Problem

DeepSeek V4 loses coherence on long sessions. MiMo Code's research showed that periodic state extraction at intervals keeps context lean and enables crash recovery.

Fix

New v4-checkpoint-writer hook (ToolGuard tier) that:

1. Tracks V4 sessions from message.updated events
2. Every 20 tool calls, writes a checkpoint file to .omo/checkpoints/{sessionID}.json
3. The checkpoint contains: session ID, model ID, tool call count, last tool name, timestamp
4. Non-V4 sessions are completely unaffected (zero overhead)

Checkpoint file format:

{
  "sessionID": "ses_abc123",
  "modelID": "deepseek/deepseek-v4-pro",
  "toolCallCount": 20,
  "lastToolName": "bash",
  "timestamp": "2026-06-19T17:30:00.000Z"
}

TDD Evidence

4 tests covering all paths:

• V4 session at 20 tool calls -> checkpoint file written with correct data
• V4 session at 19 tool calls -> NO checkpoint (interval not reached)
• Non-V4 session at 20 tool calls -> NO checkpoint (V4-only)
• V4 session at 40 tool calls -> 2nd checkpoint written (overwrites with count=40)

All existing tool-guard composition tests still pass.

Research backing

From MiMo Code's architecture (which beat Claude Code at >200-step tasks):

• Checkpoint writer: subagent extracts state at 20/45/70% budget
• This is a simplified version: periodic file-based checkpoints (no background agent)
• Complements PR feat(agents): add DeepSeek V4 Pro/Flash Sisyphus prompt routing #5403's earlier compaction (35% threshold) by providing a recovery trail

Files

• packages/omo-opencode/src/hooks/v4-checkpoint-writer/hook.ts (new) -- hook implementation
• packages/omo-opencode/src/hooks/v4-checkpoint-writer/index.ts (new) -- barrel export
• packages/omo-opencode/src/hooks/v4-checkpoint-writer/hook.test.ts (new) -- 4 tests
• packages/omo-opencode/src/hooks/index.ts -- barrel export added
• packages/omo-opencode/src/plugin/hooks/create-tool-guard-hooks.ts -- hook registered
• packages/omo-opencode/src/config/schema/hooks.ts -- hook name added to HookNameSchema

Usage

The hook is always on (can be disabled via disabled_hooks):

{
  "disabled_hooks": ["v4-checkpoint-writer"]
}

Complements PR #5403 (DeepSeek V4 Sisyphus + compaction) and PR #5437 (V4 verification gate).

Conflict note

This PR and #5437 (V4 verification gate) both modify the same 3 files: hooks.ts, hooks/index.ts, and create-tool-guard-hooks.ts. Whichever merges first, the other will need a rebase to resolve merge conflicts in those files. If you'd like me to batch-rebase one onto the other before merging, let me know.

Summary by cubic

Add a checkpoint writer hook for DeepSeek V4 that saves session state every 20 tool calls to keep long sessions coherent and enable crash recovery. Non‑V4 sessions are unaffected.

• New Features
  • Added v4-checkpoint-writer that tracks V4 sessions from message.updated events and writes checkpoints every 20 tool calls to <workspace>/.omo/checkpoints/{sessionID}.json (or ~/.omo/checkpoints when no workspace), including session ID, model ID, tool call count, last tool, and timestamp.
  • Enabled by default; disable via disabled_hooks: ["v4-checkpoint-writer"].
  • Registered in the hook factory and schema; export added. Tests cover interval behavior (20/19/40 calls) and non‑V4 sessions.

^{Written for commit 205693a. Summary will update on new commits.}