Advanced Hasura GraphQL MCP Server

Instructions

Implementation Reference

• src/index.ts:339-367 (handler)

  The main handler function for the 'preview_table_data' tool. It fetches the GraphQL introspection schema, finds the table object type, selects scalar and enum fields, builds a GraphQL query to preview limited rows, executes it via makeGqlRequest, and returns the result as formatted JSON text.

  async ({ tableName, limit }) => {
    console.log(`[INFO] Executing tool 'preview_table_data' for table: ${tableName}, limit: ${limit}`);
    try {
      const schema = await getIntrospectionSchema();
      const tableType = schema.types.find(t => t.name === tableName && t.kind === 'OBJECT') as IntrospectionObjectType | undefined;
      if (!tableType) {
          throw new Error(`Table (Object type) '${tableName}' not found in schema.`);
      }
      const scalarFields = tableType.fields
        ?.filter(f => { 
            let currentType = f.type;
            while (currentType.kind === 'NON_NULL' || currentType.kind === 'LIST') currentType = currentType.ofType;
            return currentType.kind === 'SCALAR' || currentType.kind === 'ENUM';
        })
        .map(f => f.name) || [];
       if (scalarFields.length === 0) {
           console.warn(`[WARN] No scalar fields found for table ${tableName}...`);
           scalarFields.push('__typename');
       }
      const fieldsString = scalarFields.join('\n          ');
      const query = gql` query PreviewData($limit: Int!) { ${tableName}(limit: $limit) { ${fieldsString} } }`;
      const variables = { limit };
      const result = await makeGqlRequest(query, variables);
      return { content: [{ type: "text", text: JSON.stringify(result, null, 2) }] };
    } catch (error: any) {
      console.error(`[ERROR] Tool 'preview_table_data' failed: ${error.message}`);
      throw error;
    }
  }

• src/index.ts:335-338 (schema)

  Zod input schema for the tool, defining 'tableName' as a required string and 'limit' as an optional positive integer defaulting to 5.

  {
    tableName: z.string().describe("The exact name of the table..."),
    limit: z.number().int().positive().optional().default(5).describe("Optional. Maximum number of rows..."),
  },

• src/index.ts:332-368 (registration)

  The server.tool() registration call for 'preview_table_data', including the tool name, description, input schema, and handler function.

  server.tool(
    "preview_table_data",
    "Fetch esa limited sample of rows (default 5) from a specified table...",
    {
      tableName: z.string().describe("The exact name of the table..."),
      limit: z.number().int().positive().optional().default(5).describe("Optional. Maximum number of rows..."),
    },
    async ({ tableName, limit }) => {
      console.log(`[INFO] Executing tool 'preview_table_data' for table: ${tableName}, limit: ${limit}`);
      try {
        const schema = await getIntrospectionSchema();
        const tableType = schema.types.find(t => t.name === tableName && t.kind === 'OBJECT') as IntrospectionObjectType | undefined;
        if (!tableType) {
            throw new Error(`Table (Object type) '${tableName}' not found in schema.`);
        }
        const scalarFields = tableType.fields
          ?.filter(f => { 
              let currentType = f.type;
              while (currentType.kind === 'NON_NULL' || currentType.kind === 'LIST') currentType = currentType.ofType;
              return currentType.kind === 'SCALAR' || currentType.kind === 'ENUM';
          })
          .map(f => f.name) || [];
         if (scalarFields.length === 0) {
             console.warn(`[WARN] No scalar fields found for table ${tableName}...`);
             scalarFields.push('__typename');
         }
        const fieldsString = scalarFields.join('\n          ');
        const query = gql` query PreviewData($limit: Int!) { ${tableName}(limit: $limit) { ${fieldsString} } }`;
        const variables = { limit };
        const result = await makeGqlRequest(query, variables);
        return { content: [{ type: "text", text: JSON.stringify(result, null, 2) }] };
      } catch (error: any) {
        console.error(`[ERROR] Tool 'preview_table_data' failed: ${error.message}`);
        throw error;
      }
    }
  );