API: Add abort/cancel turn endpoint

[NathanFlurry]

Description

The Sandbox Agent API needs an abort/cancel turn endpoint so that clients can stop an in-progress turn without terminating the entire session.

Currently, the only way to stop a running agent is POST /v1/sessions/{sessionId}/terminate, which destroys the session entirely. There's no way to cancel just the current turn and continue the conversation.

Proposed API

Add POST /v1/sessions/{sessionId}/abort that:

1. Cancels the in-progress turn (stops the LLM request / agent subprocess work)
2. Keeps the session alive so new messages can be sent
3. Emits a session.ended event with reason: "terminated" for the current turn, or a new event type like turn.aborted

Agent support

• OpenCode already has a native session.abort() API that does exactly this
• Claude Code supports Escape to cancel via stdin
• Codex — needs investigation
• Amp — needs investigation

The daemon would need to translate the universal abort into the appropriate agent-specific mechanism.

Related

• Gigacode ESC cancel: needs to call this endpoint once it exists (see linked gigacode issue)

🤖 Generated with Claude Code

[Glavin001]

Was just about to create a new issue for asking how to stop a streaming session.

I'm new to Sandbox Agent, however, would be happy to contribute with some guidance to get started (e.g. if you already have an idea on how best to implement this with say Claude Code Agent, etc). Thanks!