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

Annotations already declare the safe-read profile (readOnly, idempotent, non-destructive), so the description's added value is the return-shape disclosure: 'Returns the image itself, downscaled.' That downscaling caveat is real behavioral information an agent cannot get from annotations or the schema, and it matters when judging image quality for alt text. No output schema exists, so it does leave the exact return format (bytes vs. URL vs. base64) unspecified.

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 action and the cardinality constraint, then the usage condition, then the return shape. Nothing is padded or restated.

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 one-parameter read tool with full annotation coverage and no output schema, the description supplies the rationale, the source of the id, and the downscaled-return caveat. The only minor gap is the concrete return format, which matters somewhat for an image-fetching tool.

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% and the single parameter's schema description already covers the accepted forms (gid, URL, or id from get_resource). The description restates the get_resource provenance but adds no format or syntax detail beyond the schema, so the baseline 3 applies.

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 ('Fetch ONE image') and immediately scopes the input source ('by image id from get_resource'), distinguishing it from the sibling get_resource that produces the id. The emphasis on ONE signals a single-item fetch, not a listing.

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?

Explicitly instructs when to call it ('Call this before writing alt text') and gives the failure mode that motivates it ('alt text guessed from the product title describes the product rather than the picture'). This effectively routes the agent toward update_image_alt as the downstream step.

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