is_legal

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

With no annotations provided, the description carries the behavioral burden. It discloses the return format ({legal:boolean}) and the parse_error edge case, which adds useful context beyond the bare purpose. It does not state side effects, but the verb 'check' implies non-mutation.

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, front-loaded with the primary purpose and followed by return details. There is no fluff or redundant information—every sentence earns its place.

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?

The description covers the return values and the parse-error case, which is important given no output schema is shown. The requirement of an active game is implied by 'current position' but not explicitly stated, which is a minor gap for a simple 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?

The description clarifies that the 'uci' parameter is a UCI move string, adding meaning to the otherwise bare schema field. However, it does not specify the exact UCI format or provide examples, so the compensation for 0% schema coverage is minimal but sufficient for a single parameter.

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 clearly states the action ('Check if a UCI move is legal') and the resource ('in the current position'), distinguishing it from sibling tools that manipulate or list moves. This is a specific verb+resource combination with no ambiguity.

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 when to use it—to validate a move before adding it—but does not explicitly contrast with alternatives like add_move or state when not to use it. There is no explicit context or exclusions, so it falls at the implied-usage level.

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