dotnet/runtime · error · RuntimeError

Unknown host OS.

Error message

Unknown host OS.

What it means

Raised by determine_jit_name() in jitutil.py at the final return when host_os is not 'osx', 'linux', or 'windows'. This is the host-side OS check used to pick the JIT file extension (.dylib/.so/.dll) and prefix.

Solutions

1. Pass host_os as exactly one of: osx, linux, windows.
2. Detect and normalize the host OS before calling (map 'darwin'->'osx', 'win32'->'windows').
3. Extend the function if you genuinely need to support another host OS.

Example fix

// before
name = determine_jit_name(host_os='darwin')  # 'Unknown host OS.'
// after
host_os = {'darwin':'osx','win32':'windows','linux':'linux'}.get(platform.system().lower(),'linux')
name = determine_jit_name(host_os=host_os)

VALID_HOST_OS = {"osx", "linux", "windows"}
if host_os not in VALID_HOST_OS:
    raise SystemExit(f"host_os must be one of {sorted(VALID_HOST_OS)}, got {host_os!r}")

def is_supported_host_os(s: str) -> bool:
    return s in {"osx", "linux", "windows"}

When it happens

Trigger: Calling determine_jit_name with host_os set to any value other than osx/linux/windows (None, 'mac', 'win', 'freebsd', etc.).

Common situations: platform.system() mapping producing an unexpected string; caller passing a capitalized OS; script run on an unsupported host OS without overriding host_os.

Related errors

• Unknown target OS.
• Unknown OS.
• Unsupported OS.
• Azure failed to download
• {coreclr_args.mode}

AI-assisted analysis of dotnet/runtime@60108ba66e (2026-08-10). Data as JSON: /api/errors/ab459cc7ac8ccd82. Report an issue: GitHub.

Appendix: source

Thrown at src/coreclr/scripts/jitutil.py:515

        elif target_arch.startswith("wasm"):
            jit_os_name = "universal"
        elif target_os == "windows":
            jit_os_name = "win"
        elif target_os == "osx" or target_os == "linux":
            jit_os_name = "unix"
        else:
            raise RuntimeError("Unknown target OS.")

        jit_base_name = 'clrjit_{}_{}_{}'.format(jit_os_name, target_arch, host_arch)

    if host_os == "osx":
        return "lib" + jit_base_name + ".dylib"
    elif host_os == "linux":
        return "lib" + jit_base_name + ".so"
    elif host_os == "windows":
        return jit_base_name + ".dll"
    else:
        raise RuntimeError("Unknown host OS.")


def get_deepest_existing_directory(path):
    """ Given a path, find the deepest existing directory containing it. This
        might be the path itself, or a parent directory. If no such directory
        is found, None is returned.

    Args:
        path (str) : path to check

    Returns:
        As described above
    """
    path = os.path.abspath(path)
    lastPath = ""

    # When os.path.dirname() is called on the root directory ("C:\\" on Windows or "/" on Linux),
    # if returns itself.

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