2 files changed, 33 insertions, 1 deletions
diff --git a/src/features/chat/logic/tool-summary.test.ts b/src/features/chat/logic/tool-summary.test.ts
index b6c7e0b..c0caa0d 100644
--- a/src/features/chat/logic/tool-summary.test.ts
+++ b/src/features/chat/logic/tool-summary.test.ts
@@ -311,6 +311,21 @@ describe("tool summaries", () => {
     });
     expect(() => view("mcp", { integer: 1n })).not.toThrow();
   });
+  it("rejects ancestor cycles but preserves shared non-cyclic tool values", () => {
+    const nested: Record<string, unknown> = { name: "nested" };
+    const root = { nested };
+    nested.parent = root;
+    expect(view("mcp", { payload: root }).callDetail).toEqual({
+      kind: "fields",
+      fields: [{ key: "payload", value: "[unserializable input]", multiline: false }],
+    });
+    const shared = { name: "shared" };
+    const payload = { first: shared, second: shared, values: [shared, shared] };
+    const detail = view("mcp", { payload }).callDetail;
+    expect(detail.kind).toBe("fields");
+    if (detail.kind !== "fields") throw new Error("Expected fields");
+    expect(detail.fields[0]?.value).toBe(JSON.stringify(payload, null, 2));
+  });
   it("caps and whitespace-collapses header strings but retains full detail", () => {
     const command = `  echo\n${"x".repeat(300)}`;
     const card = view("run_shell", { command });
diff --git a/src/features/chat/logic/tool-summary.ts b/src/features/chat/logic/tool-summary.ts
index f6d1569..2380adc 100644
--- a/src/features/chat/logic/tool-summary.ts
+++ b/src/features/chat/logic/tool-summary.ts
@@ -74,7 +74,24 @@ function asArgs(input: unknown): Args {
 function serialize(input: unknown, pretty = false): string {
   if (typeof input === "string") return input;
   try {
-    return JSON.stringify(input, null, pretty ? 2 : undefined) ?? String(input);
+    // Detect ancestor cycles before the native serializer recurses. Bun's
+    // pretty JSON cycle detection can otherwise consume seconds of UI time.
+    // Track ancestors, not every visited object: shared non-cyclic values are valid.
+    const ancestors: object[] = [];
+    return (
+      JSON.stringify(
+        input,
+        function (this: unknown, _key, value: unknown) {
+          if (value !== null && typeof value === "object") {
+            while (ancestors.length && ancestors.at(-1) !== this) ancestors.pop();
+            if (ancestors.includes(value)) throw new TypeError("Circular tool input");
+            ancestors.push(value);
+          }
+          return value;
+        },
+        pretty ? 2 : undefined,
+      ) ?? String(input)
+    );
   } catch {
     return "[unserializable input]";
   }