Exploring MCP Tool Quality

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.

For a single tool, prefer the typed tools — posthog:query-mcp-tool-stats (calls, errors, p50/p95, users, sessions, intents), posthog:query-mcp-tool-failures (top error messages by harness), and posthog:query-mcp-tool-daily-stats (day-by-day trend). Each takes a toolName + dateRange, runs the same query runner as the tool-detail UI, and is gated behind the mcp-analytics flag — no hand-written SQL needed.

HogQL via posthog:execute-sql is the path for cross-tool questions — the "which tool errors most" ranking below has no typed tool, so rank with SQL, then drill into the worst tool with posthog:query-mcp-tool-stats and posthog:query-mcp-tool-failures. 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.

The two rules that matter most

• Always use the effective tool name. New-SDK events wrap the real tool in a single-exec call, so grouping on raw $mcp_tool_name collapses everything under the wrapper. Use:

  coalesce(nullIf(toString(properties.$mcp_exec_tool_call_name), ''), toString(properties.$mcp_tool_name))
  

• Always read $mcp_is_error via toBool(...) and cast $mcp_duration_ms via toFloat(...). The properties are strings.

Always set a time range — these queries scan events otherwise.

Workflow: which tool has the highest error rate

This is the canonical "which tool errors most" question. Rank tools by error rate, but guard against small-sample noise with a HAVING floor on call volume:

posthog:execute-sql
SELECT
    coalesce(nullIf(toString(properties.$mcp_exec_tool_call_name), ''), toString(properties.$mcp_tool_name)) AS tool,
    count() AS total_calls,
    countIf(toBool(properties.$mcp_is_error)) AS errors,
    round(countIf(toBool(properties.$mcp_is_error)) * 100.0 / count(), 1) AS error_rate_pct
FROM events
WHERE event = '$mcp_tool_call'
    AND coalesce(nullIf(toString(properties.$mcp_exec_tool_call_name), ''), toString(properties.$mcp_tool_name)) != ''
    AND timestamp >= now() - INTERVAL 30 DAY
GROUP BY tool
HAVING total_calls >= 20
ORDER BY error_rate_pct DESC, total_calls DESC
LIMIT 20

Report both rate and volume — a 100% error rate over 3 calls is rarely the real story; a 12% rate over 50,000 calls is. Offer to pull the top $mcp_error_message values for the worst tool (see below).

Workflow: tool-quality matrix

One row per tool with error rate, latency percentiles, and reach — mirrors the tool-quality screen. The ready-to-run query is in models-mcp.md under "Tool-quality matrix".

Workflow: why is a tool failing

For one tool's top failure buckets (grouped by harness), call posthog:query-mcp-tool-failures with the toolName — it's the typed equivalent of the query below. Failures come from the same source as the error rate: errored $mcp_tool_call events ($mcp_is_error), scoped by the effective tool name. Failures are grouped by $mcp_error_type (a semantic bucket: internal, validation, api_4xx, api_5xx, permission, timeout, rate_limited, missing_context) and the HTTP $mcp_error_status when present. To see individual errored calls inside a bucket — with the captured $mcp_error_message, session id, harness, and intent — pass the bucket's raw error_type/error_status to posthog:query-mcp-tool-failure-occurrences ($mcp_error_message is empty on events captured before message capture shipped):

posthog:execute-sql
SELECT
    concat(
        coalesce(nullIf(toString(properties.$mcp_error_type), ''), 'unknown'),
        if(empty(coalesce(toString(properties.$mcp_error_status), '')), '',
           concat(' (HTTP ', coalesce(toString(properties.$mcp_error_status), ''), ')'))
    ) AS failure,
    count() AS n
FROM events
WHERE event = '$mcp_tool_call'
    AND toBool(properties.$mcp_is_error)
    AND coalesce(nullIf(toString(properties.$mcp_exec_tool_call_name), ''), toString(properties.$mcp_tool_name)) = '<tool>'
    AND timestamp >= now() - INTERVAL 30 DAY
GROUP BY failure ORDER BY n DESC LIMIT 10

$mcp_error_type is only populated on newer SDK/server paths — a chunk of errored calls carry neither type nor status and fall into the unknown bucket.

Workflow: slowest tools

Swap the aggregate for latency percentiles (quantile(0.95)(toFloat(properties.$mcp_duration_ms))) and order by p95_ms. The matrix query already returns p50_ms / p95_ms.

Constructing UI links

• Dashboard: https://app.posthog.com/project/<project_id>/mcp-analytics/dashboard
• Tool quality: https://app.posthog.com/project/<project_id>/mcp-analytics/tool-quality

Always surface a UI link so the user can verify visually.

Tips

• Report error rate and call volume together; a HAVING total_calls >= N floor stops tools with very few calls from topping the list spuriously
• Exclude errored calls from latency percentiles only when asked — failed calls are often the slow ones, and dropping them hides the problem
• $mcp_client_name lets you cut quality by harness (Claude Code vs Cursor vs …); the canonical bucketing multiIf is in models-mcp.md
• Harness bucketing is resolved server-side by products/mcp_analytics/backend/mcp_harness.py — that's the source of truth, and posthog:query-mcp-harness-breakdown runs it. If your hand-written SQL disagrees with the screen, your bucketing has drifted from mcp_harness.py; prefer the typed tool over re-deriving it

Related skills

• exploring-mcp-sessions — drill into a single agent run and its tool sequence
• exploring-mcp-intent-clusters — group agent goals and see which intents drive the errors