diff --git a/src/oauth-protected-resource/responses.ts b/src/oauth-protected-resource/responses.ts
index 356224d..279ac18 100644
--- a/src/oauth-protected-resource/responses.ts
+++ b/src/oauth-protected-resource/responses.ts
@@ -1,4 +1,9 @@
-import { getAuthUrl, getResourceMetadataUrl, getResourceUrl } from './url.js'
+import {
+  getAuthUrl,
+  getResourceMetadataUrl,
+  getResourceUrl,
+  percentEncodeQuotes,
+} from './url.js'
 import type {
   ResourceMetadataOptions,
   UnauthorizedResponseOptions,
@@ -6,8 +11,8 @@ import type {
 
 /**
  * `401` response with a `WWW-Authenticate: Bearer resource_metadata="..."` header (RFC 9728).
- * Auto-constructs the metadata URL from `X-Forwarded-*` headers.
- * Pass `resourceMetadataUrl` to override for custom setups.
+ * The metadata URL defaults to the Edge Functions derivation and throws off
+ * platform; pass `resourceMetadataUrl` to override for custom setups.
  *
  * @category Middleware
  */
@@ -15,8 +20,9 @@ export function unauthorizedResponse(
   req: Request,
   options?: UnauthorizedResponseOptions,
 ): Response {
-  const metadataUrl =
-    options?.resourceMetadataUrl ?? getResourceMetadataUrl(req)
+  const metadataUrl = percentEncodeQuotes(
+    options?.resourceMetadataUrl ?? getResourceMetadataUrl(req),
+  )
   return new Response(JSON.stringify({ error: 'Unauthorized' }), {
     status: 401,
     headers: {
@@ -29,7 +35,8 @@ export function unauthorizedResponse(
 /**
  * RFC 9728 OAuth Protected Resource Metadata response.
  * Advertises the authorization server, resource URI, and bearer methods supported.
- * Auto-constructs URLs from `X-Forwarded-*` headers.
+ * URLs default to the Edge Functions derivation and throw off platform; pass
+ * `resource` / `authorizationServers` to override.
  *
  * @category Middleware
  */
diff --git a/src/oauth-protected-resource/url.ts b/src/oauth-protected-resource/url.ts
index 988642d..8f0fc2a 100644
--- a/src/oauth-protected-resource/url.ts
+++ b/src/oauth-protected-resource/url.ts
@@ -28,6 +28,18 @@ export function trimTrailingSlash(value: string): string {
   return value.endsWith('/') ? value.slice(0, -1) : value
 }
 
+/**
+ * Percent-encodes `"` and `\` — invalid URL code points that would otherwise
+ * break out of the RFC 9110 quoted-string in `WWW-Authenticate`. Applied to
+ * every advertised URL, so the header, the metadata document, and the ctx
+ * contribution agree on one spelling (RFC 9728 §3.3 requires an exact match).
+ *
+ * @internal
+ */
+export function percentEncodeQuotes(value: string): string {
+  return value.replace(/["\\]/g, (c) => (c === '"' ? '%22' : '%5C'))
+}
+
 /**
  * The externally-visible origin of a Supabase Edge Function.
  *
@@ -76,6 +88,10 @@ function edgeOrigin(req: Request): string {
  * The two forms differ only for a request at a sub-path of the function: the
  * canonical one reports the function, the reconstructed one the sub-path.
  *
+ * @throws {EnvError} `MISSING_RESOURCE_SERVER` on a root path with no slug —
+ * there is no function segment to restore, and a bare `/functions/v1`
+ * identifies no resource.
+ *
  * @internal
  */
 function edgeResourcePath(req: Request): string {
@@ -86,6 +102,9 @@ function edgeResourcePath(req: Request): string {
     METADATA_SUFFIX_PATTERN,
     '',
   )
+  if (received === '' || received === '/') {
+    throw Errors[MissingResourceServerError]()
+  }
   return `${EDGE_FUNCTIONS_PATH_PREFIX}${received}`
 }
 
@@ -96,7 +115,8 @@ function edgeResourcePath(req: Request): string {
  * There is no environment fallback — `SUPABASE_URL` names the Supabase project,
  * not this endpoint — so off Edge Functions it throws.
  *
- * @throws {EnvError} `MISSING_RESOURCE_SERVER` off Edge Functions.
+ * @throws {EnvError} `MISSING_RESOURCE_SERVER` off Edge Functions, or on a
+ * root path with no `SUPABASE_FUNCTION_SLUG` — see {@link edgeResourcePath}.
  *
  * @internal
  */
@@ -170,7 +190,7 @@ export function resolveUrlOption(
   fallback: (req: Request) => string,
 ): string {
   const value = typeof option === 'function' ? option(req) : option
-  return trimTrailingSlash(value ?? fallback(req))
+  return percentEncodeQuotes(trimTrailingSlash(value ?? fallback(req)))
 }
 
