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

Annotations cover the safety profile (readOnly=false, destructive=true, idempotent=false), and the description adds real behavior: text is placed on image posts but not on video key frames, the image is auto-banked into the brand library, and the result surfaces via post.mediaLinks. It doesn't explain why the call is destructive or what gets overwritten on the post's existing media, leaving a small gap.

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-loaded with the core action and input modes, then the conditional text behavior, then the exclusion and alternative. Dense and mostly waste-free, though the final sentence about showing the user mediaLinks drifts slightly into presentation guidance for a tool definition.

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 usefully explains what the response contains (post.mediaLinks) and how media is banked. Preconditions, exclusions, alternatives, and input modes are all covered, so an agent has everything needed to call it correctly.

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 all six parameters are already documented, including the exactly-one-of constraint on the three media inputs. The description restates these modes but adds no syntax or format detail beyond what the schema provides, 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?

States a specific verb (attaches) and resource (an image to an existing image_post or video_post awaiting media), plus the three accepted input forms. It distinguishes itself from create_adhoc_post and from the text_post/carousel case, so an agent can identify it without opening the schema.

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 names when-not conditions (text_post/carousel, the user's own verbatim posts) and routes to the correct alternative, create_adhoc_post's mediaLibraryAssetId. The precondition that the post must be waiting on media is stated up front.

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