fix(mcp): strip root_path before matching the per-server MCP route spelling - #35576

TLDR

Problem this solves:

• The per-server MCP 401 challenge decides which URL spelling the client used by matching _original_path against the root-relative /{server}/mcp shape
• _original_path (and scope["path"]) are raw request-line paths, so on a SERVER_ROOT_PATH deployment they still carry the prefix and that match always fails
• A client connecting on /litellm/{server}/mcp is therefore pointed at the standard-pattern discovery document, whose resource is {base}/litellm/mcp/{server} rather than the URL it called, and a strict RFC 9728 section 3 client aborts before the MCP request fires

How it solves it:

• Removes root_path from the path before the spelling match, on a segment boundary, the same way litellm.proxy.auth.auth_utils.get_request_route already does for the rest of the MCP auth path
• Root-mounted deployments are unaffected; the strip is a no-op when there is no root_path

Relevant issues

• Fixes the per-server MCP OAuth 401 challenge advertising the wrong protected-resource document on a SERVER_ROOT_PATH (sub-path) deployment
• Restores the RFC 9728 section 3 exact-match property: the resource a client discovers now equals the MCP URL it connected to, in both the /{server}/mcp and /mcp/{server} spellings
• Adds a regression test that pins both spellings under SERVER_ROOT_PATH through the real process_mcp_request caller

Surfaced by the discussion on #35226, which reported the same class of resource mismatch. That PR proposes a new opt-in env var to derive the discovery path from the request; this change instead fixes the root-path normalization the existing code already relies on, which covers the sub-path deployment without new configuration

Linear ticket

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Screenshots / Proof of Fix

Live proxy on a sub-path deployment, config below, started with

SERVER_ROOT_PATH=/litellm LITELLM_MASTER_KEY=sk-1234 \
  python litellm/proxy/proxy_cli.py --config mcp_rootpath_config.yaml --port 4111

mcp_servers:
  github:
    url: "https://upstream.example/mcp"
    transport: "http"
    auth_type: "oauth2"
    oauth2_flow: "authorization_code"
    client_id: "gateway-managed-client"
    client_secret: "gateway-managed-secret"
    authorization_url: "https://upstream.example/oauth/authorize"
    token_url: "https://upstream.example/oauth/token"

Before (this branch's base, ba480a619f) both spellings collapse onto the standard-pattern document:

$ for path in /litellm/github/mcp /litellm/mcp/github; do
    curl -s -i -X POST "http://localhost:4111${path}" \
      -H 'Content-Type: application/json' -H 'Accept: application/json, text/event-stream' \
      -d '{"jsonrpc":"2.0","id":1,"method":"tools/list"}' | grep -iE '^HTTP/|^www-authenticate'
  done

HTTP/1.1 401 Unauthorized
www-authenticate: Bearer resource_metadata="http://localhost:4111/litellm/.well-known/oauth-protected-resource/litellm/mcp/github"
HTTP/1.1 401 Unauthorized
www-authenticate: Bearer resource_metadata="http://localhost:4111/litellm/.well-known/oauth-protected-resource/litellm/mcp/github"

Following the challenge a /litellm/github/mcp client was sent to shows the mismatch it aborts on:

$ curl -s "http://localhost:4111/litellm/.well-known/oauth-protected-resource/litellm/mcp/github"
{"resource": "http://localhost:4111/litellm/mcp/github", ...}

After (this branch) each spelling keeps its own document:

$ for path in /litellm/github/mcp /litellm/mcp/github; do
    curl -s -i -X POST "http://localhost:4111${path}" \
      -H 'Content-Type: application/json' -H 'Accept: application/json, text/event-stream' \
      -d '{"jsonrpc":"2.0","id":1,"method":"tools/list"}' | grep -iE '^HTTP/|^www-authenticate'
  done

HTTP/1.1 401 Unauthorized
www-authenticate: Bearer resource_metadata="http://localhost:4111/litellm/.well-known/oauth-protected-resource/litellm/github/mcp"
HTTP/1.1 401 Unauthorized
www-authenticate: Bearer resource_metadata="http://localhost:4111/litellm/.well-known/oauth-protected-resource/litellm/mcp/github"

and each document's resource is exactly the URL the client connected to:

$ curl -s "http://localhost:4111/litellm/.well-known/oauth-protected-resource/litellm/github/mcp"
{"resource": "http://localhost:4111/litellm/github/mcp", "authorization_servers": ["http://localhost:4111/litellm/mcp"], ...}

$ curl -s "http://localhost:4111/litellm/.well-known/oauth-protected-resource/litellm/mcp/github"
{"resource": "http://localhost:4111/litellm/mcp/github", "authorization_servers": ["http://localhost:4111/litellm/mcp"], ...}

Type

🐛 Bug Fix

Changes

oauth_utils.py gains get_route_relative_request_path, which reads _original_path (falling back to scope["path"]) and removes the deployment's root_path when the raw path is that prefix or continues past it on a / boundary, so /litellmfoo is not truncated under root_path=/litellm. get_passthrough_resource_metadata_url now compares that normalized path instead of the raw one

Nothing else changes. On a root-mounted proxy root_path is empty and the helper returns the raw path unchanged, so the emitted metadata URL is byte-identical to today's

The regression test lives in the existing TestAggregateGatewayDcrChallenge class next to the non-root-path spelling test it mirrors, and drives the real process_mcp_request entry point rather than the helper, so it fails if either the challenge or the spelling selection regresses. It reverts to the pre-fix assertion failure when the normalization is removed

QA runbook

1. uv run --no-sync pytest tests/test_litellm/proxy/_experimental/mcp_server/auth/ tests/test_litellm/proxy/_experimental/mcp_server/test_discoverable_endpoints.py -q
2. Write the mcp_servers config above to mcp_rootpath_config.yaml
3. SERVER_ROOT_PATH=/litellm LITELLM_MASTER_KEY=sk-1234 python litellm/proxy/proxy_cli.py --config mcp_rootpath_config.yaml --port 4111
4. Run the two curl loops from the proof section and confirm the two spellings now advertise different resource_metadata URLs
5. GET each advertised URL and confirm resource equals the MCP URL from step 4
6. Restart with SERVER_ROOT_PATH unset and confirm the challenges are unchanged from before this PR (/.well-known/oauth-protected-resource/github/mcp and /.well-known/oauth-protected-resource/mcp/github)

Final Attestation

• The tests check the right things, including the edge cases, and regressions in the respective real-world customer use-cases are not possible after this PR