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

Annotations already cover the safety profile (readOnlyHint=false, destructiveHint=false, openWorldHint=false), so credit here goes to extra context: adding to a lane affects only the people in it, times are local to the place, and flights/transports carry a separate arrival point with its own local time. That is real behavioral detail beyond the annotations.

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?

Three sentences, front-loaded with the core action and the lane caveat, and each sentence carries distinct information about scoping, time zones, and multi-leg trips. It's dense and slightly run-on, but nothing 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?

With 15 parameters, annotations, and an output schema, the description covers the non-obvious parts: lane targeting, local-time semantics, and arrival places. Minor gaps remain — no mention of how date/endDate interact for multi-day stays, or of validation behavior.

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 80%, so the baseline is 3, but the description adds provenance and semantics the schema doesn't: laneId comes from get_trip's splits, placeProviderId from search_places, and that place's time zone is used unless overridden. The endPlaceProviderId/endPlace pairing for flights is also explained.

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 uses the specific verb 'Adds' with the resource 'activity' and immediately scopes it: the shared plan by default, or a lane for a subset of people. That lane/whole-plan distinction also separates it from update_activity and move_activity, which operate on an existing activity.

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 routes the agent to other tools for the data it needs — laneId 'from get_trip's splits', placeProviderId 'from search_places' — and explains when to leave laneId out ('for everyone'). It doesn't state when to prefer this over update_activity or a list/wishlist tool, but the intent is clear.

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