Session Execution Agent Skill

Session Execution

Read docs/SESSION_EXECUTION.md before working in this area. It explains the architecture for reliable command execution with stdout/stderr separation.

Key Concepts

Two execution modes:

• Foreground (exec): Runs in main shell, state persists. Uses temp files for output capture.
• Background (execStream/startProcess): Runs in subshell via FIFOs. Labelers prefix output in background.

Binary prefix contract:

• Stdout: \x01\x01\x01 prefix per line
• Stderr: \x02\x02\x02 prefix per line
• Log parser reconstructs streams from these prefixes

Completion signaling:

• Exit code written to <id>.exit file via atomic tmp + mv
• Hybrid fs.watch + polling detects completion (robust on tmpfs/overlayfs)
• Background mode uses labelers.done marker to ensure output is fully captured

When Developing

• Understand why foreground uses temp files (bash waits for redirects to complete)
• Understand why background uses FIFOs (concurrent streaming without blocking shell)
• Test silent commands (cd, variable assignment) - these historically caused hangs
• Test large output - buffering issues can cause incomplete logs

When Reviewing

Correctness checks:

• Verify exit code handling is atomic (write to .tmp then mv)
• Check FIFO cleanup in error paths
• Ensure labelers.done is awaited before reading final output (background mode)

Race condition analysis:

Session execution has a mutex that serializes command execution per session. Before flagging race conditions:

• Check if operations happen within the same session (mutex protects)
• Check if operations are per-session vs cross-session (cross-session races are real)
• Refer to docs/CONCURRENCY.md for the full concurrency model

Common false positives:

• "Concurrent reads/writes to session state" - mutex serializes these
• "FIFO operations might race" - labelers are per-command, not shared

Actual concerns to watch for:

• Cross-session operations without proper isolation
• Cleanup operations that might affect still-running commands
• File operations outside the mutex-protected section

Key Files

• packages/sandbox-container/src/session.ts - Session class with exec/execStream
• packages/sandbox-container/src/managers/SessionManager.ts - Mutex and lifecycle
• packages/sandbox/src/clients/CommandClient.ts - SDK interface to session commands