docs(mcp-analytics): forbid the standalone $identify event

[gesh]

Problem

• The MCP Analytics spec lets an SDK send $identify, and the JS and Python SDKs no longer do.
• Stateless MCP servers sent it on almost every request, so its volume tracked tool-call volume.
• The event did nothing. It carries no $anon_distinct_id, so ingestion merges no person from it.
• Every other MCP event already carries the same distinct_id, $set and $groups.
• feat(mcp): stop sending standalone $identify events posthog-js#5306 and feat(mcp): stop sending standalone $identify events posthog-python#1067 removed it from both SDKs.

Changes

• The Person identity requirement now says the SDK SHALL NOT send a standalone $identify, for any MCP revision or session mode.
• Per-event identity does not change: $set and $groups on identified events, $process_person_profile: false on anonymous ones.
• A new scenario checks that an identified call sends no $identify.
• The identified-call scenario in acceptance/public/mcp-analytics.feature now asserts no $identify. It reuses the harness phrase no event named "…" should be enqueued.
• The change is proposed, applied and archived under openspec/changes/archive/2026-10-19-drop-mcp-identify-event/.

Note

Insights built on $identify from MCP servers stop receiving that event. Person properties are unaffected.

Testing

• openspec validate --specs --strict --no-interactive with CI's pinned 1.4.1 passes all 64 specs.
• Not run: the acceptance scenario. PostHog/posthog-sdk-test-harness has no MCP step bindings yet, so no MCP scenario in that feature runs today.

🤖 Agent context

• Autonomy: human-driven, agent-assisted. The author reviewed the diff before the push.
• Tool and model: Claude Code, claude-opus-5-5.
• Skills: writing-pr-descriptions.

🤖 Generated with Claude Code