Exploring MCP tool quality

Any MCP server instrumented with PostHog's MCP analytics SDK emits a $mcp_tool_call event on the shared events table every time an agent invokes a tool. There is no dedicated ClickHouse table — every field lives as a $mcp_* property on events, and every tool-quality metric (error rate, latency percentiles, reach) is an aggregation over this one event. This is the data behind the MCP analytics dashboard and tool-quality screens.

HogQL via posthog:execute-sql is the primary path. There are no typed tools for tool quality — it is all SQL. The full property schema and the canonical query recipes live in the shared MCP data reference: products/posthog_ai/skills/querying-posthog-data/references/models-mcp.md. That reference is the single source of truth for the $mcp_* schema and the effective-tool-name idiom used below — this skill inlines only the headline "which tool errors most" query for convenience; pull the matrix, latency, and harness recipes from the reference rather than re-deriving them. Read it before writing queries.