Claude plugin validation failed? Check the package first

When Claude refuses a plugin with a validation error, the cause is almost always inside the package, not in Claude. These are real errors reproduced on Claude Code v2.1.296 (October 2026) with deliberately broken packages, and the fixes that clear them.

Error 1: the marketplace file is invalid

Failed to parse marketplace file at .../marketplace.json: Invalid schema: owner: Invalid input: expected object, received undefined

A marketplace's .claude-plugin/marketplace.json must include an owner object. A file with only name and plugins fails at the add step, before any plugin is even read. Fix: add an owner block, for example "owner": {"name": "Your Name", "email": "you@example.com"}.

Error 2: the plugin manifest fails validation

Failed to install plugin ... has an invalid manifest file at .../plugin.json. Validation errors: name: Plugin name cannot contain spaces. Use kebab-case (e.g., "my-plugin")

The CLI reads .claude-plugin/plugin.json and validates each field. The name must be kebab-case: lowercase letters, numbers and hyphens. A name like "bad one with spaces!!!" is rejected with the exact field named. Fix: rename to the kebab-case form (bad-one) and re-install. The same mechanism catches other malformed fields, so read the validation lines one at a time; each names its field.

The non-error that surprises people

A plugin directory with NO .claude-plugin/plugin.json at all does not fail. It installs, with the name taken from the marketplace entry and the version shown as "unknown" in claude plugin list. The manifest is optional for a minimal plugin; it becomes necessary when you want a proper version, description or metadata. So "missing manifest" is not why an install fails - look for a present-but-invalid manifest instead.

The check order

1. Read the validation lines: every one names a field and the rule it broke.
2. Validate the JSON itself (a trailing comma breaks parse before schema checks).
3. Marketplace problems appear at marketplace add; plugin problems appear at install. Note which step failed.
4. Fix the package, then claude plugin marketplace update <name> and retry - a cached copy keeps the old error alive otherwise. More on stale-cache symptoms in marketplace not found.

Building a plugin and want to start from a valid base? claude plugin init scaffolds one - see create a minimal Claude Code plugin.

Sources

• Plugins reference - Claude Code Docs
• Errors reproduced on Claude Code v2.1.296, October 11, 2026