fix: handle SSE responses in MCP bridge and add Commander-level tests

The bridge now parses SSE text/event-stream responses (extracting data:
lines) in addition to plain JSON. Also sends correct Accept header
per MCP streamable HTTP spec. Added tests for SSE handling and
command option parsing (-p/--project).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/cli/src/commands/mcp.ts

@@ -21,7 +21,7 @@ function postJsonRpc(
     const parsed = new URL(url);
     const headers: Record<string, string> = {
       'Content-Type': 'application/json',
-      'Accept': 'application/json',
+      'Accept': 'application/json, text/event-stream',
     };
     if (sessionId) {
       headers['mcp-session-id'] = sessionId;
@@ -95,6 +95,25 @@ function sendDelete(
   });
 }
 
+/**
+ * Extract JSON-RPC messages from an HTTP response body.
+ * Handles both plain JSON and SSE (text/event-stream) formats.
+ */
+function extractJsonRpcMessages(contentType: string | undefined, body: string): string[] {
+  if (contentType?.includes('text/event-stream')) {
+    // Parse SSE: extract data: lines
+    const messages: string[] = [];
+    for (const line of body.split('\n')) {
+      if (line.startsWith('data: ')) {
+        messages.push(line.slice(6));
+      }
+    }
+    return messages;
+  }
+  // Plain JSON response
+  return [body];
+}
+
 /**
  * STDIO-to-Streamable-HTTP MCP bridge.
  *
@@ -126,10 +145,16 @@ export async function runMcpBridge(opts: McpBridgeOptions): Promise<void> {
 
       if (result.status >= 400) {
         stderr.write(`MCP bridge error: HTTP ${result.status}: ${result.body}\n`);
-        // Still forward the response body — it may contain a JSON-RPC error
       }
 
-      stdout.write(result.body + '\n');
+      // Handle both plain JSON and SSE responses
+      const messages = extractJsonRpcMessages(result.headers['content-type'], result.body);
+      for (const msg of messages) {
+        const trimmedMsg = msg.trim();
+        if (trimmedMsg) {
+          stdout.write(trimmedMsg + '\n');
+        }
+      }
     } catch (err) {
       stderr.write(`MCP bridge error: ${err instanceof Error ? err.message : String(err)}\n`);
     }

src/cli/tests/commands/mcp.test.ts

@@ -94,8 +94,14 @@ beforeAll(async () => {
               responseBody = JSON.stringify({ jsonrpc: '2.0', id: rpc.id, error: { code: -32601, message: 'Method not found' } });
           }
 
-          res.writeHead(200, { 'Content-Type': 'application/json' });
+          // Respond in SSE format for /projects/sse-project/mcp
-          res.end(responseBody);
+          if (req.url?.includes('sse-project')) {
+            res.writeHead(200, { 'Content-Type': 'text/event-stream' });
+            res.end(`event: message\ndata: ${responseBody}\n\n`);
+          } else {
+            res.writeHead(200, { 'Content-Type': 'application/json' });
+            res.end(responseBody);
+          }
         } catch {
           res.writeHead(400, { 'Content-Type': 'application/json' });
           res.end(JSON.stringify({ error: 'Invalid JSON' }));
@@ -389,6 +395,30 @@ describe('MCP STDIO Bridge', () => {
     expect(lines).toHaveLength(1);
   });
 
+  it('handles SSE (text/event-stream) responses', async () => {
+    recorded.length = 0;
+    const stdin = new Readable({ read() {} });
+    const { stdout, stdoutChunks } = createMockStreams();
+
+    const initMsg = JSON.stringify({
+      jsonrpc: '2.0', id: 1, method: 'initialize',
+      params: { protocolVersion: '2024-11-05', capabilities: {}, clientInfo: { name: 'test', version: '1.0' } },
+    });
+
+    pushAndEnd(stdin, [initMsg]);
+
+    await runMcpBridge({
+      projectName: 'sse-project', // triggers SSE response from mock server
+      mcplocalUrl: `http://localhost:${mockPort}`,
+      stdin, stdout, stderr: new Writable({ write(_, __, cb) { cb(); } }),
+    });
+
+    // Should extract JSON from SSE data: lines
+    const output = stdoutChunks.join('').trim();
+    const parsed = JSON.parse(output);
+    expect(parsed.result.serverInfo.name).toBe('test-server');
+  });
+
   it('URL-encodes project name', async () => {
     recorded.length = 0;
     const stdin = new Readable({ read() {} });