Creating Custom Providers

Genkit’s plugin system allows you to create custom model providers to integrate any AI service. Whether you’re using a proprietary API, a custom inference server, or wrapping an existing SDK, you can build a Genkit plugin that provides first-class integration.

When to Build a Custom Provider

Consider building a custom provider when:

You want to integrate a model service not yet supported by Genkit
You have a proprietary or internal AI API
You’re running custom model infrastructure
You want to wrap an existing ML framework
You need special authentication or request handling

Plugin Architecture

A Genkit plugin provides:

Initialization - Set up clients, validate configuration
Action Registration - Define available models and embedders
Action Resolution - Handle dynamic model requests
Action Listing - Provide discoverable model metadata

Basic Plugin Structure

Simple Plugin Example

Here’s a minimal custom provider:

import { genkitPluginV2, GenkitPluginV2 } from 'genkit/plugin';
import { modelRef, ModelReference } from 'genkit/model';
import { z } from 'genkit';

export interface MyProviderOptions {
  apiKey?: string;
  baseUrl?: string;
}

function myProviderPlugin(options?: MyProviderOptions): GenkitPluginV2 {
  return genkitPluginV2({
    name: 'my-provider',
    
    // Initialize plugin, return known models
    async init() {
      return [
        // Return model actions here (or empty array)
      ];
    },
    
    // Resolve models on-demand
    async resolve(actionType, actionName) {
      if (actionType === 'model') {
        // Define and return model action
        return defineMyModel(actionName, options);
      }
      return undefined;
    },
    
    // List available models
    async list() {
      // Return model metadata
      return [];
    },
  });
}

export const myProvider = myProviderPlugin;

Defining Models

Model Action

A model action implements the interface for generating responses:

import { model } from 'genkit/plugin';
import { 
  GenerateRequest, 
  GenerateResponseData,
  StreamingCallback,
} from 'genkit';
import { 
  GenerationCommonConfigSchema,
  ModelInfo,
} from 'genkit/model';
import { z } from 'genkit';

const MyModelConfigSchema = GenerationCommonConfigSchema.extend({
  // Add custom config options
  customParam: z.string().optional(),
});

function defineMyModel(name: string, options?: MyProviderOptions) {
  const modelInfo: ModelInfo = {
    label: `My Provider - ${name}`,
    supports: {
      multiturn: true,
      tools: true,
      media: false,
      systemRole: true,
      output: ['text', 'json'],
    },
  };

  return model({
    name: `my-provider/${name}`,
    configSchema: MyModelConfigSchema,
    info: modelInfo,
  }, async (request: GenerateRequest, streamingCallback?: StreamingCallback): Promise<GenerateResponseData> => {
    // 1. Convert Genkit request to your API format
    const apiRequest = convertToApiFormat(request);
    
    // 2. Call your AI service
    const apiResponse = await callMyApi(apiRequest, options);
    
    // 3. Convert API response to Genkit format
    return convertToGenkitFormat(apiResponse);
  });
}

Model Reference Helper

Provide a helper for creating model references:

export const myProvider = myProviderPlugin as {
  (options?: MyProviderOptions): GenkitPluginV2;
  model(name: string, config?: any): ModelReference<typeof MyModelConfigSchema>;
};

(myProvider as any).model = (
  name: string,
  config?: any
): ModelReference<typeof MyModelConfigSchema> => {
  return modelRef({
    name: `my-provider/${name}`,
    config,
    configSchema: MyModelConfigSchema,
  });
};

Request/Response Conversion

Converting Genkit Requests

import { GenerateRequest, MessageData, Part } from 'genkit';

function convertToApiFormat(request: GenerateRequest) {
  const messages = request.messages.map(message => ({
    role: convertRole(message.role),
    content: message.content.map(part => convertPart(part)),
  }));

  return {
    model: request.model,
    messages,
    temperature: request.config?.temperature,
    max_tokens: request.config?.maxOutputTokens,
    // Map other config options...
  };
}

function convertRole(role: string): string {
  switch (role) {
    case 'user': return 'user';
    case 'model': return 'assistant';
    case 'system': return 'system';
    default: return role;
  }
}

function convertPart(part: Part) {
  if (part.text) {
    return { type: 'text', text: part.text };
  }
  if (part.media) {
    return { type: 'image_url', image_url: { url: part.media.url } };
  }
  if (part.toolRequest) {
    return {
      type: 'tool_use',
      id: part.toolRequest.ref,
      name: part.toolRequest.name,
      input: part.toolRequest.input,
    };
  }
  if (part.toolResponse) {
    return {
      type: 'tool_result',
      tool_use_id: part.toolResponse.ref,
      content: part.toolResponse.output,
    };
  }
  throw new Error('Unsupported part type');
}

