mcp: wait/attempt discovery of POST messages endpoint before POST to avoid 405

pentestagent/mcp/transport.py

@@ -229,6 +229,15 @@ class SSETransport(MCPTransport):
         if not self.session:
             raise RuntimeError("Transport not connected")
 
+        # Ensure we have a POST endpoint. If discovery hasn't completed yet,
+        # try a quick synchronous discovery attempt before posting so we don't
+        # accidentally POST to the SSE listen endpoint which returns 405.
+        if not self._post_url:
+            try:
+                await self._discover_post_url(timeout=2.0)
+            except Exception:
+                pass
+
         post_target = self._post_url or self.url
 
         try:
@@ -258,6 +267,44 @@ class SSETransport(MCPTransport):
         except Exception as e:
             raise RuntimeError(f"SSE request failed: {e}") from e
 
+    async def _discover_post_url(self, timeout: float = 2.0) -> None:
+        """Attempt a short GET to the SSE endpoint to find the advertised POST URL.
+
+        This is a fallback used when the background listener hasn't yet
+        extracted the `endpoint` event. It reads a few lines with a short
+        timeout and sets `self._post_url` if found.
+        """
+        if not self.session:
+            return
+
+        try:
+            async with self.session.get(self.url, timeout=timeout) as resp:
+                if resp.status != 200:
+                    return
+                # Read up to a few lines looking for `data:`
+                for _ in range(20):
+                    line = await resp.content.readline()
+                    if not line:
+                        break
+                    try:
+                        text = line.decode(errors="ignore").strip()
+                    except Exception:
+                        continue
+                    if text.startswith("data:"):
+                        endpoint = text.split("data:", 1)[1].strip()
+                        from urllib.parse import urlparse
+
+                        p = urlparse(self.url)
+                        if endpoint.startswith("http"):
+                            self._post_url = endpoint
+                        elif endpoint.startswith("/"):
+                            self._post_url = f"{p.scheme}://{p.netloc}{endpoint}"
+                        else:
+                            self._post_url = f"{p.scheme}://{p.netloc}/{endpoint.lstrip('/')}"
+                        return
+        except Exception:
+            return
+
     async def disconnect(self):
         """Close the HTTP session."""
         # Cancel listener and close SSE response