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

Annotations already cover safety (readOnlyHint, openWorldHint), so the description focuses on what they cannot convey: results are region-dependent based on request origin, it is editorial rather than a true chart, and extra_fetch costs 1 credit per item up to page size with a 1-credit base cost. That cost and geography disclosure is genuinely useful behavior not derivable from annotations.

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 statement, followed by interpretation caveats and then the cost-bearing parameter. Slightly wordy framing ('Worth knowing what this is and is not') but every sentence carries substantive information.

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?

For a read-only listing tool with no output schema, the description supplies the crucial interpretive context (region-shaped results, editorial provenance, credit cost) an agent needs to set expectations. It does not describe the response shape or pagination mechanics, but the cursor parameter covers paging.

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 75%, so count/cursor/extra_fetch are already documented. The description still adds value by restating the credit cost model for extra_fetch and listing exactly which fields it fills (publish date, caption, duration), which the schema only summarizes as 'fields the list page leaves null'. cache_max_age remains undocumented in both places.

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 and resource: reels trending on Instagram's public Explore grid. The caveat that this is Instagram's editorial Explore surface rather than a global chart meaningfully distinguishes it from a generic trending tool, though it never names siblings like instagram_reelsSearch or instagram_userReels.

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?

Usage is implied (discover trending reels) but there is no explicit 'use this instead of X when Y' guidance. The note that no public trending feed exists hints at why one would pick this tool, but alternatives such as instagram_reelsSearch are not addressed.

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