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

Annotations already declare the mutation/idempotent/destructive profile, but the description adds real behavioral context: the toggle semantics of mark vs unmark, the unknown-name error behavior (returns the user's activity names), and the future-day refusal. That is genuine added value beyond structured fields, though return format is left to the output schema.

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?

The core verb and example are front-loaded, and the sentence carries dense, useful information. Slightly verbose with the parenthetical asides, but every clause earns its place.

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?

An output schema exists, so return values needn't be covered, and annotations handle the safety profile. The description rounds this out with error handling and the activity-must-exist prerequisite, making it complete for calling the tool correctly.

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 all four parameters (date, done, today, activity) are already documented in the schema. The description reinforces timezone and day-restriction semantics but adds little syntax beyond what the schema states, so the baseline 3 is appropriate.

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 pair (mark/unmark) and resource (an activity day), reinforced by a concrete example ('I played football yesterday'). It explicitly distinguishes itself from the sibling log_session by stating marks are not workouts.

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?

Names the alternative (log_session) and the condition that routes to it, plus the constraint 'one day per call; past days and today only'. It also explains what to do when the activity doesn't exist, leaving little to inference.

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