diff --git a/docs/architecture.md b/docs/architecture.md
index f816a14..12fbd4f 100644
--- a/docs/architecture.md
+++ b/docs/architecture.md
@@ -447,13 +447,12 @@ builder was not used.
   process, so N Heimdall processes can present one token N times after a
   transient failure. Needs a failed-attempt marker on the row, read under the
   lock. Must land before or with Idunn onboarding.
-- `joystick:follow_up:connect-start-needs-app-auth`: connect starts are
-  unauthenticated, so anyone can name `{creator, targetId}`; the callback
-  re-check proves a binding exists, not that the caller owns it. It also owns
-  the whitespace-only `targetId` that passes both gates and the callback's
-  fail-open `''` fallback for a vanished profile (finding
-  `joystick:finding:cut-h1.s4.whitespace-targetid-passes-custody-gate`). Must
-  land before the public Joystick bot or a second creator.
+- `joystick:follow_up:connect-start-needs-app-auth` (Heimdall side done): a
+  connect start that names a connection target needs the app's own secret for
+  every managed provider, and a blank target is refused at start and at the
+  callback mint. What StreamPixels must do on its side is in
+  `notes/streampixels-connect-redeem.md`; its redeem must refuse a creator
+  connect whose target it did not start.
 - `joystick:follow_up:heimdall-idunn-onboarding` (gamecult-ops): live Heimdall
   is a hand-copied release under `heimdall.service`, not an Idunn-admitted
   generation; the recipe's Joystick lines are declared but unproven by a
diff --git a/notes/streampixels-connect-redeem.md b/notes/streampixels-connect-redeem.md
new file mode 100644
index 0000000..83f57a7
--- /dev/null
+++ b/notes/streampixels-connect-redeem.md
@@ -0,0 +1,35 @@
+# For Kiko: what StreamPixels must do for a creator connect
+
+Heimdall's side is done (this PR). A connect start that carries a `connection`
+target is accepted only with StreamPixels' own `x-heimdall-app-secret`, so only
+the StreamPixels backend can mint a state naming `{kind: "creator", targetId}`.
+Heimdall cannot see StreamPixels' session; the secret proves the caller is the
+backend, and the backend must have proved the creator-manage session itself
+before it starts the connect.
+
+What StreamPixels has to change (not done here; this repo does not touch it):
+
+1. **Start the connect only after the creator-manage check.** The route that
+   calls `POST /v1/oauth/:provider/start` with `mode: "connect"` must have
+   verified, for the signed-in StreamPixels user, that they may manage creator
+   `targetId`. Never take `targetId` from the browser unchecked.
+2. **Remember what you started.** Store, server-side and bound to the attempt
+   (the `attemptId` or browser session), the `targetId` StreamPixels asked
+   Heimdall to bind.
+3. **Redeem with that stored target, never one read back from the request or
+   the completion.** `POST /v1/apps/streampixels/auth/redeem` takes `targetId`;
+   Heimdall answers `403 completion_target_mismatch` (code left unconsumed) when
+   it differs from the signed state's. That only protects you if the value you
+   send is the one you stored at step 2. A redeem with no stored target for this
+   attempt must be refused by StreamPixels before it calls Heimdall.
+4. **Refuse a creator connect you did not start.** If a connect completion
+   arrives for a `targetId` with no matching started attempt, do not rebind that
+   creator's connector, whatever Heimdall's completion says.
+5. **A connect completion with no creator target is not a creator connect and
+   not a sign-in.** Joystick is managed-connection only: a non-creator Joystick
+   completion is an error. (Twitch/YouTube `connect` without a `connection` is
+   Heimdall's viewer-session path; treat it as that, never as a connector rebind.)
+6. Blank targets (`"   "`, zero-width characters) are refused by Heimdall at
+   start with `400 invalid_connection_target`; send real ids only.
+
+Tracked as `joystick:follow_up:connect-start-needs-app-auth`.
diff --git a/src/app-profiles.ts b/src/app-profiles.ts
index c10f704..c25dbb2 100644
--- a/src/app-profiles.ts
+++ b/src/app-profiles.ts
@@ -263,6 +263,17 @@ export function isAllowedReturnOrigin(profile: AppProfile, returnTo: string): bo
   }
 }
 
+/**
+ * A connection target names something: at least one letter or number that is
+ * not a default-ignorable code point (the Hangul and Khmer fillers are letters
+ * that render as nothing). An allowlist, so marks, private-use, unassigned and
+ * surrogate code points and symbols cannot stand in for a name. The one
+ * definition the start gate and the callback mint both use.