 /**
diff --git a/src/oauth-protected-resource/with-oauth-protected-resource.test.ts b/src/oauth-protected-resource/with-oauth-protected-resource.test.ts
index 3f4f6e4..a718bea 100644
--- a/src/oauth-protected-resource/with-oauth-protected-resource.test.ts
+++ b/src/oauth-protected-resource/with-oauth-protected-resource.test.ts
@@ -1,3 +1,6 @@
+import { createServer } from 'node:http'
+import type { AddressInfo } from 'node:net'
+
 import { afterEach, describe, expect, it, vi } from 'vitest'
 import { pipeline } from '@supabase/middleware'
 
@@ -247,6 +250,15 @@ describe('unauthorizedResponse', () => {
       `Bearer resource_metadata="${url}"`,
     )
   })
+
+  it('percent-encodes `"` in the resourceMetadataUrl override (quoted-string integrity)', () => {
+    const res = unauthorizedResponse(req('POST', '/my-fn'), {
+      resourceMetadataUrl: 'https://x.example.com/a"b/oauth-protected-resource',
+    })
+    expect(res.headers.get('WWW-Authenticate')).toBe(
+      'Bearer resource_metadata="https://x.example.com/a%22b/oauth-protected-resource"',
+    )
+  })
 })
 
 describe('withOAuthProtectedResource - resourceServer / authorizationServer', () => {
@@ -776,3 +788,129 @@ describe('withOAuthProtectedResource - off-platform defaults fail loudly', () =>
     ])
   })
 })
+
+describe('withOAuthProtectedResource - WWW-Authenticate quoted-string integrity', () => {
+  // A raw `"` in the advertised URL terminates the RFC 9110 quoted-string
+  // early, letting the remainder parse as extra auth-params (parameter
+  // injection). `"` and `\` are invalid URL code points anyway, so they are
+  // percent-encoded — in the header, the metadata document, and the ctx
+  // contribution alike, keeping the RFC 9728 §3.3 comparison intact.
+  it('percent-encodes `"` arriving via X-Forwarded-Host', async () => {
+    const res = await withOAuthProtectedResource(returns401)(
+      req('POST', '/my-fn', { 'X-Forwarded-Host': 'evil.test", scope="admin' }),
+    )
+    expect(res.headers.get('WWW-Authenticate')).toBe(
+      'Bearer resource_metadata="http://evil.test%22, scope=%22admin/functions/v1/my-fn/oauth-protected-resource"',
+    )
+  })
+
+  it('percent-encodes `"` and `\\` in a configured resourceServer', async () => {
+    const res = await withOAuthProtectedResource(
+      {
+        resourceServer: 'https://api.example.com/m"c\\p',
+        authorizationServer: 'https://auth.example.com',
+      },
+      returns401,
+    )(req('POST', '/my-fn'))
+    expect(res.headers.get('WWW-Authenticate')).toBe(
+      'Bearer resource_metadata="https://api.example.com/m%22c%5Cp/oauth-protected-resource"',
+    )
+  })
+
+  it('advertises the same encoded resource in the metadata document (§3.3 agreement)', async () => {
+    const res = await withOAuthProtectedResource(passthrough)(
+      req('GET', '/my-fn/oauth-protected-resource', {
+        'X-Forwarded-Host': 'evil.test"h',
+      }),
+    )
+    const body = await res.json()
+    expect(body.resource).toBe('http://evil.test%22h/functions/v1/my-fn')
+  })
+})
+
+describe('withOAuthProtectedResource - 401 enrichment resilience', () => {
+  it('enriches a 401 whose body the handler already consumed', async () => {
+    const handler = async () => {
+      const res = new Response('denied', { status: 401 })
+      await res.text()
+      return res
+    }
+    const res = await withOAuthProtectedResource(handler)(req('POST', '/my-fn'))
+    expect(res.status).toBe(401)
+    expect(res.headers.get('WWW-Authenticate')).toMatch(/^Bearer /)
+  })
+
+  it('enriches the 401 in place, so fetch-carried fields (.url, .redirected) survive', async () => {
+    let issued: Response | undefined
+    const handler = async () => {
+      issued = new Response(null, { status: 401 })
+      return issued
+    }
+    const res = await withOAuthProtectedResource(handler)(req('POST', '/my-fn'))
+    expect(res).toBe(issued)
+  })
+
+  it('enriches a fetch()-proxied 401, whose headers are immutable', async () => {
+    const upstream = createServer((_req, res) => {
