MCPInstall

session-execution

Use when working on or reviewing session execution, command handling, shell state, FIFO-based streaming, or stdout/stderr separation. Relevant for session.ts, command handlers, exec/execStream, or anything involving shell process management. (project)

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Install

Works with every agent the skills CLI supports.

npx skills add https://github.com/cloudflare/sandbox-sdk --skill session-execution
  1. 01

    Install

    Run the command in your project. The CLI asks which agents to install for.

  2. 02

    Describe the task

    Ask your agent for something this skill covers. It loads session-execution automatically when relevant.

  3. 03

    Review the result

    The agent follows the SKILL.md steps, scripts and references bundled with the skill.

SKILL.md

The instructions your agent reads when the skill is triggered.

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:

  1. Check if operations happen within the same session (mutex protects)
  2. Check if operations are per-session vs cross-session (cross-session races are real)
  3. 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

FAQ

What does the session-execution skill do?

Use when working on or reviewing session execution, command handling, shell state, FIFO-based streaming, or stdout/stderr separation. Relevant for session.ts, command handlers, exec/execStream, or anything involving shell process management. (project)

How do I install session-execution?

Run npx skills add https://github.com/cloudflare/sandbox-sdk --skill session-execution in your project. The skills CLI detects the agents you use and copies the skill into the right folder. For Claude Code you can also place the skill folder at ~/.claude/skills/session-execution/ (personal) or .claude/skills/session-execution/ (project).

Which AI agents can use session-execution?

Any agent that supports the open Agent Skills format (a folder with a SKILL.md file), including Claude Code, Codex, Cursor, GitHub Copilot, Gemini CLI, Windsurf, Cline, OpenCode and more.

Is session-execution safe to install?

No partner security audit is available yet. Skills can run commands and scripts, so read the SKILL.md and any bundled files before installing.

Who maintains session-execution?

It is published by cloudflare in cloudflare/sandbox-sdk, and it is an official skill from the maker of the technology it covers. It has 58 recorded installs.

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