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

Annotations already declare the mutation profile (readOnlyHint=false, idempotentHint=true, destructiveHint=false), so the bar is lower. The description adds two things annotations do not carry: the operation is self-scoped ("for yourself") and requires a Premium API key. It does not state what happens on a repeat call beyond the idempotent hint.

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?

Two short sentences, zero filler, with the core action front-loaded and the prerequisite trailing. Nothing needs trimming and nothing essential is buried.

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 one-param mutation with a documented schema and full annotations, the definition covers purpose, scope, and the auth gate. It lacks only edge-case behavior (already-complete lessons, error responses), and no output schema exists to cover return values.

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% and the slug parameter is fully documented in the schema (format, length bounds, accepted identifier forms). The description adds only the implicit constraint that the slug should reference a published lesson, which is modest value on top of the schema.

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?

States a specific verb (mark complete) and resource (lesson), and scopes it to the caller ("for yourself") and to published lessons. It is clearly distinguishable from the read-oriented siblings like get_lesson and list_lessons, though it does not explicitly name an alternative.

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?

"Premium API key required" supplies a real prerequisite, but the description gives no when-to-use guidance, no mention of what happens if the lesson is already complete, and no routing to or away from any sibling tool. Usage is implied by the name and the premium note.

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