Converting API Responses

import { GenerateResponseData, MessageData } from 'genkit';

function convertToGenkitFormat(apiResponse: any): GenerateResponseData {
  const message: MessageData = {
    role: 'model',
    content: [],
  };

  // Handle text responses
  if (apiResponse.content?.text) {
    message.content.push({ text: apiResponse.content.text });
  }

  // Handle tool calls
  if (apiResponse.tool_calls) {
    for (const toolCall of apiResponse.tool_calls) {
      message.content.push({
        toolRequest: {
          name: toolCall.function.name,
          input: toolCall.function.arguments,
          ref: toolCall.id,
        },
      });
    }
  }

  return {
    message,
    finishReason: apiResponse.stop_reason || 'stop',
    usage: {
      inputTokens: apiResponse.usage?.prompt_tokens || 0,
      outputTokens: apiResponse.usage?.completion_tokens || 0,
      totalTokens: apiResponse.usage?.total_tokens || 0,
    },
  };
}

Streaming Support

Implementing Streaming

async function generateWithStreaming(
  request: GenerateRequest,
  streamingCallback?: StreamingCallback
): Promise<GenerateResponseData> {
  if (!streamingCallback) {
    // Non-streaming path
    return generateNonStreaming(request);
  }

  // Call streaming API
  const stream = await callStreamingApi(request);
  
  let fullText = '';
  let chunkIndex = 0;

  for await (const chunk of stream) {
    const text = chunk.delta?.content || '';
    fullText += text;

    // Send chunk to callback
    streamingCallback({
      index: chunkIndex++,
      content: [{ text }],
    });
  }

  // Return final response
  return {
    message: {
      role: 'model',
      content: [{ text: fullText }],
    },
    finishReason: 'stop',
  };
}

Tool (Function) Support

Converting Tool Definitions

import { ToolDefinition } from 'genkit/model';

function convertTools(tools?: ToolDefinition[]) {
  if (!tools) return undefined;

  return tools.map(tool => ({
    type: 'function',
    function: {
      name: tool.name,
      description: tool.description,
      parameters: tool.inputSchema || {},
    },
  }));
}

Handling Tool Responses

See the request/response conversion examples above for handling toolRequest and toolResponse parts.

Embedder Support

Defining an Embedder

import { embedder } from 'genkit/plugin';
import { EmbedderReference, embedderRef } from 'genkit';

function defineMyEmbedder(name: string, options?: MyProviderOptions) {
  return embedder({
    name: `my-provider/${name}`,
    configSchema: z.object({
      dimensions: z.number().optional(),
    }),
    info: {
      dimensions: 768,
      supports: {
        input: ['text'],
      },
    },
  }, async (input) => {
    // Call your embedding API
    const response = await callEmbeddingApi(input.content, options);
    
    return {
      embeddings: [response.embedding],
    };
  });
}

// Add embedder helper
(myProvider as any).embedder = (
  name: string,
  config?: any
): EmbedderReference => {
  return embedderRef({
    name: `my-provider/${name}`,
    config,
  });
};

Advanced Features

Dynamic Model Discovery

List available models from your API:

async function listActions() {
  try {
    // Call your API to get available models
    const models = await fetchAvailableModels();
    
    return models.map(m => 
      modelActionMetadata({
        name: `my-provider/${m.id}`,
        info: {
          label: m.name,
          supports: m.capabilities,
        },
      })
    );
  } catch (error) {
    console.error('Failed to list models:', error);
    return [];
  }
}

