diff --git a/docs/research/request-timeout-boundaries.md b/docs/research/request-timeout-boundaries.md
index 8b8ec4d..38f2607 100644
--- a/docs/research/request-timeout-boundaries.md
+++ b/docs/research/request-timeout-boundaries.md
@@ -14,6 +14,16 @@ HTTPX timeout extension immediately before HTTPCore dispatch:
 - malformed maps, unknown keys, booleans, negative values, and non-finite
   numbers fail through the generic `EgressNotAllowedError` boundary.
 
+The trusted policy-construction boundary accepts only the exact
+`EgressTimeoutPolicy` type. Subclass polymorphism is not a supported extension
+mechanism because transport binding later invokes `as_httpcore_timeout()`: a
+subclass could otherwise replace that reviewed export path after startup
+validation. Applications that supplied an `EgressTimeoutPolicy` subclass must
+migrate to an exact instance configured through the documented immutable timeout
+fields. This secure-default boundary keeps declarative values authoritative; it
+does not claim to sandbox arbitrary trusted Python code executing in the host
+process.
+
 Policy maxima must be greater than zero. A request may still choose zero as an
 immediate, stricter timeout. The sanitized mapping is detached from caller-owned
 state and preserves unrelated safe extensions, including the validated TLS
@@ -49,6 +59,8 @@ response data.
 
 ## Security properties
 
+- **Exact trusted policy type:** construction rejects timeout-policy subclasses
+  before any later transport export can dynamically dispatch subclass code.
 - **No timeout disablement:** missing and `None` phase values become finite.
 - **No weaker override:** a request cannot exceed the immutable policy cap.
 - **Stricter caller control:** non-negative values below the cap are retained.
diff --git a/src/egressweave/policy.py b/src/egressweave/policy.py
index 80afe95..f92dfbb 100644
--- a/src/egressweave/policy.py
+++ b/src/egressweave/policy.py
@@ -178,7 +178,7 @@ def __post_init__(self) -> None:
         """Validate and canonicalize every immutable policy field."""
         if not isinstance(self.allow_local, bool):
             raise TypeError("allow_local must be a boolean")
-        if not isinstance(self.request_timeout_policy, EgressTimeoutPolicy):
+        if type(self.request_timeout_policy) is not EgressTimeoutPolicy:
             raise TypeError(
                 "request_timeout_policy must be an EgressTimeoutPolicy"
             )
@@ -511,4 +511,4 @@ def allows_http_method(self, method: str) -> bool:
             normalized = _normalize_allowed_method(method)
         except (TypeError, ValueError):
             return False
-        return normalized in self.allowed_methods
\ No newline at end of file
+        return normalized in self.allowed_methods
diff --git a/tests/test_timeout_policy_type_boundary.py b/tests/test_timeout_policy_type_boundary.py
new file mode 100644
index 0000000..180b9b3
--- /dev/null
+++ b/tests/test_timeout_policy_type_boundary.py
@@ -0,0 +1,33 @@
+"""Regression contracts for the exact request-timeout policy type boundary."""
+
+from __future__ import annotations
+
+import pytest
+
+from egressweave import EgressPolicy, EgressTimeoutPolicy
+
+
+class _HostileTimeoutPolicy(EgressTimeoutPolicy):
+    """Model a subclass that can replace the reviewed timeout export method."""
+
+    def as_httpcore_timeout(self) -> dict[str, float]:
+        """Fail if a later transport dynamically dispatches this override."""
+        raise AssertionError("subclass-controlled timeout export executed")
+
+
+def test_host_policy_rejects_timeout_policy_subclass() -> None:
+    """Reject non-exact timeout policy types at trusted policy construction."""
+    with pytest.raises(TypeError, match="request_timeout_policy"):
+        EgressPolicy.from_hosts(
+            "api.example.com",
+            request_timeout_policy=_HostileTimeoutPolicy(),
+        )
+
+
+def test_exact_authority_policy_rejects_timeout_policy_subclass() -> None:
+    """Apply the same exact-type boundary to the authority-pair constructor."""
+    with pytest.raises(TypeError, match="request_timeout_policy"):
+        EgressPolicy.from_authorities(
+            [("api.example.com", 443)],
+            request_timeout_policy=_HostileTimeoutPolicy(),
+        )
diff --git a/tests/test_timeout_policy_type_documentation.py b/tests/test_timeout_policy_type_documentation.py
new file mode 100644
index 0000000..bbe60c6
--- /dev/null
+++ b/tests/test_timeout_policy_type_documentation.py
@@ -0,0 +1,27 @@
+"""Documentation contracts for exact request-timeout policy configuration."""
+
+from __future__ import annotations
+
+from pathlib import Path
+
+REPOSITORY_ROOT = Path(__file__).resolve().parents[1]
+TIMEOUT_GUIDE_PATH = REPOSITORY_ROOT / "docs" / "research" / "request-timeout-boundaries.md"
+
+
+def _read(path: Path) -> str:
+    """Return one repository text file as normalized UTF-8 prose."""
+    return " ".join(path.read_text(encoding="utf-8").split())
+
+
+def test_timeout_guide_requires_exact_reviewed_policy_type() -> None:
+    """Explain why timeout-policy subclass polymorphism is not supported."""
+    guide = _read(TIMEOUT_GUIDE_PATH)
+
+    for fragment in (
+        "exact `EgressTimeoutPolicy` type",
+        "subclass",
+        "trusted policy-construction boundary",
+        "`as_httpcore_timeout()`",
+        "must migrate",
+    ):
+        assert fragment in guide