diff --git a/docs/service-contract.md b/docs/service-contract.md
index 47fa7fa..001b34b 100644
--- a/docs/service-contract.md
+++ b/docs/service-contract.md
@@ -495,14 +495,18 @@ Important behavior:
   response without a new refresh token revokes the credential instead of
   keeping the old token
 - `410 {"error":"managed_credential_revoked","provider":...}`: the provider
-  rejected the grant (`invalid_grant`), or the identity holds no access token.
+  rejected the grant (HTTP 400 whose OAuth `error` is exactly `invalid_grant`),
+  or the identity holds no access token.
   The stored tokens are cleared and an audit event `managed_credential_revoked`
   is written; the owner must reconnect. `404 managed_credential_not_found`
   means no linked identity exists
 - `502 {"error":"managed_credential_refresh_failed","provider":...}`: the
   refresh failed for any reason that is not a grant verdict (invalid client,
-  provider 5xx, timeout, malformed body). Stored tokens are untouched and a
-  later request may succeed
+  a 401 or 5xx whatever its body says, timeout, a 2xx without a string
+  `access_token` or carrying an `error`, an empty or non-JSON body). Stored
+  tokens are untouched and a later request may succeed. An answer without
+  `expires_in` stores the expiry as unknown, and an unknown expiry is never
+  due, so it does not trigger another refresh
 - app-local binding, diagnostics, subscription setup, and runtime behavior stay
   app-owned
 
diff --git a/src/managed-credential.ts b/src/managed-credential.ts
index 4da4b5a..6a06727 100644
--- a/src/managed-credential.ts
+++ b/src/managed-credential.ts
@@ -113,7 +113,10 @@ export async function refreshManagedCredential(
       refreshTokenEncrypted = locked.refreshTokenEncrypted!;
     }
 
-    const tokenExpiresAt = refreshed.expiresAt ?? locked.tokenExpiresAt;
+    // An answer without expires_in leaves the expiry unknown. The old expiry is
+    // already due, so keeping it would refresh again on every resolve; an
+    // unknown expiry is never due (dueForRefresh).
+    const tokenExpiresAt = refreshed.expiresAt;
     return {
       kind: "rotate",
       accessTokenEncrypted: ctx.tokenCustody.encrypt(refreshed.accessToken),
diff --git a/src/oauth.ts b/src/oauth.ts
index c46976e..9593b54 100644
--- a/src/oauth.ts
+++ b/src/oauth.ts
@@ -89,7 +89,7 @@ function readOAuthErrorCode(text: string): OAuthErrorCode | undefined {
  */
 export class ProviderResponseError extends Error {
   readonly status: number;
-  readonly code: "provider_status" | "provider_body_not_json";
+  readonly code: "provider_status" | "provider_body_not_json" | "provider_body_not_token_set";
   /**
    * The RFC 6749 section 5.2 error code of a refusal, when the provider sent
    * one from that closed set. The only value taken from a provider's body.
@@ -109,7 +109,14 @@ export class ProviderResponseError extends Error {
 async function readProviderJson<T>(response: Response, failure: string): Promise<T> {
   const text = await response.text();
   if (!response.ok) {
-    throw new ProviderResponseError(response.status, "provider_status", failure, readOAuthErrorCode(text));
+    // RFC 6749 section 5.2 sends a grant verdict as 400 only. The same JSON
+    // behind a 401, a 5xx or a gateway page is not one.
+    throw new ProviderResponseError(
+      response.status,
+      "provider_status",
+      failure,
+      response.status === 400 ? readOAuthErrorCode(text) : undefined
+    );
   }
   if (!text) {
     return {} as T;
@@ -173,13 +180,28 @@ export async function requestProviderTokens(
     init.signal = signal;
   }
   const label = grant.grant_type === "refresh_token" ? "refresh" : "exchange";
-  const tokenResponse = await fetchJson<{
+  const failure = `${descriptor.displayName} token ${label} failed`;
+  const response = await fetch(descriptor.tokenEndpoint, init);
+  const answer = await readProviderJson<unknown>(response, failure);
+  // A 2xx is a token set only when it is an object holding a string access
+  // token and no error.
+  if (
+    typeof answer !== "object" ||
+    answer === null ||
+    Array.isArray(answer) ||
+    typeof (answer as { access_token?: unknown }).access_token !== "string" ||
+    !(answer as { access_token: string }).access_token ||
+    (answer as { error?: unknown }).error !== undefined
+  ) {
+    throw new ProviderResponseError(response.status, "provider_body_not_token_set", failure);
+  }
+  const tokenResponse = answer as {
     access_token: string;
     refresh_token?: string;
     token_type?: string;
     expires_in?: number;
     scope?: string | string[];
-  }>(descriptor.tokenEndpoint, init, `${descriptor.displayName} token ${label} failed`);
+  };
 
