File Upload - FastMCP

New in version 3.2.0 FileUpload adds drag-and-drop file upload to any server. Users upload files through an interactive UI, bypassing the LLM context window entirely. The LLM can then list and read uploaded files through model-visible tools.

The FileUpload provider shown in Goose, with a drag-and-drop zone for uploading files

from fastmcp import FastMCP
from fastmcp.apps.file_upload import FileUpload

mcp = FastMCP("My Server")
mcp.add_provider(FileUpload())

This registers four tools:

Tool	Visibility	Purpose
`file_manager`	Model	Opens the drag-and-drop upload UI
`store_files`	App only	Called by the UI when the user clicks Upload
`list_files`	Model	Returns metadata for all uploaded files
`read_file`	Model	Returns a file’s contents by name

The LLM sees file_manager, list_files, and read_file. It calls file_manager to show the upload interface, then uses list_files and read_file to work with whatever the user uploaded. store_files is app-only — the UI calls it directly and the LLM never needs to know about it.

Configuration

FileUpload(
    name="Files",                    # App name (used in tool routing)
    max_file_size=10 * 1024 * 1024,  # 10 MB default, enforced server-side
    title="File Upload",             # Heading shown in the UI
    description="Drop files to...",  # Description text below the heading
    drop_label="Drop files here",    # Label inside the drop zone
)

The max_file_size limit is enforced both in the UI (the DropZone rejects oversized files) and on the server (the store_files tool validates before calling on_store).

Storage scoping

By default, files are stored in memory and scoped by MCP session ID. Each session gets its own isolated file store — files uploaded in one conversation aren’t visible in another. This works with stdio, SSE, and stateful HTTP transports, where sessions persist across requests.

In stateless HTTP mode, each request creates a new session object with a new ID. Files stored during one request (e.g. the UI upload) will be invisible to the next request (e.g. the LLM calling list_files). You must override _get_scope_key to use a stable identifier like a user ID from your auth token.

For stateless deployments, override _get_scope_key to return a stable identifier. For example, to scope files by authenticated user:

from fastmcp.apps.file_upload import FileUpload

class UserScopedUpload(FileUpload):
    def _get_scope_key(self, ctx):
        return ctx.access_token["sub"]

For process-wide shared storage (all users see all files):

class SharedUpload(FileUpload):
    def _get_scope_key(self, ctx):
        return "__shared__"

Custom storage

The default implementation stores files in memory for the lifetime of the server process. For persistent storage, subclass FileUpload and override three methods. Each receives the current Context, giving you access to session IDs, auth tokens, and request metadata for partitioning and authorization.

import base64

from fastmcp.apps.file_upload import FileUpload

class S3Upload(FileUpload):
    def on_store(self, files, ctx):
        user_id = ctx.access_token["sub"]
        for f in files:
            s3.put_object(
                Bucket="uploads",
                Key=f"{user_id}/{f['name']}",
                Body=base64.b64decode(f["data"]),
            )
        return self.on_list(ctx)

    def on_list(self, ctx):
        user_id = ctx.access_token["sub"]
        objects = s3.list_objects(Bucket="uploads", Prefix=f"{user_id}/")
        return [
            {
                "name": obj["Key"].split("/", 1)[1],
                "type": "application/octet-stream",
                "size": obj["Size"],
                "size_display": f"{obj['Size']} B",
                "uploaded_at": obj["LastModified"].isoformat(),
            }
            for obj in objects.get("Contents", [])
        ]

    def on_read(self, name, ctx):
        user_id = ctx.access_token["sub"]
        obj = s3.get_object(Bucket="uploads", Key=f"{user_id}/{name}")
        content = obj["Body"].read()
        return {
            "name": name,
            "size": obj["ContentLength"],
            "type": obj["ContentType"],
            "uploaded_at": obj["LastModified"].isoformat(),
            "content": content.decode("utf-8"),
        }

Each file dict passed to on_store contains name, size, type, and data (base64-encoded content). The return value from on_store and on_list should be a list of summary dicts with name, type, size, size_display, and uploaded_at fields — these populate the file list in the UI. on_read returns a dict with file metadata and either content (decoded text) or content_base64 (a base64 preview for binary files).

​Configuration

​Storage scoping

​Custom storage

Configuration

Storage scoping

Custom storage