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

Annotations only declare readOnly=false, destructive=false, openWorld=true; the description adds materially useful behavior: a fixed 10-credit cost, subscription-dependent output size, and a returned project id intended for wait_for_image_project (async job pattern). Not covered: whether credits are refunded on failure, expected job duration, or the output artifact format — though the output schema covers the return shape.

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?

Two short paragraphs with zero filler: policy guardrails, then mechanics (cost, output, next step). The core action is stated only after the policy preamble, which slightly buries the purpose, but the total length is well matched to a tool of this complexity.

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?

Given an output schema and 100% schema coverage, the description supplies what the structured fields cannot: cost, subscription-dependent sizing, the async follow-up call, and legal/ethical restrictions. An agent can select, invoke, and continue the workflow from this description plus the schema.

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 100% and the nested assets object (face_swap_mode enum, face_mappings, source_file_path, target_file_path) is fully documented inline, so the baseline is 3. The description's phrase 'source face image or selected face mappings' echoes the schema's two modes without adding format or constraint detail.

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 ('Swaps faces in a supplied photo') plus the two input modes (source face image or face mappings), which is far more than the title alone. It does not distinguish itself from close siblings such as edit_face_in_photo, swap_head_in_photo, or swap_face_in_video, so an agent must infer the photo-vs-video split from the name.

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 first paragraph sets hard usage constraints (own likeness only; no impersonation, deception, sexual content, or minors), and the second names the follow-up tool (wait_for_image_project), effectively documenting the workflow. However, it gives no guidance on when to pick this over alternatives like edit_face_in_photo or swap_head_in_photo, and the exclusions are content policy rather than tool-selection guidance.

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