feat(server): pre-validation request hook for spec-default injection

Adds `pre_validation_hooks` to `serve()`, `create_mcp_server()`,
`create_a2a_server()`, and `create_tool_caller()`. The per-tool hook
runs on the raw wire dict before `validate_request` (schema) and
`model_validate` (Pydantic), so it can supply spec-mandated defaults
that absent pre-v3 buyers omit. Hook exceptions surface as
INVALID_REQUEST. Includes 7 new pytest tests.

Closes #614

https://claude.ai/code/session_015hHQqXRbn2jX9WTu564gjZ

Author: claude

src/adcp/decisioning/serve.py

@@ -373,6 +373,7 @@ def serve(
     advertise_all: bool = False,
     mock_ad_server: Any | None = None,
     enable_debug_endpoints: bool = False,
+    pre_validation_hooks: dict[str, Any] | None = None,
     **serve_kwargs: Any,
 ) -> None:
     """One-call wrapper — build the handler and serve over MCP.
@@ -435,6 +436,24 @@ def serve(
         responses="strict")`` to enable schema-driven request/response
         validation against the bundled AdCP JSON schemas — sellers who
         want their server to enforce wire conformance turn it on here.
+    :param pre_validation_hooks: Optional dict mapping AdCP tool name to
+        a ``(tool_name, raw_args) -> raw_args`` callable. The hook runs
+        on the raw wire dict **before** schema + Pydantic validation —
+        use it to apply spec-mandated defaults for pre-v3 buyers that
+        omit required fields. Example::
+
+            serve(
+                router,
+                pre_validation_hooks={
+                    "get_products": lambda n, a: {
+                        **a, "buying_mode": a.get("buying_mode", "brief")
+                    },
+                },
+            )
+
+        Hook exceptions surface as ``INVALID_REQUEST`` on the wire.
+        The hook must return a new dict; mutating the input in-place is
+        a bug — the original is captured separately for context echo.
     """
     # Local import to avoid a circular at module-load time. Adopter
     # serves never run during foundation imports anyway.
@@ -504,6 +523,8 @@ def serve(
 
     server_name = name or type(platform).__name__
     debug_traffic_source = mock_ad_server.get_traffic if mock_ad_server is not None else None
+    if pre_validation_hooks is not None:
+        serve_kwargs["pre_validation_hooks"] = pre_validation_hooks
     _adcp_serve(
         handler,
         name=server_name,

src/adcp/server/a2a_server.py

@@ -143,6 +143,7 @@ def __init__(
         message_parser: MessageParser | None = None,
         advertise_all: bool = False,
         validation: ValidationHookConfig | None = SERVER_DEFAULT_VALIDATION,
+        pre_validation_hooks: dict[str, Any] | None = None,
         test_controller_account_resolver: Any | None = None,
     ) -> None:
         self._handler = handler
@@ -169,7 +170,10 @@ def __init__(
             name = tool_def["name"]
             if name == "comply_test_controller" and test_controller is None:
                 continue
-            self._tool_callers[name] = create_tool_caller(handler, name, validation=validation)
+            hook = (pre_validation_hooks or {}).get(name)
+            self._tool_callers[name] = create_tool_caller(
+                handler, name, validation=validation, pre_validation_hook=hook
+            )
 
         if test_controller is not None:
             self._register_test_controller(test_controller)
@@ -758,6 +762,7 @@ def create_a2a_server(
     message_parser: MessageParser | None = None,
     advertise_all: bool = False,
     validation: ValidationHookConfig | None = SERVER_DEFAULT_VALIDATION,
+    pre_validation_hooks: dict[str, Any] | None = None,
     context_builder: Any | None = None,
     auth: BearerTokenAuth | None = None,
     public_url: str | None = None,
@@ -884,6 +889,7 @@ def create_a2a_server(
         message_parser=message_parser,
         advertise_all=advertise_all,
         validation=validation,
+        pre_validation_hooks=pre_validation_hooks,
         test_controller_account_resolver=test_controller_account_resolver,
     )
 

src/adcp/server/mcp_tools.py

@@ -1870,6 +1870,7 @@ def create_tool_caller(
     method_name: str,
     *,
     validation: ValidationHookConfig | None = None,
+    pre_validation_hook: Callable[[str, dict[str, Any]], dict[str, Any]] | None = None,
 ) -> Callable[..., Any]:
     """Create a tool caller function for an ADCP handler method.
 
