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

Annotations only provide readOnlyHint=true and openWorldHint=true, so the description carries the burden of behavioral context. It discloses that the tool flags catastrophic backtracking, missing anchors, unescaped dots, dangerous character ranges, and alternation precedence mistakes, and explicitly instructs the agent to 'Check hasBlockingIssue first.' The example (a+)+$ demonstrates real-world impact. This goes well beyond what annotations convey.

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 front-loaded with the core purpose and critical usage warning, then expands into rationale and ownership. The 'WHY DELEGATE THIS' section is somewhat extra but earns its place by reinforcing the risk of manual inspection. The structure is clear, though slightly longer than strictly necessary.

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 tool with no output schema, the description supplies key output guidance via 'Check hasBlockingIssue first' and enumerates common issues it detects. It also explains why the tool is necessary and when to invoke it. It does not fully describe the output format, but for an explainer tool this is sufficient for correct selection and invocation.

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 only parameter, pattern, already has a clear schema description: 'A JavaScript regex, bare or as /pattern/flags.' Since schema coverage is 100%, the description adds little extra semantic value beyond confirming the parameter is a regex. The tool description does not use the parameter specifically, so baseline 3 applies.

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 opens with a specific verb-resource pair: 'Explain a regular expression in plain English and detect catastrophic backtracking.' It clearly states both primary functions, and the title reinforces this. It distinguishes itself from all listed siblings, which are unrelated tools, so no further differentiation is needed.

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 an explicit, imperative trigger: 'ALWAYS call this before putting a pattern where it will run against untrusted input.' It also offers a concrete example of when skipping this would be harmful. It does not name alternative tools, but sibling tools are unrelated, so alternatives are not relevant here. The 'WHY DELEGATE THIS' section further justifies delegation.

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