sgl-project/sglang · error · RuntimeError

failed to build {crate.python_module} with Cargo

Error message

failed to build {crate.python_module} with Cargo

What it means

`cargo build` failed or could not be spawned while building the crate; the original OSError/CalledProcessError is chained. The Cargo stderr appears in the traceback's __cause__.

Source

Thrown at python/sglang/srt/rust_extensions/loader.py:350

    command = [
        "cargo",
        "build",
        "--release",
        "--locked",
        "--package",
        crate.package,
    ]
    if crate.features:
        command.extend(("--features", ",".join(crate.features)))

    environment = os.environ.copy()
    environment["CARGO_TARGET_DIR"] = os.fspath(target_dir)
    environment["PYO3_PYTHON"] = sys.executable
    logger.info("Building %s with `%s`", crate.python_module, " ".join(command))
    try:
        subprocess.run(command, cwd=crate.workspace, env=environment, check=True)
    except (OSError, subprocess.CalledProcessError) as exc:
        raise RuntimeError(f"failed to build {crate.python_module} with Cargo") from exc

    release_dir = target_dir / "release"
    if target := environment.get("CARGO_BUILD_TARGET"):
        release_dir = target_dir / target / "release"
    artifact = release_dir / _cargo_library_filename(crate.library)
    if not artifact.is_file():
        raise FileNotFoundError(
            f"Cargo completed but did not produce the expected artifact {artifact}"
        )
    return artifact


def _cargo_library_filename(library: str) -> str:
    if sys.platform == "win32":
        return f"{library}.dll"
    if sys.platform == "darwin":
        return f"lib{library}.dylib"
    return f"lib{library}.so"

View on GitHub (pinned to 0132848349)

Solutions

1. Read the chained CalledProcessError/underlying cargo output for the compile error and fix it
2. Run `cargo build --locked` manually in the workspace to reproduce
3. Ensure network access for crates.io (or a vendored/offline mirror) and a compatible Rust version
4. Use the prebuilt wheel to avoid compiling

subprocess.run(["cargo", "build", "--locked"], cwd=workspace, check=True)  # preflight the build

try:
    load_rust_extension(...)
except RuntimeError as e:
    cause = e.__cause__
    log_cargo_output(cause)
    raise

When it happens

Trigger: load_rust_extension triggering a source build where cargo exits non-zero (compile error, dependency fetch failure) or cannot execute (missing binary).

Common situations: Rust compile errors in the crate; offline environment preventing dependency downloads; cargo absent from PATH; incompatible rustc version vs pyo3.

Related errors

• Rust sources under {crate.workspace} changed during the buil
• Rust workspace for {python_module} was not found at {workspa
• {lockfile} is required for reproducible `cargo build --locke
• {manifest} declares python-module {python_module!r} but must
• Cargo completed but did not produce the expected artifact {a

AI-assisted analysis of sgl-project/sglang@0132848349 (2026-08-28). Data as JSON: /api/errors/6dcb2b5b098d49f5. Report an issue: GitHub.