fix(acp): a CLI that stops reading before the first request reports its exit text

The union run flaked test_alias_keeps_the_former_gh_extension_diagnosis: a CLI that dies at once
(the old `gh copilot` extension) can close its stdin before ACPClient writes `initialize`, and the
write raised a bare BrokenPipeError, skipping the early-exit path that reports the CLI's stderr
(and the alias's gh-extension diagnosis). Pre-existing on main's CopilotACPClient. A broken pipe on
a request write now waits briefly for the exit and falls into that path. Test drives the race
deterministically (the fake CLI closes stdin before the client's first write).

agent/acp_client.py

@@ -451,8 +451,12 @@ class ACPClient:
         def _request(method: str, params: dict[str, Any], *, text_parts: list[str] | None = None,
                      reasoning_parts: list[str] | None = None) -> Any:
             request_id = next(request_ids)
-            proc.stdin.write(json.dumps({"jsonrpc": "2.0", "id": request_id, "method": method, "params": params}) + "\n")
-            proc.stdin.flush()
+            try:
+                proc.stdin.write(json.dumps({"jsonrpc": "2.0", "id": request_id, "method": method, "params": params}) + "\n")
+                proc.stdin.flush()
+            except BrokenPipeError:  # the CLI stopped reading (dying): report its exit text, not a bare EPIPE
+                with contextlib.suppress(subprocess.TimeoutExpired):
+                    proc.wait(timeout=5)
             deadline = session_deadline
             while time.monotonic() < deadline and proc.poll() is None:
                 try:

tests/agent/test_acp_client_invariants.py

@@ -14,6 +14,7 @@ import json
 import os
 import subprocess
 import sys
+import time
 from pathlib import Path
 from unittest.mock import patch
 
@@ -304,6 +305,36 @@ def test_cli_death_is_reported_as_a_crash_not_a_timeout(tmp_path):
             pass
 
 
+_STDIN_CLOSING_ACP_CLI = """import os, sys, time
+os.close(0)  # stops reading: the client's first write hits a closed pipe
+open(sys.argv[1], "w").close()
+time.sleep(0.3)
+print('fatal: agent cannot start', file=sys.stderr)
+sys.exit(2)
+"""
+
+
+def test_cli_that_stops_reading_is_reported_with_its_exit_text(tmp_path):
+    """A CLI dying before it reads the first request: the write raised a bare BrokenPipeError, so the
+    user never saw the CLI's own crash text (and the error read as a client bug)."""
+    server = tmp_path / "stdin_closing_acp.py"
+    server.write_text(_STDIN_CLOSING_ACP_CLI, encoding="utf-8")
+    closed = tmp_path / "stdin-closed"
+    client = _AcmeClient(command=sys.executable, args=[str(server), str(closed)], acp_cwd=str(tmp_path))
+    real_spawn = client._spawn
+
+    def spawn_then_wait_for_the_close():
+        proc = real_spawn()
+        deadline = time.monotonic() + 30
+        while not closed.exists() and time.monotonic() < deadline:
+            time.sleep(0.01)
+        return proc
+
+    client._spawn = spawn_then_wait_for_the_close
+    with pytest.raises(RuntimeError, match="exited early: fatal: agent cannot start"):
+        with client._session(30):
+            pass
+
 def test_model_discovery_does_not_allow_file_requests(tmp_path):
     """``list_models`` opens a session only to read the catalog: an fs request on that path is refused."""
     target = tmp_path / "should-not-be-read.txt"