MCP Test MCP

# Resources & Templates > Expose data sources and dynamic content generators to your MCP client. export const VersionBadge = ({version}) => { return `<code className="version-badge-container">` `<p className="version-badge">` `<span className="version-badge-label">`New in version:  `<code className="version-badge-version">`{version}`</code>` `</p>` `</code>`; }; Resources represent data or files that an MCP client can read, and resource templates extend this concept by allowing clients to request dynamically generated resources based on parameters passed in the URI. FastMCP simplifies defining both static and dynamic resources, primarily using the `@mcp.resource` decorator. ## What Are Resources? Resources provide read-only access to data for the LLM or client application. When a client requests a resource URI: 1. FastMCP finds the corresponding resource definition. 2. If it's dynamic (defined by a function), the function is executed. 3. The content (text, JSON, binary data) is returned to the client. This allows LLMs to access files, database content, configuration, or dynamically generated information relevant to the conversation. ## Resources ### The `@resource` Decorator The most common way to define a resource is by decorating a Python function. The decorator requires the resource's unique URI. ```python import json from fastmcp import FastMCP mcp = FastMCP(name="DataServer") # Basic dynamic resource returning a string @mcp.resource("resource://greeting") def get_greeting() -> str: """Provides a simple greeting message.""" return "Hello from FastMCP Resources!" # Resource returning JSON data (dict is auto-serialized) @mcp.resource("data://config") def get_config() -> dict: """Provides application configuration as JSON.""" return { "theme": "dark", "version": "1.2.0", "features": ["tools", "resources"], } ``` **Key Concepts:** * **URI:** The first argument to `@resource` is the unique URI (e.g., `"resource://greeting"`) clients use to request this data. * **Lazy Loading:** The decorated function (`get_greeting`, `get_config`) is only executed when a client specifically requests that resource URI via `resources/read`. * **Inferred Metadata:** By default: * Resource Name: Taken from the function name (`get_greeting`). * Resource Description: Taken from the function's docstring. #### Decorator Arguments You can customize the resource's properties using arguments in the `@mcp.resource` decorator: ```python from fastmcp import FastMCP mcp = FastMCP(name="DataServer") # Example specifying metadata @mcp.resource( uri="data://app-status", # Explicit URI (required) name="ApplicationStatus", # Custom name description="Provides the current status of the application.", # Custom description mime_type="application/json", # Explicit MIME type tags={"monitoring", "status"}, # Categorization tags meta={"version": "2.1", "team": "infrastructure"} # Custom metadata ) def get_application_status() -> dict: """Internal function description (ignored if description is provided above).""" return {"status": "ok", "uptime": 12345, "version": mcp.settings.version} # Example usage ``` <Card icon="code" title="@resource Decorator Arguments"> <ParamField body="uri" type="str" required> The unique identifier for the resource </ParamField> <ParamField body="name" type="str | None"> A human-readable name. If not provided, defaults to function name </ParamField> <ParamField body="description" type="str | None"> Explanation of the resource. If not provided, defaults to docstring </ParamField> <ParamField body="mime_type" type="str | None"> Specifies the content type. FastMCP often infers a default like `text/plain` or `application/json`, but explicit is better for non-text types </ParamField> <ParamField body="tags" type="set[str] | None"> A set of strings used to categorize the resource. These can be used by the server and, in some cases, by clients to filter or group available resources. </ParamField> <ParamField body="enabled" type="bool" default="True"> A boolean to enable or disable the resource. See [Disabling Resources](#disabling-resources) for more information </ParamField> <ParamField body="annotations" type="Annotations | dict | None"> An optional `Annotations` object or dictionary to add additional metadata about the resource. `<Expandable title="Annotations attributes">` `<ParamField body="readOnlyHint" type="bool | None">` If true, the resource is read-only and does not modify its environment. `</ParamField>` `<ParamField body="idempotentHint" type="bool | None">` If true, reading the resource repeatedly will have no additional effect on its environment. `</ParamField>` `</Expandable>` `</ParamField>` <ParamField body="meta" type="dict[str, Any] | None"> <VersionBadge version="2.11.0" /> Optional meta information about the resource. This data is passed through to the MCP client as the`_meta` field of the client-side resource object and can be used for custom metadata, versioning, or other application-specific purposes. `</ParamField>` `</Card>` ### Return Values FastMCP automatically converts your function's return value into the appropriate MCP resource content: * **`str`**: Sent as `TextResourceContents` (with `mime_type="text/plain"` by default). * **`dict`, `list`, `pydantic.BaseModel`**: Automatically serialized to a JSON string and sent as `TextResourceContents` (with `mime_type="application/json"` by default). * **`bytes`**: Base64 encoded and sent as `BlobResourceContents`. You should specify an appropriate `mime_type` (e.g., `"image/png"`, `"application/octet-stream"`). * **`None`**: Results in an empty resource content list being returned. ### Disabling Resources <VersionBadge version="2.8.0" /> You can control the visibility and availability of resources and templates by enabling or disabling them. Disabled resources will not appear in the list of available resources or templates, and attempting to read a disabled resource will result in an "Unknown resource" error. By default, all resources are enabled. You can disable a resource upon creation using the `enabled` parameter in the decorator: ```python @mcp.resource("data://secret", enabled=False) def get_secret_data(): """This resource is currently disabled.""" return "Secret data" ``` You can also toggle a resource's state programmatically after it has been created: ```python @mcp.resource("data://config") def get_config(): return {"version": 1} # Disable and re-enable the resource get_config.disable() get_config.enable() ``` ### Accessing MCP Context <VersionBadge version="2.2.5" /> Resources and resource templates can access additional MCP information and features through the `Context` object. To access it, add a parameter to your resource function with a type annotation of `Context`: ```python from fastmcp import FastMCP, Context mcp = FastMCP(name="DataServer") @mcp.resource("resource://system-status") async def get_system_status(ctx: Context) -> dict: """Provides system status information.""" return { "status": "operational", "request_id": ctx.request_id } @mcp.resource("resource://{name}/details") async def get_details(name: str, ctx: Context) -> dict: """Get details for a specific name.""" return { "name": name, "accessed_at": ctx.request_id } ``` For full documentation on the Context object and all its capabilities, see the [Context documentation](/servers/context). ### Async Resources Use `async def` for resource functions that perform I/O operations (e.g., reading from a database or network) to avoid blocking the server. ```python import aiofiles from fastmcp import FastMCP mcp = FastMCP(name="DataServer") @mcp.resource("file:///app/data/important_log.txt", mime_type="text/plain") async def read_important_log() -> str: """Reads content from a specific log file asynchronously.""" try: async with aiofiles.open("/app/data/important_log.txt", mode="r") as f: content = await f.read() return content except FileNotFoundError: return "Log file not found." ``` ### Resource Classes While `@mcp.resource` is ideal for dynamic content, you can directly register pre-defined resources (like static files or simple text) using `mcp.add_resource()` and concrete `Resource` subclasses. ```python from pathlib import Path from fastmcp import FastMCP from fastmcp.resources import FileResource, TextResource, DirectoryResource mcp = FastMCP(name="DataServer") # 1. Exposing a static file directly readme_path = Path("./README.md").resolve() if readme_path.exists(): # Use a file:// URI scheme readme_resource = FileResource( uri=f"file://{readme_path.as_posix()}", path=readme_path, # Path to the actual file name="README File", description="The project's README.", mime_type="text/markdown", tags={"documentation"} ) mcp.add_resource(readme_resource) # 2. Exposing simple, predefined text notice_resource = TextResource( uri="resource://notice", name="Important Notice", text="System maintenance scheduled for Sunday.", tags={"notification"} ) mcp.add_resource(notice_resource) # 3. Using a custom key different from the URI special_resource = TextResource( uri="resource://common-notice", name="Special Notice", text="This is a special notice with a custom storage key.", ) mcp.add_resource(special_resource, key="resource://custom-key") # 4. Exposing a directory listing data_dir_path = Path("./app_data").resolve() if data_dir_path.is_dir(): data_listing_resource = DirectoryResource( uri="resource://data-files", path=data_dir_path, # Path to the directory name="Data Directory Listing", description="Lists files available in the data directory.", recursive=False # Set to True to list subdirectories ) mcp.add_resource(data_listing_resource) # Returns JSON list of files ``` **Common Resource Classes:** * `TextResource`: For simple string content. * `BinaryResource`: For raw `bytes` content. * `FileResource`: Reads content from a local file path. Handles text/binary modes and lazy reading. * `HttpResource`: Fetches content from an HTTP(S) URL (requires `httpx`). * `DirectoryResource`: Lists files in a local directory (returns JSON). * (`FunctionResource`: Internal class used by `@mcp.resource`). Use these when the content is static or sourced directly from a file/URL, bypassing the need for a dedicated Python function. #### Custom Resource Keys <VersionBadge version="2.2.0" /> When adding resources directly with `mcp.add_resource()`, you can optionally