Your MCP Server Passes Every Test — and Claude Still Rejects the Tool

We shipped what looked like a routine improvement to one of our MCP tools: a declared outputSchema, generated from our existing Zod types. Server-side smoke tests passed. The structured output validated cleanly against JSON Schema 2020-12 with an independent validator. We deployed.

Then Claude Desktop refused to call the tool at all.

This post is a write-up of that incident—what failed, why all our tests missed it, and what an actual end-to-end test for an MCP server needs to look like. One caveat up front: this is n=1, observed on our specific setup (@modelcontextprotocol/sdk ^1.10, zodToJsonSchema, Claude Desktop as the client, June 2026). Treat it as a mechanism worth knowing about, not a universal law.

"MCP tool not showing up": outputSchema rejected by the client ingest layer

That heading is the literal symptom, because it's what you'll be searching for at 11 p.m. The variants we tried ourselves: MCP tool not showing up, outputSchema rejected, MCP tool ingest failed, Claude Desktop tool error request_id.