diff --git a/argus_skill/agent_cli/_prompt_delivery.py b/argus_skill/agent_cli/_prompt_delivery.py
index 65904acf9..9206dc2bd 100644
--- a/argus_skill/agent_cli/_prompt_delivery.py
+++ b/argus_skill/agent_cli/_prompt_delivery.py
@@ -5,7 +5,9 @@
 import os
 import subprocess
 import tempfile
+from contextlib import contextmanager
 from pathlib import Path
+from typing import Iterator, TextIO
 
 from ..core.sandbox import sandboxed_child_env
 from ._sandbox_commands import (
@@ -114,6 +116,28 @@ def _opencode_full_access_env() -> dict[str, str]:
 class PromptDeliveryMixin:
     """Deliver large role prompts without exposing them in process arguments."""
 
+    @staticmethod
+    @contextmanager
+    def _prompt_stdin(prompt: str | None) -> Iterator[TextIO | int]:
+        """Provide finite stdin without writing into a live child's pipe.
+
+        CLIs may emit startup output before reading their prompt. A synchronous
+        pipe write before starting stdout/stderr readers deadlocks under that
+        backpressure. A temporary file preserves stdin delivery and EOF without
+        a writer thread, prompt argv exposure, or a pipe-capacity dependency.
+        The child inherits its own handle; the parent closes its copy at spawn.
+        """
+        if prompt is None:
+            yield subprocess.DEVNULL
+            return
+        with tempfile.TemporaryFile(mode="w+", encoding="utf-8", errors="replace") as stream:
+            stream.write(prompt)
+            if not prompt.endswith("\n"):
+                stream.write("\n")
+            stream.seek(0)
+            yield stream
+
+
     @staticmethod
     def _write_prompt(*, process: subprocess.Popen[str], prompt: str) -> None:
         if process.stdin is None:
diff --git a/argus_skill/agent_cli/_run_exec.py b/argus_skill/agent_cli/_run_exec.py
index 459feffce..758f91bab 100644
--- a/argus_skill/agent_cli/_run_exec.py
+++ b/argus_skill/agent_cli/_run_exec.py
@@ -234,35 +234,29 @@ def _spawn_turn_process(
                     prompt_path,
                 )
         try:
-            process = subprocess.Popen(
-                command,
-                stdin=subprocess.PIPE,
-                stdout=subprocess.PIPE,
-                stderr=subprocess.PIPE,
-                text=True,
-                # Pin UTF-8 explicitly: without this, text mode uses the OS locale
-                # encoding, which is cp1252 on Windows and raises UnicodeEncodeError
-                # when the prompt or streamed model output contains non-Latin-1
-                # characters (e.g. "\u2192", CJK, emoji). errors="replace" keeps the
-                # reader from crashing on malformed bytes mid-stream.
-                encoding="utf-8",
-                errors="replace",
-                bufsize=1,
-                cwd=options.working_dir or None,
-                env=self._child_env(options, executable=command[0]),
-                **background_subprocess_kwargs(),
-            )
+            with self._prompt_stdin(stdin_prompt) as child_stdin:
+                process = subprocess.Popen(
+                    command,
+                    stdin=child_stdin,
+                    stdout=subprocess.PIPE,
+                    stderr=subprocess.PIPE,
+                    text=True,
+                    # Pin UTF-8 explicitly: without this, text mode uses the OS locale
+                    # encoding, which is cp1252 on Windows and raises UnicodeEncodeError
+                    # when the prompt or streamed model output contains non-Latin-1
+                    # characters (e.g. "\u2192", CJK, emoji). errors="replace" keeps the
+                    # reader from crashing on malformed bytes mid-stream.
+                    encoding="utf-8",
+                    errors="replace",
+                    bufsize=1,
+                    cwd=options.working_dir or None,
+                    env=self._child_env(options, executable=command[0]),
+                    **background_subprocess_kwargs(),
+                )
         except BaseException:
             if prompt_path is not None:
                 prompt_path.unlink(missing_ok=True)
             raise
-        if stdin_prompt is not None:
-            self._write_prompt(
-                process=process,
-                prompt=stdin_prompt,
-            )
-        else:
-            self._close_stdin(process)
         return command, process, None, prompt_path
 
