diff --git a/solstone/think/talents.py b/solstone/think/talents.py
index fdac75115..71bd7d341 100644
--- a/solstone/think/talents.py
+++ b/solstone/think/talents.py
@@ -566,6 +566,16 @@ def prepare_config(request: dict) -> dict:
     talent_path = Path(config["path"]) if config.get("path") else None
     sources = config.get("sources", {})
     talent_cwd = config.get("cwd")
+    # Capture the security-relevant fields from the talent definition BEFORE the
+    # request merge. access_tier selects tool capability; type steers provider/
+    # model resolution and the local-lane runtime promise. A request may not
+    # override either (same as cwd). Pin on PRESENCE, not just value:
+    # access_tier is populated only for cogitate talents (absent otherwise), so
+    # a request that introduces access_tier on a talent that declares none is
+    # itself the conflict to reject.
+    definition_has_access_tier = "access_tier" in config
+    definition_access_tier = config.get("access_tier")
+    definition_type = config.get("type")
 
     # Merge request values (request overrides talent defaults)
     config.update({k: v for k, v in request.items() if v is not None})
@@ -576,6 +586,23 @@ def prepare_config(request: dict) -> dict:
             f"({talent_cwd!r} != {request_cwd!r})"
         )
 
+    request_access_tier = request.get("access_tier")
+    if request_access_tier is not None and (
+        not definition_has_access_tier
+        or request_access_tier != definition_access_tier
+    ):
+        raise ValueError(
+            f"Request overrides 'access_tier' for talent '{name}' are not allowed "
+            f"({definition_access_tier!r} != {request_access_tier!r})"
+        )
+
+    request_type = request.get("type")
+    if request_type is not None and request_type != definition_type:
+        raise ValueError(
+            f"Request overrides 'type' for talent '{name}' are not allowed "
+            f"({definition_type!r} != {request_type!r})"
+        )
+
     cwd_value = config.get("cwd")
     if cwd_value == "journal":
         try:
diff --git a/tests/test_prepare_config_pin.py b/tests/test_prepare_config_pin.py
new file mode 100644
index 000000000..caeca6931
--- /dev/null
+++ b/tests/test_prepare_config_pin.py
@@ -0,0 +1,36 @@
+# SPDX-License-Identifier: AGPL-3.0-only
+# Copyright (c) 2026 sol pbc
+
+"""prepare_config pins security-relevant fields against request override.
+
+access_tier selects tool capability and type steers provider/model
+resolution; neither may be overridden by a request body (the same rule the
+cwd guard already enforces). The `partner` talent is type=cogitate with
+access_tier=synthesis, so a request tier of "normal" is a privilege
+escalation that must be refused.
+"""
+
+import pytest
+
+from solstone.think.talents import prepare_config
+
+
+def test_request_cannot_override_access_tier():
+    with pytest.raises(ValueError, match="access_tier"):
+        prepare_config({"name": "partner", "access_tier": "normal"})
+
+
+def test_request_cannot_override_type():
+    with pytest.raises(ValueError, match="type"):
+        prepare_config({"name": "partner", "type": "generate"})
+
+
+def test_matching_access_tier_is_a_noop():
+    config = prepare_config({"name": "partner", "access_tier": "synthesis"})
+    assert config["access_tier"] == "synthesis"
+
+
+def test_no_override_leaves_definition_values():
+    config = prepare_config({"name": "partner"})
+    assert config["access_tier"] == "synthesis"
+    assert config["type"] == "cogitate"