str = "claude-";
+
+#[derive(Clone, Debug, PartialEq, Eq, DatabaseEntry)]
+#[cultcache(type = "gamecult.model_connector.claude_connector_config.v1")]
+pub struct ClaudeConnectorConfig {
+    /// A loopback socket address.
+    #[cultcache(key = 0)]
+    pub listen: String,
+    #[cultcache(key = 1)]
+    pub replay_store: PathBuf,
+    #[cultcache(key = 2)]
+    pub claude_binary: PathBuf,
+    #[cultcache(key = 3)]
+    pub claude_binary_sha256: [u8; 32],
+    #[cultcache(key = 4)]
+    pub work_dir: PathBuf,
+    #[cultcache(key = 5)]
+    pub config_dir: PathBuf,
+    /// Where each call's system prompt is staged: a directory the connector
+    /// owns, mode 0700.
+    #[cultcache(key = 6)]
+    pub prompt_dir: PathBuf,
+    #[cultcache(key = 7)]
+    pub instance: String,
+    /// The name of the systemd credential that holds the child's token.
+    #[cultcache(key = 8)]
+    pub token_credential: String,
+    #[cultcache(key = 9)]
+    pub deadline_ms: u64,
+    #[cultcache(key = 10)]
+    pub max_line_bytes: usize,
+    #[cultcache(key = 11)]
+    pub max_total_bytes: usize,
+    #[cultcache(key = 12)]
+    pub stderr_tail_bytes: usize,
+    #[cultcache(key = 13)]
+    pub max_frame_bytes: usize,
+    #[cultcache(key = 14)]
+    pub max_connections: usize,
+    #[cultcache(key = 15)]
+    pub socket_timeout_ms: u64,
+    #[cultcache(key = 16)]
+    pub max_expiry_skew_ms: u64,
+    /// Exactly one caller until claude-handle-caller-binding lands.
+    #[cultcache(key = 17)]
+    pub callers: Vec<CallerConfig>,
+    /// What each allowed model costs; keys are canonical model ids.
+    #[cultcache(key = 18)]
+    pub prices: BTreeMap<String, ModelPrice>,
+}
+
+/// A refusal. It names the field or stage that failed and nothing the
+/// operator or the file typed.
+#[derive(Debug, Error)]
+pub enum ConfigError {
+    #[error("invalid configuration field {0}")]
+    Invalid(&'static str),
+    #[error("the configuration store could not be read as a claude connector document")]
+    Store,
+    #[error("the prompt directory is not a directory owned by this process with mode 0700")]
+    PromptDir,
+    #[error("the listener could not be bound")]
+    Listen,
+    /// Something the document names exists but cannot be used now.
+    #[error("the configured {0} cannot be used")]
+    Unusable(&'static str),
+    /// The serve loop failed after it started.
+    #[error("the serve loop failed at {0}")]
+    Serving(&'static str),
+}
+
+/// The kit's serve errors carry stored paths, ids and numbers; none of that
+/// text is shown. Each maps to the configuration field or stage it names.
+impl From<ServeError> for ConfigError {
+    fn from(error: ServeError) -> Self {
+        match error {
+            ServeError::InvalidConfig(field) | ServeError::BackendUnready(field) => {
+                Self::Unusable(field)
+            }
+            ServeError::FrameBound { .. } => Self::Invalid("max_frame_bytes"),
+            ServeError::ConnectionKey(_) => Self::Unusable("connection_key_file"),
+            ServeError::Service(error) => match error {
+                ServiceError::ReplayStore(_) => Self::Unusable("replay_store"),
+                ServiceError::UnpricedModel(_) => Self::Invalid("prices"),
+                ServiceError::InvalidAdmission
+                | ServiceError::DuplicateCaller
+                | ServiceError::SharedCallerKey => Self::Invalid("callers"),
+                _ => Self::Serving("service"),
+            },
+            ServeError::ServicePoisoned => Self::Serving("service"),
+            ServeError::Listen(_) => Self::Serving("listener"),
+            ServeError::Thread(_) | ServeError::Connection(_) => Self::Serving("connection"),
+            ServeError::FrameSize | ServeError::FrameEncoding => Self::Serving("frame"),
+            ServeError::Clock => Self::Serving("clock"),
+        }
+    }
+}
+
+impl ClaudeConnectorConfig {
+    /// The one validation: `config init`, `config check` and `serve` all pass
+    /// through it. Returns the listen address.
+    pub fn validate(&self) -> Result<SocketAddr, ConfigError> {
+        let listen = self
+            .listen
+            .parse::<SocketAddr>()
+            .ok()
+            .filter(|address| address.ip().is_loopback())
+            .ok_or(ConfigError::Invalid("listen"))?;
+        let paths = [
+            ("replay_store", &self.replay_store),
+            ("claude_binary", &self.claude_binary),
+            ("work_dir", &self.work_dir),
+            ("config_dir", &self.config_dir),
+            ("prompt_dir", &self.prompt_dir),
+        ];
+        for (field, path) in paths {
+            if !path.is_absolute() {
+                return Err(ConfigError::Invalid(field));
+            }
+        }
+        if self.claude_binary_sha256 == [0; 32] {
+            return Err(ConfigError::Invalid("claude_binary_sha256"));
+        }
+        if !is_trimmed_nonempty(&self.instance) {
+            return Err(ConfigError::Invalid("instance"));
+        }
+        if !is_credential_name(&self.token_credential) {
+            return Err(ConfigError::Invalid("token_credential"));
+        }
+        if self.deadline_ms == 0
+            || self.max_line_bytes == 0
+            || self.max_total_bytes < self.max_line_bytes
+            || self.stderr_tail_bytes == 0
+        {
+            return Err(ConfigError::Invalid("child limits"));
+        }
+        if self.max_frame_bytes == 0
+            || self.max_connections == 0
+            || self.socket_timeout_ms == 0
+            || self.max_expiry_skew_ms == 0
+        {
+            return Err(ConfigError::Invalid("serve limits"));
+        }
+        let [caller] = self.callers.as_slice() else {
+            return Err(ConfigError::Invalid("callers"));
+        };
+        if !is_trimmed_nonempty(&caller.caller_runtime_id) {
+            return Err(ConfigError::Invalid("caller_runtime_id"));
+        }
+        if !caller.connection_key_file.is_absolute() {
+            return Err(ConfigError::Invalid("connection_key_file"));
+        }
+        if caller.connection_key_epoch == 0 {
+            return Err(ConfigError::Invalid("connection_key_epoch"));
+        }
+        if caller.max_concurrent_requests == 0
+            || caller.max_payload_bytes < MIN_PAYLOAD_BYTES
+            || caller.max_output_tokens == 0
+            || caller.spend_ceiling_micro_usd == 0
+            || caller.spend_window_ms == 0
+        {
+            return Err(ConfigError::Invalid("caller bounds"));
+        }
+        if caller.allowed_models.is_empty() {
+            return Err(ConfigError::Invalid("allowed_models"));
+        }
+        let mut seen = BTreeSet::new();
+        for model in &caller.allowed_models {
+            if !is_canonical_model(model)
+                || !seen.insert(model.as_str())
+                || !self.prices.contains_key(model)
+            {
+                return Err(ConfigError::Invalid("allowed_models"));
+            }
+        }
+        if !self
+            .prices
+            .iter()
+            .all(|(model, price)| is_canonical_model(model) && is_fully_priced(price))
+        {
+            return Err(ConfigError::Invalid("prices"));
+        }
+        Ok(listen)
+    }
+
+    fn backend_config(&self) -> ClaudeCodeConfig {
+        ClaudeCodeConfig {
+            claude_binary: self.claude_binary.clone(),
+            claude_binary_sha256: self.claude_binary_sha256,
+            work_dir: self.work_dir.clone(),
+            config_dir: self.config_dir.clone(),
+            prompt_dir: self.prompt_dir.clone(),
+            token_credential: self.token_credential.clone(),
+            instance: self.instance.clone(),
+            limits: ClaudeCodeLimits {
+                deadline: Duration::from_millis(self.deadline_ms),
+                max_line_bytes: self.max_line_bytes,
+                max_total_bytes: self.max_total_bytes,
+                stderr_tail_bytes: self.stderr_tail_bytes,
+            },
+        }
+    }
+
+    fn serve_limits(&self) -> ServeLimits {
+        ServeLimits {
+            max_frame_bytes: self.max_frame_bytes,
+            max_connections: self.max_connections,
+            socket_timeout_ms: self.socket_timeout_ms,
+            max_expiry_skew_ms: self.max_expiry_skew_ms,
+        }
+    }
+}
+
+fn is_trimmed_nonempty(text: &str) -> bool {
+    !text.is_empty() && text.trim() == text
+}
+
+/// A model id the CLI echoes back unchanged: `claude-<family>-<major>-<minor>`
+/// with an optional `-YYYYMMDD` suffix, the family lowercase letters. Aliases
+/// and bare prefixes are refused because the stream's `init.model` must equal
+/// the requested model.
+fn is_canonical_model(model: &str) -> bool {
+    let Some(rest) = model.strip_prefix(MODEL_PREFIX) else {
+        return false;
+    };
+    let digits = |part: &str, most: usize| {
+        (1..=most).contains(&part.len()) && part.bytes().all(|byte| byte.is_ascii_digit())
+    };
+    let mut parts = rest.split('-');
+    let family = parts.next().unwrap_or("");
+    let major = parts.next().unwrap_or("");
+    let minor = parts.next().unwrap_or("");
+    let date = parts.next();
+    !family.is_empty()
+        && family.bytes().all(|byte| byte.is_ascii_lowercase())
+        && digits(major, 2)
+        && digits(minor, 2)
+        && date.is_none_or(|date| date.len() == 8 && digits(date, 8))
+        && parts.next().is_none()
+}
+
+/// A price is a fuse only if every component it bills is nonzero: a zero rate
+/// lets a call cost nothing against the caller's ceiling.
+fn is_fully_priced(price: &ModelPrice) -> bool {
+    let long = price.long_context.map(|long| long.rates);
+    [Some(price.rates), long]
+        .into_iter()
+        .flatten()
+        .all(|rates| {
+            [
+                rates.input,
+                rates.cache_write_short,
+                rates.cache_write_long,
+                rates.cache_read,
+                rates.output,
+            ]
+            .iter()
+            .all(|rate| *rate > 0)
+        })
+}
+
+/// One file name under `$CREDENTIALS_DIRECTORY`, never a path.
+fn is_credential_name(name: &str) -> bool {
+    !name.is_empty()
+        && name != "."
+        && name != ".."
+        && name
+            .bytes()
+            .all(|byte| byte.is_ascii_alphanumeric() || matches!(byte, b'-' | b'.' | b'_'))
+}
+
+/// Reads the document without writing: a shared snapshot of exactly one
