fix(agents): land #38935 from @MumuTW

Co-authored-by: MumuTW <MumuTW@users.noreply.github.com>

src/agents/cache-trace.test.ts

@@ -144,4 +144,35 @@ describe("createCacheTrace", () => {
     expect(source.bytes).toBe(6);
     expect(source.sha256).toBe(crypto.createHash("sha256").update("U0VDUkVU").digest("hex"));
   });
+
+  it("handles circular references in messages without stack overflow", () => {
+    const lines: string[] = [];
+    const trace = createCacheTrace({
+      cfg: {
+        diagnostics: {
+          cacheTrace: {
+            enabled: true,
+          },
+        },
+      },
+      env: {},
+      writer: {
+        filePath: "memory",
+        write: (line) => lines.push(line),
+      },
+    });
+
+    const parent: Record<string, unknown> = { role: "user", content: "hello" };
+    const child: Record<string, unknown> = { ref: parent };
+    parent.child = child; // circular reference
+
+    trace?.recordStage("prompt:images", {
+      messages: [parent] as unknown as [],
+    });
+
+    expect(lines.length).toBe(1);
+    const event = JSON.parse(lines[0]?.trim() ?? "{}") as Record<string, unknown>;
+    expect(event.messageCount).toBe(1);
+    expect(event.messageFingerprints).toHaveLength(1);
+  });
 });

src/agents/cache-trace.ts

@@ -104,7 +104,7 @@ function getWriter(filePath: string): CacheTraceWriter {
   return getQueuedFileWriter(writers, filePath);
 }
 
-function stableStringify(value: unknown): string {
+function stableStringify(value: unknown, seen: WeakSet<object> = new WeakSet()): string {
   if (value === null || value === undefined) {
     return String(value);
   }
@@ -117,30 +117,40 @@ function stableStringify(value: unknown): string {
   if (typeof value !== "object") {
     return JSON.stringify(value) ?? "null";
   }
+  if (seen.has(value)) {
+    return JSON.stringify("[Circular]");
+  }
+  seen.add(value);
   if (value instanceof Error) {
-    return stableStringify({
-      name: value.name,
-      message: value.message,
-      stack: value.stack,
-    });
+    return stableStringify(
+      {
+        name: value.name,
+        message: value.message,
+        stack: value.stack,
+      },
+      seen,
+    );
   }
   if (value instanceof Uint8Array) {
-    return stableStringify({
-      type: "Uint8Array",
-      data: Buffer.from(value).toString("base64"),
-    });
+    return stableStringify(
+      {
+        type: "Uint8Array",
+        data: Buffer.from(value).toString("base64"),
+      },
+      seen,
+    );
   }
   if (Array.isArray(value)) {
     const serializedEntries: string[] = [];
     for (const entry of value) {
-      serializedEntries.push(stableStringify(entry));
+      serializedEntries.push(stableStringify(entry, seen));
     }
     return `[${serializedEntries.join(",")}]`;
   }
   const record = value as Record<string, unknown>;
   const serializedFields: string[] = [];
   for (const key of Object.keys(record).toSorted()) {
-    serializedFields.push(`${JSON.stringify(key)}:${stableStringify(record[key])}`);
+    serializedFields.push(`${JSON.stringify(key)}:${stableStringify(record[key], seen)}`);
   }
   return `{${serializedFields.join(",")}}`;
 }