shouldFallbackToNewSession does not handle -32602 from session/load, causing crash with cursor-agent

[lygshanwj]

Problem

When using acpx with cursor-agent (Cursor IDE CLI) in prompt mode (persistent sessions), session reconnection fails with:

AcpRuntimeError: acpx exited with code 1: code=ACP_TURN_FAILED

The exec mode works fine because it always creates a new session via session/new.

Root Cause

1. cursor-agent reports agentCapabilities.loadSession = true during ACP initialize
2. When acpx tries to restore a previous session via session/load, cursor-agent returns JSON-RPC error -32602 (Invalid params) — its session/load implementation appears incomplete
3. shouldFallbackToNewSession() in src/session-runtime/connect-load.ts only handles -32603 (Internal Error) for the fallback-to-new-session logic, but does not handle -32602
4. The -32602 error propagates up uncaught, causing the process to exit with code 1

Expected Behavior

When session/load returns -32602 (Invalid params) or -32601 (Method not found), acpx should gracefully fallback to session/new instead of crashing.

Suggested Fix

The constant SESSION_CONTROL_UNSUPPORTED_ACP_CODES (containing -32601 and -32602) already exists in the codebase but is not used in shouldFallbackToNewSession. The fix is to check it before the sessionHasAgentMessages guard:

 function shouldFallbackToNewSession(error, record) {
   if (error instanceof TimeoutError || error instanceof InterruptedError) return false;
   if (isAcpResourceNotFoundError(error)) return true;
+  const acpErr = extractAcpError(error);
+  if (acpErr && SESSION_CONTROL_UNSUPPORTED_ACP_CODES.has(acpErr.code)) return true;
   if (!sessionHasAgentMessages(record)) {
     if (isAcpQueryClosedBeforeResponseError(error)) return true;
-    if (extractAcpError(error)?.code === -32603) return true;
+    if (acpErr?.code === -32603) return true;
   }
   return false;
 }

Why this works

• SESSION_CONTROL_UNSUPPORTED_ACP_CODES = new Set([-32601, -32602]) already exists in the codebase
• -32601 = JSON-RPC Method not found (agent doesn't support the method at all)
• -32602 = JSON-RPC Invalid params (method exists but params validation fails — this is what cursor-agent returns for session/load)
• Checking these codes unconditionally (not gated behind sessionHasAgentMessages) ensures that even sessions with prior agent messages can gracefully recover when the agent simply doesn't support session/load
• As a bonus, the extractAcpError(error) call is now cached in a local variable to avoid redundant extraction

Steps to Reproduce

1. Configure acpx with cursor-agent as the backend
2. Run acpx cursor exec "hello" — works (uses session/new)
3. Run acpx cursor prompt and send a message — works (first run, creates new session)
4. Run acpx cursor prompt again — crashes (tries session/load on existing session, gets -32602)

Workaround

Manually patch the compiled dist/session-*.js file to add the SESSION_CONTROL_UNSUPPORTED_ACP_CODES check, or delete ~/.acpx/sessions/* before each run (forces fresh sessions every time).

Environment

• acpx: 0.3.0
• cursor-agent (Cursor CLI): 2026.03.11-6dfa30c
• OS: Windows 10
• Node.js: v24.14.0
• Integration: OpenClaw gateway calling acpx in prompt mode