diff --git a/packages/core/src/oauth/oauth.test.ts b/packages/core/src/oauth/oauth.test.ts
index a742523fa4..bf680e42f3 100644
--- a/packages/core/src/oauth/oauth.test.ts
+++ b/packages/core/src/oauth/oauth.test.ts
@@ -122,6 +122,53 @@ describe("OAuthService.refreshToken", () => {
     expect(result.errorCode).toBe("auth_error");
   });
 
+  it.each([
+    {
+      name: "invalid_grant",
+      body: { error: "invalid_grant" },
+      expected: "auth_error",
+    },
+    {
+      name: "invalid_token",
+      body: { error: "invalid_token" },
+      expected: "auth_error",
+    },
+    {
+      name: "invalid_client",
+      body: { error: "invalid_client" },
+      expected: "unknown_error",
+    },
+    {
+      name: "invalid_request",
+      body: { error: "invalid_request" },
+      expected: "unknown_error",
+    },
+    {
+      name: "a non-string error field",
+      body: { error: 42 },
+      expected: "unknown_error",
+    },
+    { name: "no error field", body: {}, expected: "unknown_error" },
+  ])("maps a 400 $name to a $expected", async ({ body, expected }) => {
+    const { service } = createDeps();
+    fetchMock.mockResolvedValue(jsonResponse(body, 400));
+
+    const result = await service.refreshToken("rt", "us");
+
+    expect(result.success).toBe(false);
+    expect(result.errorCode).toBe(expected);
+  });
+
+  it("maps a 400 with an unparseable body to an unknown_error", async () => {
+    const { service } = createDeps();
+    fetchMock.mockResolvedValue(new Response("", { status: 400 }));
+
+    const result = await service.refreshToken("rt", "us");
+
+    expect(result.success).toBe(false);
+    expect(result.errorCode).toBe("unknown_error");
+  });
+
   it("maps 5xx to a server_error", async () => {
     const { service } = createDeps();
     fetchMock.mockResolvedValue(jsonResponse({}, 503));
@@ -133,7 +180,7 @@ describe("OAuthService.refreshToken", () => {
 
   it("maps other 4xx to an unknown_error", async () => {
     const { service } = createDeps();
-    fetchMock.mockResolvedValue(jsonResponse({}, 400));
+    fetchMock.mockResolvedValue(jsonResponse({}, 404));
 
     const result = await service.refreshToken("rt", "us");
 
diff --git a/packages/core/src/oauth/oauth.ts b/packages/core/src/oauth/oauth.ts
index a6ea202d24..5149630174 100644
--- a/packages/core/src/oauth/oauth.ts
+++ b/packages/core/src/oauth/oauth.ts
@@ -61,6 +61,15 @@ interface PendingOAuthFlow {
   abortController?: AbortController;
 }
 
+async function parseOAuthErrorCode(response: Response): Promise<string | null> {
+  try {
+    const body = (await response.json()) as { error?: unknown };
+    return typeof body.error === "string" ? body.error : null;
+  } catch {
+    return null;
+  }
+}
+
 @injectable()
 export class OAuthService {
   private pendingFlow: PendingOAuthFlow | null = null;
@@ -216,8 +225,18 @@ export class OAuthService {
       });
 
       if (!response.ok) {
-        // 401/403 are auth errors - the token is invalid
-        const isAuthError = response.status === 401 || response.status === 403;
+        // 401/403 are always auth failures. A 400 is only a dead refresh token
+        // when the OAuth error is invalid_grant/invalid_token; other 400s like
+        // invalid_client or invalid_request are config bugs and must not log the
+        // user out, or they would be unable to log back in with the same broken
+        // config.
+        const oauthErrorCode =
+          response.status === 400 ? await parseOAuthErrorCode(response) : null;
+        const isAuthError =
+          response.status === 401 ||
+          response.status === 403 ||
+          oauthErrorCode === "invalid_grant" ||
+          oauthErrorCode === "invalid_token";
         // 5xx are server errors - should be retried
         const isServerError = response.status >= 500;
         this.log.warn(