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

With no annotations, the description carries the burden and does well: it discloses that done=true affects every session of a multi-session habit, that done=false clears the day (an overwrite/erase behavior an agent must know about), and that sessions overrides done. It omits return shape and any permission/error behavior, which keeps it short of a 5.

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?

One dense sentence that front-loads the primary action and then covers the edge cases in order of likely need. It is slightly run-on but no clause 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 4-parameter mutation tool with no annotations and no output schema, the description covers the tricky semantics well but leaves gaps: what 'id' identifies, the expected date format, and whether logging is idempotent or overwrites prior entries. Adequate but not complete.

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 only 50% (id and date have no schema descriptions), so the description must compensate, and it does for the ambiguous parts: it establishes that date defaults to today and clarifies the interaction between done and sessions beyond the schema's own wording. The id parameter and the accepted date format are still undocumented.

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 a habit done') plus the core scoping (a date, defaulting to today). An agent can tell this is a per-day habit log rather than a habit definition change, though it never names update_habit/create_habit as the alternative, so sibling differentiation is left implicit.

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 does explain the three usage modes clearly: done=true marks all sessions, done=false clears the day, and sessions logs a partial count. That is useful implied guidance, but there is no explicit statement of when to reach for update_habit or create_habit instead, nor any prerequisite context.

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