Validate the manifest, reject on failure, and your plugin client is non-conformant

Agent Plugins 1.0.0 ships a JSON Schema for plugin.json . It sets additionalProperties: false . So the obvious loader is four lines: const manifest = JSON . parse ( await readFile ( join ( dir , ' plugin.json ' ))); if ( ! validate ( manifest )) return reject ( ' invalid manifest ' ); That loader is wrong, and the specification says so in a sentence most people never reach. §5.2: Clients MUST report and ignore each unknown field and MUST continue loading the plugin if the manifest otherwise satisfies this section. An unknown top-level field is a schema violation you have to tolerate . §8.1 says the same for an extensions field that isn't an object. Every other schema violation is fatal. So a validator gives you one boolean where the spec wants three different outcomes, and the natural implementation is non-conformant in exactly two cases and correct everywhere else. That is the kind of bug that doesn't show up in your tests. It shows up as a plugin that works in one client and not another, six months later, in someone else's bug tracker. This has already happened, repeatedly I went looking before building anything. In the last few months: Codex loaded any directory with a root plugin.json through its Agent Plugins loader, which had no hook support. Every hook in .codex-plugin/plugin.json silently stopped running. Two plugins were dead for a week before anyone noticed. oh-my-pi routed packages declaring an agent-plugins.org $schema to a strict provider that dropped any SKILL.md with an extra frontmatter key. Downstream, a plugin went from 33 skills to 3. The fix was to delete $schema from the manifest, so conforming to the standard cost them the standard. dotnet/skills shipped manifests with no $schema and with skills , agents and mcpServers as top-level fields. Kiro refused them. Adding $schema got past the rejection and then loaded the package with every functional component excluded. VS Code , the largest shipping client, has no validation surface at all. Its trou