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

Annotations already declare the write/non-idempotent/open-world profile, so the description's job is to add context — and it does: replies are public, they appear under the account's identity, and nested replies are rejected. It doesn't mention error behavior or rate limits, but for a mutation tool with full annotation coverage this is a solid addition.

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?

Three short sentences, zero filler, and the operating instruction (which id to pass) is front-loaded immediately after the one-line purpose. Every sentence carries a distinct constraint.

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?

Covers purpose, mode selection, visibility and the top-level-only restriction for a 3-parameter mutation tool with no output schema. Slightly short of complete because it doesn't say what a successful response looks like or how conflicting media_id + comment_id is handled at runtime.

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 only 33% (media_id and comment_id are undocumented), and the description compensates by assigning each one a concrete mode and implying mutual exclusivity with "or". The required `message` parameter is left to the schema, which is acceptable given its own description and length bounds.

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 (post a reply / add a top-level comment) and resource, and the two-mode structure (comment_id vs media_id) makes it distinct from siblings like ig_delete_comment, ig_hide_comment and ig_list_comments. An agent can tell exactly what this tool produces 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 routes the caller: use comment_id to reply under a comment, media_id for a new top-level comment, plus a platform constraint (replies only to top-level comments) that pre-empts a failed call. No sibling tool is named as an alternative, which keeps it just below a 5.

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