     def _stream_turn_output(
diff --git a/tests/agent_cli/test_prompt_stdin_backpressure.py b/tests/agent_cli/test_prompt_stdin_backpressure.py
new file mode 100644
index 000000000..b71f7ff8c
--- /dev/null
+++ b/tests/agent_cli/test_prompt_stdin_backpressure.py
@@ -0,0 +1,104 @@
+"""Prompt delivery under real input/output backpressure."""
+
+import hashlib
+import json
+import sys
+import time
+from concurrent.futures import ThreadPoolExecutor
+
+import pytest
+
+from argus_skill.agent_cli import _run_exec
+from argus_skill.agent_cli.agent_cli_runner import AgentCliRunner, RunnerOptions
+
+
+def run_fixture(monkeypatch, tmp_path, body, prompt, *, options=None, callback=None):
+    runner = AgentCliRunner(agent_bin=sys.executable, backend="codex", event_callback=callback)
+    monkeypatch.setattr(
+        runner, "_build_command", lambda **kw: [sys.executable, "-X", "utf8", "-u", "-c", body]
+    )
+    processes = []
+    original = _run_exec.subprocess.Popen
+
+    def capture(*args, **kwargs):
+        proc = original(*args, **kwargs)
+        processes.append(proc)
+        return proc
+
+    monkeypatch.setattr(_run_exec.subprocess, "Popen", capture)
+    start = time.monotonic()
+    with ThreadPoolExecutor(max_workers=1) as executor:
+        future = executor.submit(
+            runner.run_exec,
+            prompt=prompt,
+            resume_thread_id=None,
+            options=options or RunnerOptions(working_dir=str(tmp_path)),
+            run_label="engineer-r1",
+        )
+        try:
+            result = future.result(timeout=5)
+            return result, time.monotonic() - start, processes
+        finally:
+            for proc in processes:
+                runner._terminate_process(proc)
+            # If an assertion or timeout fired, terminating the identified root
+            # releases a blocked stdin write before joining the executor.
+            if not future.done():
+                future.result(timeout=5)
+
+
+@pytest.mark.parametrize("stream", [1, 2])
+@pytest.mark.parametrize("prompt", ["p" * 262144, "中文🙂\n" * 65536], ids=["ascii", "unicode"])
+def test_large_prompt_and_startup_output_cannot_deadlock(monkeypatch, tmp_path, stream, prompt):
+    received = tmp_path / "received.json"
+    body = (
+        "import os,sys,json,hashlib;from pathlib import Path;"
+        f"os.write({stream},b'x'*262144+b'\\n');data=sys.stdin.read();"
+        f"Path({str(received)!r}).write_text(json.dumps({{'hash':hashlib.sha256(data.encode()).hexdigest(),'length':len(data)}}));"
+        "print(json.dumps({'type':'turn.completed'}),flush=True)"
+    )
+    result, elapsed, processes = run_fixture(monkeypatch, tmp_path, body, prompt)
+    expected = prompt if prompt.endswith("\n") else prompt + "\n"
+    assert json.loads(received.read_text()) == {
+        "hash": hashlib.sha256(expected.encode()).hexdigest(),
+        "length": len(expected),
+    }
+    assert result.turn_completed and result.exit_code == 0 and elapsed < 3
+    assert all(p.stdin is None for p in processes)
+
+
+@pytest.mark.parametrize("reason", ["stop", "hard_idle"])
+def test_child_not_reading_input_can_be_stopped(monkeypatch, tmp_path, reason):
+    start = time.monotonic()
+    options = RunnerOptions(working_dir=str(tmp_path), watchdog_hard_idle_seconds=1)
+    if reason == "stop":
+        options.external_interrupt_reason_provider = lambda: (
+            "daemon stop requested" if time.monotonic() - start > 0.6 else None
+        )
+    result, elapsed, processes = run_fixture(
+        monkeypatch, tmp_path, "import time;time.sleep(20)", "p" * 262144, options=options
+    )
+    assert elapsed < 3 and result.turn_failed
+    assert (
+        "daemon stop requested" if reason == "stop" else "hard idle timeout"
+    ) in result.fatal_error
+    assert all(p.poll() is not None for p in processes)
+
+
+def test_spawn_failure_closes_temporary_stdin(monkeypatch, tmp_path):
+    inputs = []
+
+    def fail(*args, **kwargs):
+        inputs.append(kwargs["stdin"])
+        assert kwargs["stdin"].read() == "fixture\n"
+        raise OSError("injected spawn error")
+
+    monkeypatch.setattr(_run_exec.subprocess, "Popen", fail)
+    runner = AgentCliRunner(agent_bin=sys.executable, backend="codex")
+    with pytest.raises(OSError, match="injected spawn"):
+        runner.run_exec(
+            prompt="fixture",
+            resume_thread_id=None,
+            options=RunnerOptions(working_dir=str(tmp_path)),
+        )
+    assert inputs and inputs[0].closed