# Custom Providers

Extensions can register custom model providers via `pi.registerProvider()`. This enables:

- **Proxies** - Route requests through corporate proxies or API gateways
- **Custom endpoints** - Use self-hosted or private model deployments
- **OAuth/SSO** - Add authentication flows for enterprise providers
- **Custom APIs** - Implement streaming for non-standard LLM APIs

## Example Extensions

See these complete provider examples:

- [`examples/extensions/custom-provider-anthropic/`](../examples/extensions/custom-provider-anthropic/)
- [`examples/extensions/custom-provider-gitlab-duo/`](../examples/extensions/custom-provider-gitlab-duo/)

## Table of Contents

- [Example Extensions](#example-extensions)
- [Quick Reference](#quick-reference)
- [Override Existing Provider](#override-existing-provider)
- [Register New Provider](#register-new-provider)
- [Unregister Provider](#unregister-provider)
- [OAuth Support](#oauth-support)
- [Custom Streaming API](#custom-streaming-api)
- [Context Overflow Errors](#context-overflow-errors)
- [Testing Your Implementation](#testing-your-implementation)
- [Config Reference](#config-reference)
- [Model Definition Reference](#model-definition-reference)

## Quick Reference

Extensions can register either a complete pi-ai `Provider` or use the legacy provider-config form. Prefer a complete provider when custom authentication, filtering, refresh, or streaming behavior is required. Pi composes `models.json` overrides above registered native providers.

```typescript
import { createProvider, openAICompletionsApi } from "@earendil-works/pi-ai";
import type { ExtensionAPI } from "@earendil-works/pi-coding-agent";

export default function (pi: ExtensionAPI) {
  pi.registerProvider(createProvider({
    id: "native-local",
    name: "Native Local",
    baseUrl: "http://localhost:8080/v1",
    auth: {
      apiKey: {
        name: "Local server API key",
        async login(interaction) {
          return {
            type: "api_key",
            key: await interaction.prompt({ type: "secret", message: "API key" })
          };
        },
        async resolve({ credential }) {
          return credential?.key
            ? { auth: { apiKey: credential.key }, source: "stored API key" }
            : undefined;
        }
      }
    },
    models: [],
    api: openAICompletionsApi()
  }));

  // Legacy provider-config form:
  // Override baseUrl for existing provider
  pi.registerProvider("anthropic", {
    baseUrl: "https://proxy.example.com"
  });

  // Register new provider with models
  pi.registerProvider("my-provider", {
    name: "My Provider",
    baseUrl: "https://api.example.com",
    apiKey: "$MY_API_KEY",
    api: "openai-completions",
    models: [
      {
        id: "my-model",
        name: "My Model",
        reasoning: false,
        input: ["text", "image"],
        cost: { input: 0, output: 0, cacheRead: 0, cacheWrite: 0 },
        contextWindow: 128000,
        maxTokens: 4096
      }
    ]
  });
}
```

The extension factory can also be `async`. For dynamic model discovery, fetch and register models in the factory instead of `session_start`. pi waits for the factory before startup continues, so the provider is available during interactive startup and to `pi --list-models`.

## Override Existing Provider

The simplest use case: redirect an existing provider through a proxy.

```typescript
// All Anthropic requests now go through your proxy
pi.registerProvider("anthropic", {
  baseUrl: "https://proxy.example.com"
});

// Add custom headers to OpenAI requests
pi.registerProvider("openai", {
  headers: {
    "X-Custom-Header": "value"
  }
});

// Both baseUrl and headers
pi.registerProvider("google", {
  baseUrl: "https://ai-gateway.corp.com/google",
  headers: {
    "X-Corp-Auth": "$CORP_AUTH_TOKEN"  // env var or literal
  }
});
```

When only `baseUrl` and/or `headers` are provided (no `models`), all existing models for that provider are preserved with the new endpoint.

## Register New Provider

To add a completely new provider, specify `models` along with the required configuration.

