Claude Code MCP Shows Connected but 0 Tools: "Invalid result for tools/list" (ttlMs / cacheScope), Reproduced and Fixed
An MCP server shows as connected but exposes 0 tools, and the log says Invalid result for tools/list with ttlMs and cacheScope failing validation. Reproduced with a stub server on Claude Code 2.1.280, 2.1.285 and 2.1.288: the cause is not extra fields being rejected. The server negotiated MCP 2026-07-28 and then left out fields that revision requires (resultType, ttlMs, cacheScope); adding unknown fields works fine. Whether stdio uses the new protocol is decided by a remote flag that is off by default, so the same version breaks for some users and not others, and 2.1.280 fails the same way once negotiation is on. MCP_PROTOCOL_NEGOTIATION=legacy (also works in settings.json env) restores the tools; servers fix it by adding the three fields.
mcpclaude-code+4
