When a model passes `acp_command="copilot"` (or any other binary name) in a
`delegate_task` tool call, `_build_child_agent` unconditionally sets
`effective_provider = "copilot-acp"`, which routes the subagent through
`CopilotACPClient`. That client spawns the named binary via subprocess; if it
isn't on PATH, every retry raises RuntimeError and an asyncio cleanup race
during error delivery can take the entire gateway down.
This is a real failure mode on headless deploys (Railway / Fly / VPS / Docker)
where `copilot` / `claude` / etc. aren't installed. The schema does say
"Do NOT set unless the user explicitly told you an ACP CLI is installed,"
but models occasionally pass it anyway — particularly for X (Twitter) search
prompts where Grok seems to associate ACP with "search assistance."
Reproduction:
- Headless install (no `copilot` binary on PATH)
- Set provider to xai-oauth + model grok-4.3
- Telegram prompt: "Search X for crypto twitter trends"
- Grok decides to delegate and passes `acp_command="copilot"`
- Subagent crashes 3x, gateway crashes on the 3rd retry teardown
Fix: validate the binary exists on PATH via `shutil.which` before honoring
the override. If missing, log a warning and fall through to the parent's
default transport. No behavior change when the binary IS present (covered
by `test_build_child_agent_honors_acp_command_when_binary_present`).
Tests:
- `test_build_child_agent_ignores_acp_command_when_binary_missing`
- `test_build_child_agent_honors_acp_command_when_binary_present`
Verified on Python 3.11 (macOS) and 3.12 (Debian 13 container).
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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