If the model list comes from a remote endpoint, use an async extension factory:

```typescript
import type { ExtensionAPI } from "@earendil-works/pi-coding-agent";

export default async function (pi: ExtensionAPI) {
  const response = await fetch("http://localhost:1234/v1/models");
  const payload = (await response.json()) as {
    data: Array<{
      id: string;
      name?: string;
      context_window?: number;
      max_tokens?: number;
    }>;
  };

  pi.registerProvider("local-openai", {
    baseUrl: "http://localhost:1234/v1",
    apiKey: "$LOCAL_OPENAI_API_KEY",
    api: "openai-completions",
    models: payload.data.map((model) => ({
      id: model.id,
      name: model.name ?? model.id,
      reasoning: false,
      input: ["text"],
      cost: { input: 0, output: 0, cacheRead: 0, cacheWrite: 0 },
      contextWindow: model.context_window ?? 128000,
      maxTokens: model.max_tokens ?? 4096,
    })),
  });
}
```

This registers the fetched models before startup finishes.

```typescript
pi.registerProvider("my-llm", {
  baseUrl: "https://api.my-llm.com/v1",
  apiKey: "$MY_LLM_API_KEY",  // env var reference
  api: "openai-completions",  // which streaming API to use
  models: [
    {
      id: "my-llm-large",
      name: "My LLM Large",
      reasoning: true,        // supports extended thinking
      input: ["text", "image"],
      cost: {
        input: 3.0,           // $/million tokens
        output: 15.0,
        cacheRead: 0.3,
        cacheWrite: 3.75
      },
      contextWindow: 200000,
      maxTokens: 16384
    }
  ]
});
```

When `models` is provided, it **replaces** all existing models for that provider.

`apiKey` and custom header values use the same config value syntax as `models.json`: `!command` at the start executes a command for the whole value, `$ENV_VAR` and `${ENV_VAR}` interpolate environment variables, `$$` emits a literal `$`, and `$!` emits a literal `!`.

## Unregister Provider

Use `pi.unregisterProvider(name)` to remove a provider that was previously registered via `pi.registerProvider(name, ...)`:

```typescript
// Register
pi.registerProvider("my-llm", {
  baseUrl: "https://api.my-llm.com/v1",
  apiKey: "$MY_LLM_API_KEY",
  api: "openai-completions",
  models: [
    {
      id: "my-llm-large",
      name: "My LLM Large",
      reasoning: true,
      input: ["text", "image"],
      cost: { input: 3.0, output: 15.0, cacheRead: 0.3, cacheWrite: 3.75 },
      contextWindow: 200000,
      maxTokens: 16384
    }
  ]
});

// Later, remove it
pi.unregisterProvider("my-llm");
```

Unregistering removes that provider's dynamic models, API key fallback, OAuth provider registration, and custom stream handler registrations. Any built-in models or provider behavior that were overridden are restored.

Calls made after the initial extension load phase are applied immediately, so no `/reload` is required.

### API Types

The `api` field determines which streaming implementation is used:

| API | Use for |
|-----|---------|
| `anthropic-messages` | Anthropic Claude API and compatibles |
| `openai-completions` | OpenAI Chat Completions API and compatibles |
| `openai-responses` | OpenAI Responses API |
| `azure-openai-responses` | Azure OpenAI Responses API |
| `openai-codex-responses` | OpenAI Codex Responses API |
| `mistral-conversations` | Mistral SDK Conversations/Chat streaming |
| `google-generative-ai` | Google Generative AI API |
| `google-vertex` | Google Vertex AI API |
| `bedrock-converse-stream` | Amazon Bedrock Converse API |

Most OpenAI-compatible providers work with `openai-completions`. Use model-level `thinkingLevelMap` for model-specific thinking levels, and `compat` for provider quirks. The `xhigh` and `max` levels are opt-in, require non-null map entries, and may be separated by unsupported holes:

```typescript
models: [{
  id: "custom-model",
  // ...
  reasoning: true,
  thinkingLevelMap: {              // map pi levels to provider values; null hides unsupported levels
    minimal: null,
    low: null,
    medium: null,
    high: "default",
    xhigh: null,
    max: "max"
  },
  compat: {
    supportsDeveloperRole: false,   // use "system" instead of "developer"
    supportsReasoningEffort: true,
    maxTokensField: "max_tokens",   // instead of "max_completion_tokens"
    requiresToolResultName: true,   // tool results need name field
    thinkingFormat: "qwen",        // top-level enable_thinking: true
    cacheControlFormat: "anthropic" // Anthropic-style cache_control markers
  }
}]
```

Use `openrouter` for OpenRouter-style `reasoning: { effort }` controls. Use `together` for Together-style `reasoning: { enabled }` controls; with `supportsReasoningEffort`, it also sends `reasoning_effort`. Use `qwen-chat-template` for local Qwen-compatible servers that read `chat_template_kwargs.enable_thinking` and need `preserve_thinking`.
Use `cacheControlFormat: "anthropic"` for OpenAI-compatible providers that expose Anthropic-style prompt caching via `cache_control` on the system prompt, last tool definition, and last user, assistant, or tool-result text content.

