fix: Per RFC 9728, enable dynamic discovery of OAuth endpoints in MCP using path-aware discovery

src/mcp/oauth.rs

@@ -14,6 +14,8 @@ use url::Url;
 struct ProtectedResourceMetadata {
     #[serde(default)]
     authorization_servers: Vec<String>,
+    #[serde(default)]
+    scopes_supported: Vec<String>,
 }
 
 #[derive(Debug, Deserialize)]
@@ -187,46 +189,122 @@ async fn register_client(endpoint: &str, redirect_uri: &str) -> Result<String> {
 }
 
 async fn discover_oauth_metadata(server_url: &str) -> Result<OAuthServerMetadata> {
-    let base = extract_base_url(server_url)?;
     let client = Client::new();
+    let mut tried: Vec<String> = Vec::new();
 
-    // RFC 9728: try protected resource metadata first; it points to the auth server
-    let pr_url = format!("{base}/.well-known/oauth-protected-resource");
-    if let Ok(resp) = client.get(&pr_url).send().await
-        && resp.status().is_success()
-        && let Ok(pr) = resp.json::<ProtectedResourceMetadata>().await
-        && let Some(auth_server) = pr.authorization_servers.first()
-    {
-        let as_url = format!("{auth_server}/.well-known/oauth-authorization-server");
+    // RFC 9728 @ 5.1: an unauthenticated request should yield a 401 whose
+    // WWW-Authenticate challenge advertises the protected resource metadata URL.
+    let mut pr_urls = Vec::new();
+    if let Some(url) = probe_resource_metadata_url(&client, server_url).await {
+        pr_urls.push(url);
+    }
+
+    // RFC 9728 @ 3.1: path-aware well-known URL, then root as legacy fallback.
+    pr_urls.extend(well_known_urls(server_url, "oauth-protected-resource")?);
+    pr_urls.dedup();
+
+    for pr_url in &pr_urls {
+        tried.push(pr_url.clone());
+        let Ok(resp) = client.get(pr_url).send().await else {
+            continue;
+        };
+        if !resp.status().is_success() {
+            continue;
+        }
+        let Ok(pr) = resp.json::<ProtectedResourceMetadata>().await else {
+            continue;
+        };
+        let Some(issuer) = pr.authorization_servers.first() else {
+            continue;
+        };
+        // RFC 8414 @ 3.1: for issuers with a path component the well-known
+        // segment is inserted BEFORE the path (with the legacy appended form
+        // and root as fallbacks).
+        for as_url in well_known_urls(issuer, "oauth-authorization-server")? {
+            tried.push(as_url.clone());
             if let Ok(resp) = client.get(&as_url).send().await
                 && resp.status().is_success()
-            && let Ok(meta) = resp.json::<OAuthServerMetadata>().await
+                && let Ok(mut meta) = resp.json::<OAuthServerMetadata>().await
             {
+                // Some auth servers (e.g. GitHub) omit scopes_supported from
+                // their metadata; fall back to the resource's advertised scopes.
+                if meta.scopes_supported.is_empty() {
+                    meta.scopes_supported = pr.scopes_supported.clone();
+                }
                 return Ok(meta);
             }
         }
+    }
 
-    let as_url = format!("{base}/.well-known/oauth-authorization-server");
-    let resp = client
-        .get(&as_url)
-        .send()
-        .await
-        .with_context(|| format!("Failed to reach {as_url}"))?;
-
-    if resp.status().is_success() {
+    // Last resort: the MCP server itself may host authorization server metadata.
+    for as_url in well_known_urls(server_url, "oauth-authorization-server")? {
+        tried.push(as_url.clone());
+        if let Ok(resp) = client.get(&as_url).send().await
+            && resp.status().is_success()
+        {
             return resp
                 .json::<OAuthServerMetadata>()
                 .await
                 .with_context(|| format!("Failed to parse OAuth metadata from {as_url}"));
         }
+    }
 
     Err(anyhow!(
         "Could not discover OAuth metadata for '{server_url}'.\n\
-         Tried:\n  {pr_url}\n  {as_url}\n\
-         Ensure the server supports MCP OAuth discovery, or consult its documentation."
+         Tried:\n  {}\n\
+         Ensure the server supports MCP OAuth discovery, or consult its documentation.",
+        tried.join("\n  ")
     ))
 }
 
