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21st_magic_component_builder

Generate React UI component snippets from user requests—e.g., buttons, forms, or tables. Use this tool to quickly create component code, then manually integrate snippets into your codebase for tailored UI development.

Instructions

"Use this tool when the user requests a new UI component—e.g., mentions /ui, /21 /21st, or asks for a button, input, dialog, table, form, banner, card, or other React component. This tool ONLY returns the text snippet for that UI component. After calling this tool, you must edit or add files to integrate the snippet into the codebase."

Input Schema

TableJSON Schema
NameRequiredDescriptionDefault
messageYesFull users message
searchQueryYesGenerate a search query for 21st.dev (library for searching UI components) to find a UI component that matches the user's message. Must be a two-four words max or phrase

Implementation Reference

  • Implements the core logic of the '21st_magic_component_builder' tool by sending a POST request to '/api/create-ui' with the user's message and search query, then returning the generated UI component text.
    async execute({ message, searchQuery }: z.infer<typeof this.schema>) {
      try {
        const { data } = await twentyFirstClient.post<CreateUiResponse>(
          "/api/create-ui",
          {
            message,
            searchQuery,
          }
        );
    
        return {
          content: [
            {
              type: "text" as const,
              text: data.text,
            },
          ],
        };
      } catch (error) {
        console.error("Error executing tool", error);
        throw error;
      }
    }
  • Defines the input schema using Zod for the tool's parameters: 'message' (user's full message) and 'searchQuery' (query for 21st.dev UI components).
    schema = z.object({
      message: z.string().describe("Full users message"),
      searchQuery: z
        .string()
        .describe(
          "Generate a search query for 21st.dev (library for searching UI components) to find a UI component that matches the user's message. Must be a two-four words max or phrase"
        ),
    });
  • src/index.ts:14-14 (registration)
    Registers the CreateUiTool instance with the MCP server.
    new CreateUiTool().register(server);
Behavior3/5

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 explains that the tool returns text snippets and requires manual integration afterward, which is useful context. However, it doesn't disclose important behavioral traits like whether this is a read-only operation, if it makes external API calls, potential rate limits, or error handling for invalid requests.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Conciseness5/5

Is the description appropriately sized, front-loaded, and free of redundancy?

The description is extremely concise and well-structured with only three sentences, each serving a distinct purpose: trigger conditions, tool limitation, and required follow-up actions. There's zero wasted text, and the most important information (when to use the tool) is front-loaded.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Completeness3/5

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

Given no annotations and no output schema, the description provides adequate context for a simple tool but has gaps. It explains the tool's purpose and usage well but doesn't describe what the output looks like (format, structure, or content of returned snippets) or address potential limitations or error conditions that would help an agent use it correctly.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Parameters3/5

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

With 100% schema description coverage, the baseline is 3. The description doesn't add any parameter-specific information beyond what's already in the schema (message and searchQuery parameters are fully documented in the schema). No additional syntax, format, or usage details for parameters are provided in the description.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Purpose5/5

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 specifies the exact trigger conditions (mentions of /ui, /21, /21st, or specific component types) and distinguishes it from sibling tools by focusing on returning component snippets rather than inspiration or logo searches.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Usage Guidelines5/5

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 specific trigger examples, and distinguishes it from alternatives by stating 'This tool ONLY returns the text snippet' and requiring follow-up actions. It clearly defines when to use this tool versus other approaches.

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

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