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

Annotations already disclose write semantics (readOnlyHint=false), non-destructiveness, and open-world behavior, so the bar is lower. The description adds genuinely useful traits beyond that: the internal unit conversion (km→m, min→s) applied before building the request body, and a documented live failure mode (HTTP 500 ValueInstantiationException when milliseconds are omitted) — non-obvious operational context an agent cannot get from structured fields.

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 critical constraint (startDatetime format) is front-loaded, which is good, but the same timestamp example is stated twice in near-identical form, and the final clause mixes instruction with a debugging anecdote in a run-on sentence. The content is dense and mostly earns its place; the duplication is real waste.

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 six-parameter, fully-required write tool with no output schema and no schema descriptions, the definition covers the risky fields but leaves half the surface undocumented — typeKey validity, timeZone format, and activityName constraints. An agent could call it correctly for the timestamp but would be guessing on three required inputs.

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 description coverage is 0%, so the description carries the full burden for six required parameters. It compensates well for three of them (distanceKm, durationMin, startDatetime, including exact format and conversion semantics), but timeZone, typeKey, and activityName are never explained — notably which type keys are valid and what timezone string format is expected.

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+resource ('Creates a manual activity') and names the primary inputs (distance in km, duration in minutes), so the agent knows exactly what the tool produces. It does not explicitly differentiate itself from the sibling 'create_manual_activity_from_json', which is the nearest alternative and is left unaddressed.

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?

There is no when-to-use guidance, no prerequisites, and no mention of the obvious alternative 'create_manual_activity_from_json' or when the JSON-based variant is preferable. The description explains mechanics only, leaving tool selection entirely to inference from the name.

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