Add FileInputDescriptor and fileInputs capability for file upload SEP

Implements declarative file input metadata so servers can tell clients which
tool arguments / elicitation form fields should receive a native file picker.

- FileInputDescriptor: Pydantic model with accept (MIME patterns) and max_size (bytes)
- FileInputsCapability: client capability gate
- Tool.input_files: maps arg names to FileInputDescriptor
- ElicitRequestFormParams.requested_files: symmetric for elicitation
- StringArraySchema: new PrimitiveSchemaDefinition member for multi-file fields

All models use alias_generator=to_camel for snake_case ↔ camelCase on the wire.
Files are transmitted as RFC 2397 data URIs: data:<mediatype>;name=<filename>;base64,<data>

https://claude.ai/code/session_01JxhHWiXrXgE4JWC27dznRN

Author: claude

src/mcp/types/__init__.py

@@ -59,6 +59,8 @@
     ElicitResult,
     EmbeddedResource,
     EmptyResult,
+    FileInputDescriptor,
+    FileInputsCapability,
     FormElicitationCapability,
     GetPromptRequest,
     GetPromptRequestParams,
@@ -249,6 +251,7 @@
     "ClientTasksRequestsCapability",
     "CompletionsCapability",
     "ElicitationCapability",
+    "FileInputsCapability",
     "FormElicitationCapability",
     "LoggingCapability",
     "PromptsCapability",
@@ -303,6 +306,7 @@
     "Task",
     "TaskMetadata",
     "RelatedTaskMetadata",
+    "FileInputDescriptor",
     "Tool",
     "ToolAnnotations",
     "ToolChoice",

src/mcp/types/_types.py

@@ -222,6 +222,16 @@ class SamplingToolsCapability(MCPModel):
     """
 
 
+class FileInputsCapability(MCPModel):
+    """Capability for declarative file inputs on tools and elicitation forms.
+
+    When a client declares this capability, servers may include ``input_files``
+    on Tool definitions and ``requested_files`` on form-mode elicitation
+    requests. Servers must not send those fields unless this capability is
+    present.
+    """
+
+
 class FormElicitationCapability(MCPModel):
     """Capability for form mode elicitation."""
 
@@ -323,6 +333,8 @@ class ClientCapabilities(MCPModel):
     """Present if the client supports listing roots."""
     tasks: ClientTasksCapability | None = None
     """Present if the client supports task-augmented requests."""
+    file_inputs: FileInputsCapability | None = None
+    """Present if the client supports declarative file inputs for tools and elicitation."""
 
 
 class PromptsCapability(MCPModel):
@@ -1150,6 +1162,28 @@ class ToolExecution(MCPModel):
     """
 
 
+class FileInputDescriptor(MCPModel):
+    """Describes a single file input argument for a tool or elicitation form.
+
+    Provides optional hints for client-side file picker filtering and validation.
+    All fields are advisory; servers must still validate inputs independently.
+    """
+
+    accept: list[str] | None = None
+    """MIME type patterns the server will accept for this input.
+
+    Supports exact types (``"image/png"``) and wildcard subtypes (``"image/*"``).
+    If omitted, any file type is accepted.
+    """
+
+    max_size: int | None = None
+    """Maximum file size in bytes (decoded size, per file).
+
+    Servers should reject larger files with JSON-RPC ``-32602`` (Invalid Params)
+    and the structured reason ``"file_too_large"``.
+    """
+
+
 class Tool(BaseMetadata):
     """Definition for a tool the client can call."""
 
@@ -1174,6 +1208,20 @@ class Tool(BaseMetadata):
 
     execution: ToolExecution | None = None
 
+    input_files: dict[str, FileInputDescriptor] | None = None
+    """Declares which arguments in ``input_schema`` are file inputs.
+
+    Keys must match property names in ``input_schema["properties"]`` and the
+    corresponding schema properties must be ``{"type": "string", "format": "uri"}``
+    or an array thereof. Servers must not include this field unless the client
+    declared the ``file_inputs`` capability during initialization.
+
+    Clients should render a native file picker for these arguments and encode
+    selected files as RFC 2397 data URIs of the form
+    ``data:<mediatype>;name=<filename>;base64,<data>`` where the ``name=``
+    parameter (percent-encoded) carries the original filename.
+    """
+
 
 class ListToolsResult(PaginatedResult):
     """The server's response to a tools/list request from the client."""
@@ -1649,6 +1697,20 @@ class ElicitRequestFormParams(RequestParams):
     Only top-level properties are allowed, without nesting.
     """
 
+    requested_files: dict[str, FileInputDescriptor] | None = None
+    """Declares which fields in ``requested_schema`` are file inputs.
+
+    Keys must match property names in ``requested_schema["properties"]`` and the
+    corresponding schema properties must be a string schema with ``format: "uri"``
+    or an array of such string schemas. Servers must not include this field unless
+    the client declared the ``file_inputs`` capability during initialization.
+
+    Clients should render a native file picker for these fields and encode
+    selected files as RFC 2397 data URIs of the form
+    ``data:<mediatype>;name=<filename>;base64,<data>`` where the ``name=``
+    parameter (percent-encoded) carries the original filename.
+    """
+
 
 class ElicitRequestURLParams(RequestParams):
     """Parameters for URL mode elicitation requests.

