modelcontextprotocol · Aaron-Oh · Jul 23, 2026 · cubic-dev-ai · Jul 23, 2026 · cubic-dev-ai
diff --git a/examples/stories/schema_validators/README.md b/examples/stories/schema_validators/README.md
@@ -1,9 +1,10 @@
 # schema-validators
 
-Four ways to type a tool parameter so `MCPServer` derives the JSON-Schema
+Five ways to type a tool parameter so `MCPServer` derives the JSON-Schema
 `inputSchema` and validates arguments before your handler runs: a pydantic
-`BaseModel`, a `TypedDict`, a `@dataclass`, and a bare `dict[str, Any]`. The
-client lists the tools, resolves each `who` schema, and round-trips a call.
+`BaseModel`, a `TypedDict`, a `@dataclass`, a bare `dict[str, Any]`, and a
+pydantic model built at runtime with `create_model`. The client lists the
+tools, resolves each `who` schema, and round-trips a call.
 
 ## Run it
 
@@ -25,6 +26,13 @@ uv run python -m stories.schema_validators.client --http --server server_lowleve
 - `server.py` — `who.name` vs `who["name"]`: pydantic and dataclass parameters
   arrive as **instances** (attribute access); TypedDict and `dict[str, Any]`
   arrive as plain dicts.
+- `server.py` `greet_dynamic` — the parameter type is not written in the source
+  at all: `create_model` builds it at runtime from a JSON Schema dict (the shape
+  you'd get from OpenAPI, a config file, or a DB row), then hands it to
+  `@mcp.tool()` like any `BaseModel`; the published schema is identical to the
+  `greet_pydantic` variant. A `create_model` result is opaque to static type
+  checkers, so a `TYPE_CHECKING` branch aliases it to a declared model of the
+  same shape — the runtime still uses the dynamic class.
 - `client.py` — the listed `inputSchema` for the three typed variants nests a
   `$defs`/`$ref` object with a `name` property; `greet_dict` publishes only
   `{"type": "object", "additionalProperties": true}` — no field validation.

diff --git a/examples/stories/schema_validators/client.py b/examples/stories/schema_validators/client.py
@@ -10,9 +10,17 @@ async def main(target: Target, *, mode: str = "auto") -> None:
     async with Client(target, mode=mode) as client:
         listed = await client.list_tools()
         by_name = {t.name: t for t in listed.tools}
-        assert set(by_name) == {"greet_pydantic", "greet_typeddict", "greet_dataclass", "greet_dict"}
-
-        for name in ("greet_pydantic", "greet_typeddict", "greet_dataclass"):
+        assert set(by_name) == {
+            "greet_pydantic",
+            "greet_typeddict",
+            "greet_dataclass",
+            "greet_dict",
+            "greet_dynamic",
+        }
+
+        # greet_dynamic's parameter is a runtime create_model() class, but MCPServer
+        # reflects over it exactly like the hand-written BaseModel — same $defs/$ref shape.
+        for name in ("greet_pydantic", "greet_typeddict", "greet_dataclass", "greet_dynamic"):
             schema = by_name[name].input_schema
             assert schema["required"] == ["who"], schema
             # MCPServer emits a $defs/$ref pair; lowlevel inlines. Resolve either.

diff --git a/examples/stories/schema_validators/server.py b/examples/stories/schema_validators/server.py
@@ -1,9 +1,9 @@
-"""Four ways to type a tool parameter so MCPServer derives and enforces inputSchema."""
+"""Five ways to type a tool parameter so MCPServer derives and enforces inputSchema."""
 
 from dataclasses import dataclass
-from typing import Any
+from typing import TYPE_CHECKING, Any
 
-from pydantic import BaseModel
+from pydantic import BaseModel, create_model
 
 # pydantic requires typing_extensions.TypedDict (not typing.TypedDict) on Python < 3.12
 # when a TypedDict is used as a field/parameter type.
@@ -29,6 +29,28 @@ class PersonDC:
     title: str = "friend"
 
 
+# The four types above are declared in source. This fifth one is not: a caller
+# holding an external JSON Schema (from OpenAPI, a config file, a DB row) builds
+# the pydantic model at runtime with create_model, then hands it to @mcp.tool()
+# exactly like a hand-written BaseModel. MCPServer reflects over it the same way.
+PERSON_JSON_SCHEMA: dict[str, Any] = {
+    "properties": {"name": {"type": "string"}, "title": {"type": "string", "default": "friend"}},
+    "required": ["name"],
+}
+if TYPE_CHECKING:
+    # A create_model() result is opaque to static tools: its fields don't exist
+    # until runtime, and a runtime variable can't appear in a type annotation.
+    # Alias it to a declared model of the same shape so type checkers can see
+    # `name`/`title`; at runtime the dynamic class below is what @mcp.tool() sees.
+    PersonDynamic = PersonModel
+else:
+    _dynamic_fields: dict[str, Any] = {
+        field_name: (str, ... if field_name in PERSON_JSON_SCHEMA["required"] else field_schema.get("default"))
+        for field_name, field_schema in PERSON_JSON_SCHEMA["properties"].items()
+    }
+    PersonDynamic = create_model("PersonDynamic", **_dynamic_fields)
+
+
 def build_server() -> MCPServer:
     mcp = MCPServer("schema-validators-example")
 
@@ -52,6 +74,16 @@ def greet_dict(who: dict[str, Any]) -> str:
         """`who` is a free-form object — any dict passes; the handler must check it."""
         return f"Hello {who['name']}, my {who.get('title', 'friend')}"
 
+    @mcp.tool()
+    def greet_dynamic(who: PersonDynamic) -> str:
+        """`who`'s type was built at runtime by create_model, not declared in source.
+
+        It validates and behaves like the ``PersonModel`` variant; the only
+        difference is that its class is assembled from a JSON Schema dict at import
+        time rather than written out as a ``class`` statement.
+        """
+        return f"Hello {who.name}, my {who.title}"
+
     return mcp
 
 

diff --git a/examples/stories/schema_validators/server_lowlevel.py b/examples/stories/schema_validators/server_lowlevel.py
@@ -1,4 +1,4 @@
-"""Same four tools via lowlevel.Server — inputSchema is hand-written JSON Schema."""
+"""Same five tools via lowlevel.Server — inputSchema is hand-written JSON Schema."""
 
 from typing import Any
 
@@ -21,7 +21,7 @@
         description=f"Greet ({variant} input shape)",
         input_schema={"type": "object", "properties": {"who": PERSON_SCHEMA}, "required": ["who"]},
     )
-    for variant in ("pydantic", "typeddict", "dataclass")
+    for variant in ("pydantic", "typeddict", "dataclass", "dynamic")
 ]
 TOOLS.append(
     types.Tool(