+/// record, with the exact key, type and schema id, in canonical positional
+/// MessagePack, then validated.
+pub fn load_config(path: &Path) -> Result<ClaudeConnectorConfig, ConfigError> {
+    let store = SingleFileMessagePackBackingStore::new(path);
+    let config = store
+        .with_read_only_shared_snapshot(|entries| {
+            let [envelope] = entries.as_slice() else {
+                anyhow::bail!("not exactly one record");
+            };
+            if envelope.key != CONFIG_KEY
+                || envelope.r#type != ClaudeConnectorConfig::TYPE
+                || envelope.schema_id.as_deref() != Some(ClaudeConnectorConfig::TYPE)
+            {
+                anyhow::bail!("wrong typed envelope");
+            }
+            let config: ClaudeConnectorConfig = rmp_serde::from_slice(&envelope.payload)?;
+            if rmp_serde::to_vec(&config)? != envelope.payload {
+                anyhow::bail!("not canonical positional MessagePack");
+            }
+            Ok(config)
+        })
+        .map_err(|_| ConfigError::Store)?;
+    config.validate()?;
+    Ok(config)
+}
+
+/// Validates, then writes the document as the store's only record.
+pub fn write_config(path: &Path, config: &ClaudeConnectorConfig) -> Result<(), ConfigError> {
+    config.validate()?;
+    let mut cache = CultCache::new();
+    cache
+        .register_entry_type::<ClaudeConnectorConfig>()
+        .map_err(|_| ConfigError::Store)?;
+    cache
+        .add_generic_backing_store(SingleFileMessagePackBackingStore::new(path))
+        .map_err(|_| ConfigError::Store)?;
+    cache
+        .pull_all_backing_stores()
+        .map_err(|_| ConfigError::Store)?;
+    cache
+        .put(CONFIG_KEY, config)
+        .map_err(|_| ConfigError::Store)?;
+    Ok(())
+}
+
+/// The prompt directory holds every system prompt in flight: it must be a
+/// real directory (not a link) that this process owns and nobody else can
+/// enter.
+#[cfg(unix)]
+pub fn check_prompt_dir(dir: &Path) -> Result<(), ConfigError> {
+    use std::os::unix::fs::{MetadataExt, PermissionsExt};
+    let metadata = std::fs::symlink_metadata(dir).map_err(|_| ConfigError::PromptDir)?;
+    // SAFETY: geteuid has no preconditions and cannot fail.
+    let uid = unsafe { libc::geteuid() };
+    if !metadata.is_dir()
+        || metadata.uid() != uid
+        || metadata.permissions().mode() & 0o7777 != 0o700
+    {
+        return Err(ConfigError::PromptDir);
+    }
+    Ok(())
+}
+
+/// The child's stderr is the log's business by size only: its bytes are never
+/// written anywhere the connector controls.
+struct LogSink<W>(std::sync::Mutex<W>);
+
+impl<W: std::io::Write + Send> StderrSink for LogSink<W> {
+    fn stderr_tail(&self, tail: &[u8]) {
+        if let Ok(mut log) = self.0.lock() {
+            let _ = writeln!(log, "claude-connector: child stderr, {} bytes", tail.len());
+        }
+    }
+}
+
+/// Everything `serve` needs true before it binds, in one place so that
+/// `config check` passes exactly what `serve` will start: the prompt
+/// directory, the other directories, the credential, the pin, the connection
+/// key and the frame and connection bounds. Returns the admitted callers.
+fn readiness_of(config: &ClaudeConnectorConfig) -> Result<Vec<CallerAdmission>, ConfigError> {
+    config.validate()?;
+    #[cfg(unix)]
+    check_prompt_dir(&config.prompt_dir)?;
+    let admissions = load_admissions(&config.callers)?;
+    config.serve_limits().validate(&admissions)?;
+    let backend = config.backend_config();
+    readiness(
+        &backend,
+        &CredentialTokenSource::from_environment(backend.token_credential.clone()),
+    )
+    .map_err(ConfigError::Unusable)?;
+    Ok(admissions)
+}
+
+/// Whether `serve` would start with this document and this environment.
+pub fn check_ready(config: &ClaudeConnectorConfig) -> Result<(), ConfigError> {
+    readiness_of(config).map(drop)
+}
+
+/// Loads the document and serves until the listener fails.
+#[cfg(unix)]
+pub fn serve_config(path: &Path) -> Result<(), ConfigError> {
+    let config = load_config(path)?;
+    let admissions = readiness_of(&config)?;
+    let listen = config.validate()?;
+    let backend = Arc::new(ClaudeCodeBackend::new(
+        config.backend_config(),
+        Box::new(LogSink(std::sync::Mutex::new(std::io::stderr()))),
+    ));
+    let listener = TcpListener::bind(listen).map_err(|_| ConfigError::Listen)?;
+    serve(
+        listener,
+        config.serve_limits(),
+        admissions,
+        config.prices.clone(),
+        &config.replay_store,
+        backend,
+    )?;
+    Ok(())
+}
+
+#[cfg(test)]
+mod tests {
+    use std::sync::{Arc, Mutex};
+
+    use super::*;
+
+    /// A log that a test can read back.
+    #[derive(Clone, Default)]
+    struct Captured(Arc<Mutex<Vec<u8>>>);
+
+    impl std::io::Write for Captured {
+        fn write(&mut self, bytes: &[u8]) -> std::io::Result<usize> {
+            self.0.lock().unwrap().extend_from_slice(bytes);
+            Ok(bytes.len())
+        }
+        fn flush(&mut self) -> std::io::Result<()> {
+            Ok(())
+        }
+    }
+
+    #[test]
+    fn serve_errors_name_a_field_or_stage_and_never_a_stored_value() {
+        const CANARY: &str = "canary-path-8e2a";
+        let io = || std::io::Error::other(CANARY);
+        let cases: Vec<(ServeError, &str)> = vec![
+            (ServeError::InvalidConfig("callers"), "callers"),
+            (ServeError::BackendUnready("work_dir"), "work_dir"),
+            (
+                ServeError::FrameBound {
+                    max_frame_bytes: 4099,
+                    payload_bytes: 4096,
+                    needed_frame_bytes: 9999,
+                },
+                "max_frame_bytes",
+            ),
+            (ServeError::ConnectionKey(io()), "connection_key_file"),
+            (
+                ServeError::Service(ServiceError::ReplayStore(format!("{CANARY}/replay.cc"))),
+                "replay_store",
+            ),
+            (
+                ServeError::Service(ServiceError::UnpricedModel(CANARY.to_string())),
+                "prices",
+            ),
+            (
+                ServeError::Service(ServiceError::InvalidAdmission),
+                "callers",
+            ),
+            (
+                ServeError::Service(ServiceError::DuplicateCaller),
+                "callers",
+            ),
+            (
+                ServeError::Service(ServiceError::SharedCallerKey),
+                "callers",
+            ),
+            (ServeError::Listen(io()), "listener"),
+        ];
+        for (error, field) in cases {
+            let shown = ConfigError::from(error);
+            let text = format!("{shown} {shown:?}");
+            assert!(text.contains(field), "{text}");
+            for stored in [CANARY, "4099", "9999", "replay.cc"] {
+                assert!(!text.contains(stored), "{text}");
+            }
+        }
+    }
+
+    #[test]
+    fn the_log_gets_the_size_of_the_childs_stderr_and_never_its_bytes() {
+        let log = Captured::default();
+        let sink = LogSink(Mutex::new(log.clone()));
+        sink.stderr_tail(b"canary-stderr-text, 22 bytes");
+        let written = String::from_utf8(log.0.lock().unwrap().clone()).unwrap();
+        assert_eq!(
+            written,
+            "claude-connector: child stderr, 28 bytes
+"
+        );
+        assert!(!written.contains("canary"));
+    }
+
+    #[cfg(unix)]
+    mod prompt_dir {
+        use std::os::unix::fs::{PermissionsExt, chown, symlink};
+        use std::path::PathBuf;
+
+        use super::super::check_prompt_dir;
+
+        struct Dir(PathBuf);
+
+        impl Dir {
+            fn new(name: &str) -> Self {
+                let path = std::env::temp_dir()
+                    .join(format!("claude-prompt-dir-{}-{name}", std::process::id()));
+                let _ = std::fs::remove_dir_all(&path);
+                std::fs::create_dir_all(&path).unwrap();
+                Self(path)
+            }
+            fn mode(&self, mode: u32) {
+                std::fs::set_permissions(&self.0, std::fs::Permissions::from_mode(mode)).unwrap();
+            }
+        }
+
+        impl Drop for Dir {
+            fn drop(&mut self) {
+                let _ = std::fs::set_permissions(&self.0, std::fs::Permissions::from_mode(0o700));
+                let _ = std::fs::remove_dir_all(&self.0);
+            }
+        }
+
+        #[test]
+        fn only_mode_0700_passes() {
+            let dir = Dir::new("modes");
+            dir.mode(0o700);
+            assert!(check_prompt_dir(&dir.0).is_ok());
+            // Each of these leaves the owner able to enter, or adds a bit
+            // beyond the owner's three; all are refused.
+            for mode in [
+                0o500, 0o300, 0o100, 0o600, 0o770, 0o707, 0o750, 0o755, 0o1700, 0o2700, 0o4700,
+            ] {
+                dir.mode(mode);
+                assert!(check_prompt_dir(&dir.0).is_err(), "{mode:o}");
+            }
+        }
+
+        #[test]
+        fn a_link_to_a_good_directory_is_refused() {
+            let dir = Dir::new("link-target");
+            dir.mode(0o700);
+            let link =
+                std::env::temp_dir().join(format!("claude-prompt-dir-{}-link", std::process::id()));
+            let _ = std::fs::remove_file(&link);
+            symlink(&dir.0, &link).unwrap();
+            let refused = check_prompt_dir(&link).is_err();
+            std::fs::remove_file(&link).unwrap();
+            assert!(refused);
+        }
+
+        #[test]
+        fn a_directory_owned_by_another_user_is_refused() {
+            // SAFETY: geteuid has no preconditions and cannot fail.
+            if unsafe { libc::geteuid() } != 0 {
+                eprintln!("skipped: only root can give a directory away");
+                return;
+            }
+            let dir = Dir::new("owner");
+            dir.mode(0o700);
+            assert!(check_prompt_dir(&dir.0).is_ok());
+            chown(&dir.0, Some(1234), None).unwrap();
+            assert!(check_prompt_dir(&dir.0).is_err());
+        }
+    }
+}
diff --git a/crates/claude-connector/src/launch.rs b/crates/claude-connector/src/launch.rs
index 22c8813..c94134c 100644
--- a/crates/claude-connector/src/launch.rs
+++ b/crates/claude-connector/src/launch.rs
@@ -14,8 +14,8 @@ use zeroize::Zeroizing;
 use crate::config::ClaudeCodeConfig;
 use crate::handle::Plan;
 
