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

Annotations already carry readOnlyHint=true, idempotentHint=true and destructiveHint=false, so the safety profile is covered. The description adds meaningful context beyond that: the call does not read or ingest anything, it only forecasts a cost, which implies no content is fetched and no credits are consumed by the estimate itself. It stops short of saying outright that estimating is free or what happens if sourceType is unsupported.

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?

Two short sentences with no filler. The positive statement comes first and the exclusion-plus-alternative follows, so the most decision-relevant content is front-loaded.

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 single-parameter, read-only estimator this covers what matters: which types are priced, which are free, and where to go for the free ones. There is no output schema, and the description says enough about the return value ('what X would cost in credits') that an agent knows to expect a credit figure, though it does not specify precision or how the estimate relates to actual charges.

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% and there is only one parameter, so the baseline would be 3. The description adds value by enumerating the valid sourceType values in plain prose (youtube, tiktok, audio, research) and implicitly marking everything else as out of scope — useful for a bare string parameter with no enum constraint.

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: it reports the credit cost of reading a given source type. It also names the four source types in scope, which separates it cleanly from sibling estimators like estimate_image_cost, estimate_scrape_cost and estimate_video_cost.

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 says when NOT to use it: article, PDF and plain text pages are free, and the agent should call add_knowledge_from_url or add_knowledge instead. That is a concrete alternative plus the condition that selects it, which is exactly the routing guidance an agent needs.

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