uv sync errors: no pyproject, no solution, --locked, requires-python (tested)

Common uv sync errors: No pyproject.toml found (use uv pip install -r for requirements files), No solution found (conflicting pins), The lockfile needs to be updated, but --locked was provided (run uv lock and commit it; --frozen silently installs the old lock) and an interpreter outside requires-python.

1. Four uv sync errors, reproduced

uv 0.13.5 on Linux, each case in a fresh folder:

1. a folder with only a requirements.txt;
2. a pyproject.toml whose [project] has dependencies = ["requests>=2", "requests<2"];
3. dependencies = ["requests"], then uv lock, then "six" added to dependencies without re-locking (for --frozen only the last 3 lines are shown, then .venv/bin/python -c "import six");
4. requires-python = ">=3.14", synced with -p 3.11.

$ uv sync   # folder with only requirements.txt
error: No `pyproject.toml` found in current directory or any parent directory

$ uv sync   # conflicting requirements
Using CPython 3.11.17
Creating virtual environment at: .venv
error: No solution found when resolving dependencies
  cause: Because your project depends on requests>=2 and requests<2, we can conclude that your project's requirements are unsatisfiable.

$ uv sync --locked   # pyproject.toml changed after uv lock
Using CPython 3.11.17
Creating virtual environment at: .venv
Resolved 7 packages in 120ms
error: The lockfile at `uv.lock` needs to be updated, but `--locked` was provided.

hint: To update the lockfile, run `uv lock`.

$ uv sync --frozen
 + idna==3.20
 + requests==2.34.2
 + urllib3==2.8.0
ModuleNotFoundError: No module named 'six'

$ uv sync -p 3.11   # requires-python = ">=3.14"
Using CPython 3.11.17
error: The requested interpreter resolved to Python 3.11.17, which is incompatible with the project's Python requirement: `>=3.14` (from `project.requires-python`)

2. What each one means and the fix

• No pyproject.toml found: uv sync only works on a uv project. For a plain requirements file use uv pip install -r requirements.txt (see uv pip install requirements.txt), or convert it with uv init then uv add -r requirements.txt.
• No solution found when resolving dependencies: the cause: line names the conflicting requirements. Loosen one of the pins; there is no flag to sync anyway, since an environment that breaks a requirement is what uv refuses to build.
• The lockfile needs to be updated, but --locked was provided: pyproject.toml changed since uv.lock was written (common in CI). Run uv lock locally and commit uv.lock. --frozen does not error, but it installs the old lockfile: six was silently missing.
• Incompatible with the project's Python requirement: the interpreter (here forced with -p 3.11, or picked from .python-version) is outside requires-python. Use a matching version (uv python install 3.14, uv sync -p 3.14) or relax requires-python.

Related: uv pip install vs uv add · uv pip compile · uv SSL errors.

Written with AI assistance. Measured on 2026-10-23 with uv 0.13.5 and a uv-managed CPython 3.11.17 on Linux. Commands were run verbatim and the output pasted unedited (--frozen output trimmed to its last 3 lines, as stated). PlainTables is our extension; uv is free.