Missing terminal tool_call when custom tool throws

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1

Where does the bug appear (feature/product)?

Cursor SDK

Describe the Bug

When a custom MCP tool throws or otherwise never returns, the SDK does not emit a terminal tool_call event. The transcript only persists tool calls on completed or error, so a running call whose tool never finishes is never written. I lose failed delegations entirely from the durable record.

Steps to Reproduce

Run a conversation where custom tools (e.g. delegate) throw or hang without returning. Inspect the persisted transcript. In pr-delivery-loop I see 33 tool calls, all completed, with zero record of four delegations that failed. The only trace is model prose, not structured events.

Expected Behavior

Every started tool_call gets a terminal completed or error event in the transcript, even when the tool throws or the turn is cancelled.

Version Information

issue-tracker app embedding @cursor/sdk on linux 6.12.67-linuxkit, Node in Cursor agent session, model composer.

5

Hey, thanks for the detailed report. The breakdown is, as always, top-notch.

Confirmed, what you described is reproducible. In the local SDK runtime, the terminal tool_call event really doesn’t make it into the transcript when a custom/MCP tool throws an exception or hangs, and finished calls get stamped as completed with no way to get status: "error". So 33 tool calls, all completed with four failed delegations is exactly the behavior we’re seeing.

I’ve passed this to the team. I can’t share an ETA yet, but I’ll reply in this thread when there’s an update.

If you have a minimal repro script (a custom tool that throws + a snippet that reads run.messages()), please attach it. It’s not required to reproduce, but it’d still be helpful.