Call an app function

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

Annotations already establish idempotency and non-read-only behavior. The description adds valuable details beyond that: argument validation against declared params, failure responses including the declared schema, and success replies containing the return value plus a per-write receipt. These behaviors are not present in annotations, enriching the agent's expectations without contradiction.

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 description is two sentences, front-loaded with the core purpose, then adding behavioral details. Every clause adds information: purpose, no-UI nature, validation, error handling, and return content. There is no redundancy or filler.

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 tool with 5 parameters, nested args, and an output schema, the description covers the essential operational aspects: what it does, how args are validated, how failures are reported, and what the success reply contains. It omits nuances like the actor enum's effect, but the output schema and overall clarity keep it reasonably 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?

With only 20% schema description coverage, the description carries a heavier burden. It does clarify that 'args' are checked against the app's declared parameters, giving meaning to that field. However, it does not explain 'actor' or add any semantics beyond what the names imply for 'app' and 'function'. This is adequate but not fully compensating for the low coverage.

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 starts with a specific verb-resource pairing: 'Run a function an app declares (manifest.functions)'. The added phrase 'data in, data out, no UI needed' immediately distinguishes this from UI-oriented siblings like open_app and app_html, making the tool's scope unmistakable.

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?

The description provides clear context that this is for headless function execution ('no UI needed') and even gives retry guidance after failures ('failures return the declared schema so the retry needs no extra read'). However, it does not explicitly name alternative tools or state when not to use it, stopping short of full exclusion guidance.

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