fix(mcp): capability-gate tools/list so prompt-only MCP servers can connect (#44550)

Port from anomalyco/opencode#31271: only call tools/list when the server
advertises the 'tools' capability in InitializeResult.capabilities.

Previously, _discover_tools() unconditionally called session.list_tools()
right after initialize. Prompt-only / resource-only servers (which omit
the tools capability per the MCP spec) raise McpError(-32601 Method not
found), which aborted the connection — burning all 3 initial-connect
retries and permanently failing the server even though its prompts and
resources were perfectly usable. The 180s keepalive had the same problem:
it probed with list_tools(), so even a successfully connected prompt-only
server would be torn down on the first keepalive cycle.

Changes:
- MCPServerTask._advertises_tools(): capability check with a legacy
  fallback (no captured InitializeResult -> behave as before)
- _discover_tools(): skip tools/list for non-tool servers
- keepalive: use the universal ping request for non-tool servers
- _refresh_tools(): guard against tools/list_changed from non-tool servers

E2E verified with a real stdio prompt-only FastMCP-style server: on main
it fails all 3 connection attempts with Method-not-found; with this fix
it connects, lists prompts, answers ping keepalives, and shuts down
cleanly.

tools/mcp_tool.py

@@ -1202,6 +1202,26 @@ class MCPServerTask:
         """Check if this server uses HTTP transport."""
         return "url" in self._config
 
+    def _advertises_tools(self) -> bool:
+        """Whether the server advertises the ``tools`` capability.
+
+        Per the MCP spec, ``InitializeResult.capabilities.tools`` is non-None
+        iff the server implements the ``tools/*`` request family. Prompt-only
+        or resource-only servers omit it, and calling ``tools/list`` against
+        them raises ``McpError(-32601 Method not found)`` — which previously
+        killed the connection during discovery and made every keepalive fail.
+        (Ported from anomalyco/opencode#31271.)
+
+        Returns True when no capability info was captured (legacy fallback:
+        preserve the old always-call-list_tools behavior rather than regress
+        any server that was working before this gate).
+        """
+        init_result = self.initialize_result
+        caps = getattr(init_result, "capabilities", None) if init_result is not None else None
+        if caps is None:
+            return True
+        return getattr(caps, "tools", None) is not None
+
     # ----- Dynamic tool discovery (notifications/tools/list_changed) -----
 
     async def _refresh_tools_task(self):
@@ -1273,6 +1293,12 @@ class MCPServerTask:
         """
         from tools.registry import registry
 
+        if not self._advertises_tools():
+            # A server that doesn't implement tools/* should never send
+            # tools/list_changed, but guard anyway — calling tools/list
+            # would raise McpError(-32601).
+            return
+
         async with self._refresh_lock:
             # Capture old tool names for change diff
             old_tool_names = set(self._registered_tool_names)
@@ -1360,12 +1386,22 @@ class MCPServerTask:
 
                 # Timeout — no lifecycle event fired.  Send a keepalive
                 # to exercise the connection and detect stale sockets.
+                # Prompt-only / resource-only servers don't implement
+                # ``tools/list`` (McpError -32601), so use the universal
+                # ``ping`` request for them instead — otherwise every
+                # keepalive cycle would trigger a spurious reconnect.
                 if self.session:
                     try:
-                        await asyncio.wait_for(
-                            self.session.list_tools(),
-                            timeout=30.0,
-                        )
+                        if self._advertises_tools():
+                            await asyncio.wait_for(
+                                self.session.list_tools(),
+                                timeout=30.0,
+                            )
+                        else:
+                            await asyncio.wait_for(
+                                self.session.send_ping(),
+                                timeout=30.0,
+                            )
                     except Exception as exc:
                         logger.warning(
                             "MCP server '%s' keepalive failed, "
@@ -1778,9 +1814,25 @@ class MCPServerTask:
                         )
 
     async def _discover_tools(self):
-        """Discover tools from the connected session."""
+        """Discover tools from the connected session.
+
+        Capability-gated: prompt-only / resource-only MCP servers don't
+        implement ``tools/list``, and calling it raises ``McpError(-32601)``,
+        which previously aborted the connection — those servers could never
+        stay connected for their prompts/resources. Skip the call when the
+        server doesn't advertise the ``tools`` capability.
+        (Ported from anomalyco/opencode#31271.)
+        """
         if self.session is None:
             return
+        if not self._advertises_tools():
+            logger.info(
+                "MCP server '%s': does not advertise 'tools' capability — "
+                "skipping tools/list (prompts/resources remain available)",
+                self.name,
+            )
+            self._tools = []
+            return
         async with self._rpc_lock:
             tools_result = await self.session.list_tools()
         self._tools = (