Lore

A timeline of the FastAPI codebase, derived from git history (git log since the first commit on 5 December 2018).

Eras

Genesis (Dec 2018 – mid 2019)

The first commit landed on 5 December 2018 by Sebastián Ramírez (tiangolo). The framework was a single-author project: 2,193 commits in 2019 alone, almost all by the original author. The initial design choices already in place — Starlette as the ASGI core, Pydantic v1 for validation, a decorator-driven path-operation API, automatic OpenAPI generation, and a bundled Swagger UI — have not changed in seven years.

Stabilisation and ecosystem (2020 – 2021)

Activity slowed in calendar terms (725 commits in 2020, 482 in 2021) but the API surface stabilised. The Annotated[...] parameter style was added during this stretch; deprecated parameters (regex=, Form and File quirks) gained their deprecated() markers. The repo grew translation infrastructure (docs/<lang>/...), a translations workflow under .github/workflows/notify-translations.yml, and the scripts/people.py automation that rebuilds the contributor pages.

Pydantic v2 migration (mid 2023 – 2024)

The largest single technical change in the project. Pydantic 2 shipped with a different API for fields, validators, and JSON-schema generation, and FastAPI maintained both v1 and v2 paths through fastapi/_compat/v1.py and fastapi/_compat/v2.py for over a year. The compatibility module is still there (see fastapi/_compat/__init__.py and fastapi/_compat/v2.py), but Pydantic v1 support has now been removed — fastapi/utils.py:create_model_field raises PydanticV1NotSupportedError if a v1 model is encountered, and fastapi/exceptions.py defines that error class.

Tooling expansion (2024)

2024 was the busiest year on record at 1,999 commits, much of it from github-actions[bot] and dependabot[bot] (see By the numbers). Concrete additions visible in the tree:

• The ty type checker (Astral) added alongside mypy — see scripts/lint.sh.
• prek replaces pre-commit as the default runner.
• playwright and zizmor join the dev dependency group (pyproject.toml).
• The fastapi-cli extraction completed — fastapi/cli.py is now a five-line shim that imports from the separate fastapi-cli package.
• Translation automation grew an LLM-driven helper at scripts/translate.py, using pydantic-ai.

Modern feature additions (2025 – 2026)

Recent commits show the framework picking up first-class Server-Sent Events support — fastapi/sse.py exposes EventSourceResponse, ServerSentEvent, and format_sse_event() with the OpenAPI 3.2 SSE schema baked in (see the _SSE_EVENT_SCHEMA constant). The routing layer now keeps a per-endpoint context cache (_endpoint_context_cache in fastapi/routing.py) so validation errors can quote a path, function name, file, and line number. EndpointContext, DependencyScopeError, and the scope: Literal["function", "request"] field on Dependant were all added during this stretch.

The latest commit on master (d8a2c1eda, 30 April 2026) is 📝 Update release notes, made by github-actions[bot]. Recent feature PRs include 15462 (layout polish), 15458 (interactive tabs in the opinions section), and 15457 (FastAPI Conf '26 announcement).

Longest-standing code

fastapi/__init__.py, fastapi/applications.py, and fastapi/routing.py all date from the very first commit (2018-12-05). They have been refactored heavily but never replaced — the public-API exports in __init__.py (FastAPI, APIRouter, Depends, HTTPException, Body, Query, Path, Cookie, Header, File, Form, Security, Request, Response, WebSocket, …) have been stable for the lifetime of the project. The BackgroundTasks re-export from fastapi/background.py is similarly original.

Deprecated features still visible in tree

• UJSONResponse and ORJSONResponse in fastapi/responses.py are explicitly deprecated; the framework now serializes through Pydantic.
• The regex= parameter on Path/Query/Header/Cookie/Body is deprecated in favour of pattern= (see Param.__init__ in fastapi/params.py).
• fastapi.utils.generate_operation_id_for_path is deprecated and unused internally; generate_unique_id replaced it.
• on_startup / on_shutdown are kept for compatibility via the vendored _DefaultLifespan in fastapi/routing.py. Starlette removed its own copy; FastAPI re-vendored the class to maintain backwards compatibility (referenced in a comment to Starlette PR #3117).
• pydantic.v1 model support: removed. PydanticV1NotSupportedError (fastapi/exceptions.py) is now raised at model-creation time.

Major rewrites

• Pydantic v2 migration — see era above.
• fastapi-cli extraction — the top-level fastapi command moved into the standalone fastapi-cli package; fastapi/cli.py shrank to a five-line wrapper.
• fastapi-slim distribution — the fastapi-slim/ directory hosts an alternate PyPI distribution built from the same sources, exercised by .github/workflows/test-redistribute.yml.

Growth trajectory

xychart-beta
    title "Commits per year"
    x-axis ["2019", "2020", "2021", "2022", "2023", "2024", "2025", "2026 YTD"]
    y-axis "Commits"
    bar [2193, 725, 482, 665, 780, 1999, 1144, 597]

The framework was hyperactive in its first year, settled into 500–800 commits/year through 2020-2023, then accelerated again from 2024 onward — a mix of human contributions and the bot-driven release/translation/dependency cadence described in By the numbers.