You add a new MCP server, or a new tool to an existing one. You ask the agent to use it and it says the tool isn't available — or worse, it confidently reports that "only the HTTP server is connected" and the new stdio servers aren't live. The instinct is to blame the MCP server or bounce the runtime. Resist it. The agent's narration about its own toolset is not ground truth, and a malformed test invocation will happily manufacture a fake "MCP unreachable" symptom. Here's the broke → tried → fixed of a debugging session that burned time on exactly this.
Problem
We'd just added a second stdio MCP server and a new tool. To smoke-test it, we hand-typed a quick probe at the agent runtime and asked it to use the new capability. The agent came back with this:
Those tools aren't in my live toolset right now — only the
Home Assistant (HTTP/SSE) server is connected. I'll read the
