fix(shell): validate shell yields against the PTY ceiling

The PTY layer clamps any yield above its maximum, so a larger configured value bought nothing and looked effective. Bound both yield settings by that constant instead, and take the exec default from it.

strix/config/settings.py

@@ -4,6 +4,7 @@ from __future__ import annotations
 
 from typing import Literal
 
+from agents.sandbox.session.pty_types import PTY_YIELD_TIME_MS_MAX
 from pydantic import AliasChoices, Field
 from pydantic_settings import BaseSettings, SettingsConfigDict
 
@@ -110,19 +111,27 @@ class ShellSettings(BaseSettings):
     Raising these defaults lets one call return a meaningful result instead of a
     no-op round-trip. An explicit ``yield_time_ms`` from the model always wins.
 
-    The SDK's PTY layer clamps every yield to 30s, so a larger value here would
-    be silently ineffective; keep both yields at or below that ceiling.
+    The SDK's PTY layer clamps every yield to ``PTY_YIELD_TIME_MS_MAX``, so both
+    yields are validated against that ceiling instead of being silently reduced.
     """
 
     model_config = _BASE_CONFIG
 
-    # Default yield for exec_command when the model omits yield_time_ms. 30s is
-    # the most the PTY layer honours, so this sits right at that ceiling.
-    exec_yield_ms: int = Field(default=30_000, gt=0, alias="STRIX_SHELL_EXEC_YIELD_MS")
+    # Default yield for exec_command when the model omits yield_time_ms. The
+    # default sits at the ceiling: waiting is cheaper than another poll turn.
+    exec_yield_ms: int = Field(
+        default=PTY_YIELD_TIME_MS_MAX,
+        gt=0,
+        le=PTY_YIELD_TIME_MS_MAX,
+        alias="STRIX_SHELL_EXEC_YIELD_MS",
+    )
     # Default yield for an empty (polling) write_stdin call. The SDK already
     # floors an empty poll at 5s; this trades a little latency for far fewer turns.
     write_stdin_poll_yield_ms: int = Field(
-        default=20_000, gt=0, alias="STRIX_SHELL_WRITE_STDIN_POLL_YIELD_MS"
+        default=20_000,
+        gt=0,
+        le=PTY_YIELD_TIME_MS_MAX,
+        alias="STRIX_SHELL_WRITE_STDIN_POLL_YIELD_MS",
     )
     # Cap on a bare `sleep N` hand-wait (seconds); larger sleeps are clamped.
     max_sleep_seconds: int = Field(default=60, gt=0, alias="STRIX_SHELL_MAX_SLEEP_SECONDS")

tests/test_agent_factory_shell.py

@@ -8,11 +8,12 @@ from typing import Any, cast
 
 import pytest
 from agents.sandbox.errors import InvalidManifestPathError
+from agents.sandbox.session.pty_types import PTY_YIELD_TIME_MS_MAX
 from agents.tool import CustomTool, FunctionTool
 from pydantic import BaseModel, ValidationError
 
 from strix.agents import factory
-from strix.config import load_settings
+from strix.config import ShellSettings, load_settings
 
 
 def _capturing_exec_tool(captured: dict[str, str]) -> FunctionTool:
@@ -363,3 +364,12 @@ async def test_invalid_workdir_is_rendered_as_a_message() -> None:
     assert isinstance(result, str)
     assert "workdir must be a path inside /workspace" in result
     assert "'../etc'" in result
+
+
+@pytest.mark.parametrize("field", ["exec_yield_ms", "write_stdin_poll_yield_ms"])
+def test_shell_settings_reject_a_yield_above_the_pty_ceiling(field: str) -> None:
+    """A yield the PTY layer would clamp is a misconfiguration, not a longer wait."""
+    with pytest.raises(ValidationError):
+        ShellSettings(**{field: PTY_YIELD_TIME_MS_MAX + 1})
+
+    assert getattr(ShellSettings(**{field: PTY_YIELD_TIME_MS_MAX}), field) == PTY_YIELD_TIME_MS_MAX