ProxyOAuthServerProvider fails with Zod validation error when using Ory Hydra as OAuth provider

Describe the bug
The ProxyOAuthServerProvider's client registration fails with a Zod schema validation error when using Hydra as the upstream OAuth provider. The error occurs because Hydra returns "contacts": null in the client registration response, but the MCP SDK's OAuthClientInformationFullSchema expects contacts to be an array.

To Reproduce
Steps to reproduce the behavior:

Configure ProxyOAuthServerProvider with Hydra endpoints (e.g., registrationUrl pointing to Hydra's /oauth2/register)
Attempt client registration through the MCP server
Registration request succeeds at Hydra but fails during MCP SDK's response parsing

Expected behavior
Client registration should succeed when using standards-compliant OAuth2 providers like Hydra that return valid OAuth client information responses.

Logs

Unexpected error registering client: ZodError: [
    {
      "code": "invalid_type",
      "expected": "array",
      "received": "null",
      "path": [
        "contacts"
      ],
      "message": "Expected array, received null"
    }
  ]
  at Object.registerClient 
  (/node_modules/@modelcontextprotocol/sdk/src/server/auth/providers/proxyProvider.ts:113:51)

Additional context
Hydra returns valid RFC 7591 compliant client registration responses where optional fields like contacts can be null. The MCP SDK's Zod schema appears to be stricter than the OAuth2 specification requires for these optional fields.

Still an issue

Still an issue.

Hacky workaround is to use something like this:

/**
 * Creates a fetch wrapper that strips `null` values from JSON responses.
 *
 * Some OAuth servers return `null` for optional fields (e.g. client_uri,
 * logo_uri, client_secret) instead of omitting them. The MCP SDK's Zod
 * schemas use `.optional()` which accepts `undefined` but not `null`,
 * causing validation to fail. This wrapper intercepts successful JSON
 * responses and removes null-valued keys before the SDK parses them.
 */
export const createNullStrippingFetch = (): typeof fetch => {
  return async (input, init) => {
    const response = await fetch(input, init);

    if (!response.ok) {
      return response;
    }

    const contentType = response.headers.get('content-type');

    if (!contentType?.includes('application/json')) {
      return response;
    }

    const body = await response.json();

    const sanitized = Object.fromEntries(
      Object.entries(body as Record<string, unknown>).filter(
        ([, value]) => value !== null,
      ),
    );

    return new Response(JSON.stringify(sanitized), {
      headers: response.headers,
      status: response.status,
      statusText: response.statusText,
    });
  };
};

@punkpeye

@punkpeye's null-stripping fetch wrapper works — confirmed on my end with Ory Hydra. The core issue is that Hydra returns "contacts": null instead of omitting the field, and OAuthClientInformationFullSchema uses .optional() which accepts undefined but rejects null. The wrapper that converts null values to undefined before Zod validation is a solid workaround until the SDK handles this natively. 👍

fix: add Envoy DCR sanitizer for Hydra MCP SDK compatibility

fix(auth): normalize null token-response members at parse sites, not …

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ProxyOAuthServerProvider fails with Zod validation error when using Ory Hydra as OAuth provider #754

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ProxyOAuthServerProvider fails with Zod validation error when using Ory Hydra as OAuth provider #754

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jeffdyke commented on Nov 12, 2025

punkpeye commented on Jan 29, 2026

sstklen commented on Feb 22, 2026

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