PrefectHQ/fastmcp · error · RuntimeError

Failed to sync dependencies: {e.stderr}

Error message

Failed to sync dependencies: {e.stderr}

What it means

prepare() finishes with `uv sync --project <dir>` to install everything declared in the generated project, and raises this RuntimeError on nonzero exit. Earlier `uv add` steps only record dependencies (--no-sync); this final sync is where actual install/resolve errors surface, including lockfile and wheel-build failures. uv's stderr is in the message.

Source

Thrown at fastmcp_slim/fastmcp/utilities/mcp_server_config/v1/environments/uv.py:269

                )
            except subprocess.CalledProcessError as e:
                logger.error(f"Failed to add editable packages: {e.stderr}")
                raise RuntimeError(
                    f"Failed to add editable packages: {e.stderr}"
                ) from e

        # Final sync to install everything
        logger.info("Installing dependencies...")
        try:
            subprocess.run(
                ["uv", "sync", "--project", str(output_dir)],
                check=True,
                capture_output=True,
                text=True,
            )
        except subprocess.CalledProcessError as e:
            logger.error(f"Failed to sync dependencies: {e.stderr}")
            raise RuntimeError(f"Failed to sync dependencies: {e.stderr}") from e

        logger.info(f"Environment prepared successfully in {output_dir}")

View on GitHub (pinned to 1f02114297)

Solutions

1. Read e.stderr for the failing package: add missing system build tools/headers (gcc, python3-dev) or pin a version with prebuilt wheels for your Python.
2. Fix transitive conflicts by loosening the offending pin reported in the resolution error.
3. Delete the generated environment and retry fresh: `rm -rf <output_dir>` (or the fastmcp-env dir) so uv regenerates pyproject.toml/uv.lock.
4. Clear uv's cache if installs corrupt: `uv cache clean`, then re-run.
5. Try a different `python` version in the environment config where all your deps ship wheels.

Example fix

// before (build failure on Python 3.13)
"python": "3.13", "dependencies": ["somepkg==1.2"]  # no cp313 wheel, needs old Cython

// after
"python": "3.12", "dependencies": ["somepkg>=1.3"]  # wheels available

import subprocess, tempfile

# dry-run the full install in a scratch project before prepare()
with tempfile.TemporaryDirectory() as d:
    subprocess.run(["uv", "init", "--project", d, "--name", "probe"], check=True, capture_output=True)
    r = subprocess.run(["uv", "add", "fastmcp", *(cfg.environment.dependencies or []), "--project", d], capture_output=True, text=True)
    if r.returncode != 0:
        raise SystemExit(f"sync would fail: {r.stderr}")

try:
    await env.prepare(output_dir)
except RuntimeError as e:
    if str(e).startswith("Failed to sync dependencies"):
        # regenerate the env project from scratch, then retry once
        shutil.rmtree(output_dir, ignore_errors=True)
        await env.prepare(output_dir)
    else:
        raise

When it happens

Trigger: prepare() when the combined dependency set (dependencies + requirements + editable + fastmcp) resolves at add-time but fails at install: transitive conflicts discovered during lock, a wheel/source build failure, unavailable Python matching the pinned version, or a corrupted cache/lockfile in the persistent output_dir.

Common situations: A dependency has no wheel for the pinned Python (e.g. older package on 3.13) and source build fails for missing compilers/headers; conflicting transitive pins; a stale fastmcp-env project dir with a hand-edited pyproject.toml or broken uv.lock; disk full or read-only cache directory.

Related errors

• Failed to pin Python version: {e.stderr}
• Failed to add dependencies: {e.stderr}
• Failed to add requirements: {e.stderr}
• Failed to add editable packages: {e.stderr}
• uv is not installed. Please install it with: curl -LsSf http

AI-assisted analysis of PrefectHQ/fastmcp@1f02114297 (2026-08-29). Data as JSON: /api/errors/04ef09c6abb106a3. Report an issue: GitHub.