docs: update CHANGELOG.md for 0.55.0

Author: github-actions[bot]

CHANGELOG.md

@@ -4,6 +4,71 @@ All notable changes to this project are documented in this file.
 This changelog is automatically generated from GitHub Releases.
 
 ---
+## [0.55.0](https://github.com/koxudaxi/datamodel-code-generator/releases/tag/0.55.0) - 2026-03-10
+
+## Breaking Changes
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+### Dependency Changes
+* Pydantic v1 runtime support removed - Pydantic v2 is now required as a runtime dependency. Users running datamodel-code-generator with Pydantic v1 installed must upgrade to Pydantic v2. The previously deprecated v1 compatibility layer has been completely removed. (#3025)
+
+### Default Behavior Changes
+* Default output model switched from Pydantic v1 to v2 - Running `datamodel-codegen` without specifying `--output-model-type` now generates Pydantic v2 models (`pydantic_v2.BaseModel`) instead of Pydantic v1 models (`pydantic.BaseModel`). Users who depend on the previous default behavior must now explicitly specify `--output-model-type pydantic.BaseModel` to continue generating Pydantic v1 compatible code. (#3029)
+
+### Code Generation Changes
+* Generated model syntax changed for default output - The default generated code now uses Pydantic v2 syntax including `RootModel` instead of `__root__` fields, native union syntax (`str | None`) instead of `Optional[str]`, and Pydantic v2 validator/serializer decorators. Existing code that consumes generated models may need updates to work with the new Pydantic v2 output format. (#3029)
+* Pydantic v1 output support removed - The `pydantic.BaseModel` output model type has been completely removed. Generated code now only supports Pydantic v2 patterns including `RootModel` instead of `__root__`, `model_rebuild()` instead of `update_forward_refs()`, and `model_config` instead of `class Config`. Users generating Pydantic v1 models must migrate to v2 output. (#3031)
+
+### Custom Template Update Required
+* Pydantic v1 templates removed - The following Jinja2 templates have been deleted and users with custom templates extending them must migrate to v2 equivalents:
+  - `pydantic/BaseModel.jinja2` → use `pydantic_v2/BaseModel.jinja2`
+  - `pydantic/BaseModel_root.jinja2` → use `pydantic_v2/RootModel.jinja2`
+  - `pydantic/Config.jinja2` → removed (v2 uses `model_config` dict)
+(#3031)
+
+### API/CLI Changes
+* `--output-model-type pydantic.BaseModel` removed - The `pydantic.BaseModel` value for `--output-model-type` is no longer valid. Use `pydantic_v2.BaseModel` instead (now the default). (#3031)
+* Pydantic v1 compatibility utilities removed from Python API - The following functions were removed from `datamodel_code_generator.util`: `model_dump()`, `model_validate()`, `get_fields_set()`, `model_copy()`. Use Pydantic v2 methods directly (`obj.model_dump()`, `cls.model_validate()`, etc.). (#3031)
+* `datamodel_code_generator.model.pydantic` module removed - The entire Pydantic v1 model module including `BaseModel`, `CustomRootType`, `DataModelField`, `DataTypeManager`, and `dump_resolve_reference_action` has been removed. Use `datamodel_code_generator.model.pydantic_v2` instead. (#3031)
+* Pydantic v2 now required at runtime - The minimum pydantic dependency changed from `pydantic>=1.5` to `pydantic>=2,<3`. Users with pydantic v1 installed must upgrade to pydantic v2 before using datamodel-code-generator (#3027)
+* Removed internal pydantic compatibility utilities - The following functions/classes were removed from `datamodel_code_generator.util`: `get_pydantic_version()`, `is_pydantic_v2()`, `model_validator()`, `field_validator()`, and `ConfigDict`. Users who imported these internal utilities directly must update their code to use pydantic's native APIs (#3027)
+* Removed `datamodel_code_generator.pydantic_patch` module - The entire pydantic compatibility patching module was removed. Any code importing from this module will fail (#3027)
+* Removed `packaging` dependency - The `packaging` library is no longer a dependency. Code that relied on it being transitively available should add it explicitly (#3027)
+
+## What's Changed
+* Extract shared pydantic base module for v2/dataclass/msgspec by @koxudaxi in https://github.com/koxudaxi/datamodel-code-generator/pull/3022
+* Remove pydantic v1 runtime compat layer by @koxudaxi in https://github.com/koxudaxi/datamodel-code-generator/pull/3025
+* test: make integration helpers explicit about output model by @koxudaxi in https://github.com/koxudaxi/datamodel-code-generator/pull/3028
+* feat: switch default output model to pydantic v2 by @koxudaxi in https://github.com/koxudaxi/datamodel-code-generator/pull/3029
+* test: align v2 parser baselines before v1 output removal by @koxudaxi in https://github.com/koxudaxi/datamodel-code-generator/pull/3032
+* feat: remove pydantic v1 output support by @koxudaxi in https://github.com/koxudaxi/datamodel-code-generator/pull/3031
+* docs: remove pydantic v1 references by @koxudaxi in https://github.com/koxudaxi/datamodel-code-generator/pull/3030
+* Remove pydantic v1 runtime compat shims and update dependencies by @koxudaxi in https://github.com/koxudaxi/datamodel-code-generator/pull/3027
+* docs: remove final pydantic v1 traces by @koxudaxi in https://github.com/koxudaxi/datamodel-code-generator/pull/3036
+* Fix all-export generation for hyphenated directories by @an5t in https://github.com/koxudaxi/datamodel-code-generator/pull/3033
+* Fix exact imports for reused tree-scope models by @koxudaxi in https://github.com/koxudaxi/datamodel-code-generator/pull/3042
+* fix #3034 and #3035 by introducing `ValidatedDefault` by @keyz in https://github.com/koxudaxi/datamodel-code-generator/pull/3040
+* Avoid TYPE_CHECKING imports for Ruff modular output by @koxudaxi in https://github.com/koxudaxi/datamodel-code-generator/pull/3043
+* Add deprecated decorators for dataclass output by @koxudaxi in https://github.com/koxudaxi/datamodel-code-generator/pull/3044
+* Fix for #3045 by @ashipilov in https://github.com/koxudaxi/datamodel-code-generator/pull/3046
+* Refactor generate tests to use assertion helper by @koxudaxi in https://github.com/koxudaxi/datamodel-code-generator/pull/3047
+
+## New Contributors
+* @an5t made their first contribution in https://github.com/koxudaxi/datamodel-code-generator/pull/3033
+* @keyz made their first contribution in https://github.com/koxudaxi/datamodel-code-generator/pull/3040
+* @ashipilov made their first contribution in https://github.com/koxudaxi/datamodel-code-generator/pull/3046
+
+**Full Changelog**: https://github.com/koxudaxi/datamodel-code-generator/compare/0.54.1...0.55.0
+
+---
+
 ## [0.54.1](https://github.com/koxudaxi/datamodel-code-generator/releases/tag/0.54.1) - 2026-03-04
 
 ## What's Changed