feat(mcp): add SSE transport support for MCP client (salvage #19135)

tools/mcp_tool.py

@@ -2,9 +2,9 @@
 """
 MCP (Model Context Protocol) Client Support
 
-Connects to external MCP servers via stdio or HTTP/StreamableHTTP transport,
-discovers their tools, and registers them into the hermes-agent tool registry
-so the agent can call them like any built-in tool.
+Connects to external MCP servers via stdio, HTTP/StreamableHTTP, or SSE
+transport, discovers their tools, and registers them into the hermes-agent
+tool registry so the agent can call them like any built-in tool.
 
 Configuration is read from ~/.hermes/config.yaml under the ``mcp_servers`` key.
 The ``mcp`` Python package is optional -- if not installed, this module is a
@@ -29,7 +29,11 @@
         headers:
           Authorization: "Bearer sk-..."
         timeout: 180
-      analysis:
+      searxng:
+        url: "http://localhost:8000/sse"
+        transport: sse       # use SSE transport instead of Streamable HTTP
+        timeout: 180
+        connect_timeout: 10
         command: "npx"
         args: ["-y", "analysis-server"]
         sampling:                    # server-initiated LLM requests
@@ -44,6 +48,7 @@
 
 Features:
     - Stdio transport (command + args) and HTTP/StreamableHTTP transport (url)
+    - SSE transport (transport: sse) for MCP servers using the SSE protocol
     - Automatic reconnection with exponential backoff (up to 5 retries)
     - Environment variable filtering for stdio subprocesses (security)
     - Credential stripping in error messages returned to the LLM
@@ -191,6 +196,12 @@ def _write_stderr_log_header(server_name: str) -> None:
         from mcp.types import LATEST_PROTOCOL_VERSION
     except ImportError:
         logger.debug("mcp.types.LATEST_PROTOCOL_VERSION not available -- using fallback protocol version")
+    # SSE transport client (for MCP servers using SSE transport instead of Streamable HTTP)
+    try:
+        from mcp.client.sse import sse_client
+    except ImportError:
+        sse_client = None
+        logger.debug("mcp.client.sse.sse_client not available -- SSE transport disabled")
     # Sampling types -- separated so older SDK versions don't break MCP support
     try:
         from mcp.types import (
@@ -1210,6 +1221,37 @@ async def _run_http(self, config: dict):
         if _MCP_NOTIFICATION_TYPES and _MCP_MESSAGE_HANDLER_SUPPORTED:
             sampling_kwargs["message_handler"] = self._make_message_handler()
 
+        # SSE transport (for MCP servers that implement the SSE transport protocol
+        # rather than Streamable HTTP). Configure with ``transport: sse`` in the
+        # mcp_servers entry in config.yaml.
+        if config.get("transport") == "sse":
+            if sse_client is None:
+                raise ImportError(
+                    f"MCP server '{self.name}' requires SSE transport but "
+                    "mcp.client.sse.sse_client is not available. "
+                    "Upgrade the mcp package to get SSE support."
+                )
+            async with sse_client(
+                url=url,
+                headers=headers or None,
+                timeout=float(connect_timeout),
+                sse_read_timeout=float(config.get("timeout", _DEFAULT_TOOL_TIMEOUT)),
+            ) as (read_stream, write_stream):
+                async with ClientSession(
+                    read_stream, write_stream, **sampling_kwargs
+                ) as session:
+                    await session.initialize()
+                    self.session = session
+                    await self._discover_tools()
+                    self._ready.set()
+                    reason = await self._wait_for_lifecycle_event()
+                    if reason == "reconnect":
+                        logger.info(
+                            "MCP server '%s': reconnect requested — "
+                            "tearing down SSE session", self.name,
+                        )
+            return
+
         if _MCP_NEW_HTTP:
             # New API (mcp >= 1.24.0): build an explicit httpx.AsyncClient
             # matching the SDK's own create_mcp_http_client defaults.
@@ -2965,7 +3007,7 @@ def get_mcp_status() -> List[dict]:
         active_servers = dict(_servers)
 
     for name, cfg in configured.items():
-        transport = "http" if "url" in cfg else "stdio"
+        transport = cfg.get("transport", "http") if "url" in cfg else "stdio"
         server = active_servers.get(name)
         if server and server.session is not None:
             entry = {