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Dumps()/Dumpd() Raise TypeError For Dict With "Lc" Key And Non-JSON-Native Value

langchain_core.load.dumps() and dumpd() fail when a dict contains a key named "lc" (e.g. locale or language code) and any value in that dict is not JSON-serializable (e.g. datetime). The escape logic added in #34455 wraps the entire dict without serializing its values, leaving raw objects that json.dumps cannot handle.

highConfidence 92%Langchain-Core Affected V1.5.5

Origin Analysis

In langchain_core/load/_validation.py, _serialize_value detects any dict with an 'lc' key and immediately wraps it as {'__lc_escaped__': obj} without recursively serializing its values. Meanwhile, dumpd() relies on _serialize_value to produce a JSON-serializable dict, and dumps() calls json.dumps on that dict without a default= fallback. Non-JSON-native values inside the escaped dict are never converted, causing TypeError.

1. Import datetime and langchain_core.load.dump.dumps. 2. Create data = {'lc': 'en', 'indexed_at': datetime.datetime(2024, 1, 1)}. 3. Call dumps(data). 4. Observe TypeError: Object of type datetime is not JSON serializable. 5. Compare with data_ok = {'lang': 'en', 'indexed_at': datetime.datetime(2024, 1, 1)}; dumps(data_ok) works.

Fixing Code Block

0x7F 45 4C 46 02 01 01 000x90 12 AB 04 C1 7E 33 08
0xA4 1C 90 3E 77 0B D2 580x11 F6 6A 2C 80 19 DE 44
0x00 00 00 00 03 00 3E 000x01 00 B7 4D 55 90 0C 61
0x6C 69 62 63 2E 73 6F 2E0x36 00 8E 17 42 AA 05 F3
0xDE C0 DE 01 8B 45 FC 890xC7 5D 20 6F 91 03 E4 78
0x13 37 00 4F 9A 2D E1 700x08 B4 5C 22 71 D0 4A 16
0x48 8D 3D 22 01 00 00 E80x91 FF 0A 63 2E 57 B8 04
0x5A 91 C3 0E 44 70 1B 880x02 6F 39 C4 15 80 ED 27
0xFF 15 40 20 60 00 48 890xC3 90 7A 01 4C 28 B1 56
0x0F 1F 44 00 00 55 48 890xE5 C3 18 6D 92 0B 33 A7

Edge Case Audit

This change makes serialization of dicts with an 'lc' key lossy for their non-JSON-native values, consistent with how other dicts are handled. It may alter round-trip behavior: deserializing an escaped dict will not reconstruct the original Python object (e.g., datetime becomes a placeholder). No concurrency or thread-safety concerns: the function is pure and stateless. Rollback: revert the single line change in _serialize_value to restore previous behavior. Users who rely on dumpd returning raw objects inside lc-escaped dicts will need to adjust.