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

The annotations already establish the safety profile (a non-read-only, idempotent, non-destructive, closed-world mutation), so the description only needs to add state behavior. It does add one such detail: the tool acts on an in-progress activity and apparently records no duration. It does not say what happens if no exercise is open, or whether the end time is rejected when it is earlier than the start time.

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 definition is a single front-loaded sentence with no padding. Its brevity works against it only in the trailing parenthetical, which introduces ambiguity rather than clarification.

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 state-closing mutation with one fully documented optional parameter and no output schema, the definition is minimally sufficient. It leaves out what happens when there is no open exercise, and it does not make clear what '(sin duración)' means for the recorded entry, which is the main open question for an agent calling this tool.

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?

There is one optional parameter, and the schema describes it at 100% coverage, including the accepted formats, the default of 'right now', and the rule against future times. The description adds nothing about 'time', which is acceptable because the schema already handles the parameter, but the baseline is 3 rather than 4 for a non-empty parameter set.

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: 'Marca el final de una actividad apuntada en curso' frames this tool as closing an already-logged activity, which makes it easy to tell apart from the log_exercise and delete_entry siblings. The one weakness is the ambiguous parenthetical '(sin duración)', which could be read either as a constraint on the activity type or as a note on the recorded result.

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?

Saying that the activity is 'en curso' implies a prerequisite: an open entry must already exist. That prerequisite is only implied, though; the description never says what to do if nothing is in progress, and it never explicitly contrasts this tool with log_exercise or delete_entry. An agent can infer the context, but it has to do so itself.

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