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

Annotations already cover the safety profile (readOnlyHint, openWorldHint), and the description adds meaningful behavior beyond them: the data window (latest 12 posts), the exclusion rule (pinned posts excluded), and the cost (2 credits). It stops short of auth requirements or rate-limit behavior, but the added constraints are genuinely useful.

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 tool's identity, then a compact enumeration of outputs, then the cost line. Every sentence carries information and nothing is redundant. Slightly dense in the output enumeration but well organized.

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 compensates by enumerating the return fields and the data window, which is enough for an agent to know what it will get. Annotations cover the read-only nature. Minor gaps remain around pagination or refresh behavior, but the definition is adequate for correct invocation.

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 100% with both user_id and username fully documented in the schema, including that either can be passed. The description adds no parameter syntax or format detail beyond what the schema already provides, so the 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 resource and computed outputs: engagement rate (mean/median), posting cadence, format mix, best posting hours, and a benchmark percentile. This computed-metrics role is clearly distinct from siblings like instagram_posts or instagram_profile that return raw content. 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?

The description implies usage ('engagement rate calculator' over the latest 12 posts) but never states when to choose it over siblings or when it is inappropriate. There is no explicit routing to alternatives and no prerequisites. Usage is inferable but not guided.

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