Authentication Handling

function createAuthenticatedClient(options?: MyProviderOptions) {
  const apiKey = options?.apiKey || process.env.MY_PROVIDER_API_KEY;
  
  if (!apiKey) {
    throw new Error(
      'API key required. Set MY_PROVIDER_API_KEY or pass apiKey option.'
    );
  }

  return new MyApiClient({
    apiKey,
    baseUrl: options?.baseUrl || 'https://api.example.com',
  });
}

Custom Configuration Schema

const CustomConfigSchema = GenerationCommonConfigSchema.extend({
  // Provider-specific options
  stylePreset: z.enum(['vivid', 'natural']).optional(),
  qualityLevel: z.enum(['standard', 'hd']).optional(),
  safetyLevel: z.number().min(0).max(5).optional(),
});

Error Handling

import { GenkitError } from 'genkit';

async function callApiWithErrorHandling(request: any) {
  try {
    return await myApiClient.generate(request);
  } catch (error: any) {
    // Map API errors to Genkit errors
    if (error.status === 429) {
      throw new GenkitError({
        status: 'RESOURCE_EXHAUSTED',
        message: 'Rate limit exceeded',
      });
    }
    
    if (error.status === 401) {
      throw new GenkitError({
        status: 'UNAUTHENTICATED',
        message: 'Invalid API key',
      });
    }
    
    throw new GenkitError({
      status: 'INTERNAL',
      message: error.message || 'Unknown error',
    });
  }
}

Complete Example: Simple HTTP API Provider

import { genkitPluginV2, GenkitPluginV2 } from 'genkit/plugin';
import { model, modelActionMetadata } from 'genkit/plugin';
import { 
  GenerateRequest, 
  GenerateResponseData,
  MessageData,
  modelRef,
  z,
} from 'genkit';
import { 
  GenerationCommonConfigSchema,
  ModelInfo,
} from 'genkit/model';

export interface SimpleApiOptions {
  apiKey?: string;
  baseUrl?: string;
}

const ConfigSchema = GenerationCommonConfigSchema.extend({
  temperature: z.number().min(0).max(1).optional(),
});

function defineSimpleModel(name: string, options?: SimpleApiOptions) {
  const modelInfo: ModelInfo = {
    label: `Simple API - ${name}`,
    supports: {
      multiturn: true,
      tools: false,
      media: false,
      systemRole: true,
      output: ['text'],
    },
  };

  return model({
    name: `simple-api/${name}`,
    configSchema: ConfigSchema,
    info: modelInfo,
  }, async (request: GenerateRequest): Promise<GenerateResponseData> => {
    const apiKey = options?.apiKey || process.env.SIMPLE_API_KEY;
    const baseUrl = options?.baseUrl || 'https://api.example.com';

    // Convert messages to simple text
    const prompt = request.messages
      .map(m => m.content.map(p => p.text).join(''))
      .join('\n');

    // Call API
    const response = await fetch(`${baseUrl}/generate`, {
      method: 'POST',
      headers: {
        'Authorization': `Bearer ${apiKey}`,
        'Content-Type': 'application/json',
      },
      body: JSON.stringify({
        model: name,
        prompt,
        temperature: request.config?.temperature,
        max_tokens: request.config?.maxOutputTokens,
      }),
    });

    if (!response.ok) {
      throw new Error(`API error: ${response.statusText}`);
    }

    const data = await response.json();

    // Convert to Genkit format
    const message: MessageData = {
      role: 'model',
      content: [{ text: data.text }],
    };

    return {
      message,
      finishReason: 'stop',
      usage: {
        inputTokens: data.usage?.prompt_tokens || 0,
        outputTokens: data.usage?.completion_tokens || 0,
      },
    };
  });
}

function simpleApiPlugin(options?: SimpleApiOptions): GenkitPluginV2 {
  return genkitPluginV2({
    name: 'simple-api',
    
    async init() {
      // Return predefined models
      return [
        defineSimpleModel('base', options),
        defineSimpleModel('advanced', options),
      ];
    },
    
    async resolve(actionType, actionName) {
      if (actionType === 'model') {
        return defineSimpleModel(actionName, options);
      }
      return undefined;
    },
    
    async list() {
      return [
        modelActionMetadata({
          name: 'simple-api/base',
          info: { label: 'Simple API - Base' },
        }),
        modelActionMetadata({
          name: 'simple-api/advanced',
          info: { label: 'Simple API - Advanced' },
        }),
      ];
    },
  });
}

export const simpleApi = simpleApiPlugin as {
  (options?: SimpleApiOptions): GenkitPluginV2;
  model(name: string): any;
};

