Feature 7: Session Termination

Implementation approach: ACP extension, referencing existing ACP RFD

Summary

The legacy session REST API had an explicit terminate endpoint. ACP only has session/cancel (turn cancellation, not session kill) and DELETE /v1/rpc (connection close, not session termination). Need explicit session destroy/terminate semantics.

Current v1 State

session/cancel — cancels an in-flight prompt turn only
DELETE /v1/rpc — closes the HTTP connection, does not terminate the session
_sandboxagent/session/detach — detaches a session from a connection (multi-client visibility)
No session termination/deletion exists
rfds-vs-extensions.md: "Session Termination: Not covered by ACP. Only implement if product explicitly requires termination semantics beyond session/cancel"
extensibility-status.md: Documents _sandboxagent/session/terminate as proposed but not implemented

v1 Implementation

HTTP Endpoint

legacy session terminate endpoint

Handler (from `router.rs`)

The terminate handler:

Looked up the session by ID
Killed the agent subprocess (SIGTERM then SIGKILL after grace period)
Emitted a session.ended event with reason: Terminated, terminated_by: Daemon
Cleaned up session state

v1 Types

#[derive(Debug, Clone, Serialize, Deserialize, JsonSchema, ToSchema)]
pub struct SessionEndedData {
    pub reason: SessionEndReason,
    pub terminated_by: TerminatedBy,
    pub message: Option<String>,
    pub exit_code: Option<i32>,
    pub stderr: Option<StderrOutput>,
}

#[derive(Debug, Clone, PartialEq, Serialize, Deserialize, JsonSchema, ToSchema)]
#[serde(rename_all = "snake_case")]
pub enum SessionEndReason {
    Completed,
    Error,
    Terminated,
}

#[derive(Debug, Clone, Serialize, Deserialize, JsonSchema, ToSchema)]
#[serde(rename_all = "snake_case")]
pub enum TerminatedBy {
    Agent,
    Daemon,
}

ACP RFD Reference

Per ~/misc/acp-docs/, session termination is listed as an RFD topic. The existing ACP spec does not define a session/terminate or session/delete method.

Implementation Plan

ACP Extension Method

_sandboxagent/session/terminate

Client -> Runtime request:

{
  "jsonrpc": "2.0",
  "id": "t-1",
  "method": "_sandboxagent/session/terminate",
  "params": {
    "sessionId": "session-uuid"
  }
}

Response:

{
  "jsonrpc": "2.0",
  "id": "t-1",
  "result": {
    "terminated": true,
    "reason": "terminated",
    "terminatedBy": "daemon"
  }
}

Behavior

Client sends _sandboxagent/session/terminate request
Runtime identifies the session and its owning agent process
For shared-process agents (Codex, OpenCode): send a cancel/terminate signal to the agent process for that specific session
For per-turn subprocess agents (Claude, Amp): kill the subprocess if running, mark session as terminated
Emit _sandboxagent/session/ended to all connected clients watching that session
Method is idempotent: repeated calls on an already-ended session return success without side effects

Files to Modify

File	Change
`server/packages/sandbox-agent/src/acp_runtime/mod.rs`	Add `_sandboxagent/session/terminate` handler; add session removal from registry; add process kill logic
`server/packages/sandbox-agent/src/acp_runtime/mock.rs`	Add mock terminate support
`sdks/typescript/src/client.ts`	Add `terminateSession(sessionId)` method
`server/packages/sandbox-agent/tests/v1_api.rs`	Add session termination test

Docs to Update

Doc	Change
`docs/sdks/typescript.mdx`	Document `terminateSession` method
`research/acp/spec.md`	Add `_sandboxagent/session/terminate` to extension methods list
`research/acp/rfds-vs-extensions.md`	Update session termination row

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