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

Beyond the annotations (readOnlyHint=false, destructiveHint=true, idempotentHint=false), the description reveals that filling can also replace an existing slot and that creator-linked slots retain a source guard. This is useful behavioral information not discoverable from annotations or the schema.

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 tightly written sentences deliver the action, target, allowed sources, exclusivity constraint, and the creator-linked guard. There is no boilerplate, repetition, or wasted wording.

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 plus a fully covered schema and explicit ownership constraints give an agent enough to select and call the tool correctly. It lacks any return-value guidance, and since there is no output schema, a small note on expected result 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 description coverage is 100%, and each parameter already has a meaningful description, including the 'Use exactly one source field' constraint and the oneOf mutual-exclusion structure. The description reinforces source ownership and public URL but does not add material parameter-level meaning beyond the schema.

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 precise verb phrase, 'Fill or replace one reference slot', names the exact resource (an existing owned avatar/model), and enumerates the three allowed source kinds (workspace file, generation result, public image URL). This makes it immediately distinguishable from sibling tools like create_avatar_from_references or upload_avatar_from_chat_file.

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 clearly scopes when the tool applies: modifying an existing owned avatar/model, filling or replacing one slot, with exactly one of three owned/public sources. It does not explicitly name sibling alternatives or state when not to use it, so it misses the top bar but provides clear usage context.

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