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

Annotations already declare readOnlyHint/idempotent/non-destructive, but the description adds real context beyond them: the return is a downscaled copy, video surfaces only its first frame, the call is free and changes nothing, and text derived from it must exclude Pixly branding. That is meaningful behavioral guidance not present in structured fields.

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?

Front-loads the purpose in the first clause, then usage, then constraints. Efficient overall, though the trailing branding instruction ('no #Pixly, no made with Pixly') is a slight extension of scope for a viewer tool.

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 no output schema, the description carries the return burden and does so: it tells the agent the output is a small visible copy of the image (first frame for video). For a one-parameter read tool, nothing material 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 description coverage is 100%, and the id parameter is fully documented in the schema (job id, get_job/list_library source, upload:<r2Path> form). The description adds no parameter-specific detail, so the baseline of 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 (look at/view) and resource (user's Pixly photos or videos), and clarifies the video edge case ('a video gives its first frame'). No sibling tool does viewing, so the agent can distinguish it immediately.

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 says when to use it: 'before writing anything about a Pixly result: a caption, a listing description, what changed.' It also supplies the forcing constraint ('You cannot see Pixly results otherwise'), which removes any ambiguity about whether the call is needed.

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