21st.dev Magic AI Agent

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

No annotations are provided, so the description carries the full burden. It discloses key behavioral traits: the tool returns only a text snippet (not integrated code) and requires post-call file editing. However, it lacks details on error handling, rate limits, authentication needs, or what happens if inputs are invalid. For a tool with 5 parameters and no annotations, this leaves gaps in understanding its full behavior.

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 appropriately sized and front-loaded, starting with usage triggers and core functionality. It uses three concise sentences with zero waste, clearly stating purpose, limitation, and required follow-up. However, it could be slightly more structured by separating usage conditions from post-call instructions for better readability.

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?

Given the complexity (5 parameters, no annotations, no output schema), the description is partially complete. It covers when to use the tool and its output nature but lacks details on error cases, return format, or how parameters influence the snippet generation. Without an output schema, it should ideally hint at what the snippet contains (e.g., code, documentation), leaving some contextual gaps for effective agent use.

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%, so the schema already documents all 5 parameters thoroughly. The description does not add any parameter-specific information beyond what's in the schema (e.g., it doesn't explain how 'searchQuery' relates to '21st.dev' or clarify parameter interactions). With high schema coverage, the baseline is 3, as the description doesn't compensate with extra semantic context.

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 tool's purpose: to return a text snippet for a UI component when users request new UI components. It specifies the verb 'returns' and resource 'text snippet for that UI component,' and mentions React components as examples. However, it doesn't explicitly differentiate from sibling tools like '21st_magic_component_refiner' or '21st_magic_component_inspiration,' which likely have related but distinct purposes.

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 provides explicit usage guidelines: 'Use this tool when the user requests a new UI component' and lists specific triggers (e.g., mentions of /ui, /21, /21st, or component names like button, input). It also states 'This tool ONLY returns the text snippet' and instructs on post-call actions ('you must edit or add files to integrate'), clearly outlining when and how to use it versus alternatives like manual coding or other tools.

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