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

Annotations already declare readOnlyHint=false, destructiveHint=false and openWorldHint=true, so the safety profile is covered. The description adds one genuinely useful behavioral fact: the photo must come from list_my_media, implying ownership scoping. It does not say whether the old caption is overwritten, whether the change is reversible, or what is returned, which matters for a mutation.

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?

A single short sentence with no filler, front-loaded with the action and target. It is efficient, though arguably too terse for a mutation tool — nothing is wasted but little is offered.

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?

The tool is simple (2 params, full schema coverage, annotations present, no output schema), so little is required. Still, an agent calling a write tool benefits from knowing that the caption is replaced wholesale and roughly what the call returns; both are absent, leaving it merely adequate.

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 schema already documents both parameters including the 5000-character limit and the mediaId provenance. The description merely restates the mediaId source, adding no syntax, format, or validation detail beyond structured data; 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 ('changes') and a specific resource ('caption of a photo'), and anchors the tool to the list_my_media workflow so an agent knows exactly which object class it targets. It is clear, though it doesn't contrast against any near-alternative beyond the read tool it references.

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 phrase 'from list_my_media' implies the prerequisite step (fetch mediaIds first) which is useful workflow guidance. However, there is no explicit when-to-use vs when-not, no statement about permissions, and no alternative named — usage is inferred rather than stated.

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