For Anthropic-compatible providers using `api: "anthropic-messages"`, set `compat.forceAdaptiveThinking: true` on models or providers whose upstream model requires adaptive thinking (`thinking.type: "adaptive"` plus `output_config.effort`). Built-in adaptive Claude models set this automatically. Set `compat.allowEmptySignature: true` only for providers that emit empty thinking signatures and expect `signature: ""` on replay.

> Migration note: Mistral moved from `openai-completions` to `mistral-conversations`.
> Use `mistral-conversations` for native Mistral models.
> If you intentionally route Mistral-compatible/custom endpoints through `openai-completions`, set `compat` flags explicitly as needed.

### Auth Header

If your provider expects `Authorization: Bearer <key>` but doesn't use a standard API, set `authHeader: true`:

```typescript
pi.registerProvider("custom-api", {
  baseUrl: "https://api.example.com",
  apiKey: "$MY_API_KEY",
  authHeader: true,  // adds Authorization: Bearer header
  api: "openai-completions",
  models: [...]
});
```

The key is resolved for each request. An explicit request `Authorization` header takes precedence over the generated value.

## OAuth Support

Add OAuth/SSO authentication that integrates with `/login`:

```typescript
import type { OAuthCredentials, OAuthLoginCallbacks } from "@earendil-works/pi-ai";

pi.registerProvider("corporate-ai", {
  baseUrl: "https://ai.corp.com/v1",
  api: "openai-responses",
  models: [...],
  oauth: {
    name: "Corporate AI (SSO)",

    async login(callbacks: OAuthLoginCallbacks): Promise<OAuthCredentials> {
      const method = await callbacks.onSelect({
        message: "Select login method:",
        options: [
          { id: "browser", label: "Browser OAuth" },
          { id: "device", label: "Device code" }
        ]
      });
      if (!method) throw new Error("Login cancelled");

      let code: string;
      if (method === "device") {
        callbacks.onDeviceCode({
          userCode: "ABCD-1234",
          verificationUri: "https://sso.corp.com/device",
          intervalSeconds: 5,
          expiresInSeconds: 900
        });
        code = await pollDeviceCodeUntilComplete();
      } else {
        callbacks.onAuth({ url: "https://sso.corp.com/authorize?..." });
        code = await callbacks.onPrompt({ message: "Enter SSO code:" });
      }

      // Exchange for tokens (your implementation)
      const tokens = await exchangeCodeForTokens(code);

      return {
        refresh: tokens.refreshToken,
        access: tokens.accessToken,
        expires: Date.now() + tokens.expiresIn * 1000
      };
    },

    async refreshToken(credentials: OAuthCredentials): Promise<OAuthCredentials> {
      const tokens = await refreshAccessToken(credentials.refresh);
      return {
        refresh: tokens.refreshToken ?? credentials.refresh,
        access: tokens.accessToken,
        expires: Date.now() + tokens.expiresIn * 1000
      };
    },

    getApiKey(credentials: OAuthCredentials): string {
      return credentials.access;
    }
  }
});
```

After registration, users can authenticate via `/login corporate-ai`.

### OAuthLoginCallbacks

The `callbacks` object provides UI-neutral interactions for the provider-owned flow:

```typescript
interface OAuthLoginCallbacks {
  // Open URL in browser (for OAuth redirects)
  onAuth(params: { url: string }): void;

  // Show device code (for device authorization flow)
  onDeviceCode(params: {
    userCode: string;
    verificationUri: string;
    intervalSeconds?: number;
    expiresInSeconds?: number;
  }): void;

  // Show transient progress
  onProgress?(message: string): void;

  // Prompt user for input (for manual token entry)
  onPrompt(params: { message: string }): Promise<string>;

  // Show an interactive selector, e.g. to choose browser OAuth vs device code
  onSelect(params: {
    message: string;
    options: { id: string; label: string }[];
  }): Promise<string | undefined>;
}
```

### OAuthCredentials

Credentials are persisted in `~/.pi/agent/auth.json`:

```typescript
interface OAuthCredentials {
  refresh: string;   // Refresh token (for refreshToken())
  access: string;    // Access token (returned by getApiKey())
  expires: number;   // Expiration timestamp in milliseconds
}
```

