fix(oauth): serve RFC 9728 resource metadata at the path-based locations Claude.ai fetches (#1915)

Claude.ai's connector setup derives the protected-resource metadata URL
from the MCP server URL and fetches it before any 401 challenge:
  /.well-known/oauth-protected-resource/api/extensions/ext/mcp-server/mcp
  /api/extensions/ext/mcp-server/mcp/.well-known/oauth-protected-resource
Both were 404 (only the root document our WWW-Authenticate header points
at existed), which the dialog reported as "Authorization with Accounted
failed". One shared builder now serves all three locations; the
path-based route answers 404 for any path other than the MCP endpoint.


Claude-Session: https://claude.ai/code/session_018wCdzRTatKiDByKB8hCNT6

Co-authored-by: Jakob Wennberg <311770904+jakobwennberg-oss@users.noreply.github.com>
Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

DECISIONS.md

@@ -1245,3 +1245,4 @@ One line per decision: `[YYYY-MM-DD] <decision>: <why>`. Appended by agents and
 [2026-08-25] Payslip YTD ("Ackumulerat") stays a stored snapshot on salary_run_employees, refreshed at approve + book, rather than being recomputed at PDF-render time: an employee who re-downloads a lonebesked must see the figures it had when it was issued, and a render-time sum would silently restate delivered payslips after any backdated correction. The same change widens the counted prior-run statuses from booked-only to approved/paid/booked (corrected stays excluded: its correction run replaces the whole month), because the original snapshot-at-calculate-time rule froze a YTD that was missing every month not yet booked when next month's run was prepared.
 [2026-08-25] ROT/RUT BegartBelopp truncates to whole kronor (truncateToWholeKronor), not half-up: the deduction is capped at 50%/30% of arbetskostnaden. At the RUT cap, half-up manufactured begart > betalt and blocked correct invoices; for ROT below the cap it over-requested past the cap and the 1513 fordran, so those files now ask 1 kr less (skeptic-verified on PR #1910).
 [2026-08-25] Woo bulk revenue template = per-rate account choice, no hardcoded varor/tjanster preset: BAS 2026 has no standard 30xx goods/services subdivision (3040-series is company-specific), so presets would invent accounts; chosen accounts are validated against the company chart instead, and only diffs from the 3001-series default are sent.
+[2026-08-26] RFC 9728 protected-resource metadata is served at THREE locations (root, path-based /.well-known/oauth-protected-resource/<mcp path>, and <mcp url>/.well-known/oauth-protected-resource): Claude.ai's connector setup derives the metadata URL from the server URL and fetches it before any 401, so the root document our WWW-Authenticate header points at was not enough ('Authorization with Accounted failed' with only 404s in the logs). One builder, three routes; the path-based route answers 404 for any path other than the MCP endpoint so no phantom resource is advertised.

app/.well-known/oauth-protected-resource/[...path]/__tests__/route.test.ts

@@ -0,0 +1,62 @@
+import { afterEach, describe, expect, it, vi } from 'vitest'
+import { GET } from '../route'
+
+// RFC 9728 path-based discovery: Claude.ai's connector setup fetches
+// /.well-known/oauth-protected-resource/<mcp path> before any 401 and treats
+// a 404 as "Authorization failed" (seen in production 2026-08-26).
+
+function call(url: string, path: string[]) {
+  return GET(new Request(url, { headers: { host: new URL(url).host } }), {
+    params: Promise.resolve({ path }),
+  })
+}
+
+describe('path-based MCP protected-resource discovery', () => {
+  afterEach(() => {
+    vi.unstubAllEnvs()
+  })
+
+  it('serves the MCP endpoint metadata at the RFC 9728 path-based location', async () => {
+    vi.stubEnv('NEXT_PUBLIC_APP_URL', 'https://app.accounted.se')
+    const response = await call(
+      'https://app.accounted.se/.well-known/oauth-protected-resource/api/extensions/ext/mcp-server/mcp',
+      ['api', 'extensions', 'ext', 'mcp-server', 'mcp']
+    )
+    expect(response.status).toBe(200)
+    const body = await response.json()
+    expect(body.resource).toBe('https://app.accounted.se/api/extensions/ext/mcp-server/mcp')
+    expect(body.authorization_servers).toEqual(['https://app.accounted.se'])
+    expect(body.scopes_supported).toEqual(['mcp'])
+  })
+
+  it('reflects the accounted namespace exactly like the root document', async () => {
+    vi.stubEnv('NEXT_PUBLIC_APP_URL', 'https://app.accounted.se')
+    const response = await call(
+      'https://app.accounted.se/.well-known/oauth-protected-resource/api/extensions/ext/mcp-server/mcp?tool_namespace=accounted',
+      ['api', 'extensions', 'ext', 'mcp-server', 'mcp']
+    )
+    const body = await response.json()
+    expect(body.resource).toBe(
+      'https://app.accounted.se/api/extensions/ext/mcp-server/mcp?tool_namespace=accounted'
+    )
+  })
+
+  it('never echoes an arbitrary namespace value', async () => {
+    vi.stubEnv('NEXT_PUBLIC_APP_URL', 'https://app.accounted.se')
+    const response = await call(
+      'https://app.accounted.se/.well-known/oauth-protected-resource/api/extensions/ext/mcp-server/mcp?tool_namespace=evil%22',
+      ['api', 'extensions', 'ext', 'mcp-server', 'mcp']
+    )
+    const body = await response.json()
+    expect(body.resource).toBe('https://app.accounted.se/api/extensions/ext/mcp-server/mcp')
+  })
+
+  it('answers 404 for any other path so no phantom resource is advertised', async () => {
+    vi.stubEnv('NEXT_PUBLIC_APP_URL', 'https://app.accounted.se')
+    const response = await call(
+      'https://app.accounted.se/.well-known/oauth-protected-resource/api/v1/companies',
+      ['api', 'v1', 'companies']
+    )
+    expect(response.status).toBe(404)
+  })
+})

