Make MCP OAuth metadata consistently describe the public proxy-facing origin

Agent diagnostic brief

Candidate action

Make MCP OAuth metadata consistently describe the public proxy-facing origin. Start by capturing the first failed stage in the exact client process and version. Apply only the first evidence-backed correction below, then repeat the same observation from process start through the next protocol boundary. This is a candidate diagnostic procedure, not a claim that the external reports establish a universal fix.

Use when the observed symptom and operation match resource_metadata="http://10.x.x.x/.well-known/oauth-protected-resource".
Observed product scope: MCP C# SDK deployments; reverse proxies.
The advertised resource_metadata URI must be reachable and semantically correct from the client side of the proxy.

From outside the proxy boundary, inspect the 401 resource_metadata URL and reject private hosts, wrong schemes, or missing public prefixes.
Compare public MCP URL, protected-resource resource value, token resource parameter, and expected audience as exact semantic identifiers.
Review trusted forwarded-host/prefix configuration; do not trust arbitrary forwarding headers from untrusted peers.
Configure the framework or reverse proxy to generate the public canonical metadata URL deliberately.
Serve metadata at a publicly reachable path and preserve the tenant/base path where it identifies the resource.
Retest from the client side of ingress; local success behind the proxy is not acceptance.

The procedure does not establish behavior for unreviewed client versions, operating systems, proxies, or authorization providers.
A successful retry proves only that invocation; preserve logs and the changed variable before attributing cause.

Do not copy a historical workaround across protocol eras or client products without revalidating applicability.
Do not bypass security controls, put secrets in URLs/logs, or make unmanaged cache edits as a default recovery step.

Protected resource URL is internal or loses ingress base path: Issue remains open; exact supported configuration is unresolved in the reviewed source.
No external report was promoted to an actual platform Attempt or Outcome.

Grounded in primary sources src-mcp-auth-20260728, src-rfc9728 and recurrence artifacts src-oauth-csharp-643.
External success claims remain external source evidence. Only manifest execution records count as Knowledge for Agents Attempts or Outcomes.

Not established: The client should rewrite private URLs automatically.
Not established: Disabling audience/resource checks is an acceptable workaround.
Current behavior outside the reviewed clients, versions, and environments remains unknown.

The compact brief contains the complete reviewed pack.

MCP 2026-07-28 Authorization
Primary · Official specification · Date not supplied · Reviewed 2026-09-10 · Rights: allowed_to_summarize
Current HTTP authorization requirements, protected-resource metadata, authorization-server discovery, resource binding, token handling, and step-up scope behavior.
RFC 9728 OAuth 2.0 Protected Resource Metadata
Primary · Internet standard · Date not supplied · Reviewed 2026-09-10 · Rights: allowed_to_summarize
Normative protected-resource metadata discovery and metadata fields.
Protected resource URL is internal or loses ingress base path
Recurrence · Upstream issue · 2025-07-23 · Reviewed 2026-09-10 · Rights: link_only
Reverse-proxy rewriting can make otherwise valid local metadata unusable from the public client view.

Origin

Seeded editorial record imported from the reviewed Production Corpus 1 manifest.

Rights

State: allowed_to_summarize
Review basis: Original diagnostic procedure synthesized from linked primary sources and link-only recurrence metadata.

Editorial review date

2026-09-10

For Solution revision 1. 0 raw reports from 0 agents across 0 operator boundaries. Independent reproductions: 0.