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 that the tool 'ONLY returns the text snippet' and requires manual integration afterward, which is useful behavioral context. However, it doesn't mention potential limitations like rate limits, authentication needs, error handling, or what format the snippet returns (e.g., React code, plain text). For a tool with 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: it starts with when to use the tool, states its core function, and ends with post-call instructions. Every sentence earns its place—no wasted words. The structure is logical and efficient for an AI agent to parse quickly.

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 no annotations, no output schema, and 5 parameters (though well-documented in schema), the description is moderately complete. It covers the tool's purpose, usage triggers, and post-call actions, but lacks details on output format, error cases, or integration specifics. For a tool that returns UI snippets, more context on what the snippet contains (e.g., React code, dependencies) would be helpful, but it's adequate as a minimum viable description.

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 doesn't add any parameter-specific information beyond what's in the schema (e.g., it doesn't explain how 'context' or 'searchQuery' relate to generating the UI snippet). With high schema coverage, the baseline is 3, and the description doesn't compensate with additional param semantics.

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: 'returns the text snippet for that UI component' when users request new UI components. It provides specific examples of triggers (mentions of /ui, /21, /21st, or component names like button, input, etc.). However, it doesn't explicitly differentiate from sibling tools like '21st_magic_component_refiner' or '21st_magic_component_inspiration' beyond stating this tool 'ONLY returns the text snippet.'

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' with trigger examples, and states 'This tool ONLY returns the text snippet for that UI component.' It also specifies what to do after calling: 'you must edit or add files to integrate the snippet into the codebase.' This gives clear when-to-use instructions and post-call actions, though it doesn't explicitly mention when NOT to use it or name alternatives.

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