provide a custom storage key: ```python # Creating a resource with standard URI as the key resource = TextResource(uri="resource://data") mcp.add_resource(resource) # Will be stored and accessed using "resource://data" # Creating a resource with a custom key special_resource = TextResource(uri="resource://special-data") mcp.add_resource(special_resource, key="internal://data-v2") # Will be stored and accessed using "internal://data-v2" ``` Note that this parameter is only available when using `add_resource()` directly and not through the `@resource` decorator, as URIs are provided explicitly when using the decorator. ### Notifications <VersionBadge version="2.9.1" /> FastMCP automatically sends `notifications/resources/list_changed` notifications to connected clients when resources or templates are added, enabled, or disabled. This allows clients to stay up-to-date with the current resource set without manually polling for changes. ```python @mcp.resource("data://example") def example_resource() -> str: return "Hello!" # These operations trigger notifications: mcp.add_resource(example_resource) # Sends resources/list_changed notification example_resource.disable() # Sends resources/list_changed notification example_resource.enable() # Sends resources/list_changed notification ``` Notifications are only sent when these operations occur within an active MCP request context (e.g., when called from within a tool or other MCP operation). Operations performed during server initialization do not trigger notifications. Clients can handle these notifications using a [message handler](/clients/messages) to automatically refresh their resource lists or update their interfaces. ### Annotations <VersionBadge version="2.11.0" /> FastMCP allows you to add specialized metadata to your resources through annotations. These annotations communicate how resources behave to client applications without consuming token context in LLM prompts. Annotations serve several purposes in client applications: * Indicating whether resources are read-only or may have side effects * Describing the safety profile of resources (idempotent vs. non-idempotent) * Helping clients optimize caching and access patterns You can add annotations to a resource using the `annotations` parameter in the `@mcp.resource` decorator: ```python @mcp.resource( "data://config", annotations={ "readOnlyHint": True, "idempotentHint": True } ) def get_config() -> dict: """Get application configuration.""" return {"version": "1.0", "debug": False} ``` FastMCP supports these standard annotations: | Annotation | Type | Default | Purpose | | :----------------- | :------ | :------ | :---------------------------------------------------------------- | | `readOnlyHint` | boolean | true | Indicates if the resource only provides data without side effects | | `idempotentHint` | boolean | true | Indicates if repeated reads have the same effect as a single read | Remember that annotations help make better user experiences but should be treated as advisory hints. They help client applications present appropriate UI elements and optimize access patterns, but won't enforce behavior on their own. Always focus on making your annotations accurately represent what your resource actually does. ## Resource Templates Resource Templates allow clients to request resources whose content depends on parameters embedded in the URI. Define a template using the **same `@mcp.resource` decorator**, but include `{parameter_name}` placeholders in the URI string and add corresponding arguments to your function signature. Resource templates share most configuration options with regular resources (name, description, mime\_type, tags, annotations), but add the ability to define URI parameters that map to function parameters. Resource templates generate a new resource for each unique set of parameters, which means that resources can be dynamically created on-demand. For example, if the resource template `"user://profile/{name}"` is registered, MCP clients could request `"user://profile/ford"` or `"user://profile/marvin"` to retrieve either of those two user profiles as resources, without having to register each resource individually. <Tip> Functions with `*args` are not supported as resource templates. However, unlike tools and prompts, resource templates do support `**kwargs` because the URI template defines specific parameter names that will be collected and passed as keyword arguments. </Tip> Here is a complete example that shows how to define two resource templates: ```python from fastmcp import FastMCP mcp = FastMCP(name="DataServer") # Template URI includes {city} placeholder @mcp.resource("weather://{city}/current") def get_weather(city: str) -> dict: """Provides weather information for a specific city.""" # In a real implementation, this would call a weather API # Here we're using simplified logic for example purposes return { "city": city.capitalize(), "temperature": 22, "condition": "Sunny", "unit": "celsius" } # Template with multiple parameters and annotations @mcp.resource( "repos://{owner}/{repo}/info", annotations={ "readOnlyHint": True, "idempotentHint": True } ) def