+/// Probes the MCP server with an unauthenticated request and extracts the
+/// `resource_metadata` URL from the 401 `WWW-Authenticate` challenge (RFC 9728 @ 5.1).
+async fn probe_resource_metadata_url(client: &Client, server_url: &str) -> Option<String> {
+    let resp = client.get(server_url).send().await.ok()?;
+    let header = resp.headers().get(reqwest::header::WWW_AUTHENTICATE)?;
+
+    parse_resource_metadata(header.to_str().ok()?)
+}
+
+/// Extracts the `resource_metadata` parameter value from a `WWW-Authenticate`
+/// challenge, e.g. `Bearer error="...", resource_metadata="https://..."`.
+fn parse_resource_metadata(challenge: &str) -> Option<String> {
+    let (_, rest) = challenge.split_once("resource_metadata=")?;
+    let rest = rest.trim_start();
+    let value = if let Some(stripped) = rest.strip_prefix('"') {
+        stripped.split('"').next()?
+    } else {
+        rest.split([',', ' ']).next()?
+    };
+
+    if value.is_empty() {
+        None
+    } else {
+        Some(value.to_string())
+    }
+}
+
+/// Builds candidate well-known metadata URLs for `url`, ordered by spec preference:
+/// 1. Path-aware (RFC 8414 @ 3.1 / RFC 9728 @ 3.1): `{origin}/.well-known/{suffix}{path}`
+/// 2. Legacy appended form: `{url}/.well-known/{suffix}`
+/// 3. Root: `{origin}/.well-known/{suffix}`
+///
+/// URLs without a path component yield only the root form.
+fn well_known_urls(url: &str, suffix: &str) -> Result<Vec<String>> {
+    let parsed = Url::parse(url).with_context(|| format!("Invalid URL: {url}"))?;
+    let origin = extract_base_url(url)?;
+    let path = parsed.path().trim_end_matches('/');
+
+    let mut urls = Vec::new();
+    if !path.is_empty() && path != "/" {
+        urls.push(format!("{origin}/.well-known/{suffix}{path}"));
+        urls.push(format!("{origin}{path}/.well-known/{suffix}"));
+    }
+    urls.push(format!("{origin}/.well-known/{suffix}"));
+
+    Ok(urls)
+}
+
 fn extract_base_url(url: &str) -> Result<String> {
     let parsed = Url::parse(url).with_context(|| format!("Invalid URL: {url}"))?;
     let scheme = parsed.scheme();
@@ -296,6 +374,101 @@ mod tests {
         assert!(extract_base_url("not-a-url").is_err());
     }
 
+    #[test]
+    fn well_known_urls_path_aware_first_for_url_with_path() {
+        let urls = well_known_urls(
+            "https://api.githubcopilot.com/mcp",
+            "oauth-protected-resource",
+        )
+        .unwrap();
+
+        assert_eq!(
+            urls,
+            vec![
+                "https://api.githubcopilot.com/.well-known/oauth-protected-resource/mcp",
+                "https://api.githubcopilot.com/mcp/.well-known/oauth-protected-resource",
+                "https://api.githubcopilot.com/.well-known/oauth-protected-resource",
+            ]
+        );
+    }
+
+    #[test]
+    fn well_known_urls_inserts_before_issuer_path() {
+        let urls = well_known_urls(
+            "https://github.com/login/oauth",
+            "oauth-authorization-server",
+        )
+        .unwrap();
+
+        assert_eq!(
+            urls[0],
+            "https://github.com/.well-known/oauth-authorization-server/login/oauth"
+        );
+    }
+
+    #[test]
+    fn well_known_urls_root_only_for_url_without_path() {
+        let urls = well_known_urls("https://mcp.notion.com", "oauth-authorization-server").unwrap();
+
+        assert_eq!(
+            urls,
+            vec!["https://mcp.notion.com/.well-known/oauth-authorization-server"]
+        );
+    }
+
+    #[test]
+    fn well_known_urls_ignores_trailing_slash() {
+        let urls = well_known_urls(
+            "https://api.githubcopilot.com/mcp/",
+            "oauth-protected-resource",
+        )
+        .unwrap();
+
+        assert_eq!(
+            urls[0],
+            "https://api.githubcopilot.com/.well-known/oauth-protected-resource/mcp"
+        );
+    }
+
+    #[test]
+    fn parse_resource_metadata_extracts_quoted_url() {
+        let challenge = r#"Bearer error="invalid_request", error_description="No access token was provided in this request", resource_metadata="https://api.githubcopilot.com/.well-known/oauth-protected-resource/mcp""#;
+
+        let url = parse_resource_metadata(challenge);
+
+        assert_eq!(
+            url,
+            Some(
+                "https://api.githubcopilot.com/.well-known/oauth-protected-resource/mcp"
+                    .to_string()
+            )
+        );
+    }
+
+    #[test]
+    fn parse_resource_metadata_extracts_unquoted_url() {
+        let challenge = "Bearer resource_metadata=https://example.com/.well-known/oauth-protected-resource/mcp, error=\"invalid_token\"";
+
+        let url = parse_resource_metadata(challenge);
+
+        assert_eq!(
+            url,
+            Some("https://example.com/.well-known/oauth-protected-resource/mcp".to_string())
+        );
+    }
+
+    #[test]
+    fn parse_resource_metadata_returns_none_when_absent() {
+        assert_eq!(
+            parse_resource_metadata(r#"Bearer error="invalid_token""#),
+            None
+        );
+        assert_eq!(
+            parse_resource_metadata(r#"Bearer resource_metadata="""#),
+            None
+        );
+    }
+
     #[test]
     #[serial]
     fn registered_client_id_roundtrip() {