fix(session): omit oversized tool result detail from hook payloads

Hook payloads capped content but passed detail through verbatim, letting a verbose server blow up payloads. Omit serialized detail over the payload budget while preserving small detail.

Author: TheGreatAxios

src/session/hooks.test.ts

@@ -17,6 +17,7 @@ function event(type: string, data: unknown): ReactorEmittedEvent {
 function observeOneTurnWithToolResult(
   collector: ReturnType<typeof createTurnContextCollector>,
   toolResultContent: string,
+  toolResultDetail?: unknown,
 ): void {
   collector.observe(
     event("inference.done", {
@@ -34,7 +35,11 @@ function observeOneTurnWithToolResult(
   );
   collector.observe(
     event("tool.done", {
-      result: { callId: "call-1", content: toolResultContent },
+      result: {
+        callId: "call-1",
+        content: toolResultContent,
+        ...(toolResultDetail !== undefined ? { detail: toolResultDetail } : {}),
+      },
     }),
   );
 }
@@ -64,6 +69,33 @@ describe("createTurnContextCollector tool result truncation", () => {
     const [turn] = collector.getTurns();
     expect(turn?.toolResults[0]?.content).toBe(smallOutput);
   });
+
+  test("preserves small structured detail in hook payloads", () => {
+    const collector = createTurnContextCollector(() => undefined);
+    const detail = { answer: 42 };
+
+    observeOneTurnWithToolResult(collector, "exit code 0", detail);
+
+    const [turn] = collector.getTurns();
+    expect(turn?.toolResults[0]?.detail).toEqual(detail);
+  });
+
+  test("omits oversized structured detail while keeping the content cap", () => {
+    const collector = createTurnContextCollector(() => undefined);
+    const hugeOutput = "x".repeat(HOOK_PAYLOAD_TOOL_RESULT_CHARS * 4);
+    const hugeDetail = { blob: "y".repeat(HOOK_PAYLOAD_TOOL_RESULT_CHARS * 4) };
+
+    observeOneTurnWithToolResult(collector, hugeOutput, hugeDetail);
+
+    const [turn] = collector.getTurns();
+    const result = turn?.toolResults[0];
+    expect(result?.detail).toBeUndefined();
+    const content = result?.content;
+    expect(typeof content).toBe("string");
+    expect((content as string).length).toBeLessThanOrEqual(
+      HOOK_PAYLOAD_TOOL_RESULT_CHARS + 64,
+    );
+  });
 });
 
 describe("lifecycle hook payload delivery", () => {

src/session/hooks.ts

@@ -501,13 +501,26 @@ function truncateToolResultForHookPayload(result: ToolResult): ToolResult {
     callId: result.callId,
     content,
     ...(result.isError !== undefined ? { isError: result.isError } : {}),
-    ...(result.detail !== undefined ? { detail: result.detail } : {}),
+    ...(hookPayloadDetailWithinBudget(result.detail)
+      ? { detail: result.detail }
+      : {}),
     ...(result.pendingMarker !== undefined
       ? { pendingMarker: result.pendingMarker }
       : {}),
   };
 }
 
+function hookPayloadDetailWithinBudget(detail: unknown): boolean {
+  if (detail === undefined) return false;
+  let serialized: string;
+  try {
+    serialized = JSON.stringify(detail) ?? String(detail);
+  } catch {
+    return false;
+  }
+  return serialized.length <= HOOK_PAYLOAD_TOOL_RESULT_CHARS;
+}
+
 function addUsage(a: TokenUsage, b: TokenUsage): TokenUsage {
   return {
     input: a.input + b.input,