Do not parameterize RootModel bases in generated types

pyproject.toml

@@ -109,6 +109,8 @@ docs = [
 # of the default groups so no other environment installs the client (or the
 # legacy httpx it depends on). See i18n/README.md.
 translate = ["anthropic>=0.121.0"]
+# Note: when bumping `datamodel-code-generator`, look at `scripts/codegen_templates/pydantic_v2/RootModel.jinja2`
+# if it needs updating:
 codegen = ["datamodel-code-generator==0.57.0"]
 
 [build-system]

scripts/codegen_templates/pydantic_v2/RootModel.jinja2

@@ -0,0 +1,63 @@
+{#-
+Copy of datamodel-code-generator's `pydantic_v2/RootModel.jinja2` with two changes:
+
+- The base `RootModel` class is left unparametrized instead of `RootModel[...]`.
+  The class annotates `root` anyway, and every distinct `RootModel[...]` parametrization
+  builds a model that is unused.
+- `description` and `field.docstring` go through the `format_docstring` filter. Codegen
+  only pre-formats them for its built-in templates; custom templates receive the raw text.
+  The 4-space indent matches codegen's `DOCSTRING_INDENT` / `FIELD_DOCSTRING_INDENT`.
+-#}
+{%- macro get_type_hint(_fields, use_base_type) -%}
+{%- if _fields -%}
+{#There will only ever be a single field for RootModel#}
+{%- if use_base_type -%}
+{{- _fields[0].base_type_hint}}
+{%- else -%}
+{{- _fields[0].type_hint}}
+{%- endif -%}
+{%- endif -%}
+{%- endmacro -%}
+
+
+{% for decorator in decorators -%}
+{{ decorator }}
+{% endfor -%}
+
+{#- Use base_type_hint in generic when regex_engine is set to avoid evaluating pattern before config is processed -#}
+{%- set use_base_type = config and config.regex_engine -%}
+class {{ class_name }}({{ base_class }}):  # pyright: ignore[reportMissingTypeArgument]{% if comment is defined %}  # {{ comment }}{% endif %}
+{%- if description %}
+    {{ description | format_docstring(4) }}
+{%- endif %}
+{%- if config %}
+{%- filter indent(4) %}
+{% include 'ConfigDict.jinja2' %}
+{%- endfilter %}
+{%- endif %}
+{%- for line in class_body_lines %}
+    {{ line }}
+{%- endfor %}
+{%- if not fields and not description and not config and not class_body_lines %}
+    pass
+{%- else %}
+    {%- set field = fields[0] %}
+    {%- if not field.annotated and field.field %}
+    root: {{ field.type_hint }} = {{ field.field }}
+    {%- else %}
+    {%- if field.annotated %}
+    root: {{ field.annotated }}
+    {%- else %}
+    root: {{ field.type_hint }}
+    {%- endif %}
+    {%- if not field.has_default_factory_in_field and not (field.required or (field.represented_default == 'None' and field.strip_default_none))
+            %} = {{ field.represented_default }}
+    {%- endif -%}
+    {%- endif %}
+    {%- if field.docstring %}
+    {{ field.docstring | format_docstring(4) }}
+    {%- elif field.inline_field_docstring %}
+    {{ field.inline_field_docstring }}
+
+    {%- endif %}
+{%- endif %}

scripts/gen_surface_types.py

@@ -25,6 +25,7 @@
 REPO_ROOT = Path(__file__).resolve().parent.parent
 SCHEMA_DIR = REPO_ROOT / "schema"
 TYPES_DIR = REPO_ROOT / "src" / "mcp-types" / "mcp_types"
+TEMPLATE_DIR = REPO_ROOT / "scripts" / "codegen_templates"
 
 # The result-meta serverInfo stamp: every `$defs` entry carrying this property
 # gets its typed `$ref` stripped by `make_server_info_opaque` below.
@@ -191,6 +192,7 @@ def run_codegen(schema_path: Path, output_path: Path) -> None:
             "--output-model-type", "pydantic_v2.BaseModel",
             "--target-python-version", "3.10",
             "--base-class", "mcp_types._wire_base.WireModel",
+            "--custom-template-dir", str(TEMPLATE_DIR),
             "--snake-case-field", "--remove-special-field-name-prefix",
             "--use-annotated", "--use-field-description", "--use-schema-description",
             "--enum-field-as-literal", "all",
@@ -246,7 +248,13 @@ def build(entry: dict[str, str]) -> str:
         source = raw.read_text(encoding="utf-8")
 
     source = re.sub(r"\A# generated by datamodel-codegen:\n#[^\n]*\n", "", source)
-    source = re.sub(r"^class Model\(RootModel\[Any\]\):\n {4}root: Any\n+", "", source, count=1, flags=re.MULTILINE)
+    source = re.sub(
+        r"^class Model\(RootModel\):  # pyright: ignore\[reportMissingTypeArgument\]\n {4}root: Any\n+",
+        "",
+        source,
+        count=1,
+        flags=re.MULTILINE,
+    )
     # Codegen appends `| None` to forward refs of nullable models, which is a
     # runtime TypeError on a string ref and redundant since `JSONValue` includes None.
     source = source.replace('"JSONValue" | None', '"JSONValue"')