PrefectHQ/fastmcp · error · ValueError

Could not read manifest for skill: {skill_name}

Error message

Could not read manifest for skill: {skill_name}

What it means

`get_skill_manifest` reads the `skill://{name}/_manifest` resource from a skill server and raises this ValueError when the read returns an empty result — i.e. no content items came back for the manifest URI. It indicates the server produced nothing for the manifest resource rather than an error or malformed content.

Source

Thrown at fastmcp_slim/fastmcp/utilities/skills.py:104

async def get_skill_manifest(client: Client, skill_name: str) -> SkillManifest:
    """Get the manifest for a specific skill.

    Args:
        client: Connected FastMCP client
        skill_name: Name of the skill

    Returns:
        SkillManifest with file listing

    Raises:
        ValueError: If manifest cannot be read or parsed
    """
    manifest_uri = f"skill://{skill_name}/_manifest"
    result = await client.read_resource(manifest_uri)

    if not result:
        raise ValueError(f"Could not read manifest for skill: {skill_name}")

    content = result[0]
    if isinstance(content, mcp_types.TextResourceContents):
        try:
            manifest_data = json.loads(content.text)
        except json.JSONDecodeError as e:
            raise ValueError(f"Invalid manifest JSON for skill: {skill_name}") from e
    else:
        raise ValueError(f"Unexpected manifest format for skill: {skill_name}")

    try:
        return SkillManifest(
            name=manifest_data["skill"],
            files=[
                SkillFile(path=f["path"], size=f["size"], hash=f["hash"])
                for f in manifest_data["files"]
            ],
        )

View on GitHub (pinned to 1f02114297)

Solutions

1. Verify the skill exists on the target server (list skills / check the server's skill registry) before downloading.
2. Check the skill server implementation actually registers a non-empty `_manifest` resource for the skill name.
3. Confirm the skill name spelling — a mismatched name may resolve to an empty resource instead of a not-found error.

Example fix

// before
manifest = await get_skill_manifest(client, "code-review")  # skill doesn't exist
// after
skills = await list_skills(client)
if "code-review" in skills:
    manifest = await get_skill_manifest(client, "code-review")

async def skill_manifest_available(client, skill_name: str) -> bool:
    result = await client.read_resource(f"skill://{skill_name}/_manifest")
    return bool(result)

def has_text_content(result) -> bool:
    import mcp.types as mcp_types
    return bool(result) and isinstance(result[0], mcp_types.TextResourceContents)

try:
    manifest = await get_skill_manifest(client, skill_name)
except ValueError as e:
    if "Could not read manifest" in str(e):
        raise SkillNotFoundError(skill_name) from e
    raise

When it happens

Trigger: Calling `get_skill_manifest` (directly or via `download_skill`) against a skill server that returns an empty content list for the `_manifest` resource — e.g. a server that doesn't implement the `_manifest` resource properly, returns an empty string content set, or a stub/test server misconfigured for the skill name.

Common situations: Pointing at a server that lacks the skill entirely but returns empty rather than an error, custom skill-server implementations that don't populate the manifest resource, or proxy/middleware layers that strip resource contents.

Related errors

• Invalid manifest JSON for skill: {skill_name}
• Unexpected manifest format for skill: {skill_name}
• Invalid manifest format for skill: {skill_name}
• File not found: {self.file_path}
• Skill name {skill_name!r} would escape the target directory

AI-assisted analysis of PrefectHQ/fastmcp@1f02114297 (2026-08-29). Data as JSON: /api/errors/c70c98a655ced6bf. Report an issue: GitHub.