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fastmcp.resources.base

Base classes and interfaces for FastMCP resources.

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Functions

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convert_raw_to_resource_result

Wrap a user function’s return value in a ResourceResult. Shared by Resource and ResourceTemplate so both honor the MIME type the component declares in listings. A component that advertises text/csv must not serve text/plain on read. Args:
  • raw_value: The value returned by the user’s function.
  • mime_type: The component’s declared MIME type, forwarded to content items.
  • meta: Component-level meta (e.g. ui metadata for MCP Apps CSP/permissions) propagated to each content item.

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Classes

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ResourceContent

Wrapper for resource content with optional MIME type and metadata. Accepts any value for content - strings and bytes pass through directly, other types (dict, list, BaseModel, etc.) are automatically JSON-serialized. Methods:

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to_mcp_resource_contents

Convert to MCP resource contents type. Args:
  • uri: The URI of the resource (required by MCP types)
Returns:
  • TextResourceContents for str content, BlobResourceContents for bytes

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ResourceResult

Canonical result type for resource reads. Provides explicit control over resource responses: multiple content items, per-item MIME types, and metadata at both the item and result level. Methods:

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to_mcp_result

Convert to MCP ReadResourceResult. Args:
  • uri: The URI of the resource (required by MCP types)
Returns:
  • MCP ReadResourceResult with converted contents

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InputRequiredResourceResult

The full result of a single multi-round-trip resource read (SEP-2322). InputRequiredResult is a result type, not a tools/call feature: any request may resolve to one. When a resource or resource template returns an InputRequiredResult from its body to ask the client for input, that ask is the legitimate result of this resources/read — so FastMCP wraps it in this ResourceResult subclass, mirroring InputRequiredToolResult and InputRequiredPromptResult, and it flows through the middleware chain as an ordinary return value. Invariant: the wrapped InputRequiredResult is never serialized as resource contents. contents is always empty; the wire handler (_on_read_resource) reads .input_required and returns it to the runner.

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Resource

Base class for all resources. Methods:

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from_function

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set_default_mime_type

Set default MIME type if not provided.

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set_default_name

Set default name from URI if not provided.

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read

Read the resource content. Subclasses implement this to return resource data. Supported return types:
  • str: Text content
  • bytes: Binary content
  • ResourceResult: Full control over contents and result-level meta

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convert_result

Convert a raw result to ResourceResult. This is used in two contexts:
  1. In _read() to convert user function return values to ResourceResult
  2. In tasks_result_handler() to convert Docket task results to ResourceResult
Handles ResourceResult passthrough and converts raw values using ResourceResult’s normalization. When the raw value is a plain string or bytes, the resource’s own mime_type is forwarded so that ui:// resources (and others with non-default MIME types) don’t fall back to text/plain. The resource’s component-level meta (e.g. ui metadata for MCP Apps CSP/permissions) is propagated to each content item so that hosts can read it from the resources/read response.

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to_mcp_resource

Convert the resource to an SDKResource.

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key

The globally unique lookup key for this resource.

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get_span_attributes