@@ -1899,12 +1900,28 @@ def create_tool_caller(
     server validation is a deliberate opt-in for authors who want
     dispatcher-level enforcement.
 
+    **Pre-validation hook (issue #614).** When ``pre_validation_hook`` is
+    supplied, it is called with ``(tool_name, raw_args_dict)`` and must
+    return a (possibly modified) ``dict`` that replaces the wire args
+    before schema validation and Pydantic ``model_validate`` run. Use
+    this to apply spec-mandated defaults for pre-v3 buyers that omit
+    required fields (e.g. ``buying_mode``, ``format_id`` shape coercion,
+    ``asset_type`` inference). The hook runs on every call; keep it fast.
+    Exceptions from the hook surface as ``INVALID_REQUEST`` — do not raise
+    for missing-but-defaultable fields, only for structurally unusable args.
+    The hook must return a new dict (or the original unchanged); mutating
+    the input dict in-place is a bug — the original is captured separately
+    for the context-echo path.
+
     Args:
         handler: The ADCP handler instance
         method_name: Name of the method to call
         validation: Optional :class:`ValidationHookConfig` with
             per-side modes (``strict`` / ``warn`` / ``off``). Omitting
             it disables server-side schema validation entirely.
+        pre_validation_hook: Optional callable ``(tool_name, args) -> args``
+            invoked on the raw wire dict before schema + Pydantic validation.
+            See the **Pre-validation hook** section above.
 
     Returns:
         Async callable ``call_tool(params, context=None)``. The ``context``
@@ -1938,7 +1955,22 @@ def create_tool_caller(
 
     async def call_tool(params: dict[str, Any], context: ToolContext | None = None) -> Any:
         ctx = context if context is not None else ToolContext()
-        raw_params = params  # Preserve the original dict for context echo.
+
+        if pre_validation_hook is not None:
+            try:
+                params = pre_validation_hook(method_name, params)
+            except Exception as exc:
+                raise ADCPTaskError(
+                    operation=method_name,
+                    errors=[
+                        Error(
+                            code="INVALID_REQUEST",
+                            message=f"pre_validation_hook raised {type(exc).__name__}: {exc}",
+                        )
+                    ],
+                ) from exc
+
+        raw_params = params  # Preserve the (possibly hook-modified) dict for context echo.
 
         if request_mode is not None and request_mode != "off":
             outcome = validate_request(method_name, params)
@@ -2069,6 +2101,7 @@ def __init__(
         *,
         advertise_all: bool = False,
         validation: ValidationHookConfig | None = None,
+        pre_validation_hooks: dict[str, Callable[[str, dict[str, Any]], dict[str, Any]]] | None = None,
     ):
         """Create tool set from handler.
 
@@ -2081,6 +2114,9 @@ def __init__(
                 (override-filtered advertisement).
             validation: Opt-in schema validation config applied to every
                 tool caller. See :func:`create_tool_caller`.
+            pre_validation_hooks: Optional dict mapping tool name to a
+                ``(tool_name, args) -> args`` callable. Applied before
+                schema + Pydantic validation. See :func:`create_tool_caller`.
         """
         self.handler = handler
         self._filtered_definitions = get_tools_for_handler(handler, advertise_all=advertise_all)
@@ -2089,7 +2125,10 @@ def __init__(
         # Create tool callers only for filtered tools
         for tool_def in self._filtered_definitions:
             name = tool_def["name"]
-            self._tools[name] = create_tool_caller(handler, name, validation=validation)
+            hook = (pre_validation_hooks or {}).get(name)
+            self._tools[name] = create_tool_caller(
+                handler, name, validation=validation, pre_validation_hook=hook
+            )
 
     @property
     def tool_definitions(self) -> list[dict[str, Any]]:
@@ -2123,6 +2162,7 @@ def create_mcp_tools(
     *,
     advertise_all: bool = False,
     validation: ValidationHookConfig | None = None,
+    pre_validation_hooks: dict[str, Callable[[str, dict[str, Any]], dict[str, Any]]] | None = None,
 ) -> MCPToolSet:
     """Create MCP tools from an ADCP handler.
 
