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

Goes well beyond the annotations: describes server-side processing (squared to 512 px), dedup behavior ('same picture returns the same URL'), the size/format limits, and when the result becomes visible. This adds real operational context that idempotentHint/destructiveHint alone cannot convey.

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?

Three short sentences, front-loaded with the core action and constraints, and each sentence carries information (formats, processing/dedup, visibility path). Minor terseness in 'It shows once agent_draft_edit sets avatar_url' costs a little.

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?

With an output schema, full annotation coverage, and 100% schema description coverage, the remaining burden is behavioral, and the description supplies formats, size limit, processing, idempotency semantics, and visibility timing. Nothing needed to call it correctly is missing.

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 the baseline is 3; the description adds meaning by constraining the file to PNG/JPEG/WebP at ≤10 MB and by noting the image is resized, which tells the agent what the upload will actually store. It doesn't clarify the https-only download_url requirement, but the schema pattern covers that.

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 ('stores a PNG, JPEG or WebP picture ... the host passes as a download URL'), plus the accepted formats and 10 MB ceiling. It is clearly distinct from agent_draft_edit, which only references the resulting avatar_url.

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?

Explains the workflow ('shows once agent_draft_edit sets avatar_url and the draft is published') and gives an explicit out-of-scope case ('Files that exist only on the user's computer are set in the Agent settings page'). It doesn't spell out error/precondition handling, but the routing context is clear.

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