FastAPI 0.139.2

Overview

FastAPI 0.139.2 ships on July 16, 2026 — a focused patch release addressing a concurrency edge case in the router layer. Following quickly after 0.139.1 (also July 16), this release prioritises correctness over new features.

Changes in 0.139.2

• Thread-safe router route building — The router's route-building logic has been refactored to be thread-safe. This is mainly relevant for test suites running in parallel threads (an uncommon but valid pattern). Previously, concurrent route registration could produce race conditions. PR #16013 by @tiangolo.

What Was in 0.139.1

While not a headline feature release, 0.139.1 included additional stability improvements. Both patches follow the 0.139.0 release from July 1, which introduced new API features.

Upgrade Guide

Upgrade via pip: pip install --upgrade fastapi. No breaking changes are present in this release — the thread-safety fix is fully backward-compatible. Run your test suite after upgrading to confirm the parallel-testing scenario no longer triggers the race condition.