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

Annotations already declare destructiveHint=true, and the description aligns with this by saying 'fill or replace.' It adds a behavioral nuance: 'Creator-linked slots keep their source guard,' which is not present in the schema or annotations and helps the agent understand source-ownership constraints.

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 long and front-loaded with the core action. Every word adds value—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?

The description covers the key logic (one-of sources, replace semantics) and a notable edge case (creator-linked slots) for a destructive mutation. There is no output schema, so not specifying return values is acceptable, but slightly more detail on the result (e.g., success/failure behavior) would make it fully 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?

Schema coverage is 100%, so all parameters are already well-documented with descriptions like 'Use exactly one source field.' The description mostly echoes these source types without adding new syntax or format details, so it earns the baseline score for high schema 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 clearly states the action ('fill or replace'), the target resource ('reference slot on an existing owned avatar/model'), and the three source types (workspace file, generation result, public URL). It distinguishes from sibling tools like create_avatar_from_references by explicitly mentioning 'existing' avatar/model.

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 implies when to use the tool—updating an existing avatar's reference slots—which is distinct from creating a new avatar or generating one. It does not explicitly name alternatives or exclusions, but the 'existing' qualifier provides clear contextual guidance.

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