@@ -2157,8 +2197,16 @@ async def call_tool(name: str, arguments: dict):
             every tool caller validates requests and responses against
             the bundled AdCP JSON schemas. See
             :func:`create_tool_caller` for mode semantics.
+        pre_validation_hooks: Optional dict mapping tool name to a
+            ``(tool_name, args) -> args`` callable. Applied before schema
+            + Pydantic validation. See :func:`create_tool_caller`.
 
     Returns:
         MCPToolSet with tool definitions and handlers.
     """
-    return MCPToolSet(handler, advertise_all=advertise_all, validation=validation)
+    return MCPToolSet(
+        handler,
+        advertise_all=advertise_all,
+        validation=validation,
+        pre_validation_hooks=pre_validation_hooks,
+    )

src/adcp/server/serve.py

@@ -134,6 +134,7 @@ class ServeConfig:
     advertise_all: bool = False
     max_request_size: int | None = None
     validation: ValidationHookConfig | None = None
+    pre_validation_hooks: dict[str, Callable[..., Any]] | None = None
 
     # --- Discovery manifest ---
     base_url: str | None = None
@@ -525,6 +526,7 @@ def serve(
     max_request_size: int | None = None,
     streaming_responses: bool = False,
     validation: ValidationHookConfig | None = DEFAULT_VALIDATION,
+    pre_validation_hooks: dict[str, Callable[..., Any]] | None = None,
     enable_debug_endpoints: bool = False,
     debug_traffic_source: Callable[[], dict[str, int]] | None = None,
     base_url: str | None = None,
@@ -772,6 +774,7 @@ async def force_account_status(self, account_id, status):
         max_request_size = config.max_request_size
         streaming_responses = config.streaming_responses
         validation = config.validation
+        pre_validation_hooks = config.pre_validation_hooks
         enable_debug_endpoints = config.enable_debug_endpoints
         debug_traffic_source = config.debug_traffic_source
         base_url = config.base_url
@@ -815,6 +818,7 @@ async def force_account_status(self, account_id, status):
             advertise_all=advertise_all,
             max_request_size=max_request_size,
             validation=validation,
+            pre_validation_hooks=pre_validation_hooks,
             base_url=base_url,
             specialisms=specialisms,
             description=description,
@@ -838,6 +842,7 @@ async def force_account_status(self, account_id, status):
             max_request_size=max_request_size,
             streaming_responses=streaming_responses,
             validation=validation,
+            pre_validation_hooks=pre_validation_hooks,
             base_url=base_url,
             specialisms=specialisms,
             description=description,
@@ -865,6 +870,7 @@ async def force_account_status(self, account_id, status):
             max_request_size=max_request_size,
             streaming_responses=streaming_responses,
             validation=validation,
+            pre_validation_hooks=pre_validation_hooks,
             base_url=base_url,
             specialisms=specialisms,
             description=description,
@@ -1239,6 +1245,7 @@ def _serve_mcp(
     max_request_size: int | None = None,
     streaming_responses: bool = False,
     validation: ValidationHookConfig | None = DEFAULT_VALIDATION,
+    pre_validation_hooks: dict[str, Callable[..., Any]] | None = None,
     base_url: str | None = None,
     specialisms: list[str] | None = None,
     description: str | None = None,
@@ -1260,6 +1267,7 @@ def _serve_mcp(
         advertise_all=advertise_all,
         streaming_responses=streaming_responses,
         validation=validation,
+        pre_validation_hooks=pre_validation_hooks,
         allowed_hosts=allowed_hosts,
         allowed_origins=allowed_origins,
         enable_dns_rebinding_protection=enable_dns_rebinding_protection,
@@ -1399,6 +1407,7 @@ def _serve_a2a(
     advertise_all: bool = False,
