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

Annotations already declare non-readOnly, idempotent, non-destructive, openWorld, so the description's job is incremental and it delivers: lossy image downscaling is disclosed, the no-op fallback on incompressible files is disclosed, and 'Uses one render' hints at cost/consumption. What's missing is whether the compressed output replaces the source or is returned as a new artifact.

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 what happens to the file, then the failure-mode guarantee, then the cost note. Nothing is redundant with the title or schema.

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 3-parameter mutation with no output schema, the description covers mechanism, the degraded/no-op outcome, and cost. The remaining gap is the return format (artifact reference vs inline file), which an agent would want before chaining this into a download step.

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%, so the schema already explains file.url vs base64 vs password, the low/medium/high quality trade-off, and remove_metadata. The description adds only the general notion of image downscaling, which loosely maps to level but gives no additional per-parameter semantics, so 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?

A specific verb (compress) applied to a specific resource (PDF), with the mechanism spelled out: recompressing and downscaling images plus removing unused data. No sibling (merge_pdfs, split_pdf, render_pdf, watermark_pdf) does size reduction, so the agent can route to it unambiguously.

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?

Usage is implied rather than stated: an agent can infer it's the tool for shrinking a PDF, and 'returns the original unchanged if it can't be made smaller' helpfully signals this is safe to attempt. There is no explicit when/when-not guidance, no mention of ordering relative to render_pdf or fill_pdf_form, and the level trade-off is left to the schema.

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