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

The description adds real value beyond readOnlyHint=false: the key is shown only once (non-recoverable), the free plan has a fixed $0.10/day spend cap, and it carries its own trainable injection guard. However it says nothing about idempotency or rate limits — an agent cannot tell whether a repeat call provisions a new account — which matters for an account-creation write.

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 short sentences, front-loaded with the action and its key caveat (no email, key shown once), followed by operational follow-through and plan constraints. No 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?

With no output schema, the description correctly covers the return ('the key is shown once') and the immediate next step, plus quota limits. The remaining gap is account-recreation/idempotency behavior for a provisioning tool.

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 the single optional 'username' parameter is fully described in the schema (2-32 chars of a-z 0-9 -). The description never mentions username or what happens if it is omitted, so baseline 3 applies with no added value.

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?

Names a specific verb and resource: 'Create a free Habits account and API key'. The parenthetical '(no email)' and the mention of what the key is for make the outcome unambiguous, and nothing here overlaps with check_prompt or teach_guard.

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?

Implies the trigger clearly (you need a key, and no email is required) and tells the agent exactly what to do with the result: set it as an Authorization: Bearer header for this MCP server. It stops short of stating when NOT to call it — e.g. whether calling twice creates a second account — so it sits just below a 5.

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