session.shutdown event should persist the reason from SessionEndHookInput

[d0d1]

Problem

The runtime internally tracks session end reason via SessionEndHookInput.reason, but the session.shutdown event persisted to events.jsonl only records shutdownType: "routine" | "error". That collapses several distinct non-error causes into the single label "routine" and makes post-hoc diagnosis of session lifecycle incidents much harder than it needs to be.

From the installed 1.0.32 type definitions:

// sdk/index.d.ts
export declare interface SessionEndHookInput extends BaseHookInput {
    reason: "complete" | "error" | "abort" | "timeout" | "user_exit";
    finalMessage?: string;
    error?: Error;
}

// sdk/index.d.ts
shutdown(shutdownType?: "routine" | "error", errorReason?: string): void;

// copilot-sdk/generated/session-events.d.ts
data: {
    shutdownType: "routine" | "error";
    errorReason?: string;
    // ... usage metrics
}

So the runtime appears to have more granular reason information available internally, but the persisted shutdown event does not keep it.

Concrete impact

In one 1.0.32/Linux session, I observed two session.shutdown events with shutdownType: "routine" that had materially different provenance:

Shutdown	Provenance visible from nearby events
1st	preceded by an abort event with reason: "user initiated", resumed 18 seconds later
2nd	no preceding abort, interrupted active tool execution, bash subprocess received SIGTERM / exit 143 163 ms later, resume happened 2h 44m later

At the session.shutdown event level, those two cases are indistinguishable even though the surrounding event stream shows they are not the same kind of lifecycle transition.

Context for the second shutdown

Single incident; not claiming a reproducible bug. Parameters at the time of the disruptive shutdown:

Metric	Value
Copilot CLI	1.0.32
OS	Linux
Session age	6h 12m 49.578s
Time since last user message	5h 24m 08.244s
events.jsonl size through shutdown line	45,016,499 bytes / 11,691 lines
task tool launches before shutdown	40
Cumulative API duration	38,889,407 ms (~10.8h)

I am not claiming the shutdown itself was necessarily a bug. It may have been policy, host lifecycle, resource pressure, or something else. The issue here is that the persisted shutdown event does not retain enough reason information to let users or maintainers tell which.

Request

Please persist the internal session-end reason (or equivalent) into session.shutdown events. Even a small additive field would help a lot, for example:

data: {
    shutdownType: "routine" | "error";
    reason?: "complete" | "abort" | "timeout" | "user_exit";
    errorReason?: string;
}

If the runtime can distinguish other routine-shutdown causes such as external signal receipt or platform/session eviction, a richer optional field would be even better. But even persisting the existing SessionEndHookInput.reason values would be a meaningful improvement.

Why this matters

This would make it much easier to triage session-lifecycle issues without reproducing them live, including cases that currently look similar in raw logs but may have very different causes.

Related issues

• Long-lived session shows as corrupted on resume despite valid events.jsonl #2209 — large-session resume failures (different failure mode, same session-lifecycle diagnostic wall)
• events.jsonl mutex timeout and stale inuse.*.lock files after long-lived session grows large #2609 — large-session write-path failures (different failure mode, same session-lifecycle diagnostic wall)
• CLI crashes during parallel background agent execution (OOM/TypeScript error in long sessions) #2132 — long-session crash under parallel agents (different mechanism: crash/OOM rather than a persisted session.shutdown)

Environment

• Copilot CLI 1.0.32
• Node.js v24.11.1
• Linux x86_64