diff --git a/agent_core/providers/protocol_client.py b/agent_core/providers/protocol_client.py
index 0779a24..8fd6271 100644
--- a/agent_core/providers/protocol_client.py
+++ b/agent_core/providers/protocol_client.py
@@ -27,7 +27,11 @@
 # Single source of truth for the valid ``llm.protocol`` values. Duplicating the
 # set here would let the two drift, which is how a protocol becomes buildable
 # but has no ``thinking_format`` (or vice versa).
-from agent_core.runtime.loop.model_profile import WireProtocol, is_wire_protocol
+from agent_core.runtime.loop.model_profile import (
+    ThinkingFormat,
+    WireProtocol,
+    is_wire_protocol,
+)
 
 logger = logging.getLogger(__name__)
 
@@ -37,7 +41,7 @@
 _MIN_THINKING_BUDGET = 1024
 
 
-def protocol_of(cfg: dict[str, Any]) -> str:
+def protocol_of(cfg: dict[str, Any]) -> WireProtocol:
     """Normalised ``llm.protocol`` (default ``chat_completions``).
 
     An unusable value — wrong type, or a string that names no known protocol —
@@ -84,9 +88,16 @@ def provider_label(cfg: dict[str, Any]) -> str:
     return "openai"
 
 
-def thinking_format_for_protocol(protocol: str) -> str | None:
+def thinking_format_for_protocol(protocol: str) -> ThinkingFormat | None:
     """``content_block`` for the reasoning protocols, else ``None`` (caller
-    falls back to explicit YAML / model-id inference)."""
+    falls back to explicit YAML / model-id inference).
+
+    Both this and :func:`protocol_of` return the NARROW alias, not a bare
+    ``str``: their outputs feed ``ModelProfile.thinking_format`` /
+    ``ModelProfile.protocol``, which are ``Literal`` unions, so widening to
+    ``str`` makes a value this module just validated unassignable at the very
+    field it was validated for. This file is checked at ``basic`` level, so
+    nothing here catches that — a strict host does."""
     return (
         "content_block"
         if protocol in ("anthropic", "responses", "bedrock")
diff --git a/agent_core/tool.py b/agent_core/tool.py
index 580d2a2..64c3a8e 100644
--- a/agent_core/tool.py
+++ b/agent_core/tool.py
@@ -54,6 +54,17 @@ def to_openai_schema(self) -> dict[str, Any]:
 
 def _schema_for_type(t: Any) -> dict[str, Any]:
     """Build a JSON-schema fragment for a Python type hint."""
+    # PEP 695 (``type Alias = ...``) produces a TypeAliasType, which is not a
+    # primitive, has no ``get_origin``, and is not a Pydantic model — so without
+    # this it reaches the final fallback and becomes ``{"type": "string"}``. That
+    # is the WORST failure mode available here: a tool author who names a union
+    # to describe a parameter precisely gets a schema claiming the parameter is
+    # a string, silently, with the annotation looking correct in the source.
+    # This module documents ``Any -> string`` as a deliberate floor for
+    # under-annotated parameters; an alias is the opposite of under-annotated.
+    if isinstance(t, typing.TypeAliasType):
+        return _schema_for_type(t.__value__)
+
     if t in _PRIMITIVE_TYPES:
         return {"type": _PRIMITIVE_TYPES[t]}
 
diff --git a/tests/test_tool_schema_types.py b/tests/test_tool_schema_types.py
index 1785231..1c62671 100644
--- a/tests/test_tool_schema_types.py
+++ b/tests/test_tool_schema_types.py
@@ -66,3 +66,45 @@ async def probe(items: list[dict[str, Any]]) -> str:
         return ""
 
     assert _properties(probe)["items"]["items"] == {"type": "object"}
+
+
+# ── PEP 695 aliases, the spelling used to name a union ────────────────────────
+
+
+def test_a_pep695_alias_resolves_to_what_it_aliases() -> None:
+    """``type Alias = ...`` must not degrade to ``string``.
+
+    A ``TypeAliasType`` is not a primitive, has no origin, and is not a Pydantic
+    model, so before it was unwrapped it fell through to the ``Any`` floor. The
+    result was a parameter documented as a union and SERVED as a string, with
+    nothing in the source or the schema to show the annotation had been dropped.
+    """
+    type JsonValue = dict[str, Any] | list[str] | str | float | bool
+
+    async def fn(value):  # annotation injected below
+        return ""
+    fn.__annotations__ = {"value": JsonValue, "return": str}
+    t = tool(fn)
+
+    arms = _properties(t)["value"]["anyOf"]
+    assert {arm.get("type") for arm in arms} == {
+        "object", "array", "string", "number", "boolean",
+    }
+
+
+def test_a_pep695_alias_survives_being_an_items_type() -> None:
+    """The case that motivated this: ``list[Alias]``, where the alias describes
+    what one element may be. Losing it here types every element ``string``."""
+    type Cell = str | float | bool
+
+    async def fn(rows):  # annotation injected below
+        return ""
+    fn.__annotations__ = {"rows": list[Cell], "return": str}
+    t = tool(fn)
+
+    schema = _properties(t)["rows"]
+    assert schema["type"] == "array"
+    assert {arm.get("type") for arm in schema["items"]["anyOf"]} == {
+        "string", "number", "boolean",
+    }
+    assert not _walk(schema, "rows")