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

Annotations already declare readOnlyHint=false, destructiveHint=true, idempotentHint=false and openWorldHint=true, so the safety profile is covered. The description adds the meaningful constraint that the lesson must still be Scheduled, but says nothing about what makes the operation destructive, duplicate-student behavior, or failure modes.

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?

A single sentence, front-loaded with the WRITE marker and the operation, followed by the endpoint. Nothing is wasted.

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 simple two-parameter write with no output schema, the definition covers purpose, precondition, and endpoint. It leaves unaddressed what happens when the lesson is not Scheduled or the student is already enrolled, which is a modest but real gap.

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 description coverage is 100% with only two required parameters (lesson_id, student_id), so the schema already carries the semantics. The description adds no syntax, format, or constraint detail beyond it, making 3 the correct baseline.

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 gives a specific verb+resource ('add a student to an existing lesson') plus a state precondition and the underlying endpoint, so an agent can distinguish it from create_lesson or get_lesson at a glance.

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?

It states a clear precondition for use ('the lesson must still be Scheduled'), which tells the agent when the call is valid. It does not name alternatives or say what to do when the precondition fails, so it stops short of full when/when-not guidance.

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