Bridge API

Complete API reference for the Bridge class - the core component for connecting frontend and backend adapters.

Constructor

new Bridge(frontend, backend, config?)

Creates a new Bridge instance.

Parameters:

• frontend: FrontendAdapter - Adapter for parsing input format
• backend: BackendAdapter | Router - Adapter (or router) that executes requests against an AI provider
• config?: Partial<BridgeConfig> - Optional configuration (see BridgeConfig)

Returns: Bridge instance

Example:

import { Bridge } from '@johnhenry/aimatey-core';
import { OpenAIFrontendAdapter } from '@johnhenry/aimatey-frontend/openai';
import { AnthropicBackendAdapter } from '@johnhenry/aimatey-backend/anthropic';

const bridge = new Bridge(
  new OpenAIFrontendAdapter(),
  new AnthropicBackendAdapter({
    apiKey: process.env.ANTHROPIC_API_KEY
  }),
  {
    timeout: 30000,
    retries: 3
  }
);

Methods

chat(request, options?)

Execute a chat completion request.

Parameters:

• request: any - Request in frontend adapter format
• options?: RequestOptions - Per-request overrides (see RequestOptions)

Returns: Promise<any> - Response in frontend adapter format

Throws:

• ProviderError - If the provider API returns an error
• NetworkError - If the network request fails
• ValidationError - If request validation fails
• AdapterError (CONNECTION_TIMEOUT / PROVIDER_TIMEOUT) - If the request times out
• AdapterError (ROUTING_FAILED) - If options.backend names a backend that is not registered

Example:

const response = await bridge.chat({
  model: 'gpt-4',
  messages: [
    { role: 'user', content: 'Hello!' }
  ]
});

console.log(response.choices[0].message.content);

---

chatStream(request, options?)

Execute a streaming chat completion request.

Parameters:

• request: any - Request in frontend adapter format (with stream: true)
• options?: RequestOptions - Per-request overrides (see RequestOptions)

Returns: AsyncGenerator<any> - Stream of chunks in frontend adapter format. chatStream() is an async generator, so it returns the stream synchronously - do not await the call itself.

Throws:

• ProviderError - If the provider API returns an error
• StreamError - If stream parsing fails
• ValidationError - If request validation fails
• AdapterError (ROUTING_FAILED) - If options.backend names a backend that is not registered

Example:

const stream = bridge.chatStream({
  model: 'gpt-4',
  messages: [{ role: 'user', content: 'Count to 10' }],
  stream: true
});

for await (const chunk of stream) {
  const content = chunk.choices?.[0]?.delta?.content;
  if (content) {
    process.stdout.write(content);
  }
}

---

use(middleware)

Add middleware to the bridge.

Parameters:

• middleware: Middleware - Middleware to add to the chain

Returns: Bridge (for chaining)

Example:

import { createLoggingMiddleware, createCachingMiddleware } from '@johnhenry/aimatey-middleware';

bridge
  .use(createLoggingMiddleware({ level: 'info' }))
  .use(createCachingMiddleware({ ttl: 3_600_000 })); // ttl is milliseconds

---

executeIR(request, options?)

Execute a request using the IR format directly (bypassing the frontend adapter).

Parameters:

• request: IRChatRequest - Request in IR format
• options?: RequestOptions - Per-request overrides (see RequestOptions)

Returns: Promise<IRChatResponse> - Response in IR format

Example:

const irResponse = await bridge.executeIR({
  messages: [{ role: 'user', content: 'Hello' }],
  parameters: {
    model: 'claude-haiku-4-5-20251001',
    temperature: 0.7
  },
  metadata: {
    requestId: 'req_1',
    timestamp: Date.now()
  }
});

console.log(irResponse.message.content);

---

executeIRStream(request, options?)

Execute a streaming request using IR format directly. This is an async generator - it returns the stream synchronously, so the call itself is not awaited.

Parameters:

• request: IRChatRequest - Request in IR format with streaming enabled
• options?: RequestOptions - Per-request overrides (see RequestOptions)

Returns: IRChatStream - AsyncGenerator<IRStreamChunk>

Example:

const stream = bridge.executeIRStream({
  messages: [{ role: 'user', content: 'Hello' }],
  parameters: { model: 'claude-haiku-4-5-20251001' },
  metadata: { requestId: 'req_2', timestamp: Date.now() },
  stream: true
});

for await (const chunk of stream) {
  if (chunk.type === 'content') {
    process.stdout.write(chunk.delta);
  }
}

---

clone(config)

frontend and backend are readonly - a bridge cannot be re-pointed at a different adapter after construction. To run the same request through another provider, build a second bridge; to change only configuration, clone() returns a new bridge with the same adapters and middleware and a merged config.

