langchain-ai/deepagents · error · MarketplaceError

Cannot install {plugin_id}: {exc}

Error message

Cannot install {plugin_id}: {exc}

What it means

After source materialization, install_plugin calls load_manifest; a PluginManifestError (missing or invalid manifest) is wrapped into MarketplaceError with 'Cannot install <id>: ...'. The plugin's source exists but its manifest does not parse or fails required validation, so installation is refused.

Source

Thrown at libs/code/deepagents_code/plugins/discovery.py:232

        Discovered plugin instance loaded from the cache path.

    Raises:
        MarketplaceError: If the marketplace/plugin cannot be resolved, the
            source is unsupported, or the cached plugin fails to load.
    """
    load_installed_plugins(strict=True)
    load_enabled_plugin_ids(strict=True)
    marketplace, entry = _resolve_marketplace_and_entry(plugin_id)
    rejections: list[str] = []
    source_root = materialize_plugin_source(marketplace, entry, rejections=rejections)
    if source_root is None:
        raise MarketplaceError(unresolved_source_message(plugin_id, entry, rejections))

    try:
        manifest, _manifest_path, manifest_warnings = load_manifest(
            source_root, fallback_name=entry.name
        )
    except PluginManifestError as exc:
        msg = f"Cannot install {plugin_id}: {exc}"
        raise MarketplaceError(msg) from exc

    for warning in manifest_warnings:
        logger.debug("Plugin install warning for %s: %s", plugin_id, warning)

    version = manifest.version if manifest is not None else None
    cache_path = cache_and_register_plugin(
        plugin_id,
        source_root,
        version=version,
        validate=partial(
            _validate_plugin_copy,
            plugin_id=plugin_id,
            fallback_name=entry.name,
        ),
    )

View on GitHub (pinned to a1af029e6e)

Solutions

1. Open the plugin's manifest in the source root and fix the reported field/syntax error.
2. Validate the manifest against the current schema expected by load_manifest.
3. Re-materialize the plugin source (refresh the marketplace) in case the copy is incomplete or stale.
4. If a schema change broke an older manifest, update the plugin or pin a compatible version.
5. As a user, report the broken manifest to the plugin maintainer with the embedded error text.

Example fix

// before (manifest missing version)
{"name": "widget"}
// after
{"name": "widget", "version": "1.2.0"}

import json
from pathlib import Path

def manifest_ok(root: Path) -> bool:
    mf = next(root.glob("manifest.*"), None)
    if mf is None:
        return False
    try:
        data = json.loads(mf.read_text())
    except (ValueError, OSError):
        return False
    return bool(data.get("name")) and bool(data.get("version"))

assert manifest_ok(Path("plugins/widget")), "manifest missing or invalid"

def has_valid_manifest(data: dict) -> bool:
    return isinstance(data.get("name"), str) and isinstance(data.get("version"), str)

from deepagents_code.plugins.discovery import MarketplaceError, install_plugin

try:
    install_plugin("widget@internal")
except MarketplaceError as exc:
    if str(exc).startswith("Cannot install"):
        print("fix the plugin manifest:", exc)  # embedded PluginManifestError detail
    else:
        raise

When it happens

Trigger: load_manifest(source_root, fallback_name=entry.name) raises PluginManifestError — malformed manifest file, missing required fields, invalid name/version values, or an unreadable manifest file in the materialized source root.

Common situations: A plugin author shipped a broken manifest (bad JSON/TOML, missing name/version); a manifest schema change in a newer code version rejecting older manifests; the materialized copy lost the manifest file; a release process stripping non-package files.

Related errors

• Invalid plugin name: {name!r}
• modes can only be provided when agent is a factory
• models can only be provided when agent is a factory
• -32602
• Could not parse embedded resource block. Block expected eith

AI-assisted analysis of langchain-ai/deepagents@a1af029e6e (2026-08-29). Data as JSON: /api/errors/0221205a1c57df3b. Report an issue: GitHub.