chore(mcp-analytics): make remaining MCPSession backfill columns nullable

[gesh]

Problem

MCPSession's backfill-mirror columns are dormant — the live sessions list aggregates from events on the fly and never reads them, and the only writer (the backfill-mcp-sessions job) is being removed in #60039. They're on the path to deletion, but the safe first step is relaxing their NOT NULL constraints.

#60041 already did this for session_start / session_end / duration_seconds. This completes the set for the remaining four.

Changes

DROP NOT NULL on tools_used, tool_call_count, distinct_id, mcp_client_name (instant, non-destructive metadata change). The model fields stay in place for now; physically dropping the columns is a later cleanup once these constraints are relaxed and the jobs (#60039) are gone.

How did you test this code?

Agent-authored. Verified makemigrations --check reports no drift, sqlmigrate emits only the four DROP NOT NULLs, and ruff passes.

Docs update

no

🤖 Agent context

Authored with Claude Code (Opus 4.7). Part of the incremental MCP-analytics cleanup: (1) #60039 remove the dormant backfill/summarize jobs, (2) this — finish relaxing the backfill columns to nullable (mirrors #60041), (3) a later PR removes the fields + physically drops the columns. Kept small and non-destructive on purpose.

[greptile-apps]

_{Reviews (1): Last reviewed commit: "chore(mcp): slim dormant MCPSession mode..." | Re-trigger Greptile}

[github-actions]

Migration SQL Changes

Hey 👋, we've detected some migrations on this PR. Here's the SQL output for each migration, make sure they make sense:

products/mcp_analytics/backend/migrations/0006_alter_mcpsession_distinct_id_and_more.py

BEGIN;
--
-- Alter field distinct_id on mcpsession
--
ALTER TABLE "posthog_mcp_session" ALTER COLUMN "distinct_id" DROP NOT NULL;
--
-- Alter field mcp_client_name on mcpsession
--
ALTER TABLE "posthog_mcp_session" ALTER COLUMN "mcp_client_name" DROP NOT NULL;
--
-- Alter field tool_call_count on mcpsession
--
ALTER TABLE "posthog_mcp_session" ALTER COLUMN "tool_call_count" DROP NOT NULL;
--
-- Alter field tools_used on mcpsession
--
ALTER TABLE "posthog_mcp_session" ALTER COLUMN "tools_used" DROP NOT NULL;
COMMIT;

Last updated: 2026-05-27 08:28 UTC (d506ecf)

[github-actions]

🔍 Migration Risk Analysis

We've analyzed your migrations for potential risks.

Summary: 0 Safe | 1 Needs Review | 0 Blocked

⚠️ Needs Review

May have performance impact

mcp_analytics.0006_alter_mcpsession_distinct_id_and_more
  └─ #1 ⚠️ AlterField
     Field alteration may cause table locks or data loss (check if changing type or constraints)
     model: mcpsession, field: distinct_id, field_type: CharField
  └─ #2 ⚠️ AlterField
     Field alteration may cause table locks or data loss (check if changing type or constraints)
     model: mcpsession, field: mcp_client_name, field_type: CharField
  └─ #3 ⚠️ AlterField
     Field alteration may cause table locks or data loss (check if changing type or constraints)
     model: mcpsession, field: tool_call_count, field_type: IntegerField
  └─ #4 ⚠️ AlterField
     Field alteration may cause table locks or data loss (check if changing type or constraints)
     model: mcpsession, field: tools_used, field_type: ArrayField

Last updated: 2026-05-27 08:28 UTC (d506ecf)

[greptile-apps]

_{Reviews (2): Last reviewed commit: "chore(mcp): make remaining MCPSession ba..." | Re-trigger Greptile}

[deployment-status-posthog]

Deploy status

Environment	Status	Deployed At	Workflow
dev	✅ Deployed	2026-05-27 09:21 UTC	Run
prod-us	✅ Deployed	2026-05-27 09:39 UTC	Run
prod-eu	✅ Deployed	2026-05-27 09:40 UTC	Run