   const tokenSet: OAuthTokenSet = {
     accessToken: tokenResponse.access_token,
diff --git a/tests/managed-credential.test.ts b/tests/managed-credential.test.ts
index f9a9c88..7758534 100644
--- a/tests/managed-credential.test.ts
+++ b/tests/managed-credential.test.ts
@@ -719,6 +719,152 @@ describe("refreshManagedCredential without the lock", () => {
   });
 });
 
+describe("refresh answer classification", () => {
+  const config = testConfig();
+  const custody = createTokenCustody(config);
+  const now = Date.parse("2026-10-19T12:00:00.000Z");
+  const dueAt = new Date(now).toISOString();
+
+  async function seedRow(store: InMemoryStore, provider: Provider) {
+    const created = new Date(now).toISOString();
+    const account = await store.createAccount({ createdAt: created, lastSeenAt: created });
+    return store.upsertLinkedIdentity({
+      accountId: account.id,
+      provider,
+      providerUserId: `u-${randomUUID()}`,
+      scopes: ["s"],
+      profileJson: {},
+      accessTokenEncrypted: custody.encrypt("access-0"),
+      refreshTokenEncrypted: custody.encrypt("refresh-0"),
+      tokenExpiresAt: dueAt,
+      createdAt: created,
+      updatedAt: created,
+    });
+  }
+
+  const ctxFor = (store: HeimdallStore, extra: { timeoutMs?: number } = {}) => ({
+    config,
+    store,
+    tokenCustody: custody,
+    oauthRuntimes: createOAuthRuntimeRegistry(),
+    clock: () => now,
+    ...extra,
+  });
+
+  function answer(body: string, status: number) {
+    const calls: string[] = [];
+    globalThis.fetch = (async (_input: string | URL, init?: RequestInit) => {
+      calls.push(String((init!.body as URLSearchParams).get("refresh_token")));
+      return new Response(body, { status });
+    }) as typeof fetch;
+    return calls;
+  }
+
+  afterEach(() => {
+    globalThis.fetch = originalFetch;
+  });
+
+  const grantVerdict = JSON.stringify({ error: "invalid_grant" });
+  const keepsTokens: Array<[string, string, number]> = [
+    ["an error code that only contains invalid_grant", JSON.stringify({ error: "invalid_grant_extra" }), 400],
+    ["an error code that is contained in a longer one", JSON.stringify({ error: "not_invalid_grant" }), 400],
+    ["a different error code", JSON.stringify({ error: "invalid_request" }), 400],
+    ["a 401 carrying invalid_grant", grantVerdict, 401],
+    ["a 500 carrying invalid_grant", grantVerdict, 500],
+    ["a 503 carrying invalid_grant", grantVerdict, 503],
+    ["a 2xx carrying invalid_grant", grantVerdict, 200],
+    ["a 2xx carrying an error beside an access token", JSON.stringify({ access_token: "a", error: "server_error" }), 200],
+    ["a 2xx with an empty object", "{}", 200],
+    ["a 2xx with an empty body", "", 200],
+    ["a 2xx with JSON null", "null", 200],
+    ["a 2xx with a JSON array", "[]", 200],
+    ["a 2xx with a non-string access token", JSON.stringify({ access_token: 7, refresh_token: "r" }), 200],
+    ["a 2xx with an empty access token", JSON.stringify({ access_token: "", refresh_token: "r" }), 200],
+    ["a 2xx that is not JSON", "<html>", 200],
+    ["a 5xx page", "upstream exploded", 502],
+  ];
+
+  describe.each(["twitch", "joystick"] as const)("%s", (provider) => {
+    it.each(keepsTokens)("keeps the tokens and fails typed for %s", async (_name, body, status) => {
+      const store = new InMemoryStore();
+      const row = await seedRow(store, provider);
+      const calls = answer(body, status);
+      await expect(refreshManagedCredential(ctxFor(store), row)).rejects.toBeInstanceOf(ManagedCredentialRefreshFailed);
+      expect(calls).toEqual(["refresh-0"]);
+      const after = (await store.findStoredLinkedIdentity(provider, row.providerUserId))!;
+      expect(after.accessTokenEncrypted).toBe(row.accessTokenEncrypted);
+      expect(after.refreshTokenEncrypted).toBe(row.refreshTokenEncrypted);
+      expect(after.tokenExpiresAt).toBe(dueAt);
+    });
+
+    it("keeps the tokens and fails typed when the provider never answers", async () => {
+      const store = new InMemoryStore();
+      const row = await seedRow(store, provider);
+      globalThis.fetch = ((_input: string | URL, init?: RequestInit) =>