-/// The one credential variable, copied by name from the connector's own
-/// environment into the child and nowhere else.
+/// The one credential variable: the child's environment is the only
+/// environment the token ever enters.
 pub(crate) const TOKEN_VAR: &str = "CLAUDE_CODE_OAUTH_TOKEN";
 const CHILD_PATH: &str = "/usr/local/bin:/usr/bin:/bin";
 
@@ -62,13 +62,18 @@ pub(crate) struct PromptFile(PathBuf);
 static PROMPT_FILES: AtomicU64 = AtomicU64::new(0);
 
 impl PromptFile {
-    pub(crate) fn create(instructions: &str) -> io::Result<Self> {
+    pub(crate) fn create(dir: &Path, instructions: &str) -> io::Result<Self> {
         let name = format!(
             "claude-connector-prompt-{}-{}",
             std::process::id(),
             PROMPT_FILES.fetch_add(1, Ordering::Relaxed)
         );
-        let path = std::env::temp_dir().join(name);
+        Self::create_at(dir.join(name), instructions)
+    }
+
+    /// Creates `path` and nothing that already exists there: a file, a link
+    /// (dangling or not) or a directory at the name fails, and is left alone.
+    fn create_at(path: PathBuf, instructions: &str) -> io::Result<Self> {
         let mut options = OpenOptions::new();
         options.write(true).create_new(true);
         #[cfg(unix)]
@@ -146,3 +151,57 @@ pub(crate) fn argv(
     }
     argv
 }
+
+#[cfg(all(test, unix))]
+mod tests {
+    use std::os::unix::fs::{PermissionsExt, symlink};
+
+    use super::*;
+
+    fn scratch(name: &str) -> PathBuf {
+        let dir = std::env::temp_dir().join(format!(
+            "claude-connector-launch-{name}-{}",
+            std::process::id()
+        ));
+        let _ = std::fs::remove_dir_all(&dir);
+        std::fs::create_dir_all(&dir).unwrap();
+        dir
+    }
+
+    #[test]
+    fn a_prompt_is_created_private_and_removed_on_drop() {
+        let dir = scratch("private");
+        let prompt = PromptFile::create(&dir, "the prompt").unwrap();
+        let path = prompt.path().to_path_buf();
+        assert_eq!(path.parent(), Some(dir.as_path()));
+        assert_eq!(std::fs::read_to_string(&path).unwrap(), "the prompt");
+        let mode = std::fs::metadata(&path).unwrap().permissions().mode() & 0o777;
+        assert_eq!(mode, 0o600);
+        drop(prompt);
+        assert!(!path.exists());
+        std::fs::remove_dir_all(dir).unwrap();
+    }
+
+    #[test]
+    fn a_name_already_taken_is_never_written_or_removed() {
+        let dir = scratch("taken");
+        let victim = dir.join("victim");
+        std::fs::write(&victim, "keep").unwrap();
+
+        let file = dir.join("file");
+        std::fs::write(&file, "mine").unwrap();
+        let link = dir.join("link");
+        symlink(&victim, &link).unwrap();
+        let dangling = dir.join("dangling");
+        symlink(dir.join("nowhere"), &dangling).unwrap();
+
+        for taken in [&file, &link, &dangling] {
+            assert!(PromptFile::create_at(taken.clone(), "the prompt").is_err());
+        }
+        assert_eq!(std::fs::read_to_string(&victim).unwrap(), "keep");
+        assert_eq!(std::fs::read_to_string(&file).unwrap(), "mine");
+        assert!(!dir.join("nowhere").exists());
+        assert!(link.exists() && dangling.symlink_metadata().is_ok());
+        std::fs::remove_dir_all(dir).unwrap();
+    }
+}
diff --git a/crates/claude-connector/src/lib.rs b/crates/claude-connector/src/lib.rs
index 28ad7f5..ef3e404 100644
--- a/crates/claude-connector/src/lib.rs
+++ b/crates/claude-connector/src/lib.rs
@@ -5,13 +5,17 @@
 
 mod backend;
 mod config;
+mod daemon;
 mod handle;
 mod launch;
 mod process;
 mod stream;
 
