Preserve Decimal precision through XCom serialization

[2sumtech]

A task that returns a decimal.Decimal hands the next task a different number. The serde encoder converts the value to a float before storing it, so any Decimal that needs more than 53 bits of mantissa is silently rounded on its way through XCom. The same encoder handles deferred-task trigger kwargs, so a Decimal passed to defer() is rounded too.

str keeps every significant digit, so the encoder now uses it for values that have digits after the decimal point. Whole numbers keep their existing integer encoding.

Repro on main

from decimal import Decimal

from airflow.sdk.serde import deserialize, serialize

for value in (Decimal("3.14159265358979323846"), Decimal("12345678901234567.89")):
    encoded = serialize(value)
    print(value, "->", encoded["__data__"], "->", deserialize(encoded))

# 3.14159265358979323846 -> 3.141592653589793      -> 3.141592653589793
# 12345678901234567.89   -> 1.2345678901234568e+16 -> 12345678901234568

With this change both values come back unchanged.

Compatibility

The serializer version deliberately stays at 1. deserialize has rebuilt the value with Decimal(str(data)) ever since the serializer was added in #28067, so every released Airflow reads a string payload as happily as a float one. Bumping the version would do the opposite of helping, because an older reader raises on any version above its own. Existing XCom rows written as floats keep deserializing exactly as they do today.

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Was generative AI tooling used to co-author this PR?

• Yes — Claude Code (Fable 5)

Generated-by: Claude Code (Fable 5) following the guidelines

[boring-cyborg]

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[2sumtech]

Marking this ready for review. The change is small: the serde encoder now stores non-integral Decimal values as str instead of float, so values needing more than 53 bits of precision survive XCom and trigger kwargs unchanged. Deserialization already uses Decimal(str(data)), so no version bump is needed and existing float payloads still read fine. @ashb @amoghrajesh would appreciate a look when you have time.