diff --git a/src/app.ts b/src/app.ts
index 322c6e8..f6a72e9 100644
--- a/src/app.ts
+++ b/src/app.ts
@@ -736,12 +736,12 @@ export async function buildApp(options: BuildAppOptions = {}): Promise<FastifyIn
     ...(options.oauthRuntimes ?? {}),
   };
   const app = Fastify({ logger: false });
-  // Fastify's default handlers answer with the error's message, and those
-  // messages carry input: pg and Node name the database host or the database
-  // (which a misbound URL fills with part of a secret), JSON.parse quotes the
-  // request body, and the media-type and not-found errors quote a header and
-  // the path. So every framework answer here is built from fixed text, the
-  // error code and the route's own schema, never from the message.
+  // Fastify's default handlers answer with the error's message, and messages
+  // carry input: pg and Node name the database host or the database (which a
+  // misbound URL fills with part of a secret), the default not-found answer
+  // quotes the path, and any error thrown with a 4xx status says whatever its
+  // thrower wrote. So every framework answer here is built from fixed text,
+  // the error code and the route's own schema, never from the message.
   app.setErrorHandler((error: FastifyError, request, reply) => {
     const status = error.statusCode !== undefined && error.statusCode >= 400 && error.statusCode < 600 ? error.statusCode : 500;
     const code = typeof error.code === "string" ? error.code : "error";
diff --git a/tests/app.test.ts b/tests/app.test.ts
index 79c562c..de2a896 100644
--- a/tests/app.test.ts
+++ b/tests/app.test.ts
@@ -1224,7 +1224,7 @@ describe("Heimdall service", () => {
 
 // An answer never carries an input value. The canary stands in for whatever a
 // failure could quote: part of a misbound database URL in a pg or resolver
-// error, a request body in a JSON parse error, a header or a path.
+// error, a request body, a header, a path, or a 4xx error's own message.
 describe("error answers never echo an input", () => {
   const CANARY = "CANARY";
 
@@ -1276,6 +1276,19 @@ describe("error answers never echo an input", () => {
     expect(response.body).not.toContain(CANARY);
   });
 
+  it("answers any other 4xx error by its code, not its message", async () => {
+    const app = await buildApp({ config: createTestConfig() });
+    apps.push(app);
+    app.get("/v1/probe-4xx", async () => {
+      throw Object.assign(new Error(`refused ${CANARY}input`), { statusCode: 409, code: "PROBE_REFUSED" });
+    });
+
+    const response = await app.inject({ method: "GET", url: "/v1/probe-4xx" });
+
+    expect(response.statusCode).toBe(409);
+    expect(response.json()).toEqual({ error: "invalid_request", code: "PROBE_REFUSED" });
+  });
+
   it("answers an unsupported media type without quoting the header", async () => {
     const app = await buildApp({ config: createTestConfig() });
     apps.push(app);