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

Annotations already declare readOnlyHint and idempotentHint, lowering the bar. The description adds valuable context beyond annotations: 'UTF-8 safe' and 'Supports URL-safe alphabet' disclose encoding behavior that is not present in the annotations or schema. No contradiction exists.

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 two concise sentences: the first states purpose and key features, the second gives usage guidance. Every word earns its place, with no fluff or repetition of schema fields.

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 encode/decode tool with a complete output schema and strong annotations, the description is fully adequate. It covers purpose, usage context, and key behaviors without needing to explain return values or edge cases.

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%, so the schema fully documents each parameter (mode, input, url_safe). The description does not add further parameter-level details beyond the schema, meeting the baseline of 3.

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 immediately states the core function ('Encode text to Base64 or decode Base64 back to text') with specific details like UTF-8 safety and URL-safe alphabet support. It clearly differentiates from sibling tools by naming the encode/decode operation, making the purpose unmistakable.

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?

It provides explicit use cases: 'Use whenever you need to encode/decode data URIs, tokens, or config values instead of guessing the bytes.' This gives clear context for when to apply the tool, though it does not exclude alternatives by name, which would warrant a 5.

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