+ */
+export function isUsableConnectionTarget(targetId: string): boolean {
+  return /(?![\p{Default_Ignorable_Code_Point}])[\p{L}\p{N}]/u.test(targetId);
+}
+
 /**
  * What a completed OAuth callback may mint, decided at the callback from the
  * signed state's mode and the provider's role in the profile (the start gate
@@ -289,7 +300,7 @@ export function callbackMint(
   return mode === "connect" &&
     profile.managedConnectionProviders.includes(provider) &&
     connection?.kind === "creator" &&
-    connection.targetId !== ""
+    isUsableConnectionTarget(connection.targetId)
     ? "custody_only"
     : "refused";
 }
diff --git a/src/app.ts b/src/app.ts
index 2051499..e2ba003 100644
--- a/src/app.ts
+++ b/src/app.ts
@@ -25,7 +25,7 @@ import {
   type Provider,
 } from "./contracts.js";
 import { issueAccessClaim, type IssueAccessClaimInput } from "./claims.js";
-import { builtInAppProfiles, getAppProfile, isAllowedReturnOrigin, serializeAppProfile, admitsProviderForMode, callbackMint } from "./app-profiles.js";
+import { builtInAppProfiles, getAppProfile, isAllowedReturnOrigin, serializeAppProfile, admitsProviderForMode, callbackMint, isUsableConnectionTarget } from "./app-profiles.js";
 import { renderBrowserHandoffPage } from "./browser-handoff.js";
 import { consentText, digestOfText, grantViews, GRANTS_PATH, renderConsentPage, renderGrantsPage, type ConsentAsk } from "./consent.js";
 import { type HeimdallConfig, loadConfig } from "./config.js";
@@ -702,16 +702,6 @@ export async function startOAuthFlow(
     };
   }
 
-  if (callbackMint(profile, provider, input.mode, input.connection) === "refused") {
-    return {
-      statusCode: 400,
-      body: {
-        error: "creator_connection_required",
-        detail: `${provider} on ${profile.slug} is a managed creator connection: connect needs connection.kind creator and a targetId.`,
-      },
-    };
-  }
-
   const descriptor = providerCatalog[provider];
   // A provider that never mints an account attaches an identity only through
   // a bound link: the user's own access token, since the app's word does not
@@ -811,6 +801,24 @@ export async function startOAuthFlow(
     return { statusCode: 401, body: { error: "app_auth_required" } };
   }
 
+  // After the app-secret gate, so a caller without the secret learns nothing about what a target must look like.
+  if (input.connection !== undefined && !isUsableConnectionTarget(input.connection.targetId)) {
+    return {
+      statusCode: 400,
+      body: { error: "invalid_connection_target", detail: "connection.targetId must name a target." },
+    };
+  }
+
+  if (callbackMint(profile, provider, input.mode, input.connection) === "refused") {
+    return {
+      statusCode: 400,
+      body: {
+        error: "creator_connection_required",
+        detail: `${provider} on ${profile.slug} is a managed creator connection: connect needs connection.kind creator and a targetId.`,
+      },
+    };
+  }
+
   const providerConfig = config.providers[provider];
   if (!providerConfig.clientId) {
     return {
diff --git a/tests/app.test.ts b/tests/app.test.ts
index 67d4183..20a0191 100644
--- a/tests/app.test.ts
+++ b/tests/app.test.ts
@@ -1440,6 +1440,105 @@ describe("Heimdall service", () => {
     expect(urls).toEqual([]);
   });
 
+  // Targets made only of whitespace, format characters, controls and the invisible fillers that render as nothing.
+  const BLANK_TARGETS = [
+    "   ", "\t", "\n ", "\u200B", "\u00A0\uFEFF",
+    "\u2800", "\u3164", "\u115F", "\u1160", "\uFFA0", "\u034F", "\u0000", "\u0007", " \u2800\u3164 ",
+    // Marks, private-use, unassigned and lone-surrogate code points, and the symbols that read as nothing: none is a letter or a number.
+    "\u0300", "\uE000", "\u0378", "\uD800", "\u{16FE4}", "\u{1D159}", "\u{F0000}", "\u0300\uE000 ",
+  ];
+  // A target is usable when it holds a letter or a number that is not an invisible filler.
+  const NAMING_TARGETS = ["creator-1", " creator-1 ", "Framgång", "42", "\u2800a\u3164", "\u3164\u0300x"];
+
+  describe("a connect start that names a connection target is the app backend's", () => {
+    // The target is app-backend authority: the StreamPixels backend proves the creator-manage session and states the
+    // creator it started the connect for. Anyone else naming {creator, X} would have X's connector rebound to their
+    // own OAuth account at StreamPixels' redeem.
