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

Beyond the annotations, the description discloses important behavioral details: the tool generates from the reference image alone, never from entity_id, and that combining entity_id with an image reference fails outright. It also notes that photo_media_id remains account-bound. This is meaningful behavioral context that the annotations (readOnlyHint=false, destructiveHint=false) do not capture.

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 longer than average but well-structured with clear sections and bullet-like formatting. The core purpose is front-loaded, parameter semantics are grouped logically, and the 'why not entity_id' rationale earns its place because it prevents a known failure. Only minor redundancy keeps it from a 5.

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 7-parameter tool with no output schema, the description covers the critical selection logic, the one-of requirement, failure modes, and project placement behavior. It does not discuss return values or job handling, but given that the tool has no output schema and openWorldHint=true, the description is sufficiently complete for an agent to invoke it correctly.

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 burden. It compensates well for the key parameters: character vs photo_media_id is explained in depth, prompt gets explicit guidance with an example, and project_id is clarified. Model, aspect, and include_preview are left to their enum/default values, which is acceptable but not fully documented.

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 states a specific verb and resource: 'Generate a NEW image of the SAME PERSON/character as an existing photo or portrait.' It gives concrete examples ('put me in a suit / in a park') and explicitly claims to be the 'ONLY tool' for generating another picture of a previously created character, clearly distinguishing it from siblings like generate_image and generate_character_image.

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 gives explicit when-to-use guidance ('This is the tool for...') and tells the agent exactly why not to use entity_id/character_slot_index, including a hard failure mode ('HTTP 500'). It also explains the two mutually exclusive input paths (character vs photo_media_id) and when to choose each, leaving little to inference.

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