fix: preserve exact resource URI from protected resource metadata

[Christian-Sidak]

Summary

• selectResourceURL() was returning new URL(resourceMetadata.resource), which normalizes pathless URIs like https://example.com to https://example.com/ via URL.href
• Both the authorization request and token request then serialized the resource with .href, further cementing the trailing slash
• Changed selectResourceURL() to return the original metadata string directly (preserving the exact value), and updated all serialization to use String(resource) instead of resource.href
• Updated the return type of selectResourceURL and the resource parameter in startAuthorization, executeTokenRequest, exchangeAuthorization, refreshAuthorization, and fetchToken from URL to URL | string to accommodate both metadata strings and custom validateResourceURL URL objects

Test plan

• Existing 350 tests pass (updated one test expectation that was asserting the old incorrect normalized behavior)
• Added new regression test: preserves pathless resource URI from PRM without adding trailing slash
• Manually verified: new URL("https://example.com").href === "https://example.com/" (the bug), and that returning the string directly preserves "https://example.com"

Fixes #1968

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🦋 Changeset detected

Latest commit: 60993b2

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

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@modelcontextprotocol/client	Patch

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commit: 60993b2

[Christian-Sidak]

Friendly bump -- let me know if anything needs changing.

[Christian-Sidak]

Friendly bump -- let me know if anything needs changing.

[Wesley-Miller]

Additional bump. Thanks

[Christian-Sidak]

Friendly bump -- let me know if anything needs changing.

[Christian-Sidak]

Friendly bump -- let me know if anything needs changing.

[Christian-Sidak]

Friendly bump -- let me know if anything needs changing.

[aarora79]

Endorsing this from a production deployment. We run an MCP gateway/registry that fronts many MCP servers behind a single Microsoft Entra tenant (agentic-community/mcp-gateway-registry), and we hit exactly the #1968 bug: our PRM advertises a pathless resource (verified slash-free via curl), but the client sends resource=https://.../ on /authorize + /token, so Entra rejects the exchange with AADSTS9010010.

I reviewed the diff and it resolves our case correctly:

• Returning resourceMetadata.resource verbatim from selectResourceURL (instead of new URL(...)) fixes it at the source — the URL.href normalization was the only thing adding the slash — and emitting String(resource) at both the /authorize and /token sites keeps it byte-for-byte.
• A path-qualified resource (https://gw/<server>/mcp) is preserved unchanged, and the bare origin (https://gw) now goes out without the trailing slash Entra can't match.

One production detail that makes the pathless case matter beyond a single server: because a bare-origin resource is currently unusable on Entra, a gateway is forced to register one identifierUri per MCP server (Entra App ID URIs can't end in a slash and are matched by exact string), which runs into Entra's 999-URI-per-app cap and per-server operator toil. Emitting the origin verbatim collapses that back to a single origin registration for the whole gateway. So this fix removes real operational burden, not just a one-server annoyance.

The added regression test (pathless PRM resource -> no trailing slash) and the flipped assertion match the intended behavior. +1 to merging.

Related downstream reports for cross-reference: the same AADSTS9010010 surfaced via Claude Code in anthropics/claude-code#52871, and our tracking issue agentic-community/mcp-gateway-registry#990.

cc @omrishiv

[Christian-Sidak]

Thanks @aarora79 -- great to have production confirmation. The single-origin-per-gateway use case (vs. one identifierUri per MCP server) is a clear illustration of why byte-for-byte preservation of the resource URI matters at scale. The cross-references to anthropics/claude-code#52871 and mcp-gateway-registry#990 are helpful context for maintainers.

[aarora79]

Hi @felixweinberger @pcarleton, would appreciate your attention on this one. selectResourceURL() returns new URL(resourceMetadata.resource), which normalizes a pathless https://example.com to https://example.com/ and breaks the exact-match resource parameter for Microsoft Entra ID (#1968, P2/bug/fix-proposed). Patch is 3 files, +75/−12, with a regression test and a changeset. Multiple people have confirmed it with Claude Code connecting to an MCP server via a Gateway with Entra ID being used as the IdP, please see anthropics/claude-code#52871.

[zoltan-fedor]

+1

[olliekemp]

+1

[adrianB1996]

+1

[matthewcarlisle]

+1 - this is an actual production issue. We can't connect to Azure DevOps via MCP until this is fixed. Would love to use MCP for useful things.