Fix proxy debug trace truncation previews (#313)

src/server/services/proxyDebugTraceStore.test.ts

@@ -150,4 +150,37 @@ describe('proxyDebugTraceStore', () => {
       downgradeDecision: true,
     });
   });
+
+  it('stores truncated debug payload previews as valid JSON text for json-capable databases', async () => {
+    const trace = await store.createProxyDebugTrace({
+      downstreamPath: '/v1/responses',
+      clientKind: 'codex',
+      requestedModel: 'gpt-5.4',
+      requestHeaders: {
+        authorization: `Bearer ${'x'.repeat(5000)}`,
+        'x-client': 'Codex Desktop',
+      },
+      requestBody: {
+        model: 'gpt-5.4',
+        input: [{ role: 'user', content: 'hello '.repeat(2000) }],
+      },
+      maxBodyBytes: 1024,
+    });
+
+    const detail = await store.getProxyDebugTraceDetail(trace.id);
+    const headersPayload = JSON.parse(detail?.trace.requestHeadersJson || 'null');
+    const bodyPayload = JSON.parse(detail?.trace.requestBodyJson || 'null');
+
+    expect(headersPayload).toMatchObject({
+      __metapiTruncated: true,
+    });
+    expect(typeof headersPayload.preview).toBe('string');
+    expect(headersPayload.preview).toContain('authorization');
+
+    expect(bodyPayload).toMatchObject({
+      __metapiTruncated: true,
+    });
+    expect(typeof bodyPayload.preview).toBe('string');
+    expect(bodyPayload.preview).toContain('"model": "gpt-5.4"');
+  });
 });

src/server/services/proxyDebugTraceStore.ts

@@ -25,6 +25,24 @@ export type ProxyDebugTraceSession = {
 let lastPruneAtMs = 0;
 const PRUNE_INTERVAL_MS = 5 * 60 * 1000;
 
+type TruncatedDebugPreview = {
+  __metapiTruncated: true;
+  preview: string;
+  originalBytes: number;
+  storedBytes: number;
+};
+
+function buildTruncatedDebugPreview(text: string, maxBytes: number, originalBytes: number): string {
+  const truncated = Buffer.from(text, 'utf8').subarray(0, Math.max(0, maxBytes)).toString('utf8');
+  const payload: TruncatedDebugPreview = {
+    __metapiTruncated: true,
+    preview: truncated,
+    originalBytes,
+    storedBytes: maxBytes,
+  };
+  return JSON.stringify(payload, null, 2);
+}
+
 function stringifyDebugValue(value: unknown, maxBytes: number): string | null {
   if (value == null) return null;
 
@@ -45,8 +63,7 @@ function stringifyDebugValue(value: unknown, maxBytes: number): string | null {
     return text;
   }
 
-  const truncated = buffer.subarray(0, Math.max(0, maxBytes)).toString('utf8');
-  return `${truncated}\n...[truncated ${buffer.length - maxBytes} bytes]`;
+  return buildTruncatedDebugPreview(text, maxBytes, buffer.length);
 }
 
 function normalizeHeadersValue(value: HeadersLike): Record<string, unknown> | null {