Valid MCP schemas make tools vanish silently

Claude Code #95504 is a clean operator harness catch: an MCP server can advertise a perfectly valid `inputSchema` under the MCP 2026-07-28 / SEP-2106 composition rules (`allOf`, `oneOf`, `if`/`then` at the root) and still lose the tool in Claude Code. The server stays “connected”; the tool simply never appears. Nested composition inside properties survives — only the root-level shape vanishes.

The older cousin #45106 failed loud with a 400 when top-level `oneOf`/`allOf`/`anyOf` hit the Anthropic tools API. Silence is worse: operators debug “why won’t the agent call X?” while the connection badge stays green. Treat green MCP + missing tools as a schema-shape problem before blaming auth or the model.