app/.well-known/oauth-protected-resource/[...path]/route.ts

@@ -0,0 +1,27 @@
+import { NextResponse } from 'next/server'
+import {
+  MCP_RESOURCE_PATH,
+  buildProtectedResourceMetadata,
+} from '@/lib/auth/protected-resource-metadata'
+
+/**
+ * RFC 9728 §3.1 path-based Protected Resource Metadata:
+ * `/.well-known/oauth-protected-resource{resource-path}`.
+ *
+ * Claude.ai's connector setup derives this URL from the MCP server URL and
+ * fetches it before any 401 challenge; without it the dialog reports
+ * "Authorization with Accounted failed". Only the MCP endpoint is a protected
+ * resource here, so every other path is a 404 rather than a generic answer
+ * that would advertise resources this server does not serve.
+ */
+export async function GET(
+  request: Request,
+  context: { params: Promise<{ path: string[] }> }
+) {
+  const { path } = await context.params
+  const resourcePath = '/' + (path ?? []).join('/')
+  if (resourcePath !== MCP_RESOURCE_PATH) {
+    return NextResponse.json({ error: 'not_found' }, { status: 404 })
+  }
+  return NextResponse.json(buildProtectedResourceMetadata(request))
+}

app/.well-known/oauth-protected-resource/route.ts

@@ -1,30 +1,12 @@
 import { NextResponse } from 'next/server'
-import { resolveDiscoveryBaseUrl } from '@/lib/api/v1/base-url'
+import { buildProtectedResourceMetadata } from '@/lib/auth/protected-resource-metadata'
 
 /**
- * RFC 9728: Protected Resource Metadata.
- * Tells MCP clients which authorization server to use.
- *
- * The resource/AS URLs reflect the (allowlisted) request host: MCP clients
- * validate the advertised resource against the server URL they were
- * configured with, and existing connectors point at the legacy
- * app.gnubok.se domain after the app.accounted.se cutover.
+ * RFC 9728: Protected Resource Metadata, root location. This is the URL the
+ * MCP endpoint's 401 `WWW-Authenticate` header points at. The same document
+ * is also served at the path-based and endpoint-appended locations (see
+ * lib/auth/protected-resource-metadata.ts for why all three exist).
  */
 export async function GET(request: Request) {
-  const appUrl = resolveDiscoveryBaseUrl(request)
-  const resource = new URL('/api/extensions/ext/mcp-server/mcp', appUrl)
-  // `accounted` is the COMPLETE allow-list of reflectable namespaces. We never
-  // echo the inbound parameter value: on an exact match we set the fixed
-  // literal, so a crafted tool_namespace (URL-special chars, other values) can
-  // never reach the advertised resource URL. Do not loosen this to a broader
-  // match without re-checking every downstream consumer that parses `resource`.
-  if (new URL(request.url).searchParams.get('tool_namespace') === 'accounted') {
-    resource.searchParams.set('tool_namespace', 'accounted')
-  }
-
-  return NextResponse.json({
-    resource: resource.toString(),
-    authorization_servers: [appUrl],
-    scopes_supported: ['mcp'],
-  })
+  return NextResponse.json(buildProtectedResourceMetadata(request))
 }