-pub use backend::{ClaudeCodeBackend, StderrSink};
+pub use backend::{ClaudeCodeBackend, CredentialTokenSource, StderrSink, TokenSource};
 pub use config::{ClaudeCodeConfig, ClaudeCodeLimits};
+pub use daemon::{ClaudeConnectorConfig, ConfigError, check_ready, load_config, write_config};
+#[cfg(unix)]
+pub use daemon::{check_prompt_dir, serve_config};
 pub use handle::{FsSessionStore, SessionStore};
 pub use launch::ChildEnv;
 #[cfg(unix)]
diff --git a/crates/claude-connector/src/main.rs b/crates/claude-connector/src/main.rs
new file mode 100644
index 0000000..ece39cc
--- /dev/null
+++ b/crates/claude-connector/src/main.rs
@@ -0,0 +1,241 @@
+//! The claude-connector binary.
+//!
+//!   claude-connector serve --config PATH.cc
+//!   claude-connector config init PATH.cc --flag value ...
+//!   claude-connector config check PATH.cc
+//!
+//! `serve` takes its configuration from the typed document alone. The flags of
+//! `config init` exist only to author that document. No message here echoes a
+//! value that was typed or stored; refusals name a field or a flag.
+
+use std::collections::BTreeMap;
+use std::path::PathBuf;
+use std::process::ExitCode;
+
+use claude_connector::{
+    ClaudeConnectorConfig, ConfigError, check_ready, load_config, write_config,
+};
+use model_connector::{CallerConfig, ModelPrice, Rates, frame_bytes_for_payload};
+
+fn main() -> ExitCode {
+    let args: Vec<String> = std::env::args().skip(1).collect();
+    let args: Vec<&str> = args.iter().map(String::as_str).collect();
+    let result = match args.as_slice() {
+        ["serve", "--config", path] => serve(path),
+        ["config", "init", path, flags @ ..] => init(path, flags),
+        ["config", "check", path] => check(path),
+        _ => Err(Failure::Usage),
+    };
+    match result {
+        Ok(()) => ExitCode::SUCCESS,
+        Err(failure) => {
+            eprintln!("claude-connector: {failure}");
+            ExitCode::FAILURE
+        }
+    }
+}
+
+#[derive(Debug, thiserror::Error)]
+enum Failure {
+    #[error("usage: serve --config PATH | config init PATH --flag value ... | config check PATH")]
+    Usage,
+    #[error(transparent)]
+    Config(#[from] ConfigError),
+    #[error("flag {0} is missing")]
+    Missing(&'static str),
+    #[error("flag {0} is malformed")]
+    Malformed(&'static str),
+    #[error("a flag is unknown, repeated or has no value")]
+    Flags,
+}
+
+#[cfg(unix)]
+fn serve(path: &str) -> Result<(), Failure> {
+    Ok(claude_connector::serve_config(path.as_ref())?)
+}
+
+#[cfg(not(unix))]
+fn serve(_: &str) -> Result<(), Failure> {
+    Err(Failure::Usage)
+}
+
+fn check(path: &str) -> Result<(), Failure> {
+    let config = load_config(path.as_ref())?;
+    check_ready(&config)?;
+    println!("claude connector configuration is valid and ready to serve");
+    println!("callers: {}", config.callers.len());
+    println!("allowed_models: {}", config.callers[0].allowed_models.len());
+    println!("prices: {}", config.prices.len());
+    Ok(())
+}
+
+/// The flags of `config init`, each given at most once except the two lists.
+struct Flags {
+    single: BTreeMap<&'static str, String>,
+    models: Vec<String>,
+    prices: Vec<String>,
+}
+
+const SINGLE: [&str; 22] = [
+    "--listen",
+    "--replay-store",
+    "--claude-binary",
+    "--claude-sha256",
+    "--work-dir",
+    "--config-dir",
+    "--prompt-dir",
+    "--instance",
+    "--token-credential",
+    "--caller-id",
+    "--connection-key-file",
+    "--connection-key-epoch",
+    "--spend-ceiling-micro-usd",
+    "--spend-window-ms",
+    "--deadline-ms",
+    "--max-line-bytes",
+    "--max-total-bytes",
+    "--stderr-tail-bytes",
+    "--max-concurrent-requests",
+    "--max-payload-bytes",
+    "--max-output-tokens",
+    "--socket-timeout-ms",
+];
+
+impl Flags {
+    fn parse(args: &[&str]) -> Result<Self, Failure> {
+        let mut flags = Flags {
+            single: BTreeMap::new(),
+            models: Vec::new(),
+            prices: Vec::new(),
+        };
+        let mut args = args.iter();
+        while let Some(&name) = args.next() {
+            let Some(&value) = args.next() else {
+                return Err(Failure::Flags);
+            };
+            match name {
+                "--allowed-model" => flags.models.push(value.to_string()),
+                "--price" => flags.prices.push(value.to_string()),
+                _ => {
+                    let Some(known) = SINGLE.iter().find(|known| **known == name) else {
+                        return Err(Failure::Flags);
+                    };
+                    if flags.single.insert(*known, value.to_string()).is_some() {
+                        return Err(Failure::Flags);
+                    }
+                }
+            }
+        }
+        Ok(flags)
+    }
+
+    fn text(&self, name: &'static str) -> Result<String, Failure> {
+        self.single.get(name).cloned().ok_or(Failure::Missing(name))
+    }
+
+    fn path(&self, name: &'static str) -> Result<PathBuf, Failure> {
+        self.text(name).map(PathBuf::from)
+    }
+
+    fn number<T: std::str::FromStr>(&self, name: &'static str) -> Result<T, Failure> {
+        self.text(name)?
+            .parse()
+            .map_err(|_| Failure::Malformed(name))
+    }
+
+    fn number_or<T: std::str::FromStr>(
+        &self,
+        name: &'static str,
+        default: T,
+    ) -> Result<T, Failure> {
+        match self.single.get(name) {
+            None => Ok(default),
+            Some(text) => text.parse().map_err(|_| Failure::Malformed(name)),
+        }
+    }
+}
+
+fn sha256(hex: &str) -> Option<[u8; 32]> {
+    if hex.len() != 64 || !hex.is_ascii() {
+        return None;
+    }
+    let mut digest = [0_u8; 32];
+    for (byte, pair) in digest.iter_mut().zip(hex.as_bytes().chunks(2)) {
+        *byte = u8::from_str_radix(std::str::from_utf8(pair).ok()?, 16).ok()?;
+    }
+    Some(digest)
+}
+
+/// MODEL:INPUT:OUTPUT:CACHE_WRITE:CACHE_READ:OVERHEAD, rates in micro-USD per
+/// million tokens. The one cache-write rate prices both retentions.
+fn price(text: &str) -> Option<(String, ModelPrice)> {
+    let mut parts = text.split(':');
+    let model = parts.next()?.to_string();
+    let mut next = || parts.next()?.parse::<u64>().ok();
+    let (input, output, cache_write, cache_read, overhead_tokens) =
+        (next()?, next()?, next()?, next()?, next()?);
+    if parts.next().is_some() {
+        return None;
+    }
+    Some((
+        model,
+        ModelPrice {
+            rates: Rates {
+                input,
+                cache_write_short: cache_write,
+                cache_write_long: cache_write,
+                cache_read,
+                output,
+            },
+            long_context: None,
+            overhead_tokens,
+        },
+    ))
+}
+
+fn init(path: &str, args: &[&str]) -> Result<(), Failure> {
+    let flags = Flags::parse(args)?;
+    let max_payload_bytes: usize = flags.number_or("--max-payload-bytes", 1 << 20)?;
+    let max_concurrent_requests: usize = flags.number_or("--max-concurrent-requests", 4)?;
+    let mut prices = BTreeMap::new();
+    for text in &flags.prices {
+        let (model, price) = price(text).ok_or(Failure::Malformed("--price"))?;
+        if prices.insert(model, price).is_some() {
+            return Err(Failure::Malformed("--price"));
+        }
+    }
+    let config = ClaudeConnectorConfig {
+        listen: flags.text("--listen")?,
+        replay_store: flags.path("--replay-store")?,
+        claude_binary: flags.path("--claude-binary")?,
+        claude_binary_sha256: sha256(&flags.text("--claude-sha256")?)
+            .ok_or(Failure::Malformed("--claude-sha256"))?,
+        work_dir: flags.path("--work-dir")?,
+        config_dir: flags.path("--config-dir")?,
+        prompt_dir: flags.path("--prompt-dir")?,
+        instance: flags.text("--instance")?,
+        token_credential: flags.text("--token-credential")?,
+        deadline_ms: flags.number_or("--deadline-ms", 600_000)?,
+        max_line_bytes: flags.number_or("--max-line-bytes", 4 << 20)?,
+        max_total_bytes: flags.number_or("--max-total-bytes", 64 << 20)?,
+        stderr_tail_bytes: flags.number_or("--stderr-tail-bytes", 8192)?,
+        max_frame_bytes: frame_bytes_for_payload(max_payload_bytes),
+        max_connections: max_concurrent_requests.max(8),
+        socket_timeout_ms: flags.number_or("--socket-timeout-ms", 10_000)?,
+        max_expiry_skew_ms: 300_000,
+        callers: vec![CallerConfig {
+            caller_runtime_id: flags.text("--caller-id")?,
+            connection_key_file: flags.path("--connection-key-file")?,
+            connection_key_epoch: flags.number("--connection-key-epoch")?,
+            allowed_models: flags.models.clone(),
+            max_concurrent_requests,
+            max_payload_bytes,
+            max_output_tokens: flags.number_or("--max-output-tokens", 32_768)?,
+            spend_ceiling_micro_usd: flags.number("--spend-ceiling-micro-usd")?,
+            spend_window_ms: flags.number("--spend-window-ms")?,
+        }],
+        prices,
+    };
+    write_config(path.as_ref(), &config)?;
+    Ok(())
+}
diff --git a/crates/claude-connector/src/stream.rs b/crates/claude-connector/src/stream.rs
index 7bb336b..e99935f 100644
--- a/crates/claude-connector/src/stream.rs
+++ b/crates/claude-connector/src/stream.rs
@@ -8,7 +8,8 @@ use serde::Deserialize;
 use serde_json::Value;
 
 /// A call that cannot complete, with the stage that failed. `usage` is `None`
-/// unless a result event reported it.
+/// (the kit bills its reservation) unless a result event reported it or the
+/// call never reached the provider (zero).
 #[derive(Debug, PartialEq, Eq)]
 pub(crate) struct Failure {
     pub kind: FailureKind,
@@ -17,13 +18,24 @@ pub(crate) struct Failure {
 }
 
 impl Failure {
-    pub(crate) fn unbilled(kind: FailureKind, message: &'static str) -> Self {
+    /// A failure after the child ran: what the provider billed is unknown, so
+    /// the kit bills its reservation.
+    pub(crate) fn cost_unknown(kind: FailureKind, message: &'static str) -> Self {
         Self {
             kind,
             message,
             usage: None,
         }
     }
+
+    /// A failure before anything was sent to the provider: it bills nothing.
+    pub(crate) fn never_reached(kind: FailureKind, message: &'static str) -> Self {
+        Self {
+            kind,
+            message,
+            usage: Some(sum_usage(std::iter::empty())),
+        }
+    }
 }
 
 /// What the stream said, once its result event arrived.
@@ -181,7 +193,7 @@ impl<'a> StreamState<'a> {
         let door_refusal = !success
             && matches!(
                 line.api_error_status.as_ref().and_then(Value::as_u64),
-                Some(401 | 403)
+                Some(401 | 403 | 429)
             );
         let usage = read_usage(line.model_usage_since_last_result, door_refusal);
         if !success {
@@ -252,13 +264,13 @@ impl<'a> StreamState<'a> {
 }
 
 fn fail(kind: FailureKind, message: &'static str) -> Step {
-    Step::Done(Box::new(Err(Failure::unbilled(kind, message))))
+    Step::Done(Box::new(Err(Failure::cost_unknown(kind, message))))
 }
 