     max_request_size: int | None = None,
     validation: ValidationHookConfig | None = DEFAULT_VALIDATION,
+    pre_validation_hooks: dict[str, Callable[..., Any]] | None = None,
     base_url: str | None = None,
     specialisms: list[str] | None = None,
     description: str | None = None,
@@ -1427,6 +1436,7 @@ def _serve_a2a(
         message_parser=message_parser,
         advertise_all=advertise_all,
         validation=validation,
+        pre_validation_hooks=pre_validation_hooks,
         auth=auth,
         public_url=public_url,
     )
@@ -1481,6 +1491,7 @@ def _build_mcp_and_a2a_app(
     max_request_size: int | None = None,
     streaming_responses: bool = False,
     validation: ValidationHookConfig | None = DEFAULT_VALIDATION,
+    pre_validation_hooks: dict[str, Callable[..., Any]] | None = None,
     base_url: str | None = None,
     specialisms: list[str] | None = None,
     description: str | None = None,
@@ -1523,6 +1534,7 @@ def _build_mcp_and_a2a_app(
         advertise_all=advertise_all,
         streaming_responses=streaming_responses,
         validation=validation,
+        pre_validation_hooks=pre_validation_hooks,
         allowed_hosts=allowed_hosts,
         allowed_origins=allowed_origins,
         enable_dns_rebinding_protection=enable_dns_rebinding_protection,
@@ -1576,6 +1588,7 @@ def _build_mcp_and_a2a_app(
         message_parser=message_parser,
         advertise_all=advertise_all,
         validation=validation,
+        pre_validation_hooks=pre_validation_hooks,
         auth=auth,
         public_url=public_url,
     )
@@ -1659,6 +1672,7 @@ def _serve_mcp_and_a2a(
     max_request_size: int | None = None,
     streaming_responses: bool = False,
     validation: ValidationHookConfig | None = DEFAULT_VALIDATION,
+    pre_validation_hooks: dict[str, Callable[..., Any]] | None = None,
     base_url: str | None = None,
     specialisms: list[str] | None = None,
     description: str | None = None,
@@ -1706,6 +1720,7 @@ def _serve_mcp_and_a2a(
         max_request_size=max_request_size,
         streaming_responses=streaming_responses,
         validation=validation,
+        pre_validation_hooks=pre_validation_hooks,
         base_url=base_url,
         specialisms=specialisms,
         description=description,
@@ -1787,6 +1802,7 @@ def create_mcp_server(
     advertise_all: bool = False,
     streaming_responses: bool = False,
     validation: ValidationHookConfig | None = DEFAULT_VALIDATION,
+    pre_validation_hooks: dict[str, Callable[..., Any]] | None = None,
     allowed_hosts: Sequence[str] | None = None,
     allowed_origins: Sequence[str] | None = None,
     enable_dns_rebinding_protection: bool | None = None,
@@ -1948,6 +1964,7 @@ def create_mcp_server(
         middleware=middleware,
         advertise_all=advertise_all,
         validation=validation,
+        pre_validation_hooks=pre_validation_hooks,
     )
     return mcp
 
@@ -1961,6 +1978,7 @@ def _register_handler_tools(
     middleware: Sequence[SkillMiddleware] | None = None,
     advertise_all: bool = False,
     validation: ValidationHookConfig | None = DEFAULT_VALIDATION,
+    pre_validation_hooks: dict[str, Callable[..., Any]] | None = None,
 ) -> None:
     """Register all ADCP tools from a handler onto a FastMCP server."""
     # Freeze middleware ordering at registration time. Tuple both guards
@@ -1980,7 +1998,10 @@ def _register_handler_tools(
         description = tool_def.get("description", "")
         input_schema = tool_def.get("inputSchema", {"type": "object", "properties": {}})
         output_schema = tool_def.get("outputSchema")
-        caller = create_tool_caller(handler, tool_name, validation=validation)
+        hook = (pre_validation_hooks or {}).get(tool_name)
+        caller = create_tool_caller(
+            handler, tool_name, validation=validation, pre_validation_hook=hook
+        )
         _register_tool(
             mcp,
             tool_name,