+    const returnTo = "https://streampixels.gamecult.org/auth/connect";
+    const managed = ["twitch", "youtube", "joystick"] as const;
+    const startFor = (app: FastifyInstance, provider: string, connection: unknown, headers: Record<string, string> = {}) =>
+      app.inject({
+        method: "POST",
+        url: `/v1/oauth/${provider}/start`,
+        headers,
+        payload: { appSlug: "streampixels", mode: "connect", returnTo, connection },
+      });
+    const own = { "x-heimdall-app-secret": "streampixels-secret" };
+
+    it.each(managed)("%s: a creator target is refused without the app's own secret and minted for it", async (provider) => {
+      const config = createTestConfig();
+      const app = await buildApp({ config });
+      apps.push(app);
+      const creator = { kind: "creator", targetId: "victim-creator" };
+
+      for (const headers of [{}, { "x-heimdall-app-secret": "repixelizer-secret" }, { "x-heimdall-app-secret": "wrong" }]) {
+        const refused = await startFor(app, provider, creator, headers);
+        expect([refused.statusCode, refused.json()]).toEqual([401, { error: "app_auth_required" }]);
+        expect(refused.body).not.toContain("authorizationUrl");
+      }
+      const started = await startFor(app, provider, creator, own);
+      expect(started.statusCode).toBe(201);
+      const verified = verifyJwt(stateOf(started.json()), getPublicKey(app));
+      expect(verified.valid && verified.payload).toEqual(expect.objectContaining({ connection: expect.objectContaining(creator) }));
+
+      // A direct caller (the command plane hands startOAuthFlow its own) is held to the slug too.
+      const input = { appSlug: "streampixels" as const, mode: "connect" as const, returnTo, connection: { kind: "creator" as const, targetId: "victim-creator" } };
+      const foreign = resolveAppCaller(config, "repixelizer", { "x-heimdall-app-secret": "repixelizer-secret" });
+      expect((await startOAuthFlow(getHeimdallRuntimeContext(app), provider, foreign, input)).statusCode).toBe(401);
+      expect((await startOAuthFlow(getHeimdallRuntimeContext(app), provider, null, input)).statusCode).toBe(401);
+    });
+
+    it.each(["twitch", "youtube"] as const)("%s: any connection kind needs the app's secret, not only creator", async (provider) => {
+      const app = await buildApp({ config: createTestConfig() });
+      apps.push(app);
+      const account = { kind: "account", targetId: "victim-account" };
+
+      const refused = await startFor(app, provider, account);
+      expect([refused.statusCode, refused.json()]).toEqual([401, { error: "app_auth_required" }]);
+      expect((await startFor(app, provider, account, own)).statusCode).toBe(201);
+    });
+
+    it.each(managed)("%s: a blank target is refused even for the app, whatever the blank is made of", async (provider) => {
+      const app = await buildApp({ config: createTestConfig() });
+      apps.push(app);
+
+      for (const kind of ["creator", "account"]) {
+        for (const targetId of BLANK_TARGETS) {
+          const refused = await startFor(app, provider, { kind, targetId }, own);
+          expect([refused.statusCode, refused.json().error], `${kind} ${JSON.stringify(targetId)}`).toEqual([400, "invalid_connection_target"]);
+          expect(refused.body).not.toContain("authorizationUrl");
+        }
+      }
+      for (const targetId of NAMING_TARGETS) {
+        expect((await startFor(app, provider, { kind: "creator", targetId }, own)).statusCode, JSON.stringify(targetId)).toBe(201);
+      }
+    });
+
+    it.each(managed)("%s: a blank target without the app's secret gets 401, not the 400 that would tell a stranger what the gate checks", async (provider) => {
+      const app = await buildApp({ config: createTestConfig() });
+      apps.push(app);
+
+      for (const headers of [{}, { "x-heimdall-app-secret": "wrong" }]) {
+        for (const targetId of ["   ", "\u2800"]) {
+          const refused = await startFor(app, provider, { kind: "creator", targetId }, headers);
+          expect([refused.statusCode, refused.json()], JSON.stringify(targetId)).toEqual([401, { error: "app_auth_required" }]);
+        }
+      }
+    });
+
+    it("the callback mint holds a blank creator target to the same bar", () => {
+      const streampixels = builtInAppProfiles.streampixels!;
+      for (const targetId of BLANK_TARGETS) {
+        expect(callbackMint(streampixels, "joystick", "connect", { kind: "creator", targetId }), JSON.stringify(targetId)).toBe("refused");
+      }
+      // A visible character among the blanks names a target.
+      expect(callbackMint(streampixels, "joystick", "connect", { kind: "creator", targetId: "\u2800a\u3164" })).toBe("custody_only");
+      for (const targetId of NAMING_TARGETS) {
+        expect(callbackMint(streampixels, "joystick", "connect", { kind: "creator", targetId }), JSON.stringify(targetId)).toBe("custody_only");
+      }
+    });
+  });
+
   describe("joystick connect", () => {
     const creator = { kind: "creator", targetId: "creator-1" };
     async function connect(app: FastifyInstance, extra: Record<string, unknown> = { connection: creator }) {