extensions/general/mcp-server/__tests__/protected-resource-discovery.test.ts

@@ -0,0 +1,50 @@
+import { afterEach, describe, expect, it, vi } from 'vitest'
+import { mcpServerExtension } from '../index'
+
+// Endpoint-appended RFC 9728 discovery: Claude.ai's connector setup tries
+// <server url>/.well-known/oauth-protected-resource before any 401 and turns
+// a 404 into "Authorization failed" (production, 2026-08-26).
+
+function findRoute(method: string, path: string) {
+  const route = mcpServerExtension.apiRoutes?.find((r) => r.method === method && r.path === path)
+  if (!route) throw new Error(`route ${method} ${path} not declared`)
+  return route
+}
+
+describe('GET /mcp/.well-known/oauth-protected-resource (dispatcher route)', () => {
+  afterEach(() => {
+    vi.unstubAllEnvs()
+  })
+
+  it('is declared unauthenticated and answers the same document as the root location', async () => {
+    vi.stubEnv('NEXT_PUBLIC_APP_URL', 'https://app.accounted.se')
+    const route = findRoute('GET', '/mcp/.well-known/oauth-protected-resource')
+    expect(route.skipAuth).toBe(true)
+    const response = await route.handler(
+      new Request(
+        'https://app.accounted.se/api/extensions/ext/mcp-server/mcp/.well-known/oauth-protected-resource?tool_namespace=accounted',
+        { headers: { host: 'app.accounted.se' } }
+      ),
+      undefined as never
+    )
+    expect(response.status).toBe(200)
+    const body = await response.json()
+    expect(body.resource).toBe(
+      'https://app.accounted.se/api/extensions/ext/mcp-server/mcp?tool_namespace=accounted'
+    )
+    expect(body.authorization_servers).toEqual(['https://app.accounted.se'])
+  })
+
+  it('still refuses a foreign browser origin (DNS-rebinding defense)', async () => {
+    vi.stubEnv('NEXT_PUBLIC_APP_URL', 'https://app.accounted.se')
+    const route = findRoute('GET', '/mcp/.well-known/oauth-protected-resource')
+    const response = await route.handler(
+      new Request(
+        'https://app.accounted.se/api/extensions/ext/mcp-server/mcp/.well-known/oauth-protected-resource',
+        { headers: { host: 'app.accounted.se', origin: 'https://evil.example' } }
+      ),
+      undefined as never
+    )
+    expect(response.status).toBe(403)
+  })
+})

extensions/general/mcp-server/index.ts

@@ -1,4 +1,6 @@
+import { NextResponse } from 'next/server'
 import type { Extension } from '@/lib/extensions/types'
+import { buildProtectedResourceMetadata } from '@/lib/auth/protected-resource-metadata'
 import { handleMcpRequest, tools as mcpTools } from './server'
 import { isForbiddenOrigin, forbiddenOriginResponse } from './origin-guard'
 import { registerAgentTools } from '@/lib/agent/tools/registry'
@@ -61,6 +63,20 @@ export const mcpServerExtension: Extension = {
         return new Response(null, { status: 204 })
       },
     },
+    {
+      method: 'GET',
+      path: '/mcp/.well-known/oauth-protected-resource',
+      skipAuth: true,
+      // Endpoint-appended RFC 9728 discovery. Claude.ai's connector setup
+      // derives the metadata URL from the server URL and tries both the
+      // path-based root form and this one before any 401; a 404 here reads
+      // as "Authorization failed". Public by nature: it names the
+      // authorization server and nothing tenant-specific.
+      handler: async (request: Request) => {
+        if (isForbiddenOrigin(request)) return forbiddenOriginResponse()
+        return NextResponse.json(buildProtectedResourceMetadata(request))
+      },
+    },
   ],
 
   eventHandlers: [],

lib/auth/protected-resource-metadata.ts

@@ -0,0 +1,46 @@
+import { resolveDiscoveryBaseUrl } from '@/lib/api/v1/base-url'
+
+/** Path of the one protected resource this server advertises: the MCP endpoint. */
+export const MCP_RESOURCE_PATH = '/api/extensions/ext/mcp-server/mcp'
+
+/**
+ * RFC 9728 Protected Resource Metadata for the MCP endpoint.
+ *
+ * Served from three URLs, because clients derive the location differently:
+ *   - `/.well-known/oauth-protected-resource` (root): what our 401
+ *     `WWW-Authenticate: resource_metadata=` header points at; Claude Code
+ *     and the stdio bridges follow that hint.
+ *   - `/.well-known/oauth-protected-resource/api/extensions/ext/mcp-server/mcp`
+ *     (RFC 9728 §3.1 path-based form): Claude.ai's connector setup derives
+ *     this from the server URL and fetches it BEFORE any 401, so a 404 here
+ *     reads as "Authorization failed" in the connector dialog.
+ *   - `/api/extensions/ext/mcp-server/mcp/.well-known/oauth-protected-resource`
+ *     (endpoint-appended form, tried by the same client as a fallback).
+ *
+ * The resource/AS URLs reflect the (allowlisted) request host: MCP clients
+ * validate the advertised resource against the server URL they were
+ * configured with, and existing connectors point at the legacy
+ * app.gnubok.se domain after the app.accounted.se cutover.
+ */
+export function buildProtectedResourceMetadata(request: Request): {
+  resource: string
+  authorization_servers: string[]
+  scopes_supported: string[]
+} {
+  const appUrl = resolveDiscoveryBaseUrl(request)
+  const resource = new URL(MCP_RESOURCE_PATH, appUrl)
+  // `accounted` is the COMPLETE allow-list of reflectable namespaces. We never
+  // echo the inbound parameter value: on an exact match we set the fixed
+  // literal, so a crafted tool_namespace (URL-special chars, other values) can
+  // never reach the advertised resource URL. Do not loosen this to a broader
+  // match without re-checking every downstream consumer that parses `resource`.
+  if (new URL(request.url).searchParams.get('tool_namespace') === 'accounted') {
+    resource.searchParams.set('tool_namespace', 'accounted')
+  }
+
+  return {
+    resource: resource.toString(),
+    authorization_servers: [appUrl],
+    scopes_supported: ['mcp'],
+  }
+}