+        new Promise<Response>((_resolve, reject) => {
+          init!.signal?.addEventListener("abort", () => reject(new DOMException("aborted", "AbortError")));
+        })) as typeof fetch;
+      await expect(refreshManagedCredential(ctxFor(store, { timeoutMs: 30 }), row)).rejects.toBeInstanceOf(ManagedCredentialRefreshFailed);
+      const after = (await store.findStoredLinkedIdentity(provider, row.providerUserId))!;
+      expect(after.refreshTokenEncrypted).toBe(row.refreshTokenEncrypted);
+    });
+
+    it("revokes and clears the tokens on a 400 whose error is exactly invalid_grant", async () => {
+      const store = new InMemoryStore();
+      const row = await seedRow(store, provider);
+      answer(grantVerdict, 400);
+      const outcome = await refreshManagedCredential(ctxFor(store), row);
+      expect(outcome.kind).toBe("revoked");
+      const after = (await store.findStoredLinkedIdentity(provider, row.providerUserId))!;
+      expect(after.accessTokenEncrypted).toBeUndefined();
+      expect(after.refreshTokenEncrypted).toBeUndefined();
+    });
+  });
+
+  it("a rotating provider's well-formed answer without a refresh token still revokes (rotation_missing)", async () => {
+    const store = new InMemoryStore();
+    const row = await seedRow(store, "joystick");
+    answer(JSON.stringify({ access_token: "a", expires_in: 3600 }), 200);
+    expect((await refreshManagedCredential(ctxFor(store), row)).kind).toBe("revoked");
+    expect((await store.findStoredLinkedIdentity("joystick", row.providerUserId))!.refreshTokenEncrypted).toBeUndefined();
+  });
+
+  it("an answer without expires_in stores the expiry as unknown, and the next resolve does not refresh again", async () => {
+    const store = new InMemoryStore();
+    const row = await seedRow(store, "twitch");
+    const calls = answer(JSON.stringify({ access_token: "access-1", refresh_token: "refresh-1" }), 200);
+    const first = await refreshManagedCredential(ctxFor(store), row);
+    expect(first.kind).toBe("current");
+    expect(first.identity.tokenExpiresAt).toBeUndefined();
+    const stored = (await store.findStoredLinkedIdentity("twitch", row.providerUserId))!;
+    expect(stored.tokenExpiresAt).toBeUndefined();
+    expect(custody.decrypt(stored.accessTokenEncrypted!)).toBe("access-1");
+
+    await refreshManagedCredential(ctxFor(store), stored);
+    expect(calls).toEqual(["refresh-0"]);
+  });
+
+  it("a non-rotating provider keeps the locked row's refresh token, not the snapshot's", async () => {
+    const store = new InMemoryStore();
+    const snapshot = await seedRow(store, "twitch");
+    // Another caller rotated the pair after the snapshot; its expiry is still due.
+    await store.updateLinkedIdentityCredential(snapshot.id, async () => ({
+      kind: "rotate",
+      accessTokenEncrypted: custody.encrypt("access-1"),
+      refreshTokenEncrypted: custody.encrypt("refresh-1"),
+      tokenExpiresAt: dueAt,
+      scopes: ["s"],
+      updatedAt: dueAt,
+    }));
+    const calls = answer(JSON.stringify({ access_token: "access-2", expires_in: 3600 }), 200);
+    await refreshManagedCredential(ctxFor(store), snapshot);
+    expect(calls).toEqual(["refresh-1"]);
+    const after = (await store.findStoredLinkedIdentity("twitch", snapshot.providerUserId))!;
+    expect(custody.decrypt(after.refreshTokenEncrypted!)).toBe("refresh-1");
+    expect(custody.decrypt(after.accessTokenEncrypted!)).toBe("access-2");
+  });
+});
+
 describe("live Postgres availability", () => {
   it.skipIf(!!adminUrl)("is present whenever the job requires it", () => {
     // Local workstation runs skip the live suites; the verification job sets
diff --git a/tests/provider-token-request.test.ts b/tests/provider-token-request.test.ts
index 875598e..501a6e3 100644
--- a/tests/provider-token-request.test.ts
+++ b/tests/provider-token-request.test.ts
@@ -134,10 +134,26 @@ describe("requestProviderTokens follows the provider descriptor", () => {
     expect((await requestProviderTokens("twitch", configFor(), { grant_type: "refresh_token", refresh_token: "r" })).scope).toEqual(["a", "b"]);
   });
 
