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

With no annotations provided, the description carries the full burden of behavioral disclosure. It adds valuable context: execution runs under a 100ms budget in a separate thread, and patterns that do not finish are reported as such rather than hanging. This proactively informs the agent about timeout behavior, which is critical for a tool that could potentially block. It does not detail return format or error handling, but the timeout disclosure is significant and goes beyond what annotations would typically provide.

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 sentences with zero filler. It front-loads the core purpose ('Test a pattern against input and get the actual matches') and then adds the crucial behavioral detail about the timeout. Every sentence earns its place, and the structure is clear and efficient.

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 tool with no output schema, the description covers the essential aspects: what it does, how it behaves under a timeout, and the fact that matches are returned. The only gap is the lack of detail on the exact output format or how errors (e.g., invalid pattern) are reported. However, given that the tool's purpose is straightforward and the timeout behavior is clearly disclosed, the description is fairly complete. A 4 reflects that it is adequate but could add a note on return structure.

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 67%: 'input' and 'flags' have descriptions, but 'pattern' does not. The description does not elaborate on parameter semantics beyond what the schema already states. It mentions 'pattern' and 'input' implicitly but adds no new meaning about how they should be formatted or interpreted. The baseline for this coverage level is 3, and the description does not compensate for the missing pattern description, so a 3 is 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?

The description states a specific action: 'Test a pattern against input and get the actual matches.' It clearly identifies the resource (a regex pattern) and the operation (testing and retrieving matches). This distinguishes it from sibling tools, which are mostly unrelated utilities like calc, dns, or ping, so there is no ambiguity about what this tool does.

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 implies usage: you would use this tool to validate a regex pattern against input. However, it does not explicitly state when to use this tool versus alternatives, nor does it mention any exclusions or prerequisites. Since there are no similar sibling tools, the context is clear, but the description offers no guidance on scenarios where it should not be used.

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