Parameters:

• config: Partial<BridgeConfig> - Configuration overrides

Returns: Bridge - A new bridge instance

Example:

import { OpenAIBackendAdapter } from '@johnhenry/aimatey-backend/openai';

// Same adapters, longer timeout
const patientBridge = bridge.clone({ timeout: 120000 });

// A different backend means a new bridge
const openaiBridge = new Bridge(
  bridge.frontend,
  new OpenAIBackendAdapter({ apiKey: process.env.OPENAI_API_KEY })
);

---

on(event, handler)

Subscribe to bridge events.

Parameters:

• event: BridgeEventType | '*' - Event name, or '*' for every event
• handler: BridgeEventListener - Event handler function

Returns: Bridge (for chaining). off() and once() have the same shape.

Events:

BridgeEventType declares eleven event names (see BridgeEventType), but only these six are emitted by Bridge today:

• request:start - Fired before a non-streaming request is sent
• request:success - Fired after a successful non-streaming response
• request:error - Fired when a non-streaming request fails
• stream:start - Fired when streaming starts
• stream:complete - Fired when streaming finishes
• stream:error - Fired when a stream fails

The remaining names (request:cancelled, stream:chunk, backend:selected, backend:failover, middleware:executed) are reserved and never fire.

Example:

bridge.on('request:start', (event) => {
  console.log('Request:', event.request.messages);
});

bridge.on('request:success', (event) => {
  console.log('Response tokens:', event.response?.usage?.totalTokens);
});

bridge.on('request:error', (event) => {
  console.error('Error:', event.error?.message);
});

---

Properties

frontend

Type: FrontendAdapter

Read-only

The frontend adapter instance the bridge was constructed with.

---

backend

Type: BackendAdapter | Router

Read-only

The backend adapter (or router) the bridge was constructed with.

---

config

Type: BridgeConfig

Read-only

The resolved bridge configuration.

---

getMiddleware() / getStreamingMiddleware()

There is no middlewares property. The registered middleware is read back through methods instead:

bridge.getMiddleware();          // readonly Middleware[]
bridge.getStreamingMiddleware(); // readonly StreamingMiddleware[]

---

Types

BridgeConfig

Configuration options for Bridge, passed as the third constructor argument.

interface BridgeConfig {
  /** Enable debug mode with detailed logging (default: false) */
  debug?: boolean;

  /** Global request timeout in milliseconds (default: 30000) */
  timeout?: number;

  /** Maximum retries for transient failures (default: 0) */
  retries?: number;

  /** Default model to use if the request does not specify one */
  defaultModel?: string;

  /** Router configuration, when the backend is a Router */
  routerConfig?: Partial<RouterConfig>;

  /** Add a request ID to metadata if not present (default: true) */
  autoRequestId?: boolean;

  /** Custom configuration options */
  custom?: Record<string, unknown>;
}

---

RequestOptions

Per-request overrides, passed as the second argument to chat() and chatStream().

interface RequestOptions {
  /** Request timeout in milliseconds */
  timeout?: number;

  /** AbortSignal for request cancellation */
  signal?: AbortSignal;

  /** Backend override, by registered name (router only) */
  backend?: string;

  /** Additional metadata merged into `metadata.custom` */
  metadata?: Record<string, unknown>;

  /** Skip middleware execution for this request (default: false) */
  skipMiddleware?: boolean;

  /** Custom request options */
  custom?: Record<string, unknown>;
}

Per-request backend selection

backend picks a specific backend for one request, by the name it was registered under. It only applies when the bridge’s backend is a Router - with a single backend adapter there is no routing to override and the option is ignored.

const router = new Router({ routingStrategy: 'explicit', defaultBackend: 'openai' });
router.register('openai', new OpenAIBackendAdapter({ apiKey: process.env.OPENAI_API_KEY }));
router.register('anthropic', new AnthropicBackendAdapter({ apiKey: process.env.ANTHROPIC_API_KEY }));

const bridge = new Bridge(new OpenAIFrontendAdapter(), router);

// Routed by the router's configured strategy
await bridge.chat({ model: 'gpt-4', messages });

// Forced to Anthropic for this request only
await bridge.chat({ model: 'gpt-4', messages }, { backend: 'anthropic' });

The bridge writes the override onto metadata.custom.backend, which is the channel the router reads its explicit routing decision from. It is applied last, so it takes precedence over a metadata.custom.backend already present on the request and over a backend key passed through options.metadata.

