fix(oauth): enhance error handling in proxy client

- Provide user-friendly error messages based on HTTP status codes
- Fix middleware placement for JSON body parsing
- Update tests for new error message format

Author: asoorm

client/src/lib/__tests__/oauth-proxy.test.ts

@@ -109,6 +109,7 @@ describe("OAuth Proxy Utilities", () => {
     it("should handle non-ok responses", async () => {
       (global.fetch as jest.Mock).mockResolvedValueOnce({
         ok: false,
+        status: 404,
         statusText: "Not Found",
         json: async () => ({ error: "Metadata not found" }),
       });
@@ -119,13 +120,14 @@ describe("OAuth Proxy Utilities", () => {
           mockConfig,
         ),
       ).rejects.toThrow(
-        "Failed to discover OAuth metadata: Metadata not found",
+        "Failed to discover OAuth metadata: Not found: Metadata not found",
       );
     });
 
     it("should handle responses without error details", async () => {
       (global.fetch as jest.Mock).mockResolvedValueOnce({
         ok: false,
+        status: 500,
         statusText: "Internal Server Error",
         json: async () => {
           throw new Error("Invalid JSON");
@@ -138,7 +140,7 @@ describe("OAuth Proxy Utilities", () => {
           mockConfig,
         ),
       ).rejects.toThrow(
-        "Failed to discover OAuth metadata: Internal Server Error",
+        "Failed to discover OAuth metadata: Server error (500): Internal Server Error",
       );
     });
   });
@@ -177,6 +179,7 @@ describe("OAuth Proxy Utilities", () => {
     it("should handle errors", async () => {
       (global.fetch as jest.Mock).mockResolvedValueOnce({
         ok: false,
+        status: 404,
         statusText: "Not Found",
         json: async () => ({ error: "Resource metadata not found" }),
       });
@@ -187,7 +190,7 @@ describe("OAuth Proxy Utilities", () => {
           mockConfig,
         ),
       ).rejects.toThrow(
-        "Failed to discover resource metadata: Resource metadata not found",
+        "Failed to discover resource metadata: Not found: Resource metadata not found",
       );
     });
   });
@@ -237,6 +240,7 @@ describe("OAuth Proxy Utilities", () => {
     it("should handle registration errors", async () => {
       (global.fetch as jest.Mock).mockResolvedValueOnce({
         ok: false,
+        status: 400,
         statusText: "Bad Request",
         json: async () => ({ error: "Invalid client metadata" }),
       });
@@ -250,7 +254,9 @@ describe("OAuth Proxy Utilities", () => {
           },
           mockConfig,
         ),
-      ).rejects.toThrow("Failed to register client: Invalid client metadata");
+      ).rejects.toThrow(
+        "Failed to register client: Bad Request: Invalid client metadata",
+      );
     });
   });
 
@@ -301,6 +307,7 @@ describe("OAuth Proxy Utilities", () => {
     it("should handle token exchange errors", async () => {
       (global.fetch as jest.Mock).mockResolvedValueOnce({
         ok: false,
+        status: 401,
         statusText: "Unauthorized",
         json: async () => ({ error: "invalid_grant" }),
       });
@@ -311,7 +318,9 @@ describe("OAuth Proxy Utilities", () => {
           { grant_type: "authorization_code", code: "invalid" },
           mockConfig,
         ),
-      ).rejects.toThrow("Failed to exchange authorization code: invalid_grant");
+      ).rejects.toThrow(
+        "Failed to exchange authorization code: Authentication failed: invalid_grant",
+      );
     });
 
     it("should handle network failures", async () => {

client/src/lib/oauth-proxy.ts

@@ -59,17 +59,56 @@ async function proxyFetch<T>(
     });
   }
 
-  const response = await fetch(url.toString(), {
-    method,
-    headers: getProxyHeaders(config),
-    ...(body && { body: JSON.stringify(body) }),
-  });
+  let response: Response;
+  try {
+    response = await fetch(url.toString(), {
+      method,
+      headers: getProxyHeaders(config),
+      ...(body && { body: JSON.stringify(body) }),
+    });
+  } catch (error) {
+    // Network errors (connection refused, timeout, etc.)
+    throw new Error(
+      `Network error: ${error instanceof Error ? error.message : "Failed to connect to proxy server"}`,
+    );
+  }
 
   if (!response.ok) {
-    const error = await response
+    // Try to get detailed error from response body
+    const errorData = await response
       .json()
       .catch(() => ({ error: response.statusText }));
-    throw new Error(error.error || response.statusText);
+    const errorMessage = errorData.error || response.statusText;
+    const errorDetails = errorData.details ? ` - ${errorData.details}` : "";
+
+    // Provide specific error messages based on status code
+    switch (response.status) {
+      case 400:
+        throw new Error(`Bad Request: ${errorMessage}${errorDetails}`);
+      case 401:
+        throw new Error(
+          `Authentication failed: ${errorMessage}. Check your proxy authentication token.`,
+        );
+      case 403:
+        throw new Error(
+          `Access forbidden: ${errorMessage}. You may not have permission to access this resource.`,
+        );
+      case 404:
+        throw new Error(
+          `Not found: ${errorMessage}. The OAuth endpoint may not exist or be misconfigured.`,
+        );
+      case 500:
+      case 502:
+      case 503:
+      case 504:
+        throw new Error(
+          `Server error (${response.status}): ${errorMessage}${errorDetails}`,
+        );
+      default:
+        throw new Error(
+          `Request failed (${response.status}): ${errorMessage}${errorDetails}`,
+        );
+    }
   }
 
   return await response.json();

server/src/index.ts

@@ -165,6 +165,7 @@ const updateHeadersInPlace = (
 
 const app = express();
 app.use(cors());
+app.use(express.json()); // Enable JSON body parsing globally
 app.use((req, res, next) => {
   res.header("Access-Control-Expose-Headers", "mcp-session-id");
   next();