get_repo_info(owner: str, repo: str) -> dict: """Retrieves information about a GitHub repository.""" # In a real implementation, this would call the GitHub API return { "owner": owner, "name": repo, "full_name": f"{owner}/{repo}", "stars": 120, "forks": 48 } ``` With these two templates defined, clients can request a variety of resources: * `weather://london/current` → Returns weather for London * `weather://paris/current` → Returns weather for Paris * `repos://jlowin/fastmcp/info` → Returns info about the jlowin/fastmcp repository * `repos://prefecthq/prefect/info` → Returns info about the prefecthq/prefect repository ### RFC 6570 URI Templates FastMCP implements [RFC 6570 URI Templates](https://datatracker.ietf.org/doc/html/rfc6570) for resource templates, providing a standardized way to define parameterized URIs. This includes support for simple expansion, wildcard path parameters, and form-style query parameters. #### Wildcard Parameters <VersionBadge version="2.2.4" /> Resource templates support wildcard parameters that can match multiple path segments. While standard parameters (`{param}`) only match a single path segment and don't cross "/" boundaries, wildcard parameters (`{param*}`) can capture multiple segments including slashes. Wildcards capture all subsequent path segments *up until* the defined part of the URI template (whether literal or another parameter). This allows you to have multiple wildcard parameters in a single URI template. ```python from fastmcp import FastMCP mcp = FastMCP(name="DataServer") # Standard parameter only matches one segment @mcp.resource("files://{filename}") def get_file(filename: str) -> str: """Retrieves a file by name.""" # Will only match files://<single-segment> return f"File content for: {filename}" # Wildcard parameter can match multiple segments @mcp.resource("path://{filepath*}") def get_path_content(filepath: str) -> str: """Retrieves content at a specific path.""" # Can match path://docs/server/resources.mdx return f"Content at path: {filepath}" # Mixing standard and wildcard parameters @mcp.resource("repo://{owner}/{path*}/template.py") def get_template_file(owner: str, path: str) -> dict: """Retrieves a file from a specific repository and path, but only if the resource ends with `template.py`""" # Can match repo://jlowin/fastmcp/src/resources/template.py return { "owner": owner, "path": path + "/template.py", "content": f"File at {path}/template.py in {owner}'s repository" } ``` Wildcard parameters are useful when: * Working with file paths or hierarchical data * Creating APIs that need to capture variable-length path segments * Building URL-like patterns similar to REST APIs Note that like regular parameters, each wildcard parameter must still be a named parameter in your function signature, and all required function parameters must appear in the URI template. #### Query Parameters <VersionBadge version="2.13.0" /> FastMCP supports RFC 6570 form-style query parameters using the `{?param1,param2}` syntax. Query parameters provide a clean way to pass optional configuration to resources without cluttering the path. Query parameters must be optional function parameters (have default values), while path parameters map to required function parameters. This enforces a clear separation: required data goes in the path, optional configuration in query params. ```python from fastmcp import FastMCP mcp = FastMCP(name="DataServer") # Basic query parameters @mcp.resource("data://{id}{?format}") def get_data(id: str, format: str = "json") -> str: """Retrieve data in specified format.""" if format == "xml": return f"<data id='{id}' />" return f'{{"id": "{id}"}}' # Multiple query parameters with type coercion @mcp.resource("api://{endpoint}{?version,limit,offset}") def call_api(endpoint: str, version: int = 1, limit: int = 10, offset: int = 0) -> dict: """Call API endpoint with pagination.""" return { "endpoint": endpoint, "version": version, "limit": limit, "offset": offset, "results": fetch_results(endpoint, version, limit, offset) } # Query parameters with wildcards @mcp.resource("files://{path*}{?encoding,lines}") def read_file(path: str, encoding: str = "utf-8", lines: int = 100) -> str: """Read file with optional encoding and line limit.""" return read_file_content(path, encoding, lines) ``` **Example requests:** * `data://123` → Uses default format `"json"` * `data://123?format=xml` → Uses format `"xml"` * `api://users?version=2&limit=50` → `version=2, limit=50, offset=0` * `files://src/main.py?encoding=ascii&lines=50` → Custom encoding and line limit FastMCP automatically coerces query parameter string values to the correct types based on your function's type hints (`int`, `float`, `bool`, `str`). **Query parameters vs. hidden defaults:** Query parameters expose optional configuration to clients. To hide optional parameters from clients entirely (always use defaults), simply omit them from the URI template: ```python # Clients CAN override max_results via query string @mcp.resource("search://{query}{?max_results}") def search_configurable(query: str, max_results: int = 10) -> dict: return {"query": query, "limit": max_results} # Clients CANNOT