 /// What this turn consumed, summed over every model it used (harness side
 /// calls included), or `None` when the result does not say: the field absent,
 /// of another shape, or empty. An empty report is believed only for a refusal
-/// at the door (401 or 403), which the provider cannot have billed; elsewhere
+/// at the door (401, 403 or 429), which the provider cannot have billed; elsewhere
 /// the CLI also zeroes usage on crashes after real work. `None` is billed the
 /// reservation by the kit.
 fn read_usage(reported: Option<Value>, door_refusal: bool) -> Option<Usage> {
diff --git a/crates/claude-connector/tests/config_document.rs b/crates/claude-connector/tests/config_document.rs
new file mode 100644
index 0000000..4bc35d4
--- /dev/null
+++ b/crates/claude-connector/tests/config_document.rs
@@ -0,0 +1,902 @@
+//! The claude-connector's configuration document and binary: what the
+//! document accepts, what it refuses, and that no refusal or output echoes a
+//! byte of a value that was typed, stored or read from a credential.
+
+use std::collections::BTreeMap;
+use std::path::{Path, PathBuf};
+use std::process::{Command, Output};
+use std::sync::atomic::{AtomicUsize, Ordering};
+
+use claude_connector::{ClaudeConnectorConfig, ConfigError, load_config, write_config};
+use cultcache_rs::{
+    CacheBackingStore, CultCache, CultCacheEnvelope, DatabaseEntry,
+    SingleFileMessagePackBackingStore,
+};
+use model_connector::{CallerConfig, MIN_PAYLOAD_BYTES, ModelPrice, Rates};
+
+const CANARY: &str = "canary-9f3b7c1d";
+const OPUS: &str = "claude-opus-5-1";
+const HAIKU: &str = "claude-haiku-5-5";
+
+/// An edit that should make the valid document refuse.
+type Change = Box<dyn Fn(&mut ClaudeConnectorConfig)>;
+
+static SEQUENCE: AtomicUsize = AtomicUsize::new(0);
+
+struct Scratch(PathBuf);
+
+impl Scratch {
+    fn new() -> Self {
+        let dir = std::env::temp_dir().join(format!(
+            "claude-connector-config-{}-{}",
+            std::process::id(),
+            SEQUENCE.fetch_add(1, Ordering::SeqCst)
+        ));
+        std::fs::create_dir_all(&dir).unwrap();
+        Self(dir)
+    }
+    fn path(&self, name: &str) -> PathBuf {
+        self.0.join(name)
+    }
+}
+
+impl Drop for Scratch {
+    fn drop(&mut self) {
+        let _ = std::fs::remove_dir_all(&self.0);
+    }
+}
+
+fn price() -> ModelPrice {
+    ModelPrice {
+        rates: Rates {
+            input: 3_000_000,
+            cache_write_short: 3_750_000,
+            cache_write_long: 3_750_000,
+            cache_read: 300_000,
+            output: 15_000_000,
+        },
+        long_context: None,
+        overhead_tokens: 100,
+    }
+}
+
+fn valid() -> ClaudeConnectorConfig {
+    ClaudeConnectorConfig {
+        listen: "127.0.0.1:7443".to_string(),
+        replay_store: "/var/lib/claude-connector/replay.cc".into(),
+        claude_binary: "/opt/claude/bin/claude".into(),
+        claude_binary_sha256: [7; 32],
+        work_dir: "/var/lib/claude-connector/work".into(),
+        config_dir: "/var/lib/claude-connector/home".into(),
+        prompt_dir: "/var/lib/claude-connector/prompts".into(),
+        instance: "persona-a".to_string(),
+        token_credential: "claude-token".to_string(),
+        deadline_ms: 600_000,
+        max_line_bytes: 4 << 20,
+        max_total_bytes: 64 << 20,
+        stderr_tail_bytes: 8192,
+        max_frame_bytes: 2 << 20,
+        max_connections: 8,
+        socket_timeout_ms: 10_000,
+        max_expiry_skew_ms: 300_000,
+        callers: vec![CallerConfig {
+            caller_runtime_id: "persona-a-caller".to_string(),
+            connection_key_file: "/etc/claude-connector/caller.key".into(),
+            connection_key_epoch: 1,
+            allowed_models: vec![HAIKU.to_string(), OPUS.to_string()],
+            max_concurrent_requests: 4,
+            max_payload_bytes: 1 << 20,
+            max_output_tokens: 32_768,
+            spend_ceiling_micro_usd: 50_000_000,
+            spend_window_ms: 3_600_000,
+        }],
+        prices: BTreeMap::from([(HAIKU.to_string(), price()), (OPUS.to_string(), price())]),
+    }
+}
+
+fn refused(config: &ClaudeConnectorConfig) -> &'static str {
+    match config.validate() {
+        Err(ConfigError::Invalid(field)) => field,
+        other => panic!("expected a refusal naming a field, got {other:?}"),
+    }
+}
+
+#[test]
+fn config_round_trips_through_cultcache() {
+    let scratch = Scratch::new();
+    let path = scratch.path("connector.cc");
+    let config = valid();
+    write_config(&path, &config).unwrap();
+    assert_eq!(load_config(&path).unwrap(), config);
+}
+
+#[test]
+fn a_second_record_or_the_wrong_type_is_refused() {
+    let scratch = Scratch::new();
+    let path = scratch.path("two.cc");
+    let config = valid();
+    write_config(&path, &config).unwrap();
+    let mut cache = CultCache::new();
+    cache
+        .register_entry_type::<ClaudeConnectorConfig>()
+        .unwrap();
+    cache
+        .add_generic_backing_store(SingleFileMessagePackBackingStore::new(&path))
+        .unwrap();
+    cache.pull_all_backing_stores().unwrap();
+    cache.put("extra", &config).unwrap();
+    assert!(matches!(load_config(&path), Err(ConfigError::Store)));
+
+    #[derive(Clone, Debug, PartialEq, Eq, DatabaseEntry)]
+    #[cultcache(type = "gamecult.other.document.v1")]
+    struct Other {
+        #[cultcache(key = 0)]
+        value: u32,
+    }
+    let other = scratch.path("other.cc");
+    let mut cache = CultCache::new();
+    cache.register_entry_type::<Other>().unwrap();
+    cache
+        .add_generic_backing_store(SingleFileMessagePackBackingStore::new(&other))
+        .unwrap();
+    cache.pull_all_backing_stores().unwrap();
+    cache.put("runtime", &Other { value: 1 }).unwrap();
+    assert!(matches!(load_config(&other), Err(ConfigError::Store)));
+
+    assert!(matches!(
+        load_config(&scratch.path("absent.cc")),
+        Err(ConfigError::Store)
+    ));
+}
+
+fn store_raw(path: &Path, key: &str, ty: &str, schema: Option<&str>, payload: Vec<u8>) {
+    let mut store = SingleFileMessagePackBackingStore::new(path);
+    store
+        .push(&CultCacheEnvelope {
+            key: key.to_string(),
+            r#type: ty.to_string(),
+            payload,
+            stored_at: "2026-10-29T00:00:00Z".to_string(),
+            schema_id: schema.map(str::to_string),
+        })
+        .unwrap();
+}
+
+#[test]
+fn the_envelope_must_be_exactly_the_documents_key_type_schema_and_encoding() {
+    let scratch = Scratch::new();
+    let canonical = rmp_serde::to_vec(&valid()).unwrap();
+    let ty = ClaudeConnectorConfig::TYPE;
+    let mut trailing = canonical.clone();
+    trailing.push(0xc0);
+
+    let cases = vec![
+        ("wrong key", "other", ty, Some(ty), canonical.clone()),
+        (
+            "wrong type",
+            "runtime",
+            "gamecult.other.v1",
+            Some(ty),
+            canonical.clone(),
+        ),
+        (
+            "wrong schema",
+            "runtime",
+            ty,
+            Some("gamecult.other.v1"),
+            canonical.clone(),
+        ),
+        ("not canonical", "runtime", ty, Some(ty), trailing),
+    ];
+    for (name, key, ty, schema, payload) in cases {
+        let path = scratch.path(&format!("{}.cc", name.replace(' ', "-")));
+        store_raw(&path, key, ty, schema, payload);
+        assert!(
+            matches!(load_config(&path), Err(ConfigError::Store)),
+            "{name}"
+        );
+    }
+
+    // The same bytes with every part right load.
+    let path = scratch.path("right.cc");
+    store_raw(&path, "runtime", ty, Some(ty), canonical);
+    assert_eq!(load_config(&path).unwrap(), valid());
+}
+
+#[test]
+fn bounds_sit_exactly_where_the_connector_needs_them() {
+    let mut config = valid();
+    config.max_total_bytes = config.max_line_bytes;
+    config.validate().unwrap();
+    config.callers[0].max_payload_bytes = MIN_PAYLOAD_BYTES;
+    config.validate().unwrap();
+    config.callers[0].max_payload_bytes = MIN_PAYLOAD_BYTES - 1;
+    assert_eq!(refused(&config), "caller bounds");
+
+    for set in [
+        (|c: &mut ClaudeConnectorConfig| c.max_connections = 0) as fn(&mut ClaudeConnectorConfig),
+        |c| c.socket_timeout_ms = 0,
+        |c| c.max_expiry_skew_ms = 0,
+        |c| c.max_frame_bytes = 0,
+    ] {
+        let mut config = valid();
+        set(&mut config);
+        assert_eq!(refused(&config), "serve limits");
+    }
+    let mut config = valid();
+    config.prices.insert("claude-".to_string(), price());
+    assert_eq!(refused(&config), "prices");
+}
+
+#[test]
+fn a_write_refuses_an_invalid_document_and_leaves_no_file() {
+    let scratch = Scratch::new();
+    let path = scratch.path("bad.cc");
+    let mut config = valid();
+    config.callers[0].allowed_models = vec!["opus".to_string()];
+    assert!(write_config(&path, &config).is_err());
+    assert!(!path.exists());
+}
+
+#[test]
+fn config_refuses_aliases_unpriced_models_and_second_callers() {
+    let valid_config = valid();
+    assert_eq!(valid_config.validate().unwrap().port(), 7443);
+
+    let cases: Vec<(&str, Change)> = vec![
+        (
+            "alias allowed",
+            Box::new(|c| {
+                c.callers[0].allowed_models = vec!["opus".to_string()];
+            }),
+        ),
+        (
+            "alias price key",
+            Box::new(|c| {
+                c.prices.insert("sonnet".to_string(), price());
+            }),
+        ),
+        (
+            "bare prefix",
+            Box::new(|c| {
+                c.callers[0].allowed_models = vec!["claude-".to_string()];
+            }),
+        ),
+        (
+            "family only",
+            Box::new(|c| {
+                c.callers[0].allowed_models = vec!["claude-haiku".to_string()];
+                c.prices.insert("claude-haiku".to_string(), price());
+            }),
+        ),
+        (
+            "latest alias",
+            Box::new(|c| {
+                c.callers[0].allowed_models = vec!["claude-sonnet-latest".to_string()];
+                c.prices.insert("claude-sonnet-latest".to_string(), price());
+            }),
+        ),
+        (
+            "no minor",
+            Box::new(|c| {
+                c.callers[0].allowed_models = vec!["claude-opus-5".to_string()];
+                c.prices.insert("claude-opus-5".to_string(), price());
+            }),
+        ),
+        (
+            "uppercase family",
+            Box::new(|c| {
+                c.callers[0].allowed_models = vec!["claude-Opus-5-1".to_string()];
+                c.prices.insert("claude-Opus-5-1".to_string(), price());
+            }),
+        ),
+        (
+            "short date",
+            Box::new(|c| {
+                c.callers[0].allowed_models = vec!["claude-opus-5-1-2026".to_string()];
+                c.prices.insert("claude-opus-5-1-2026".to_string(), price());
+            }),
+        ),
+        (
+            "trailing part",
+            Box::new(|c| {
+                c.callers[0].allowed_models = vec!["claude-opus-5-1-20260101-x".to_string()];
+                c.prices
+                    .insert("claude-opus-5-1-20260101-x".to_string(), price());
+            }),
+        ),
+        (
+            "unpriced allowed",
+            Box::new(|c| {
+                c.prices.remove(OPUS);
+            }),
+        ),
+        (
+            "duplicate allowed",
+            Box::new(|c| {
