diff --git a/src/app.ts b/src/app.ts
index a1d9a2f..299364a 100644
--- a/src/app.ts
+++ b/src/app.ts
@@ -1326,11 +1326,13 @@ export async function buildApp(options: BuildAppOptions = {}): Promise<FastifyIn
           ...completionPayload,
         };
       } catch (error) {
-        const message = error instanceof Error ? error.message : "OAuth callback failed.";
-        // What leaves Heimdall is fixed text. This path catches store and
-        // provider failures: a Postgres error names a constraint, the host or
-        // the database, and JSON.parse quotes the provider's response body.
-        // The full text stays in the audit event.
+        // Neither the answer nor the audit event carries the error's text.
+        // This path catches store and provider failures: a Postgres or
+        // resolver error names a constraint, the host or the database, and
+        // JSON.parse quotes the provider's response body. The audit event
+        // keeps the error's class and code.
+        const code = (error as { code?: unknown }).code;
+        const errorClass = error instanceof Error && /^[A-Za-z]{1,64}$/.test(error.name) ? error.name : "Error";
         const failure = "The provider callback could not be completed.";
         const nowIso = new Date().toISOString();
         await denyBrowserAttempt("oauth_callback_failed");
@@ -1340,7 +1342,8 @@ export async function buildApp(options: BuildAppOptions = {}): Promise<FastifyIn
           eventPayloadJson: {
             provider: request.params.provider,
             mode: statePayload.mode,
-            error: message,
+            errorClass,
+            errorCode: typeof code === "string" && /^[A-Za-z0-9_]{1,64}$/.test(code) ? code : null,
           },
           createdAt: nowIso,
         });
diff --git a/tests/app.test.ts b/tests/app.test.ts
index dbb3208..2650424 100644
--- a/tests/app.test.ts
+++ b/tests/app.test.ts
@@ -1479,3 +1479,39 @@ describe("the relayed provider error", () => {
   });
 });
 
+describe("the failed-callback audit event", () => {
+  it("keeps the error's class and code, not its text", async () => {
+    const store = new InMemoryStore();
+    const app = await buildApp({
+      config: createTestConfig(),
+      store,
+      oauthRuntimes: {
+        discord: {
+          ...createMockDiscordRuntime(),
+          async exchangeAuthorizationCode() {
+            throw Object.assign(new Error("getaddrinfo ENOTFOUND CANARYhost.invalid"), { code: "ENOTFOUND" });
+          },
+        },
+      },
+    });
+    apps.push(app);
+
+    const stateToken = await startDiscordSignIn(app);
+    const response = await app.inject({
+      method: "GET",
+      url: `/v1/oauth/discord/callback?code=test-code&state=${encodeURIComponent(stateToken)}`,
+    });
+    expect(response.statusCode).toBe(502);
+
+    const events = [...(store as unknown as { auditEvents: Map<string, { eventType: string; eventPayloadJson: unknown }> }).auditEvents.values()];
+    const failed = events.filter((event) => event.eventType === "oauth_callback_failed");
+    expect(failed).toHaveLength(1);
+    expect(failed[0]!.eventPayloadJson).toEqual({
+      provider: "discord",
+      mode: "sign_in",
+      errorClass: "Error",
+      errorCode: "ENOTFOUND",
+    });
+    expect(JSON.stringify(events)).not.toContain("CANARY");
+  });
+});