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

Annotations cover the safety profile (destructiveHint=false), and the description adds genuinely new behavioral context: it returns both standard and URL-safe variants, and it costs $0.001 in USDC on Base via x402 per successful call. The payment disclosure is a significant trait not derivable from annotations or schema, though error/failure behavior beyond "successful call" is not detailed.

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 tight sentences, front-loaded with the action, then use cases, then cost. Every sentence carries distinct information with no redundancy.

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 one-parameter tool with no output schema, the description covers purpose, use cases, cost, and the fact that two variants are returned. It could say more about the exact return shape (e.g., field names), but it 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?

With a single parameter at 100% schema coverage the baseline is 3. The description slightly exceeds it by clarifying that the input is treated as UTF-8 text, which adds encoding semantics beyond the schema's plain "The text to encode."

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 ("Encode UTF-8 text as standard base64") and adds the secondary behavior of returning the URL-safe variant. The encode direction naturally distinguishes it from the base64_decode sibling, but the sibling is never named explicitly, so it falls just short of the top tier.

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?

Gives concrete use contexts ("data URLs, headers and basic auth strings"), which tells the agent when this tool is appropriate. It does not name alternatives or state when not to use it, so it stops short of explicit routing guidance.

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