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

Annotations already declare readOnlyHint=true and openWorldHint=false, so the safety profile is known. The description adds genuinely new behavioral information beyond that: the call does not consume credits, which matters for an agent deciding whether it can poll the balance. It does not describe the returned balance format, but with no output schema that gap is minor for a zero-parameter read.

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 first sentence front-loads the purpose and the second adds the key non-obvious fact about cost. The English and Chinese sentences are direct translations of each other, so roughly half the text is redundant for any single reader, but no sentence is otherwise wasted.

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, zero-argument, read-only balance check, the description covers what is read, whose balance it is, and that it is free of charge. The only omission is what the response looks like (units, breakdown), which is a small gap given there is no output schema.

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 tool takes zero parameters, so there is nothing for the description to disambiguate; the baseline for a parameterless tool applies. Nothing in the description conflicts with the empty schema.

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 names a specific verb and resource ('Check the credit balance') and scopes it to 'the account that owns the API key', so the agent knows exactly what is being read. It does not explicitly differentiate itself from the siblings (mycel_discover, mycel_hire_agent, mycel_models, mycel_task_status), though their purposes are plainly distinct.

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?

'Does not consume credits' implies this is safe to call freely before or around paid operations, which is a useful usage cue. However, there is no explicit statement of when to call it versus the sibling tools or any preconditions, so guidance remains implied rather than stated.

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