Checkpoint serialization rejects fractions.Fraction and complex (TypeError) though Decimal is supported

[OmkumarSolanki]

Checked other resources

• This is a bug, not a usage question.
• I added a clear and descriptive title that summarizes this issue.
• I used the GitHub search to find a similar issue and didn't find it.
• I am sure that this is a bug in LangGraph rather than my code.
• The bug is not resolved by updating to the latest stable version of LangGraph.
• This is not related to the langchain-community package.
• I posted a self-contained, minimal, reproducible example. A maintainer can copy it and run it AS IS.

Related Issues / PRs

JsonPlusSerializer already supports Decimal, UUID, datetime, Path, IP addresses, etc.; Fraction and complex are the gap.

Reproduction Steps / Example Code (Python)

from decimal import Decimal
from fractions import Fraction
from langgraph.checkpoint.serde.jsonplus import JsonPlusSerializer

s = JsonPlusSerializer()

s.loads_typed(s.dumps_typed(Decimal("1.5")))   # OK -> Decimal('1.5')
s.loads_typed(s.dumps_typed(Fraction(1, 3)))    # TypeError: Type is not msgpack serializable: Fraction
s.loads_typed(s.dumps_typed(complex(1, 2)))     # TypeError: Type is not msgpack serializable: complex

Error Message and Stack Trace (if applicable)

TypeError: Type is not msgpack serializable: Fraction
TypeError: Type is not msgpack serializable: complex

Description

JsonPlusSerializer special-cases many stdlib types in _msgpack_default (libs/checkpoint/langgraph/checkpoint/serde/jsonplus.py) — including Decimal — but not fractions.Fraction or the builtin complex. Since neither is natively serializable by ormsgpack, both fall through to default and raise TypeError.

The effect: a graph whose checkpointed state contains a Fraction (exact rational arithmetic) or a complex number (scientific / signal-processing agents) cannot be checkpointed at all — put/aput raises. This is an inconsistency with the already-supported Decimal, and both have an obvious lossless representation.

I'm happy to fix this by adding Fraction and complex handlers to _msgpack_default that mirror the existing Decimal handler (encode via constructor args, reconstruct with the standard constructor), with round-trip tests. Glad to align on the approach first.

System Info

langgraph-checkpoint on main; default JsonPlusSerializer (msgpack path). Python 3.12.

[OmkumarSolanki]

I'd like to work on this. My intended approach would be to add two small handlers to _msgpack_default in jsonplus.py, mirroring the existing Decimal handler:

• complex -> encode (obj.real, obj.imag) and reconstruct with complex(real, imag)
• fractions.Fraction -> encode (obj.numerator, obj.denominator) and reconstruct with Fraction(numerator, denominator)

Both reach default today (they're not natively serializable), so this is purely additive with no back-compat impact, and I'd add round-trip tests alongside the existing serde tests. Does this align with maintainer expectations? If so, could a maintainer assign me and I'll open the PR.

[OmkumarSolanki]

While preparing the fix I ran a round-trip check across the other stdlib types _msgpack_default does not special-case, and found two more in the same category that currently raise TypeError: Type is not msgpack serializable:

• range — s.loads_typed(s.dumps_typed(range(0, 10, 2))) raises.
• the pure-path classes PurePath / PurePosixPath / PureWindowsPath — Path round-trips fine, but PurePosixPath("/a/b") raises, because the handler checks isinstance(obj, pathlib.Path) and pure paths are not Path subclasses.

Both fit the same minimal, additive pattern as the Fraction/complex handlers above:

• range → reconstruct via range(start, stop, step).
• pure paths → broaden the existing isinstance(obj, pathlib.Path) check to pathlib.PurePath. The existing handler already encodes obj.__class__ and rebuilds from obj.parts, so the exact subtype is preserved with no other change.

Happy to fold these into the same PR so it closes the serializer gap in one pass, or keep it scoped strictly to Fraction/complex — whichever you prefer. Either way I will wait for assignment before opening the PR.

[syf2211]

Hi — I'd like to continue work on this. I previously opened #8192 with handlers for Fraction and complex mirroring the existing Decimal pattern (plus round-trip tests), but it was auto-closed because the issue wasn't assigned to me. Could a maintainer assign this to 石岳峰 (@syf2211) so the PR can be reopened? Happy to keep the scope to Fraction/complex only unless you'd prefer the range/PurePath additions mentioned above.

[iamadhitya1]

Hi, I'd like to work on this. I've already implemented the fix in PR #8202 (which was auto-closed because I wasn't assigned first). The fix adds fractions.Fraction and complex to _msgpack_default() in jsonplus.py using the same EXT_CONSTRUCTOR_SINGLE_ARG pattern as decimal.Decimal, and registers both in SAFE_MSGPACK_TYPES for strict mode. Could you assign me so the PR can be reopened? Thanks

[iamadhitya1]

Following up on my earlier comment — the implementation is complete in #8202 (auto-closed only because I wasn't pre-assigned). It adds fractions.Fraction and complex to _msgpack_default() in jsonplus.py using EXT_CONSTRUCTOR_SINGLE_ARG with str(obj) as the payload, registers both in SAFE_MSGPACK_TYPES (so strict mode works too), and extends the existing test_serde_jsonplus round-trip test plus 4 focused regression tests. Ready to reopen the moment I'm assigned.