fix(FN-4930): make promptSessionAndCheck transcript diagnostics recursion-safe

Fusion-Task-Id: FN-4930
Fusion-Task-Lineage: 5fb64145-a526-4aec-846e-5771b5844b18

.changeset/fn-4930-fix-prompt-session-and-check-recursion.md

@@ -0,0 +1,5 @@
+---
+"@runfusion/fusion": patch
+---
+
+Make `promptSessionAndCheck` transcript diagnostics circular-safe to prevent stack overflows on malformed message metadata.

packages/engine/src/__tests__/pi-prompt-session-and-check-recursion.test.ts

@@ -0,0 +1,50 @@
+import { beforeEach, describe, expect, it, vi } from "vitest";
+
+const warnMock = vi.fn();
+
+vi.mock("../logger.js", () => ({
+  createLogger: () => ({
+    log: vi.fn(),
+    info: vi.fn(),
+    warn: warnMock,
+    error: vi.fn(),
+  }),
+}));
+
+describe("promptSessionAndCheck recursion guard (FN-4930)", () => {
+  beforeEach(() => {
+    warnMock.mockReset();
+    vi.resetModules();
+  });
+
+  it("does not overflow when transcript-inspection metadata has pathological toJSON", async () => {
+    const { promptSessionAndCheck } = await import("../pi.js");
+    const recursive = {
+      toJSON() {
+        return { recursive };
+      },
+    };
+
+    const state = {
+      errorMessage: "",
+      messages: [
+        {
+          role: "assistant",
+          content: "ok",
+          toolName: recursive,
+          stopReason: recursive,
+        },
+      ],
+    };
+
+    const session = {
+      prompt: vi.fn(async () => {
+        state.errorMessage = "Cannot read properties of undefined (reading 'foo')";
+      }),
+      state,
+    } as any;
+
+    await expect(promptSessionAndCheck(session, "hello")).rejects.not.toThrow(/Maximum call stack/i);
+    expect(warnMock).not.toHaveBeenCalledWith(expect.stringContaining("failed to inspect transcript"));
+  });
+});

packages/engine/src/pi.ts

@@ -221,7 +221,36 @@ function isThinkingReasoningConflictError(message: string): boolean {
   return /cannot specify both\s+['"]?thinking['"]?\s+and\s+['"]?reasoning_effort['"]?/i.test(message);
 }
 
-async function promptSessionAndCheck(session: AgentSession, prompt: string, options?: unknown): Promise<void> {
+function coercePreviewValue(value: unknown): string {
+  if (value === null) return "null";
+  if (value === undefined) return "undefined";
+  if (typeof value === "string") return value;
+  if (typeof value === "number" || typeof value === "boolean" || typeof value === "bigint") {
+    return String(value);
+  }
+  if (typeof value === "symbol") {
+    return value.description ? `symbol(${value.description})` : "symbol";
+  }
+  if (typeof value === "function") {
+    return `[function ${value.name || "anonymous"}]`;
+  }
+  return Object.prototype.toString.call(value);
+}
+
+function safePreviewJson(value: unknown): string {
+  const seen = new WeakSet<object>();
+  return JSON.stringify(value, (_key, candidate) => {
+    if (typeof candidate === "object" && candidate !== null) {
+      if (seen.has(candidate)) {
+        return "[Circular]";
+      }
+      seen.add(candidate);
+    }
+    return candidate;
+  });
+}
+
+export async function promptSessionAndCheck(session: AgentSession, prompt: string, options?: unknown): Promise<void> {
   clearSessionStateError(session);
   if (options === undefined) {
     await session.prompt(prompt);
@@ -244,13 +273,13 @@ async function promptSessionAndCheck(session: AgentSession, prompt: string, opti
             const content = m?.content;
             return {
               index: i < 0 ? idx : i,
-              role: m?.role,
+              role: coercePreviewValue(m?.role),
               contentType: Array.isArray(content) ? `array(len=${content.length})` : typeof content,
-              toolName: (m as { toolName?: unknown }).toolName,
-              stopReason: (m as { stopReason?: unknown }).stopReason,
+              toolName: coercePreviewValue((m as { toolName?: unknown }).toolName),
+              stopReason: coercePreviewValue((m as { stopReason?: unknown }).stopReason),
             };
           });
-          piLog.error(`pi state error — transcript tail (${messages.length} msgs total): ${JSON.stringify(recent)}`);
+          piLog.error(`pi state error — transcript tail (${messages.length} msgs total): ${safePreviewJson(recent)}`);
         } else {
           piLog.error(`pi state error — state.messages is not an array: ${typeof messages}`);
         }