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

With no annotations, the description carries the burden of behavioral disclosure. It states that the tool writes a new file next to the source unless an output path is set, and clarifies that it does not upload. This provides useful side-effect information, though it does not mention potential overwriting behavior, permissions, or error cases.

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?

The description is extremely concise, consisting of two sentences that front-load the core purpose and follow with a behavioral note. Every word earns its place, and there is no redundancy or fluff.

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?

Given the tool's moderate complexity, no annotations, and no output schema, the description covers the essential aspects: what it does, how output is handled, and a key non-behavior (no upload). It does not describe return values or edge cases, but those are arguably less critical for a simple compression tool, making this adequately 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 high (80%), so a baseline of 3 applies. The description adds value by clarifying the relationship between targetKB and quality ('target size (KB) or quality') and by explaining the default behavior of the output parameter ('Writes a new file next to the source unless output is set'). This goes beyond the schema's individual descriptions.

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 clearly states the tool's purpose with a specific verb ('Compress') and resource ('a local image'), and specifies the two modes of operation (target size or quality). Even though no sibling tools are listed, the purpose is unambiguous and distinct.

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 implies when to use the tool (when you need to compress a local image) and adds a useful exclusion ('Does not upload'). However, it does not explicitly discuss alternatives or when not to use it beyond the upload note, so guidance is only implied rather than fully specified.

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