(simpleApi as any).model = (name: string) => {
  return modelRef({
    name: `simple-api/${name}`,
    configSchema: ConfigSchema,
  });
};

export default simpleApi;

Using the Custom Provider

import { genkit } from 'genkit';
import { simpleApi } from './simple-api-provider';

const ai = genkit({
  plugins: [
    simpleApi({
      apiKey: process.env.SIMPLE_API_KEY,
      baseUrl: 'https://api.example.com',
    }),
  ],
});

const response = await ai.generate({
  model: simpleApi.model('base'),
  prompt: 'Hello!',
});

console.log(response.text());

Real-World Examples

Study Existing Providers

The best way to learn is by studying existing provider implementations: Simple Provider:

~/workspace/source/js/plugins/ollama/src/index.ts - Local model provider

Medium Complexity:

~/workspace/source/js/plugins/anthropic/src/index.ts - Claude provider

Advanced Provider:

~/workspace/source/js/plugins/google-genai/src/googleai/index.ts - Google AI provider
~/workspace/source/js/plugins/compat-oai/src/index.ts - OpenAI-compatible provider

Testing Your Provider

Unit Tests

import { describe, it, expect } from '@jest/globals';
import { myProvider } from './my-provider';

describe('My Provider', () => {
  it('should initialize plugin', async () => {
    const plugin = myProvider({ apiKey: 'test-key' });
    expect(plugin).toBeDefined();
  });

  it('should define model', async () => {
    const model = myProvider.model('test-model');
    expect(model.name).toBe('my-provider/test-model');
  });
});

Integration Tests

import { genkit } from 'genkit';
import { myProvider } from './my-provider';

const ai = genkit({
  plugins: [myProvider({ apiKey: process.env.TEST_API_KEY })],
});

const response = await ai.generate({
  model: myProvider.model('test-model'),
  prompt: 'Hello, world!',
});

console.log(response.text());

Best Practices

Follow naming conventions: Use provider/model-name format
Validate configuration: Check required options in constructor
Handle errors gracefully: Map API errors to Genkit errors
Support streaming: When your API supports it
Document capabilities: Accurately report model capabilities
Provide type safety: Export TypeScript types for configs
Test thoroughly: Unit and integration tests
Version your plugin: Semantic versioning
Document usage: Provide README with examples

Publishing Your Provider

Once your provider is ready:

Package structure:

my-provider/
├── src/
│   └── index.ts
├── package.json
├── tsconfig.json
└── README.md

package.json:

{
  "name": "genkitx-my-provider",
  "version": "1.0.0",
  "main": "./lib/index.js",
  "types": "./lib/index.d.ts",
  "peerDependencies": {
    "genkit": "^0.9.0"
  }
}

Publish to npm:

npm publish

Overview

Getting Started

Core Concepts

Guides

Model Providers

Deployment

Developer Tools

Documentation Index

​When to Build a Custom Provider

​Plugin Architecture

​Basic Plugin Structure

​Simple Plugin Example

​Defining Models

​Model Action

​Model Reference Helper

​Request/Response Conversion

​Converting Genkit Requests

​Converting API Responses

​Streaming Support

​Implementing Streaming

​Tool (Function) Support

​Converting Tool Definitions

​Handling Tool Responses

​Embedder Support

​Defining an Embedder

​Advanced Features

​Dynamic Model Discovery

​Authentication Handling

​Custom Configuration Schema

​Error Handling

​Complete Example: Simple HTTP API Provider

​Using the Custom Provider

​Real-World Examples

​Study Existing Providers

​Testing Your Provider

​Unit Tests

​Integration Tests

​Best Practices

​Publishing Your Provider

​Resources

​Next Steps

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