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

Annotations declare mutation, non-idempotent, and open-world. The description adds unique behavior beyond that: image posts receive auto-generated headline/subhead/cta text, video key frames get no text, and the response's post.mediaLinks should be shown to the user. This is rich behavioral context that structured data does not provide.

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?

Four sentences, each earning its place: purpose, text-placement behavior, exclusions with alternative, and response handling. The purpose is front-loaded and there is no redundant restatement of the name or title.

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?

No output schema exists, but the description explains the response shape (post.mediaLinks) and directs the agent to show the user the result. Combined with annotations and full schema coverage, the definition gives an agent everything needed to invoke the tool correctly and handle its output.

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 parameters are documented in the schema. The description restates the three input options (base64, public https URL, existing assetId) but adds no syntax, format, or constraint details beyond what the schema already provides. Baseline 3 is appropriate.

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 waiting on media), and explicitly distinguishes from siblings like create_adhoc_post and non-applicable post types. An agent can identify the exact operation and its scope without opening any 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 states when not to use it (text_post/carousel, user's own verbatim posts) and names the alternative (create_adhoc_post's mediaLibraryAssetId). Also specifies the precondition that the post must be waiting on media, leaving no ambiguity about selection.

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