tests/test_types.py

@@ -8,6 +8,9 @@
     CreateMessageRequestParams,
     CreateMessageResult,
     CreateMessageResultWithTools,
+    ElicitRequestFormParams,
+    FileInputDescriptor,
+    FileInputsCapability,
     Implementation,
     InitializeRequest,
     InitializeRequestParams,
@@ -360,3 +363,108 @@ def test_list_tools_result_preserves_json_schema_2020_12_fields():
     assert tool.input_schema["$schema"] == "https://json-schema.org/draft/2020-12/schema"
     assert "$defs" in tool.input_schema
     assert tool.input_schema["additionalProperties"] is False
+
+
+def test_file_input_descriptor_roundtrip():
+    """FileInputDescriptor serializes maxSize camelCase and accepts MIME patterns."""
+    wire: dict[str, Any] = {"accept": ["image/png", "image/*"], "maxSize": 1048576}
+    desc = FileInputDescriptor.model_validate(wire)
+    assert desc.accept == ["image/png", "image/*"]
+    assert desc.max_size == 1048576
+
+    dumped = desc.model_dump(by_alias=True, exclude_none=True)
+    assert dumped == {"accept": ["image/png", "image/*"], "maxSize": 1048576}
+
+    # Both fields are optional; empty descriptor is valid
+    empty = FileInputDescriptor.model_validate({})
+    assert empty.accept is None
+    assert empty.max_size is None
+    assert empty.model_dump(by_alias=True, exclude_none=True) == {}
+
+
+def test_tool_with_input_files():
+    """Tool.inputFiles round-trips via wire-format camelCase alias."""
+    wire: dict[str, Any] = {
+        "name": "upload_attachment",
+        "description": "Upload a file",
+        "inputSchema": {
+            "type": "object",
+            "properties": {
+                "file": {"type": "string", "format": "uri"},
+                "note": {"type": "string"},
+            },
+            "required": ["file"],
+        },
+        "inputFiles": {
+            "file": {"accept": ["application/pdf", "image/*"], "maxSize": 5242880},
+        },
+    }
+    tool = Tool.model_validate(wire)
+    assert tool.input_files is not None
+    assert set(tool.input_files.keys()) == {"file"}
+    assert isinstance(tool.input_files["file"], FileInputDescriptor)
+    assert tool.input_files["file"].accept == ["application/pdf", "image/*"]
+    assert tool.input_files["file"].max_size == 5242880
+
+    dumped = tool.model_dump(by_alias=True, exclude_none=True)
+    assert "inputFiles" in dumped
+    assert "input_files" not in dumped
+    assert dumped["inputFiles"]["file"]["maxSize"] == 5242880
+    assert dumped["inputFiles"]["file"]["accept"] == ["application/pdf", "image/*"]
+
+    # input_files defaults to None and is omitted when absent
+    plain = Tool(name="echo", input_schema={"type": "object"})
+    assert plain.input_files is None
+    assert "inputFiles" not in plain.model_dump(by_alias=True, exclude_none=True)
+
+
+def test_client_capabilities_with_file_inputs():
+    """ClientCapabilities.fileInputs round-trips as an empty capability object."""
+    caps = ClientCapabilities.model_validate({"fileInputs": {}})
+    assert caps.file_inputs is not None
+    assert isinstance(caps.file_inputs, FileInputsCapability)
+
+    dumped = caps.model_dump(by_alias=True, exclude_none=True)
+    assert dumped == {"fileInputs": {}}
+
+    # Absent by default
+    bare = ClientCapabilities.model_validate({})
+    assert bare.file_inputs is None
+    assert "fileInputs" not in bare.model_dump(by_alias=True, exclude_none=True)
+
+
+def test_elicit_form_params_with_requested_files():
+    """ElicitRequestFormParams.requestedFiles round-trips through the wire format."""
+    wire: dict[str, Any] = {
+        "mode": "form",
+        "message": "Upload your documents",
+        "requestedSchema": {
+            "type": "object",
+            "properties": {
+                "resume": {"type": "string", "format": "uri"},
+                "samples": {
+                    "type": "array",
+                    "items": {"type": "string", "format": "uri"},
+                    "maxItems": 3,
+                },
+            },
+            "required": ["resume"],
+        },
+        "requestedFiles": {
+            "resume": {"accept": ["application/pdf"], "maxSize": 2097152},
+            "samples": {"accept": ["image/*"]},
+        },
+    }
+    params = ElicitRequestFormParams.model_validate(wire)
+    assert params.requested_files is not None
+    assert isinstance(params.requested_files["resume"], FileInputDescriptor)
+    assert params.requested_files["resume"].max_size == 2097152
+    assert params.requested_files["samples"].accept == ["image/*"]
+    assert params.requested_files["samples"].max_size is None
+
+    dumped = params.model_dump(by_alias=True, exclude_none=True)
+    assert "requestedFiles" in dumped
+    assert "requested_files" not in dumped
+    assert dumped["requestedFiles"]["resume"]["maxSize"] == 2097152
+    # samples had no maxSize; ensure it's excluded, not serialized as null
+    assert "maxSize" not in dumped["requestedFiles"]["samples"]