A name that is not registered is rejected up front with an AdapterError carrying ErrorCode.ROUTING_FAILED, so a typo cannot be served by a different provider:

await bridge.chat(request, { backend: 'antropic' });
// AdapterError: Requested backend 'antropic' is not registered.
//               Registered backends: openai, anthropic

A backend that is registered but is currently unhealthy or has an open circuit breaker is not an error - the router’s normal fallback applies.

---

BridgeEventType

The event names accepted by on() / off() / once(). (BridgeEvent is a different thing - the interface describing an event object.)

type BridgeEventType =
  | 'request:start'
  | 'request:success'
  | 'request:error'
  | 'request:cancelled'
  | 'stream:start'
  | 'stream:chunk'
  | 'stream:complete'
  | 'stream:error'
  | 'backend:selected'
  | 'backend:failover'
  | 'middleware:executed';

Only request:start, request:success, request:error, stream:start, stream:complete and stream:error are actually emitted.

---

Error Handling

Bridge can throw the following errors:

Every error thrown by a bridge derives from AdapterError, which carries a machine-readable code, a category, and an isRetryable flag.

ProviderError

Thrown when the provider API returns an error response.

import { AdapterError, ProviderError } from '@johnhenry/aimatey-errors';

try {
  await bridge.chat(request);
} catch (error) {
  if (error instanceof ProviderError) {
    console.log('Provider failed:', error.provenance.backend);
    console.log('Code:', error.code);
    console.log('Message:', error.message);
  } else if (error instanceof AdapterError) {
    console.log('Adapter error:', error.code, error.category);
  }
}

ValidationError

Thrown when request validation fails. The per-field detail lives on validationDetails.

import { ValidationError } from '@johnhenry/aimatey-errors';

try {
  await bridge.chat(invalidRequest);
} catch (error) {
  if (error instanceof ValidationError) {
    console.log('Validation failed:', error.validationDetails);
  }
}

Timeouts

There is no TimeoutError class. A timeout surfaces as an AdapterError whose code is CONNECTION_TIMEOUT (the network call timed out) or PROVIDER_TIMEOUT (the provider reported a timeout).

import { AdapterError, ErrorCode } from '@johnhenry/aimatey-errors';

const bridge = new Bridge(frontend, backend, { timeout: 5000 });

try {
  await bridge.chat(request);
} catch (error) {
  if (
    error instanceof AdapterError &&
    (error.code === ErrorCode.CONNECTION_TIMEOUT || error.code === ErrorCode.PROVIDER_TIMEOUT)
  ) {
    console.log('Request timed out after 5s');
  }
}

---

Advanced Usage

Custom Backend Adapter

import type {
  BackendAdapter,
  AdapterMetadata,
  IRChatRequest,
  IRChatResponse
} from '@johnhenry/aimatey-types';

class CustomBackend implements BackendAdapter {
  readonly metadata: AdapterMetadata = {
    name: 'custom',
    version: '1.0.0',
    provider: 'Custom',
    capabilities: {
      streaming: true,
      multiModal: false,
      systemMessageStrategy: 'in-messages',
      supportsMultipleSystemMessages: true
    }
  };

  fromIR(request: IRChatRequest): unknown {
    return request;
  }

  toIR(_response: unknown, originalRequest: IRChatRequest): IRChatResponse {
    return {
      message: { role: 'assistant', content: 'Custom response' },
      finishReason: 'stop',
      usage: { promptTokens: 5, completionTokens: 5, totalTokens: 10 },
      metadata: originalRequest.metadata
    };
  }

  async execute(request: IRChatRequest): Promise<IRChatResponse> {
    return this.toIR(null, request);
  }

  async *executeStream(request: IRChatRequest) {
    yield { type: 'start' as const, sequence: 0, metadata: request.metadata };
    yield { type: 'content' as const, sequence: 1, delta: 'Custom' };
    yield { type: 'done' as const, sequence: 2, finishReason: 'stop' as const };
  }
}

const bridge = new Bridge(
  new OpenAIFrontendAdapter(),
  new CustomBackend()
);

---

Middleware Stack

import {
  createLoggingMiddleware,
  createCachingMiddleware,
  createRetryMiddleware,
  createCostTrackingMiddleware
} from '@johnhenry/aimatey-middleware';

const bridge = new Bridge(frontend, backend);

// Add middleware in order
bridge
  .use(createLoggingMiddleware({ level: 'info' }))
  .use(createRetryMiddleware({ maxAttempts: 3 }))
  .use(createCachingMiddleware({ ttl: 3_600_000 })) // ttl is milliseconds
  .use(createCostTrackingMiddleware({ dailyThreshold: 100 }));

---

See Also

• Router API - Multi-backend routing
• Middleware API - Middleware reference
• Error Handling - Error types
• Tutorial: Simple Bridge - Getting started
• Examples - Code examples