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

Annotations already declare readOnlyHint, idempotentHint, and destructiveHint=false, so the safety profile is covered; the description nonetheless explains the result's provenance hierarchy — own Instagram results when history suffices, otherwise the user's chosen time or an industry study 'said plainly'. That disclosure of data-sufficiency fallback behavior is real value beyond the annotations, though it stops short of specifying latency or how much history counts as 'enough'.

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 use case and alternative routing before the return-value detail, which is the right order. Minor waste in 'Read only', which merely repeats readOnlyHint=true and could be dropped.

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?

There is no output schema, so the description carries the burden of explaining the return value — and it does, describing both the answer and the evidence behind it plus the three-tier fallback. For a zero-parameter, read-only lookup, nothing an agent needs in order to call or interpret it is missing.

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?

The tool takes zero parameters, so there is no parameter meaning to add and the baseline of 4 applies; the description correctly implies no inputs are needed by framing it purely as an owner-scoped query.

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?

Description opens with a specific verb and resource scoped to the signed-in owner ('when THEIR posts should go out or what time works best for their own account'), and explicitly separates this from the sibling best_time_to_post ('For a city's general best times use best_time_to_post'). An agent can distinguish the two without inspecting either 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?

States the triggering condition precisely ('when the signed-in owner asks...') and names the alternative tool plus the condition that selects it (city-level general times). It also implicitly excludes use for other people's accounts by emphasizing 'THEIR' and 'their own account'.

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