capture_pane

Instructions

Implementation Reference

• server.js:134-151 (handler)

  The 'capturePane' function that executes the capture_pane tool logic. It shells out to tmux capture-pane to capture visible pane text.

  async function capturePane(args) {
    const missing = requireTmux();
    if (missing) return errorResult(missing);
    const bad = validSessionName(args.name);
    if (bad) return errorResult(bad);
    const lines = Math.max(1, Math.min(10000, Math.floor(args.lines ?? 100)));
    // `-p` prints to stdout, `-S -<N>` starts N lines before the bottom of the
    // scrollback, `-J` joins wrapped lines, `-t =<name>` targets the session's
    // active pane.
    // `=NAME:` targets the session's active window + active pane with exact name match.
    // `=NAME` (no colon) is a session target and fails on capture-pane which needs a pane.
    const r = await run(BIN.tmux, ['capture-pane', '-p', '-J', '-S', `-${lines}`, '-t', `=${args.name}:`]);
    if (r.code !== 0) return errorResult(`tmux capture-pane failed: ${r.stderr || r.stdout}`);
    // Trim trailing blank lines from the visible-pane snapshot; they're just
    // empty rows below the cursor and add noise.
    const text = r.stdout.replace(/\n+$/, '');
    return textResult({ session: args.name, lines, text });
  }

• server.js:267-280 (schema)

  The input schema registration for the capture_pane tool, defining name (required) and lines (optional, 1-10000) properties.

  {
    name: 'capture_pane',
    description: 'Capture the last N lines visible in the session\'s active pane (default 100, max 10000). Returns the joined text. The primary primitive for "is my training still running? show me the latest output".',
    annotations: { title: 'Capture pane output', readOnlyHint: true, destructiveHint: false, openWorldHint: false },
    inputSchema: {
      type: 'object',
      properties: {
        name: { type: 'string', description: 'Session name.' },
        lines: { type: 'integer', minimum: 1, maximum: 10000, description: 'Number of lines to capture (default 100).' },
      },
      required: ['name'],
      additionalProperties: false,
    },
  },

• server.js:326-334 (registration)

  The HANDLERS map that registers capture_pane -> capturePane for JSON-RPC dispatch.

  const HANDLERS = {
    list_sessions: listSessions,
    has_session: hasSession,
    list_windows: listWindows,
    capture_pane: capturePane,
    new_session: newSession,
    send_keys: sendKeys,
    kill_session: killSession,
  };

• server.js:49-64 (helper)

  The 'run' helper function that spawns a child process and captures stdout/stderr, used by capturePane to execute tmux.

  function run(cmd, args, opts = {}) {
    return new Promise((resolve) => {
      const child = spawn(cmd, args, { stdio: ['pipe', 'pipe', 'pipe'], ...opts });
      let out = Buffer.alloc(0);
      let err = Buffer.alloc(0);
      child.stdout.on('data', (d) => { out = Buffer.concat([out, d]); });
      child.stderr.on('data', (d) => { err = Buffer.concat([err, d]); });
      child.on('error', (e) => resolve({ code: -1, stdout: '', stderr: e.message }));
      child.stdin.end();
      child.on('close', (code) => resolve({
        code,
        stdout: out.toString('utf8'),
        stderr: err.toString('utf8'),
      }));
    });
  }

• server.js:69-74 (helper)

  The 'validSessionName' helper used by capturePane to validate the session name input.

  function validSessionName(name) {
    if (typeof name !== 'string' || !name.length) return 'name is required (non-empty string)';
    if (name.length > 100) return 'name too long (max 100)';
    if (/[.:\s|]/.test(name)) return "name cannot contain '.', ':', '|', or whitespace";
    return null;
  }