-  it("treats an empty 2xx body as an empty answer, not a malformed one", async () => {
-    capture(() => new Response("", { status: 200 }));
-    const tokens = await requestProviderTokens("twitch", configFor(), { grant_type: "refresh_token", refresh_token: "r" });
-    expect(tokens.accessToken).toBeUndefined();
+  it.each([
+    ["an empty body", ""],
+    ["an empty object", "{}"],
+    ["JSON null", "null"],
+    ["a JSON array", "[]"],
+    ["an error beside an access token", JSON.stringify({ access_token: "a", error: "invalid_grant" })],
+    ["an empty access token", JSON.stringify({ access_token: "" })],
+    ["a numeric access token", JSON.stringify({ access_token: 5 })],
+  ])("refuses a 2xx with %s as not a token set, carrying nothing of the body", async (_name, body) => {
+    capture(() => new Response(body, { status: 200 }));
+    const error = await requestProviderTokens("twitch", configFor(), { grant_type: "refresh_token", refresh_token: "r" }).then(
+      () => {
+        throw new Error("expected a refusal");
+      },
+      (e: unknown) => e as Error & { code?: string; status?: number; oauthError?: unknown }
+    );
+    expect(error.name).toBe("ProviderResponseError");
+    expect(error.code).toBe("provider_body_not_token_set");
+    expect(error.status).toBe(200);
+    expect(error.oauthError).toBeUndefined();
   });
 
   it("sets no refreshToken key when the provider sent none", async () => {
@@ -182,6 +198,19 @@ describe("oauthError is a closed set", () => {
     expect(`${error.message} ${error.stack} ${inspect(error, { depth: 5, showHidden: true })} ${JSON.stringify(error)}`).not.toContain("CANARY");
   });
 
+  it.each([401, 403, 429, 500, 502, 503])("takes no oauthError from a %i, whatever its body says", async (status) => {
+    const error = await failure(JSON.stringify({ error: "invalid_grant" }), status);
+    expect(error.oauthError).toBeUndefined();
+  });
+
+  it.each(["invalid_grant_extra", "not_invalid_grant", "INVALID_GRANT", " invalid_grant", "invalid_grant "])(
+    "takes the code by exact match only: %j is no code",
+    async (code) => {
+      const error = await failure(JSON.stringify({ error: code }));
+      expect(error.oauthError).toBeUndefined();
+    }
+  );
+
   it.each([
     ["an error outside the RFC set", JSON.stringify({ error: "CANARY_unknown" })],
     ["a non-string error", JSON.stringify({ error: { code: "invalid_grant" } })],