langchain-ai/deepagents · error · PluginManifestError

Invalid JSON syntax in {manifest_path}: {exc}

Error message

Invalid JSON syntax in {manifest_path}: {exc}

What it means

`load_manifest` reads the plugin manifest file and `json.loads` it. A `json.JSONDecodeError` (malformed JSON) is wrapped and re-raised as `PluginManifestError` naming the manifest path and the parser's position detail. All manifest consumers (`install_plugin`, `_validate_plugin_copy`, `_plugin_from_install_path`, `auto_update_plugins`) funnel through this, so a broken manifest blocks install/load/update.

Source

Thrown at libs/code/deepagents_code/plugins/manifest.py:278

    Args:
        root: Plugin root directory.
        fallback_name: Name to use only when deriving a manifest-less plugin.

    Returns:
        `(manifest, manifest_path, warnings)`.

    Raises:
        PluginManifestError: If the manifest exists but is invalid.
    """
    manifest_path = find_manifest_path(root)
    if manifest_path is None:
        return None, None, ()
    try:
        decoded = json.loads(manifest_path.read_text(encoding="utf-8"))
    except json.JSONDecodeError as exc:
        msg = f"Invalid JSON syntax in {manifest_path}: {exc}"
        raise PluginManifestError(msg) from exc
    except OSError as exc:
        msg = f"Could not read plugin manifest {manifest_path}: {exc}"
        raise PluginManifestError(msg) from exc
    if not isinstance(decoded, dict):
        msg = f"Plugin manifest {manifest_path} must be a JSON object"
        raise PluginManifestError(msg)
    raw = json_object(decoded)

    warnings: list[str] = []
    name = _validate_name(raw.get("name"), fallback=fallback_name)
    component_paths: dict[str, tuple[Path, ...]] = {}
    for field_name in _PATH_COMPONENT_FIELDS:
        declaration = raw.get(field_name)
        if declaration is None:
            continue
        if field_name in {"mcpServers", "hooks"} and isinstance(declaration, dict):
            continue
        paths = _resolve_component_paths(declaration, root, field_name, warnings)

View on GitHub (pinned to a1af029e6e)

Solutions

1. Open the manifest path from the error message and fix the JSON syntax at the indicated position.
2. Run `python -m json.tool <manifest>` to pinpoint the syntax error.
3. If the file was truncated, restore it from the upstream plugin repo and reinstall.
4. Avoid JSON5-style features (comments, trailing commas) — this parser is strict.

Example fix

// before (plugin.json)
{"name": "x", "version": "1.0",}
// after
{"name": "x", "version": "1.0"}

import json
from pathlib import Path
p = Path("plugin.json")
json.loads(p.read_text(encoding="utf-8"))  # raises JSONDecodeError early if malformed

def is_valid_json(text: str) -> bool:
    import json
    try:
        json.loads(text)
        return True
    except json.JSONDecodeError:
        return False

try:
    manifest, _, _ = load_manifest(root)
except PluginManifestError as exc:
    if "Invalid JSON syntax" in str(exc):
        print(f"repair manifest: {exc}")  # exc names path and parse position

When it happens

Trigger: Any call into `load_manifest` for a plugin root whose discovered manifest file (`plugin.json` or other supported path) contains syntactically invalid JSON — trailing commas, comments, unquoted keys, truncated file.

Common situations: Hand-edited manifest left a trailing comma; a merge conflict marker was committed; an interrupted download/write truncated the JSON; someone added `//` comments to what must be strict JSON.

Understand the failure class

• Parsing and encoding errors: unexpected token, malformed input — why parsers reject input and how to find the real culprit.

Related errors

• Plugin manifest {manifest_path} must be a JSON object
• Invalid plugin name: {name!r}
• Could not read plugin manifest {manifest_path}: {exc}
• Failed to parse {SERVER_ENV_PREFIX}{suffix} as JSON: {exc}.
• Invalid {SERVER_ENV_PREFIX}{suffix}: expected a JSON string

AI-assisted analysis of langchain-ai/deepagents@a1af029e6e (2026-08-29). Data as JSON: /api/errors/9b7793a4861fb0c8. Report an issue: GitHub.