PrefectHQ/fastmcp · error · RuntimeError

Failed to add dependencies: {e.stderr}

Error message

Failed to add dependencies: {e.stderr}

What it means

prepare() runs `uv add <deps> fastmcp --no-sync --project <dir>` and wraps any nonzero exit in this RuntimeError. It means uv failed to resolve or add the configured `dependencies` (plus fastmcp itself) to the generated project's pyproject.toml. uv's stderr is embedded in the message.

Source

Thrown at fastmcp_slim/fastmcp/utilities/mcp_server_config/v1/environments/uv.py:207

        dependencies = (self.dependencies or []) + ["fastmcp"]
        logger.debug(f"Adding dependencies: {', '.join(dependencies)}")
        try:
            subprocess.run(
                [
                    "uv",
                    "add",
                    *dependencies,
                    "--no-sync",
                    "--project",
                    str(output_dir),
                ],
                check=True,
                capture_output=True,
                text=True,
            )
        except subprocess.CalledProcessError as e:
            logger.error(f"Failed to add dependencies: {e.stderr}")
            raise RuntimeError(f"Failed to add dependencies: {e.stderr}") from e

        # Add requirements file if specified
        if self.requirements:
            logger.debug(f"Adding requirements from {self.requirements}")
            # Resolve requirements path relative to current directory
            req_path = Path(self.requirements).resolve()
            try:
                subprocess.run(
                    [
                        "uv",
                        "add",
                        "-r",
                        str(req_path),
                        "--no-sync",
                        "--project",
                        str(output_dir),
                    ],
                    check=True,

View on GitHub (pinned to 1f02114297)

Solutions

1. Read e.stderr: fix the exact package/spec uv reports (typo, nonexistent package, invalid specifier).
2. Relax or remove the conflicting version pin reported in the resolution error (e.g. fastmcp>=2.0 instead of fastmcp==2.3.1 if it conflicts).
3. Verify the package exists on the configured index; add private indexes via UV_INDEX_URL/--index if needed.
4. Test resolution outside fastmcp: `uv add <dep> --no-sync --project <tmpdir>` to reproduce and iterate quickly.
5. Temporarily remove entries from environment.dependencies to isolate which one breaks resolution.

Example fix

// before
"dependencies": ["fastmcp==2.3.1", "pandas<2.0"]  # conflicts with deps needing pandas>=2

// after
"dependencies": ["fastmcp>=2.0", "pandas>=2.0"]

import subprocess, tempfile, pathlib

deps = cfg.environment.dependencies or []
with tempfile.TemporaryDirectory() as d:
    subprocess.run(["uv", "init", "--project", d, "--name", "probe"], check=True, capture_output=True)
    r = subprocess.run(["uv", "add", *deps, "fastmcp", "--no-sync", "--project", d], capture_output=True, text=True)
    if r.returncode != 0:
        raise SystemExit(f"deps unresolvable: {r.stderr}")

try:
    await env.prepare(output_dir)
except RuntimeError as e:
    if str(e).startswith("Failed to add dependencies"):
        print(e)  # stderr names the unresolvable spec; fix config, then retry
    else:
        raise

When it happens

Trigger: prepare() with environment.dependencies containing a package name/spec uv cannot resolve: nonexistent package, an invalid PEP 508 specifier, or a version that conflicts with fastmcp or another entry (uv resolves all adds together).

Common situations: Typo'd package name ("reqests"); pinning fastmcp==1.x alongside an installed fastmcp 2.x wheel requirement; conflicting pins like httpx<0.24 with a dep requiring newer httpx; private packages not reachable from the configured index.

Related errors

• Failed to add requirements: {e.stderr}
• Failed to pin Python version: {e.stderr}
• Failed to add editable packages: {e.stderr}
• Failed to sync dependencies: {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/4effa35a665bb2de. Report an issue: GitHub.