Function handleSpawn

src/tools/shared/spawnMultiAgent.ts:1029–1067

* Handle spawn operation - creates a new NCode instance. * Uses in-process mode when enabled, otherwise uses tmux/iTerm2 split-pane view. * Falls back to in-process if pane backend detection fails (e.g., iTerm2 without * it2 CLI or tmux installed).

(
  input: SpawnInput,
  context: ToolUseContext,
)

 * it2 CLI or tmux installed).
 */
async function handleSpawn(
  input: SpawnInput,
  context: ToolUseContext,
): Promise<{ data: SpawnOutput }> {
  // Check if in-process mode is enabled via feature flag
  if (isInProcessEnabled()) {
    return handleSpawnInProcess(input, context)
  }

  // Pre-flight: ensure a pane backend is available before attempting pane-based spawn.
  // This handles auto-mode cases like iTerm2 without it2 or tmux installed, where
  // isInProcessEnabled() returns false but detectAndGetBackend() has no viable backend.
  // Narrowly scoped so user cancellation and other spawn errors propagate normally.
  try {
    await detectAndGetBackend()
  } catch (error) {
    // Only fall back silently in auto mode. If the user explicitly configured
    // teammateMode: 'tmux', let the error propagate so they see the actionable
    // install instructions from getTmuxInstallInstructions().
    if (getTeammateModeFromSnapshot() !== 'auto') {
      throw error
    }
    logForDebugging(
      `[handleSpawn] No pane backend available, falling back to in-process: ${errorMessage(error)}`,
    )
    // Record the fallback so isInProcessEnabled() reflects the actual mode
    // (fixes banner and other UI that would otherwise show tmux attach commands).
    markInProcessFallback()
    return handleSpawnInProcess(input, context)
  }

  // Backend is available (and now cached) - proceed with pane spawning.
  // Any errors here (user cancellation, validation, etc.) propagate to the caller.
  const useSplitPane = input.use_splitpane !== false
  if (useSplitPane) {
    return handleSpawnSplitPane(input, context)
  }
  return handleSpawnSeparateWindow(input, context)
}

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