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

Annotations already declare destructiveHint=true and readOnlyHint=false, so the mutation profile is covered. The description adds real behavioral context beyond that: the calendar hatches marked days, planned sessions there don't count as misses, and planning is prohibited. It also discloses that writes are immediate ('Saves at once'). It stops short of describing the response or any permission requirements.

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?

Front-loads the core action and packs several useful constraints into one paragraph. The heavy em-dash asides make it denser than necessary, but each clause (hatching, not-a-miss, don't-plan-there, run-of-days, clear semantics) carries distinct information.

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 destructive batch mutation with no output schema, the description covers purpose, scope, consequences, clearing, and the complementary note tool. The main omission is what the tool returns or confirms after saving.

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 75%, with the undocumented 'from' parameter implied by 'Takes a run of days'. The description adds value beyond the schema by explaining the reset semantics ('off:false / color:"none"') and constraining the color choice ('one of five colours the athlete uses for their own meaning').

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 and resource: mark calendar DAYS as 'not training' or paint them with one of five colours. The purpose is unambiguous and distinguishable from siblings like set_day_note, which the description positions as the complementary 'why' tool.

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?

Covers when to use it (trip, illness, injury), the downstream consequence ('a session planned there is not a miss, and you must not plan there'), the sibling to pair with ('to say WHY they are off, add a day note too'), and how to undo ('Clear with off:false / color:"none"'). Nothing important is left to inference.

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