Enable an extension

Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?

With no annotations, the description carries the full transparency burden and explicitly discloses the mutation: 'Adds the id to the project's `extensions` and, with a url, records it in `extensionURLs`'. It also reveals the editor fetch behavior for custom extensions via the loader URL. It stops short of a 5 because it says nothing about error conditions (unknown id, unreachable url) or idempotency of re-registering the same extension.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Is the description appropriately sized, front-loaded, and free of redundancy?

Three sentences with the purpose and prerequisite front-loaded in the first. Every sentence earns its place: even the inline built-in ID list, though dense, is necessary reference data the agent needs to invoke the tool correctly. There is no filler and no repetition of schema content.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

For a 2-parameter mutation with no output schema and no annotations, the description covers what the tool does, when it is required, how to call it in both branches, and exactly what project state it mutates. What's missing is edge-case behavior (error handling for bad IDs or failed fetches, repeated registration) and any mention of the return value, which are secondary for a simple command-like mutation.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

Schema coverage is 100%, placing the baseline at 3, but the description adds genuine value by enumerating all 11 built-in extension IDs — far beyond the schema's single 'e.g. "pen"' example. It also makes the conditional dependency explicit (url needed only for custom/third-party extensions) and justifies url's purpose ('so the editor can fetch it'), which the schema's one-liner doesn't convey.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Does the description clearly state what the tool does and how it differs from similar tools?

The description opens with a specific verb and resource — 'Register an extension on the project' — and states the concrete behavioral consequence ('its blocks load and show in the palette'). It also names the prerequisite contract ('required before using any `<id>_…` extension block'), and since no sibling tool covers extension registration, an agent can select it unambiguously.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Does the description explain when to use this tool, when not to, or what alternatives exist?

The description gives an explicit trigger condition: enabling is required before any `<id>_…` block can be used, which tells the agent when to call it. It then branches the invocation logic — built-in extensions need only `id`, custom extensions also need `url` — providing clear conditional guidance. It doesn't name alternative tools or when-not conditions, but no sibling handles extensions, so the exclusion is implicit.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.