+                c.callers[0].allowed_models = vec![HAIKU.to_string(), HAIKU.to_string()];
+            }),
+        ),
+        (
+            "no allowed",
+            Box::new(|c| c.callers[0].allowed_models.clear()),
+        ),
+        (
+            "second caller",
+            Box::new(|c| {
+                let mut second = c.callers[0].clone();
+                second.caller_runtime_id = "other".to_string();
+                c.callers.push(second);
+            }),
+        ),
+        ("no caller", Box::new(|c| c.callers.clear())),
+    ];
+    let fields = [
+        "allowed_models",
+        "prices",
+        "allowed_models",
+        "allowed_models",
+        "allowed_models",
+        "allowed_models",
+        "allowed_models",
+        "allowed_models",
+        "allowed_models",
+        "allowed_models",
+        "allowed_models",
+        "allowed_models",
+        "callers",
+        "callers",
+    ];
+    for ((name, change), field) in cases.iter().zip(fields) {
+        let mut config = valid();
+        change(&mut config);
+        assert_eq!(refused(&config), field, "{name}");
+    }
+}
+
+#[test]
+fn a_dated_canonical_id_is_allowed() {
+    let dated = "claude-opus-5-1-20260101".to_string();
+    let mut config = valid();
+    config.callers[0].allowed_models = vec![dated.clone()];
+    config.prices.insert(dated, price());
+    config.validate().unwrap();
+}
+
+#[test]
+fn a_price_with_a_zero_billed_component_is_refused() {
+    type Zero = fn(&mut Rates);
+    let zeroed: Vec<(&str, Zero)> = vec![
+        ("input", |r| r.input = 0),
+        ("cache_write_short", |r| r.cache_write_short = 0),
+        ("cache_write_long", |r| r.cache_write_long = 0),
+        ("cache_read", |r| r.cache_read = 0),
+        ("output", |r| r.output = 0),
+    ];
+    for (name, zero) in &zeroed {
+        let mut config = valid();
+        zero(&mut config.prices.get_mut(HAIKU).unwrap().rates);
+        assert_eq!(refused(&config), "prices", "{name}");
+
+        let mut config = valid();
+        let mut long = price().rates;
+        zero(&mut long);
+        config.prices.get_mut(HAIKU).unwrap().long_context = Some(model_connector::LongContext {
+            above_prompt_tokens: 200_000,
+            rates: long,
+        });
+        assert_eq!(refused(&config), "prices", "long context {name}");
+    }
+    // A zero overhead is not a rate.
+    let mut config = valid();
+    config.prices.get_mut(HAIKU).unwrap().overhead_tokens = 0;
+    config.validate().unwrap();
+}
+
+#[test]
+fn config_refuses_exposed_listeners_relative_paths_and_empty_bounds() {
+    let cases: Vec<(&str, &str, Change)> = vec![
+        (
+            "wildcard listen",
+            "listen",
+            Box::new(|c| c.listen = "0.0.0.0:7443".into()),
+        ),
+        (
+            "lan listen",
+            "listen",
+            Box::new(|c| c.listen = "192.168.1.9:7443".into()),
+        ),
+        (
+            "hostname listen",
+            "listen",
+            Box::new(|c| c.listen = "localhost:7443".into()),
+        ),
+        (
+            "replay",
+            "replay_store",
+            Box::new(|c| c.replay_store = "replay.cc".into()),
+        ),
+        (
+            "binary",
+            "claude_binary",
+            Box::new(|c| c.claude_binary = "claude".into()),
+        ),
+        ("work", "work_dir", Box::new(|c| c.work_dir = "work".into())),
+        (
+            "config",
+            "config_dir",
+            Box::new(|c| c.config_dir = "cfg".into()),
+        ),
+        (
+            "prompts",
+            "prompt_dir",
+            Box::new(|c| c.prompt_dir = "prompts".into()),
+        ),
+        (
+            "key file",
+            "connection_key_file",
+            Box::new(|c| {
+                c.callers[0].connection_key_file = "caller.key".into();
+            }),
+        ),
+        (
+            "zero pin",
+            "claude_binary_sha256",
+            Box::new(|c| c.claude_binary_sha256 = [0; 32]),
+        ),
+        (
+            "blank instance",
+            "instance",
+            Box::new(|c| c.instance = " ".into()),
+        ),
+        (
+            "credential path",
+            "token_credential",
+            Box::new(|c| c.token_credential = "a/b".into()),
+        ),
+        (
+            "credential parent",
+            "token_credential",
+            Box::new(|c| c.token_credential = "..".into()),
+        ),
+        (
+            "credential empty",
+            "token_credential",
+            Box::new(|c| c.token_credential = String::new()),
+        ),
+        (
+            "zero deadline",
+            "child limits",
+            Box::new(|c| c.deadline_ms = 0),
+        ),
+        (
+            "total below line",
+            "child limits",
+            Box::new(|c| c.max_total_bytes = c.max_line_bytes - 1),
+        ),
+        (
+            "zero frame",
+            "serve limits",
+            Box::new(|c| c.max_frame_bytes = 0),
+        ),
+        (
+            "zero epoch",
+            "connection_key_epoch",
+            Box::new(|c| c.callers[0].connection_key_epoch = 0),
+        ),
+        (
+            "tiny payload",
+            "caller bounds",
+            Box::new(|c| c.callers[0].max_payload_bytes = 16),
+        ),
+        (
+            "no ceiling",
+            "caller bounds",
+            Box::new(|c| c.callers[0].spend_ceiling_micro_usd = 0),
+        ),
+    ];
+    for (name, field, change) in cases {
+        let mut config = valid();
+        change(&mut config);
+        assert_eq!(refused(&config), field, "{name}");
+    }
+}
+
+#[test]
+fn no_refusal_echoes_a_value() {
+    let canary_path: PathBuf = format!("{CANARY}/relative").into();
+    let canaried: Vec<Change> = vec![
+        Box::new(|c| c.listen = format!("{CANARY}:1")),
+        Box::new(|c| c.instance = format!(" {CANARY} ")),
+        Box::new(|c| c.token_credential = format!("{CANARY}/x")),
+        Box::new(|c| c.callers[0].allowed_models = vec![format!("{CANARY}-model")]),
+        Box::new(|c| {
+            c.prices.insert(format!("{CANARY}-alias"), price());
+        }),
+        Box::new(|c| c.callers[0].caller_runtime_id = format!(" {CANARY}")),
+        Box::new({
+            let path = canary_path.clone();
+            move |c| c.replay_store = path.clone()
+        }),
+        Box::new({
+            let path = canary_path.clone();
+            move |c| c.callers[0].connection_key_file = path.clone()
+        }),
+        Box::new(|c| {
+            let mut second = c.callers[0].clone();
+            second.caller_runtime_id = CANARY.to_string();
+            c.callers.push(second);
+        }),
+    ];
+    for change in canaried {
+        let mut config = valid();
+        change(&mut config);
+        let error = config.validate().unwrap_err();
+        for shown in [error.to_string(), format!("{error:?}")] {
+            assert!(!shown.contains(CANARY), "{shown}");
+        }
+        let scratch = Scratch::new();
+        let write = write_config(&scratch.path("x.cc"), &config).unwrap_err();
+        assert!(!format!("{write} {write:?}").contains(CANARY));
+    }
+}
+
+// ---- the binary ----
+
+fn run(args: &[&str], envs: &[(&str, &Path)]) -> Output {
+    let mut command = Command::new(env!("CARGO_BIN_EXE_claude-connector"));
+    command.args(args).env_clear();
+    for (name, value) in envs {
+        command.env(name, value);
+    }
+    command.output().unwrap()
+}
+
+fn shown(output: &Output) -> String {
+    format!(
+        "{}{}",
+        String::from_utf8_lossy(&output.stdout),
+        String::from_utf8_lossy(&output.stderr)
+    )
+}
+
+fn init_flags<'a>(scratch: &'a Scratch, extra: &[&'a str]) -> Vec<String> {
+    let path = |name: &str| scratch.path(name).to_string_lossy().into_owned();
+    let mut flags: Vec<String> = [
+        "--listen",
+        "127.0.0.1:0",
+        "--replay-store",
+        &path("replay.cc"),
+        "--claude-binary",
+        &path("claude"),
+        "--claude-sha256",
+        &"ab".repeat(32),
+        "--work-dir",
+        &path("work"),
+        "--config-dir",
+        &path("home"),
+        "--prompt-dir",
+        &path("prompts"),
+        "--instance",
+        "persona-a",
+        "--token-credential",
+        "claude-token",
+        "--caller-id",
+        "persona-a-caller",
+        "--connection-key-file",
+        &path("caller.key"),
+        "--connection-key-epoch",
+        "1",
+        "--spend-ceiling-micro-usd",
+        "50000000",
+        "--spend-window-ms",
+        "3600000",
+        "--allowed-model",
+        HAIKU,
+        "--price",
+        &format!("{HAIKU}:1000000:5000000:1250000:100000:100"),
+    ]
+    .map(String::from)
+    .to_vec();
+    flags.extend(extra.iter().map(|flag| flag.to_string()));
+    flags
+}
+
+fn init(scratch: &Scratch, flags: &[String]) -> Output {
+    let config = scratch.path("connector.cc");
+    let mut args = vec!["config", "init", config.to_str().unwrap()];
+    args.extend(flags.iter().map(String::as_str));
+    run(&args, &[])
+}
+
+#[test]
+fn config_init_writes_the_flags_into_the_document() {
+    let scratch = Scratch::new();
+    let made = init(&scratch, &init_flags(&scratch, &[]));
+    assert!(made.status.success(), "{}", shown(&made));
+    let config = load_config(&scratch.path("connector.cc")).unwrap();
+    assert_eq!(config.callers[0].allowed_models, [HAIKU]);
+    assert_eq!(config.prices[HAIKU].rates.output, 5_000_000);
+    assert_eq!(config.prices[HAIKU].rates.cache_write_long, 1_250_000);
+    assert_eq!(config.prices[HAIKU].overhead_tokens, 100);
+    assert_eq!(config.claude_binary_sha256, [0xab; 32]);
+}
+
+/// A scratch directory in which `serve` would start: every directory, the
+/// pinned binary, the connection key and the credential exist. Returns the
+/// `config init` flags and the credential directory.
+#[cfg(unix)]
+fn prepare(scratch: &Scratch) -> (Vec<String>, PathBuf) {
+    use sha2::{Digest, Sha256};
+    use std::os::unix::fs::PermissionsExt;
+
+    for name in ["work", "home", "prompts", "credentials"] {
+        std::fs::create_dir_all(scratch.path(name)).unwrap();
+    }
+    std::fs::set_permissions(
+        scratch.path("prompts"),
+        std::fs::Permissions::from_mode(0o700),
+    )
+    .unwrap();
+    std::fs::write(scratch.path("credentials/claude-token"), CANARY).unwrap();
+    std::fs::write(scratch.path("caller.key"), "caller-secret").unwrap();
+    std::fs::write(scratch.path("claude"), b"pinned claude").unwrap();
+    let sha: String = Sha256::digest(b"pinned claude")
+        .iter()
+        .map(|byte| format!("{byte:02x}"))
+        .collect();
+    let mut flags = init_flags(scratch, &[]);
+    let at = flags
+        .iter()
+        .position(|flag| flag == "--claude-sha256")
+        .unwrap();
+    flags[at + 1] = sha;
+    (flags, scratch.path("credentials"))
+}
+
+#[cfg(unix)]
+fn check_and_serve(config: &Path, credentials: Option<&Path>) -> (Output, Output) {
+    let envs: Vec<(&str, &Path)> = credentials
+        .map(|dir| ("CREDENTIALS_DIRECTORY", dir))
+        .into_iter()
+        .collect();
+    let path = config.to_str().unwrap();
+    (
+        run(&["config", "check", path], &envs),
+        run(&["serve", "--config", path], &envs),
+    )
+}
+
+#[cfg(unix)]
+#[test]
+fn check_passes_a_document_serve_can_start_and_reports_counts_only() {
+    let scratch = Scratch::new();
+    let (flags, credentials) = prepare(&scratch);
+    let made = init(&scratch, &flags);
