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## Summary Fixes #7498 — `MESSAGE_COERCION_FAILURE` when resuming threads checkpointed before v1.0.1. **Root cause:** PR #6269 (v1.0.1) added an `_allowed_json_modules` security gate to `JsonPlusSerializer._reviver`. The gate defaults to `None`, so old `"json"`-format checkpoint blobs containing `lc=2` constructor dicts (the pre-msgpack serialization format for pydantic objects like `HumanMessage`) are now returned as raw dicts instead of being reconstructed. Those raw dicts reach `add_messages → convert_to_messages`, which sees `type="constructor"` and raises `MESSAGE_COERCION_FAILURE`. Fresh first-turn messages are unaffected because current `dumps_typed` only writes `"msgpack"` blobs. **Fix:** `_reviver` now reconstructs `lc=2` blobs whose target class is already in `SAFE_MSGPACK_TYPES` — the same curated allowlist already used by the msgpack deserialization path (includes all standard LangChain message types). Unknown classes are still blocked, preserving the security intent of #6269. ## Changes - `libs/checkpoint/langgraph/checkpoint/serde/jsonplus.py` — add `_is_safe_json_type()` helper; update `_reviver` and `_check_allowed_json_modules` to allow safe types without an explicit allowlist - `libs/checkpoint/tests/test_jsonplus.py` — two new regression tests: safe-type `lc=2` blobs revive correctly; unknown-type `lc=2` blobs stay blocked ## Test plan - [ ] `test_lc2_json_safe_type_revives_without_allowlist` — `HumanMessage`/`AIMessage` lc=2 JSON blobs round-trip to proper `BaseMessage` objects with no allowlist configured - [ ] `test_lc2_json_unknown_type_stays_blocked_without_allowlist` — `pprint.pprint` lc=2 blob still returns raw dict (not reconstructed) - [ ] `test_deserde_invalid_module` — existing behaviour unchanged - [ ] Full `test_jsonplus.py` suite: 93/93 passing Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>