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

Annotations already cover read-only, idempotent, non-destructive and closed-world, but the description adds a genuinely new contract: it never raises a protocol error and instead returns an {"error": ...} object with a fix hint, which tells the agent how to recover. It does restate read-only/idempotent, which is redundant with the annotations, keeping it out of the top band.

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?

Purpose is front-loaded in the first sentence and the usage/exclusion guidance follows logically. The inline input and output JSON examples are large and partly duplicate the schema, which costs some tightness but still earns its place as illustration.

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?

Covers trigger, exclusions, error behavior, retry safety and a sample output despite an output schema already existing. For a two-parameter read-only tool, nothing an agent needs to call it correctly is missing.

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%, so the baseline is 3, but the description adds real semantics: which segments get more or less time, and a worked example mapping ['Warm-up','Direct instruction','Practice','Exit ticket'] to concrete durations. That clarifies how the segment list is interpreted beyond the schema's name-only description.

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?

Uses a specific verb+resource ('Split a class period into timed segments') and states the weighting behavior, so the agent knows exactly what it produces. It also names the cases it does not cover (multi-day planning, spaced review), separating it from adjacent planning tools.

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?

Gives an explicit trigger ('Use when one fixed class period must be divided into segments') plus exclusions and the sibling that handles the excluded case (spaced_repetition_plan). No inference needed to choose this tool over alternatives.

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