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

Annotations only provide openWorldHint, so the description carries the burden of explaining the mutating behavior. It does this well by disclosing that the tool previews by default and only performs the write when apply:true is set. This is important, non-obvious behavioral context that goes beyond the schema.

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 two concise sentences with no filler. It front-loads the core action and endpoint, then immediately gives the critical preview/apply instruction that an agent must know before invoking.

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: four params, all schema-documented, no output schema. The description covers the endpoint, the top-level nature of the comment, and the preview/apply behavior, which together are sufficient to call it correctly. It could additionally note that replies belong to instagram_reply_to_comment, but that is a minor omission rather than a correctness gap.

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 four parameters are already documented. The description adds the key apply gate semantics and the IG_WRITE_MODE alternative, but it does not add much detail about mediaId or message beyond the schema. 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?

The description states a specific action: 'Post a new top-level comment on a media object' and includes the API endpoint. The phrase 'top-level' clearly distinguishes this from sibling tools like instagram_reply_to_comment, so an agent can select it correctly without inspecting other schemas.

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 clearly explains the preview-by-default behavior and exactly how to perform the write: 're-run with apply:true (or set IG_WRITE_MODE=apply)'. It does not explicitly name alternative tools or when not to use this tool, but the 'top-level' qualifier implies the boundary with reply tools.

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