fix: preserve JSON-safe checkpoint serialization for msgpack async saves

[vishal-bala]

fixes #181

Summary

Issue #181 reported that AsyncRedisSaver could fail during checkpoint writes with:

TypeError: Object of type HumanMessage is not JSON serializable

The failure showed up when checkpoint state contained LangChain message objects and RedisJSON received those live Python objects instead of a serialized representation.

Root Cause

The failing path was the checkpoint serializer fallback for non-JSON-safe payloads.

When a checkpoint contains binary data anywhere in its state, JsonPlusRedisSerializer.dumps_typed(...) can fall back from JSON to MessagePack. In the Redis saver, _dump_checkpoint() then deserialized that MessagePack payload back into Python objects before writing it to Redis. That worked for raw bytes, which were partially normalized, but it also rehydrated LangChain messages like HumanMessage and AIMessage as live objects.

At that point, the checkpoint document was no longer fully RedisJSON-safe, and AsyncRedisSaver eventually passed those message objects down to client.json().set(...), which raised the serialization error from the original issue.

In short: the bug was not in normal JSON checkpoint writes. It was in the msgpack fallback path, where deserialization reintroduced Python objects that RedisJSON cannot store directly.

Fix

This change keeps the existing JSON path unchanged and narrows the fix to the msgpack branch in _dump_checkpoint().

For msgpack checkpoint payloads, the saver now:

1. Deserializes the typed payload back into Python values.
2. Replaces nested raw bytes with Redis-safe marker dictionaries.
3. Runs the normalized checkpoint back through the serializer so LangChain messages are converted back into their JSON-safe constructor form.
4. Loads the resulting JSON document for Redis storage.

This preserves the expected RedisJSON representation while still allowing checkpoint payloads that require MessagePack during intermediate serialization.

Tests

Added regression coverage for both the serializer boundary and the async Redis path:

• A focused unit regression that reproduces the msgpack fallback shape with LangChain messages plus binary state and asserts the dumped checkpoint remains JSON-safe.
• An async Redis-backed regression that verifies AsyncRedisSaver can store and reload that mixed checkpoint shape without raising the original HumanMessage is not JSON serializable error.

These tests connect directly to issue #181 by exercising the exact failure mode: message objects becoming live Python instances again after MessagePack fallback.

[chatgpt-codex-connector]

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[bsbodden]

LGTM!