The v2 release of Go's encoding/json introduces several behavior updates over v1, including improved UTF-8 error reporting, a stricter approach to duplicate names, and new representations for nil slices and maps, with case-sensitive JSON-to-Go field matching and updated omitempty semantics.

Go 1.27’s jsonv2 maintains compatibility with v1, allowing gradual migration where v1 can be implemented using v2 options, supported by migration guides and options to choose between v1 or v2 semantics.

Unmarshal performance is reported at roughly a 1.36x speedup in small benchmarks, with potential larger gains in more complex scenarios.

Time duration handling in v2 lacks a default representation and will error unless a chosen representation is provided; unlike v2, v1 used to emit a numeric nanoseconds value.

The move to v2 began around 2020–2021, driven by a need for stricter standards and streaming support, involving public discussions, prototypes, and community collaboration leading to the stable v2 release.

v2 expands customization via WithMarshalers and WithUnmarshalers, enabling per-type or per-call overrides that supersede default representations.

Key v2 APIs include Marshal, MarshalWrite, MarshalEncode, Unmarshal, UnmarshalRead, UnmarshalDecode, each with options, plus a jsontext package offering Encoder/Decoder for token- and value-level processing.

v2 adopts a streaming-friendly design with MarshalerTo and UnmarshalerFrom interfaces to enable true streaming JSON processing and reduce full in-memory buffering.

V1 treated duplicate member names leniently by taking the last value, while v2 errors on the object, producing partial state unless the error is handled.

Go 1.27’s jsonv2 introduces seven breaking changes from v1 demonstrated through concrete inputs and side-by-side outputs.

Case-insensitive key matching is removed in v2, so inputs with uppercase keys are ignored and corresponding fields remain zero-valued without error.

Migration checklist recommends auditing for duplicate keys, uppercase keys, omitempty behavior on numeric/bool types, serialized time.Duration representations, and re-running golden-file tests to account for changes with nil slices/maps.