+      res.statusCode = 401
+      res.setHeader('X-Upstream', 'yes')
+      res.end('denied')
+    })
+    await new Promise<void>((resolve) => upstream.listen(0, resolve))
+    const { port } = upstream.address() as AddressInfo
+    try {
+      const handler = async () => fetch(`http://127.0.0.1:${port}/`)
+      const res = await withOAuthProtectedResource(handler)(
+        req('POST', '/my-fn'),
+      )
+      expect(res.status).toBe(401)
+      expect(res.headers.get('WWW-Authenticate')).toMatch(/resource_metadata=/)
+      expect(res.headers.get('X-Upstream')).toBe('yes')
+      expect(await res.text()).toBe('denied')
+    } finally {
+      upstream.close()
+    }
+  })
+})
+
+describe('withOAuthProtectedResource - root path (no function segment)', () => {
+  // With no slug and no path segment there is no function name to restore, so
+  // the reconstruction would advertise a bare `/functions/v1` — a URL that
+  // identifies no resource. Failing loudly matches the off-platform contract.
+  it('throws MISSING_RESOURCE_SERVER on a bare /oauth-protected-resource (edge default)', async () => {
+    setEnv('SUPABASE_PUBLIC_URL', undefined)
+    setEnv('SUPABASE_FUNCTION_SLUG', undefined)
+    await expect(
+      withOAuthProtectedResource(passthrough)(
+        req('GET', '/oauth-protected-resource'),
+      ),
+    ).rejects.toMatchObject({
+      constructor: EnvError,
+      code: MissingResourceServerError,
+      status: 500,
+    })
+  })
+
+  it('resourceMetadataResponse on a root path throws instead of advertising a bare /functions/v1', () => {
+    setEnv('SUPABASE_PUBLIC_URL', undefined)
+    setEnv('SUPABASE_FUNCTION_SLUG', undefined)
+    let thrown: unknown
+    try {
+      resourceMetadataResponse(req('GET', '/'))
+    } catch (e) {
+      thrown = e
+    }
+    expect(thrown).toBeInstanceOf(EnvError)
+    expect(thrown).toMatchObject({ code: MissingResourceServerError })
+  })
+
+  it('SUPABASE_FUNCTION_SLUG rescues a root path with a canonical identifier', async () => {
+    setEnv('SUPABASE_PUBLIC_URL', undefined)
+    setEnv('SUPABASE_FUNCTION_SLUG', 'my-fn')
+    const res = await withOAuthProtectedResource(passthrough)(
+      req('GET', '/oauth-protected-resource'),
+    )
+    const body = await res.json()
+    expect(body.resource).toBe('http://localhost/functions/v1/my-fn')
+  })
+})
diff --git a/src/oauth-protected-resource/with-oauth-protected-resource.ts b/src/oauth-protected-resource/with-oauth-protected-resource.ts
index 158c341..5c3bcea 100644
--- a/src/oauth-protected-resource/with-oauth-protected-resource.ts
+++ b/src/oauth-protected-resource/with-oauth-protected-resource.ts
@@ -166,16 +166,23 @@ export const withOAuthProtectedResource: Middleware<
         response.status === 401 &&
         !response.headers.has('WWW-Authenticate')
       ) {
-        const headers = new Headers(response.headers)
-        headers.set(
-          'WWW-Authenticate',
-          `Bearer resource_metadata="${resourceMetadataUrl}"`,
-        )
-        return new Response(response.body, {
-          status: 401,
-          statusText: response.statusText,
-          headers,
-        })
+        const challenge = `Bearer resource_metadata="${resourceMetadataUrl}"`
+        try {
+          response.headers.set('WWW-Authenticate', challenge)
+          return response
+        } catch {
+          // Headers on a fetch()-proxied response carry the immutable guard,
+          // so enrichment falls back to a copy. A copy cannot carry `.url` or
+          // `.redirected` and cannot reuse a consumed body stream, which is
+          // why in-place mutation is the primary path.
+          const headers = new Headers(response.headers)
+          headers.set('WWW-Authenticate', challenge)
+          return new Response(response.bodyUsed ? null : response.body, {
+            status: response.status,
+            statusText: response.statusText,
+            headers,
+          })
+        }
       }
 
       return response