override max_results (not in URI template) @mcp.resource("search://{query}") def search_fixed(query: str, max_results: int = 10) -> dict: return {"query": query, "limit": max_results} ``` ### Template Parameter Rules <VersionBadge version="2.2.0" /> FastMCP enforces these validation rules when creating resource templates: 1. **Required function parameters** (no default values) must appear in the URI path template 2. **Query parameters** (specified with `{?param}` syntax) must be optional function parameters with default values 3. **All URI template parameters** (path and query) must exist as function parameters Optional function parameters (those with default values) can be: * Included as query parameters (`{?param}`) - clients can override via query string * Omitted from URI template - always uses default value, not exposed to clients * Used in alternative path templates - enables multiple ways to access the same resource **Multiple templates for one function:** Create multiple resource templates that expose the same function through different URI patterns by manually applying decorators: ```python from fastmcp import FastMCP mcp = FastMCP(name="DataServer") # Define a user lookup function that can be accessed by different identifiers def lookup_user(name: str | None = None, email: str | None = None) -> dict: """Look up a user by either name or email.""" if email: return find_user_by_email(email) # pseudocode elif name: return find_user_by_name(name) # pseudocode else: return {"error": "No lookup parameters provided"} # Manually apply multiple decorators to the same function mcp.resource("users://email/{email}")(lookup_user) mcp.resource("users://name/{name}")(lookup_user) ``` Now an LLM or client can retrieve user information in two different ways: * `users://email/alice@example.com` → Looks up user by email (with name=None) * `users://name/Bob` → Looks up user by name (with email=None) This approach allows a single function to be registered with multiple URI patterns while keeping the implementation clean and straightforward. Templates provide a powerful way to expose parameterized data access points following REST-like principles. ## Error Handling <VersionBadge version="2.4.1" /> If your resource function encounters an error, you can raise a standard Python exception (`ValueError`, `TypeError`, `FileNotFoundError`, custom exceptions, etc.) or a FastMCP `ResourceError`. By default, all exceptions (including their details) are logged and converted into an MCP error response to be sent back to the client LLM. This helps the LLM understand failures and react appropriately. If you want to mask internal error details for security reasons, you can: 1. Use the `mask_error_details=True` parameter when creating your `FastMCP` instance: ```python mcp = FastMCP(name="SecureServer", mask_error_details=True) ``` 2. Or use `ResourceError` to explicitly control what error information is sent to clients: ```python from fastmcp import FastMCP from fastmcp.exceptions import ResourceError mcp = FastMCP(name="DataServer") @mcp.resource("resource://safe-error") def fail_with_details() -> str: """This resource provides detailed error information.""" # ResourceError contents are always sent back to clients, # regardless of mask_error_details setting raise ResourceError("Unable to retrieve data: file not found") @mcp.resource("resource://masked-error") def fail_with_masked_details() -> str: """This resource masks internal error details when mask_error_details=True.""" # This message would be masked if mask_error_details=True raise ValueError("Sensitive internal file path: /etc/secrets.conf") @mcp.resource("data://{id}") def get_data_by_id(id: str) -> dict: """Template resources also support the same error handling pattern.""" if id == "secure": raise ValueError("Cannot access secure data") elif id == "missing": raise ResourceError("Data ID 'missing' not found in database") return {"id": id, "value": "data"} ``` When `mask_error_details=True`, only error messages from `ResourceError` will include details, other exceptions will be converted to a generic message. ## Server Behavior ### Duplicate Resources <VersionBadge version="2.1.0" /> You can configure how the FastMCP server handles attempts to register multiple resources or templates with the same URI. Use the `on_duplicate_resources` setting during `FastMCP` initialization. ```python from fastmcp import FastMCP mcp = FastMCP( name="ResourceServer", on_duplicate_resources="error" # Raise error on duplicates ) @mcp.resource("data://config") def get_config_v1(): return {"version": 1} # This registration attempt will raise a ValueError because # "data://config" is already registered and the behavior is "error". # @mcp.resource("data://config") # def get_config_v2(): return {"version": 2} ``` The duplicate behavior options are: * `"warn"` (default): Logs a warning, and the new resource/template replaces the old one. * `"error"`: Raises a `ValueError`, preventing the duplicate registration. * `"replace"`: Silently replaces the existing resource/template with the new one. * `"ignore"`: Keeps the original resource/template and ignores the new registration attempt.