+    assert!(made.status.success(), "{}", shown(&made));
+    let checked = run(
+        &[
+            "config",
+            "check",
+            scratch.path("connector.cc").to_str().unwrap(),
+        ],
+        &[("CREDENTIALS_DIRECTORY", &credentials)],
+    );
+    assert!(checked.status.success(), "{}", shown(&checked));
+    let text = shown(&checked);
+    assert!(text.contains("callers: 1") && text.contains("allowed_models: 1"));
+    assert!(text.contains("prices: 1"));
+    for secret in ["persona-a", HAIKU, CANARY, "caller-secret"] {
+        assert!(!text.contains(secret), "{text}");
+    }
+}
+
+#[cfg(unix)]
+#[test]
+fn check_refuses_exactly_what_serve_refuses_and_names_the_field() {
+    use std::os::unix::fs::PermissionsExt;
+
+    type Break = fn(&Scratch, &Path);
+    let cases: Vec<(&str, &str, Break)> = vec![
+        ("open prompt dir", "prompt directory", |s, _| {
+            std::fs::set_permissions(s.path("prompts"), std::fs::Permissions::from_mode(0o755))
+                .unwrap();
+        }),
+        ("no key file", "connection_key_file", |s, _| {
+            std::fs::remove_file(s.path("caller.key")).unwrap();
+        }),
+        ("binary off its pin", "claude_binary_sha256", |s, _| {
+            std::fs::write(s.path("claude"), b"other bytes").unwrap();
+        }),
+        ("no credential", "token_credential", |_, credentials| {
+            std::fs::remove_file(credentials.join("claude-token")).unwrap();
+        }),
+        ("no work dir", "work_dir", |s, _| {
+            std::fs::remove_dir(s.path("work")).unwrap();
+        }),
+        ("no config dir", "config_dir", |s, _| {
+            std::fs::remove_dir(s.path("home")).unwrap();
+        }),
+        ("frame too small", "max_frame_bytes", |s, _| {
+            let path = s.path("connector.cc");
+            let mut config = load_config(&path).unwrap();
+            config.max_frame_bytes = 4099;
+            write_config(&path, &config).unwrap();
+        }),
+        ("too few connections", "connection capacity", |s, _| {
+            let path = s.path("connector.cc");
+            let mut config = load_config(&path).unwrap();
+            config.max_connections = 1;
+            write_config(&path, &config).unwrap();
+        }),
+    ];
+    for (name, field, break_it) in cases {
+        let scratch = Scratch::new();
+        let (flags, credentials) = prepare(&scratch);
+        let made = init(&scratch, &flags);
+        assert!(made.status.success(), "{name}: {}", shown(&made));
+        break_it(&scratch, &credentials);
+        let (checked, served) = check_and_serve(&scratch.path("connector.cc"), Some(&credentials));
+        assert!(!checked.status.success(), "{name} check passed");
+        assert!(!served.status.success(), "{name} serve started");
+        assert!(
+            shown(&checked).contains(field),
+            "{name}: {}",
+            shown(&checked)
+        );
+        assert_eq!(shown(&checked), shown(&served), "{name}");
+        for secret in [CANARY, "caller-secret", "other bytes"] {
+            assert!(!shown(&checked).contains(secret), "{name}");
+        }
+    }
+
+    // No credential directory at all is the same refusal.
+    let scratch = Scratch::new();
+    let (flags, _) = prepare(&scratch);
+    assert!(init(&scratch, &flags).status.success());
+    let (checked, served) = check_and_serve(&scratch.path("connector.cc"), None);
+    assert!(!checked.status.success() && !served.status.success());
+    assert!(shown(&checked).contains("token_credential"));
+    assert_eq!(shown(&checked), shown(&served));
+}
+
+#[cfg(unix)]
+#[test]
+fn serve_names_a_corrupt_replay_store_without_its_path() {
+    let scratch = Scratch::new();
+    let (mut flags, credentials) = prepare(&scratch);
+    let store = scratch.path(&format!("{CANARY}-store/replay.cc"));
+    std::fs::create_dir_all(store.parent().unwrap()).unwrap();
+    std::fs::write(&store, b"not a cultcache store").unwrap();
+    let at = flags
+        .iter()
+        .position(|flag| flag == "--replay-store")
+        .unwrap();
+    flags[at + 1] = store.to_string_lossy().into_owned();
+    assert!(init(&scratch, &flags).status.success());
+    let (_, served) = check_and_serve(&scratch.path("connector.cc"), Some(&credentials));
+    assert!(!served.status.success());
+    let text = shown(&served);
+    assert!(text.contains("replay_store"), "{text}");
+    assert!(
+        !text.contains(CANARY) && !text.contains("replay.cc"),
+        "{text}"
+    );
+}
+
+#[test]
+fn the_binary_refuses_without_echoing_what_it_was_given() {
+    let spaced = format!(" {CANARY}");
+    // Each pair replaces the value of an existing flag; the others are appended.
+    let replaced: Vec<(&str, &str)> = vec![
+        ("--listen", CANARY),
+        ("--claude-sha256", CANARY),
+        ("--claude-sha256", "abcd"),
+        ("--price", CANARY),
+        ("--instance", &spaced),
+        ("--connection-key-epoch", CANARY),
+        ("--allowed-model", CANARY),
+    ];
+    let appended: Vec<Vec<&str>> = vec![
+        vec![CANARY, CANARY],
+        vec!["--listen"],
+        vec!["--listen", "127.0.0.1:1"],
+    ];
+    let mut attempts: Vec<Vec<String>> = Vec::new();
+    for (name, value) in replaced {
+        let scratch = Scratch::new();
+        let mut flags = init_flags(&scratch, &[]);
+        let at = flags.iter().position(|flag| flag == name).unwrap();
+        flags[at + 1] = value.to_string();
+        attempts.push(flags);
+    }
+    for extra in appended {
+        let scratch = Scratch::new();
+        attempts.push(init_flags(&scratch, &extra));
+    }
+    for flags in attempts {
+        let scratch = Scratch::new();
+        let made = init(&scratch, &flags);
+        assert!(!made.status.success(), "{flags:?}");
+        assert!(!shown(&made).contains(CANARY), "{}", shown(&made));
+        assert!(!scratch.path("connector.cc").exists());
+    }
+
+    // A missing flag is refused by name.
+    let scratch = Scratch::new();
+    let mut flags = init_flags(&scratch, &[]);
+    let at = flags.iter().position(|f| f == "--instance").unwrap();
+    flags.drain(at..at + 2);
+    let made = init(&scratch, &flags);
+    assert!(!made.status.success());
+    assert!(shown(&made).contains("--instance"));
+}
+
+#[cfg(unix)]
+#[test]
+fn serve_refuses_an_unsafe_prompt_dir_and_prints_no_credential() {
+    use std::os::unix::fs::{PermissionsExt, symlink};
+
+    let scratch = Scratch::new();
+    let credentials = scratch.path("credentials");
+    std::fs::create_dir_all(&credentials).unwrap();
+    std::fs::write(credentials.join("claude-token"), CANARY).unwrap();
+    std::fs::write(scratch.path("caller.key"), "caller-secret").unwrap();
+    let made = init(&scratch, &init_flags(&scratch, &[]));
+    assert!(made.status.success(), "{}", shown(&made));
+    let config = scratch.path("connector.cc");
+    let serve = |config: &Path| {
+        run(
+            &["serve", "--config", config.to_str().unwrap()],
+            &[("CREDENTIALS_DIRECTORY", &credentials)],
+        )
+    };
+
+    // No prompt directory, then one others can enter, then a link to a good one.
+    let prompts = scratch.path("prompts");
+    let refused = serve(&config);
+    assert!(!refused.status.success());
+    assert!(
+        shown(&refused).contains("prompt directory"),
+        "{}",
+        shown(&refused)
+    );
+    std::fs::create_dir_all(&prompts).unwrap();
+    std::fs::set_permissions(&prompts, std::fs::Permissions::from_mode(0o755)).unwrap();
+    let open = serve(&config);
+    assert!(
+        shown(&open).contains("prompt directory"),
+        "{}",
+        shown(&open)
+    );
+    assert!(claude_connector::check_prompt_dir(&prompts).is_err());
+    std::fs::set_permissions(&prompts, std::fs::Permissions::from_mode(0o700)).unwrap();
+    assert!(claude_connector::check_prompt_dir(&prompts).is_ok());
+    let linked = scratch.path("linked");
+    symlink(&prompts, &linked).unwrap();
+    assert!(claude_connector::check_prompt_dir(&linked).is_err());
+
+    // A safe directory but no claude binary: the backend is not ready, and
+    // nothing the credential holds is printed.
+    let not_ready = serve(&config);
+    assert!(!not_ready.status.success());
+    for output in [&refused, &open, &not_ready] {
+        let text = shown(output);
+        assert!(
+            !text.contains(CANARY) && !text.contains("caller-secret"),
+            "{text}"
+        );
+    }
+}
+
+#[cfg(unix)]
+#[test]
+fn a_private_regular_file_is_not_a_prompt_dir() {
+    use std::os::unix::fs::PermissionsExt;
+    let scratch = Scratch::new();
+    let file = scratch.path("file");
+    std::fs::write(&file, "x").unwrap();
+    std::fs::set_permissions(&file, std::fs::Permissions::from_mode(0o700)).unwrap();
+    assert!(claude_connector::check_prompt_dir(&file).is_err());
+    assert!(claude_connector::check_prompt_dir(&scratch.path("absent")).is_err());
+}
diff --git a/crates/model-connector/src/serve.rs b/crates/model-connector/src/serve.rs
index 6d87f6e..d6723a4 100644
--- a/crates/model-connector/src/serve.rs
+++ b/crates/model-connector/src/serve.rs
@@ -23,10 +23,11 @@ use crate::service::{
 /// The only provider seam of the connector kit. A connector binary implements
 /// it over one model provider; the kit names no provider.
 pub trait ProviderBackend: Send + Sync {
-    /// Whether the provider can take work. `serve` consults it once, before it
-    /// opens the replay store or accepts a connection; the request path does
-    /// not consult it.
-    fn ready(&self) -> bool;
+    /// Whether the provider can take work; `Err` names the backend field that
+    /// is not ready, never a value. `serve` consults it once, before it opens
+    /// the replay store or accepts a connection; the request path does not
+    /// consult it.
+    fn ready(&self) -> Result<(), &'static str>;
     /// The tag of the provider this backend fronts. The service owns it from
     /// `serve` on: it names the provider on every receipt and decides which
     /// reasoning state is foreign.
@@ -59,7 +60,7 @@ impl ServeLimits {
     /// largest whose sealed frame, at its worst-case encoding, fits the frame.
     /// An admitted payload therefore always fits, and `complete` is given the
     /// same frame bound to fit every result in.
-    fn validate(&self, admissions: &[CallerAdmission]) -> Result<(), ServeError> {
+    pub fn validate(&self, admissions: &[CallerAdmission]) -> Result<(), ServeError> {
         if self.max_frame_bytes == 0
             || self.max_frame_bytes > u32::MAX as usize
             || self.max_connections == 0
@@ -171,9 +172,7 @@ pub fn serve(
     {
         return Err(ServeError::InvalidConfig("listener address"));
     }
-    if !backend.ready() {
-        return Err(ServeError::BackendUnready);
-    }
+    backend.ready().map_err(ServeError::BackendUnready)?;
     let service = Arc::new(Mutex::new(TransportService::open(
         replay_store,
         backend.provider(),
@@ -322,8 +321,8 @@ pub enum ServeError {
     ConnectionKey(#[source] std::io::Error),
     #[error(transparent)]
     Service(#[from] ServiceError),
-    #[error("provider backend is not ready")]
-    BackendUnready,
+    #[error("provider backend is not ready: {0}")]
+    BackendUnready(&'static str),
     #[error("connector service lock was poisoned")]
     ServicePoisoned,
     #[error("connector listener failed")]
@@ -379,9 +378,9 @@ mod tests {
     }
 
