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

Beyond the readOnlyHint annotation, the description reveals important behaviors: acceptance of standard and URL-safe alphabets, tolerance for missing padding and line breaks, failure on invalid input, refusal to return binary data, and local execution. These details meaningfully inform an agent's expectations about edge cases and limitations.

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?

The description is compact and front-loaded with the primary action. Each sentence earns its place by covering input formats, failure conditions, and execution context without any redundant 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?

For a simple one-parameter read-only tool, the description covers everything an agent needs: what the tool does, what input formats are accepted, when it fails, and what it will not return. The absence of an output schema is mitigated because the decoded UTF-8 text is clearly implied.

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 schema only describes the parameter as 'Base64 or base64url string,' but the description adds valuable format semantics: standard and URL-safe alphabets, optional padding, and line break handling. This enriches the parameter's meaning beyond the schema's 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?

The description states a specific verb and resource: 'Decode base64 to UTF-8 text.' This clearly differentiates it from base64_encode and other decoder siblings like url_decode and jwt_decode, leaving no ambiguity about what the tool does.

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 gives clear context for when this tool is appropriate: decoding base64 input into UTF-8 text. It also provides a de facto exclusion by stating that binary data is not returned, implying the tool should not be used when binary output is needed. However, it does not explicitly name alternative tools or provide explicit when-not-to-use guidance.

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