fix: change Resource URI fields from AnyUrl to str #1863
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Github-Issue: #1574
The Python SDK was incorrectly using Pydantic's AnyUrl for URI fields on resource types. This rejected relative paths like 'users/me' that are valid according to the MCP specification, which defines URIs as plain strings without format validation. This change updates the following types to use str instead of AnyUrl: - Resource.uri - ReadResourceRequestParams.uri - ResourceContents.uri - SubscribeRequestParams.uri - UnsubscribeRequestParams.uri - ResourceUpdatedNotificationParams.uri - FastMCP Resource.uri (base class) Client and server session methods now accept str | AnyUrl for backwards compatibility, converting to string internally. Updates migration.md with documentation of this breaking change. Github-Issue: #1574
Replace type-instantiation tests with meaningful tests that verify: - Relative URIs survive the full server-client JSON-RPC roundtrip - Custom scheme URIs work end-to-end - URIs are preserved exactly through JSON serialization These tests catch regressions more reliably - if AnyUrl is reintroduced, the integration tests will fail during serialization or URI transformation. Github-Issue: #1574
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With URI fields changed from AnyUrl to str, URIs are no longer normalized (e.g., trailing slashes not added). Update test assertions to match exact URIs being sent.
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Kludex
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Jan 16, 2026
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Summary
The Python SDK was incorrectly using Pydantic's
AnyUrlfor URI fields on resource types. This rejected relative paths likeusers/methat are valid according to the MCP specification and accepted by the TypeScript SDK.Motivation and Context
The MCP specification defines URI fields as plain strings with no JSON Schema format validation. This intentionally allows:
users/mecustom://resourcefile:///pathhttps://example.comThe Python SDK was stricter than the spec, causing interoperability issues when Python clients received resources from servers using relative URIs.
Fixes #1574
How Has This Been Tested?
tests/issues/test_1574_resource_uri_validation.pyAnyUrlBreaking Changes
This is a breaking change for users who were passing
AnyUrlobjects to URI fields. They will need to convert to strings. The client session methods (read_resource,subscribe_resource,unsubscribe_resource) accept bothstr | AnyUrlfor backwards compatibility.Updated
docs/migration.mdwith migration guidance.Types of changes
Checklist
Additional context
Types updated:
Resource.uriReadResourceRequestParams.uriResourceContents.uriSubscribeRequestParams.uriUnsubscribeRequestParams.uriResourceUpdatedNotificationParams.uriResource.uribase class