     impl ProviderBackend for CountingBackend {
-        fn ready(&self) -> bool {
+        fn ready(&self) -> Result<(), &'static str> {
             self.ready_calls.fetch_add(1, Ordering::SeqCst);
-            self.ready
+            self.ready.then_some(()).ok_or("fixture")
         }
 
         fn provider(&self) -> &str {
@@ -710,7 +709,10 @@ mod tests {
             &store.path,
             backend.clone(),
         );
-        assert!(matches!(result, Some(Err(ServeError::BackendUnready))));
+        assert!(matches!(
+            result,
+            Some(Err(ServeError::BackendUnready("fixture")))
+        ));
         assert_eq!(backend.executions.load(Ordering::SeqCst), 0);
         assert!(
             !store.path.exists(),
@@ -1016,8 +1018,8 @@ mod tests {
     }
 
     impl ProviderBackend for SizedBackend {
-        fn ready(&self) -> bool {
-            true
+        fn ready(&self) -> Result<(), &'static str> {
+            Ok(())
         }
 
         fn provider(&self) -> &str {
diff --git a/crates/model-connector/src/service.rs b/crates/model-connector/src/service.rs
index 8d4fd80..6d10cb9 100644
--- a/crates/model-connector/src/service.rs
+++ b/crates/model-connector/src/service.rs
@@ -119,7 +119,7 @@ impl CallerAdmission {
 }
 
 /// What one million tokens of each class cost, in micro-USD.
-#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Serialize, Deserialize)]
 pub struct Rates {
     pub input: u64,
     pub cache_write_short: u64,
@@ -129,7 +129,7 @@ pub struct Rates {
 }
 
 /// The rates a prompt strictly above `above_prompt_tokens` is priced at.
-#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Serialize, Deserialize)]
 pub struct LongContext {
     pub above_prompt_tokens: u64,
     pub rates: Rates,
@@ -138,7 +138,7 @@ pub struct LongContext {
 /// One model's price. Explicit zero rates are admitted (a local model is
 /// free); a missing price is not. `overhead_tokens` is added to the byte
 /// bound of a request when its worst case is reserved.
-#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Serialize, Deserialize)]
 pub struct ModelPrice {
     pub rates: Rates,
     pub long_context: Option<LongContext>,
diff --git a/crates/model-connector/tests/reasoning_state.rs b/crates/model-connector/tests/reasoning_state.rs
index f42e0f1..8c9c4af 100644
--- a/crates/model-connector/tests/reasoning_state.rs
+++ b/crates/model-connector/tests/reasoning_state.rs
@@ -38,8 +38,8 @@ impl RecordingBackend {
 }
 
 impl ProviderBackend for RecordingBackend {
-    fn ready(&self) -> bool {
-        true
+    fn ready(&self) -> Result<(), &'static str> {
+        Ok(())
     }
 
     fn provider(&self) -> &str {
@@ -291,8 +291,8 @@ struct TamperingBackend {
 }
 
 impl ProviderBackend for TamperingBackend {
-    fn ready(&self) -> bool {
-        true
+    fn ready(&self) -> Result<(), &'static str> {
+        Ok(())
     }
 
     fn provider(&self) -> &str {