diff --git a/strix/agents/factory.py b/strix/agents/factory.py
index 977d01f19..83dc88280 100644
--- a/strix/agents/factory.py
+++ b/strix/agents/factory.py
@@ -6,7 +6,7 @@
 import json
 import logging
 import re
-from typing import TYPE_CHECKING, Any
+from typing import TYPE_CHECKING, Any, cast
 
 from agents.agent import ToolsToFinalOutputResult
 from agents.sandbox import SandboxAgent
@@ -168,6 +168,11 @@ def _configure_chat_completions_filesystem_tools(toolset: Any) -> None:
 
 
 _CHARS_ESCAPE_RE = re.compile(r"\\(?:u[0-9a-fA-F]{4}|x[0-9a-fA-F]{2}|[0abtnvfr\\])")
+_PTY_SESSION_NOT_FOUND_RE = re.compile(r"PTY session not found: (?P<session_id>\d+)")
+_WRITE_STDIN_DEAD_SESSION_RESULT_TEMPLATE = (
+    "write_stdin: session {session_id} is already terminal/non-retryable "
+    "after PTY session not found."
+)
 _CHARS_ESCAPE_MAP = {
     "\\\\": "\\",
     "\\n": "\n",
@@ -234,19 +239,32 @@ async def invoke(ctx: Any, raw_input: str) -> Any:
 
 def _wrap_write_stdin(tool: FunctionTool) -> FunctionTool:
     invoke_tool = tool.on_invoke_tool
+    dead_pty_session_ids: set[int] = set()
 
     async def invoke(ctx: Any, raw_input: str) -> Any:
+        parsed: dict[str, Any] | None
         try:
-            parsed = json.loads(raw_input)
+            raw_parsed: Any = json.loads(raw_input)
         except json.JSONDecodeError:
             parsed = None
-        if isinstance(parsed, dict) and isinstance(parsed.get("chars"), str):
-            parsed["chars"] = _decode_chars_escape(parsed["chars"])
+        else:
+            parsed = cast("dict[str, Any]", raw_parsed) if isinstance(raw_parsed, dict) else None
+        session_id = parsed.get("session_id") if isinstance(parsed, dict) else None
+        chars = parsed.get("chars") if isinstance(parsed, dict) else None
+        if isinstance(session_id, int) and chars == "" and session_id in dead_pty_session_ids:
+            return _WRITE_STDIN_DEAD_SESSION_RESULT_TEMPLATE.format(session_id=session_id)
+        if isinstance(parsed, dict) and isinstance(chars, str):
+            parsed["chars"] = _decode_chars_escape(chars)
             raw_input = json.dumps(parsed)
         try:
-            return await invoke_tool(ctx, raw_input)
+            result = await invoke_tool(ctx, raw_input)
         except ValidationError as exc:
             return _format_validation_error(tool.name, exc)
+        if isinstance(result, str):
+            match = _PTY_SESSION_NOT_FOUND_RE.search(result)
+            if match is not None:
+                dead_pty_session_ids.add(int(match.group("session_id")))
+        return result
 
     tool.on_invoke_tool = invoke
     return tool
diff --git a/tests/test_agent_factory_shell.py b/tests/test_agent_factory_shell.py
index 6de932190..7f2f2c551 100644
--- a/tests/test_agent_factory_shell.py
+++ b/tests/test_agent_factory_shell.py
@@ -11,6 +11,10 @@
 from strix.agents import factory
 
 
+_MISSING_PTY_SESSION_ID = 60418
+_MISSING_PTY_RESULT = f"write_stdin failed: PTY session not found: {_MISSING_PTY_SESSION_ID}"
+
+
 def _capturing_exec_tool(captured: dict[str, str]) -> FunctionTool:
     async def invoke(_ctx: Any, raw_input: str) -> str:
         captured["raw_input"] = raw_input
@@ -24,6 +28,19 @@ async def invoke(_ctx: Any, raw_input: str) -> str:
     )
 
 
+def _capturing_write_stdin_tool(results: list[str], captured: list[str]) -> FunctionTool:
+    async def invoke(_ctx: Any, raw_input: str) -> str:
+        captured.append(raw_input)
+        return results.pop(0)
+
+    return FunctionTool(
+        name="write_stdin",
+        description="test tool",
+        params_json_schema={"type": "object", "properties": {}},
+        on_invoke_tool=invoke,
+    )
+
+
 @pytest.mark.asyncio
 async def test_wrap_exec_command_defaults_shell_to_bash() -> None:
     captured: dict[str, str] = {}
@@ -49,3 +66,19 @@ async def test_wrap_exec_command_preserves_explicit_shell(shell: str) -> None:
     )
 
     assert json.loads(captured["raw_input"])["shell"] == shell
+
+
+@pytest.mark.asyncio
+async def test_wrap_write_stdin_blocks_repeat_empty_poll_after_dead_pty() -> None:
+    captured: list[str] = []
+    wrapped = factory._wrap_write_stdin(
+        _capturing_write_stdin_tool([_MISSING_PTY_RESULT], captured)
+    )
+    raw_input = json.dumps({"session_id": _MISSING_PTY_SESSION_ID, "chars": ""})
+
+    first_result = await wrapped.on_invoke_tool(cast("Any", None), raw_input)
+    second_result = await wrapped.on_invoke_tool(cast("Any", None), raw_input)
+
+    assert first_result == _MISSING_PTY_RESULT
+    assert "non-retryable" in second_result
+    assert len(captured) == 1