Ensure `events.jsonl` honors inherited file-size limits

[ajjosh-msft2]

Summary

The Copilot CLI persists session events to:

~/.copilot/session-state/<sessionId>/events.jsonl

The CLI inherits the hosting process’s file-size limit. During sufficiently long
sessions, events.jsonl reaches that limit and the next persistence write fails:

I/O error: File too large (os error 27)

This emits SDK_SESSION_ERROR(persistence) and fails the active turn.

Request

Please ensure that events.jsonl remains within the file-size limit inherited by
the Copilot CLI while preserving supported session persistence and resume behavior.

If this behavior is owned by the bundled CLI runtime rather than the SDK wrappers,
please route the issue to the appropriate owner.

Environment

• SDK: github-copilot-sdk==1.0.7
• Bundled CLI: 1.0.71
• Host file-size limit: 50 MB
• Platform: Linux container
• Error: EFBIG / os error 27

Acceptance criteria

• events.jsonl does not exceed the inherited file-size limit.
• Long-running sessions do not fail with EFBIG due to event-log growth.
• Persisted sessions remain resumable with their expected state.

Impact

Observed in production across real user conversations, causing repeated
Service-class turn failures.

Reference

Internal tracking: https://microsoft.ghe.com/bic/mcs-dracarys/issues/7127

[github-actions]

Investigation findings

Label applied: bug

What I investigated

• events.jsonl references across all SDK language implementations and generated RPC types
• Session persistence documentation (docs/features/session-persistence.md) and the limitations table
• Compaction events (session.compaction_start / session.compaction_complete) and what they cover
• Troubleshooting and compatibility docs for any mention of file-size limits, log rotation, or EFBIG mitigations

What I found

The events.jsonl file is the CLI's append-only event log for a session. Compaction (triggered by infiniteSessions) manages the LLM context window — it summarizes conversation history so the model doesn't hit token limits — but it does not truncate or rotate events.jsonl on disk. The file grows without bound for the lifetime of a session.

The SDK documentation's "Limitations and considerations" table (session-persistence.md) does list that ~/.copilot/session-state/ must be writable, but makes no mention of:

• The fact that events.jsonl is unbounded
• That the CLI inherits the host process's RLIMIT_FSIZE and will fail with EFBIG (os error 27) once the file exceeds that limit
• Any workaround (e.g., ulimit -f unlimited, periodic session deletion, or log rotation)

The SDK_SESSION_ERROR(persistence) error that surfaces to the SDK consumer is a real, observed production failure with no documented mitigation path.

Why this is a bug

The intended behavior is that long-running sessions work correctly. The current behavior — unbounded events.jsonl growth that causes hard I/O failures in constrained environments — is not by design. The CLI should either:

1. Rotate or truncate events.jsonl to honor the inherited file-size limit, or
2. Expose a configuration option to cap/rotate the event log

Until the CLI runtime is fixed, the immediate workaround for affected deployments is to raise the file-size limit in the container before starting the process (e.g., ulimit -f unlimited) or to use short-lived sessions with periodic deleteSession + recreate cycles.

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