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

Annotations already cover the safety profile (readOnlyHint, openWorldHint, destructiveHint=false), so the bar is lower. The description adds real value beyond them: the full return payload, paywall/engagement fields, the time-window capability, and notably the billing rate ($0.003 per request), which is useful cost context an agent would otherwise lack.

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 tightly packed sentences that are front-loaded with the core purpose, followed by return shape and cost. Every clause carries information (scope, fields, time limit, billing) with no filler.

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 well by enumerating return fields and noting cost and time window. It omits pagination behavior (16-20 posts per page, page cap 101) which lives only in the schema, but otherwise everything needed to call the tool correctly is present.

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 the baseline is 3 — the schema already documents query, language, posted_ago, page, and get_sentiment. The description echoes the time window ("past day to past year") and return fields but adds no syntax or format detail beyond the schema, so it does not exceed the baseline.

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 ("Search Substack posts by keyword") and scopes it to "across every publication," which cleanly separates it from siblings like search_substack_notes, search_substack_publications, and search_substack_users. An agent can identify the correct tool without opening any 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 the search context (keyword search of posts across all publications) but gives no explicit when-to-use versus when-not guidance or routing to the four sibling search_* substack tools. Usage is inferable but not stated, matching the 'implied usage' level.

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