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

Goes beyond the annotations (readOnlyHint=false, destructiveHint=false, openWorldHint=true) by disclosing that no customer email is sent, that each call is confirmed by default (and can be switched off), that it affects other people, and that a Cotlet API key with app access is required. These are meaningful side-effect and auth disclosures an agent could not infer from the structured fields.

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 repeats the same two facts three times: 'Customers are not emailed; use message_class', then 'Sets a substitute instructor on one class', then 'changes the instructor only and sends no email; use message_class to tell the class.' Roughly half the text is redundant restatement, wasting budget despite the correct front-loaded opener.

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 low-complexity, two-parameter mutation tool with no output schema, the description covers behavior, side effects, confirmation, and auth requirements adequately. Only minor gaps remain, such as what a successful call returns or what happens to any existing instructor assignment.

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 description coverage is 100% for both parameters (class_id, instructor), so the schema already carries the parameter semantics. The description adds no format or constraint detail beyond what the schema provides, making the baseline 3 appropriate.

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 ('Put a substitute instructor on a class') and distinguishes itself from the sibling message_class by noting it does not email customers. The core purpose is clear and unambiguous, though the same idea is restated multiple times, which dilutes rather than sharpens the signal.

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?

Explicitly routes notification behavior to an alternative ('use message_class to tell the class'), which is exactly the kind of when-to-use-this-vs-that guidance an agent needs. It does not, however, frame broader exclusion conditions, so it falls short of a full when/when-not treatment.

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