## Custom Streaming API

For providers with non-standard APIs, implement `streamSimple`. Study the existing provider implementations before writing your own:

**Reference implementations:**
- [anthropic.ts](https://github.com/earendil-works/pi-mono/blob/main/packages/ai/src/providers/anthropic.ts) - Anthropic Messages API
- [mistral.ts](https://github.com/earendil-works/pi-mono/blob/main/packages/ai/src/providers/mistral.ts) - Mistral Conversations API
- [openai-completions.ts](https://github.com/earendil-works/pi-mono/blob/main/packages/ai/src/providers/openai-completions.ts) - OpenAI Chat Completions
- [openai-responses.ts](https://github.com/earendil-works/pi-mono/blob/main/packages/ai/src/providers/openai-responses.ts) - OpenAI Responses API
- [google.ts](https://github.com/earendil-works/pi-mono/blob/main/packages/ai/src/providers/google.ts) - Google Generative AI
- [amazon-bedrock.ts](https://github.com/earendil-works/pi-mono/blob/main/packages/ai/src/providers/amazon-bedrock.ts) - AWS Bedrock

### Stream Pattern

All providers follow the same pattern:

```typescript
import {
  type AssistantMessage,
  type AssistantMessageEventStream,
  type Context,
  type Model,
  type SimpleStreamOptions,
  calculateCost,
  createAssistantMessageEventStream,
} from "@earendil-works/pi-ai";

function streamMyProvider(
  model: Model<any>,
  context: Context,
  options?: SimpleStreamOptions
): AssistantMessageEventStream {
  const stream = createAssistantMessageEventStream();

  (async () => {
    // Initialize output message
    const output: AssistantMessage = {
      role: "assistant",
      content: [],
      api: model.api,
      provider: model.provider,
      model: model.id,
      usage: {
        input: 0,
        output: 0,
        cacheRead: 0,
        cacheWrite: 0,
        totalTokens: 0,
        cost: { input: 0, output: 0, cacheRead: 0, cacheWrite: 0, total: 0 },
      },
      stopReason: "pending",
      timestamp: Date.now(),
    };

    try {
      // Push start event
      stream.push({ type: "start", partial: output });

      // Make API request and process response...
      // Push content events as they arrive and set stopReason from the terminal event.
      if (output.stopReason === "pending") {
        throw new Error("Provider stream ended without a stop reason");
      }
      if (output.stopReason === "error" || output.stopReason === "aborted") {
        throw new Error(output.errorMessage || "An unknown error occurred");
      }

      // Push done event
      stream.push({
        type: "done",
        reason: output.stopReason,
        message: output
      });
      stream.end();
    } catch (error) {
      output.stopReason = options?.signal?.aborted ? "aborted" : "error";
      output.errorMessage = error instanceof Error ? error.message : String(error);
      stream.push({ type: "error", reason: output.stopReason, error: output });
      stream.end();
    }
  })();

  return stream;
}
```

### Event Types

Push events via `stream.push()` in this order:

1. `{ type: "start", partial: output }` - Stream started

2. Content events (repeatable, track `contentIndex` for each block):
   - `{ type: "text_start", contentIndex, partial }` - Text block started
   - `{ type: "text_delta", contentIndex, delta, partial }` - Text chunk
   - `{ type: "text_end", contentIndex, content, partial }` - Text block ended
   - `{ type: "thinking_start", contentIndex, partial }` - Thinking started
   - `{ type: "thinking_delta", contentIndex, delta, partial }` - Thinking chunk
   - `{ type: "thinking_end", contentIndex, content, partial }` - Thinking ended
   - `{ type: "toolcall_start", contentIndex, partial }` - Tool call started
   - `{ type: "toolcall_delta", contentIndex, delta, partial }` - Tool call JSON chunk
   - `{ type: "toolcall_end", contentIndex, toolCall, partial }` - Tool call ended

3. `{ type: "done", reason, message }` or `{ type: "error", reason, error }` - Stream ended

The `partial` field in each event contains the current `AssistantMessage` state. Update `output.content` as you receive data, then include `output` as the `partial`.

### Content Blocks

Add content blocks to `output.content` as they arrive:

```typescript
// Text block
output.content.push({ type: "text", text: "" });
stream.push({ type: "text_start", contentIndex: output.content.length - 1, partial: output });

// As text arrives
const block = output.content[contentIndex];
if (block.type === "text") {
  block.text += delta;
  stream.push({ type: "text_delta", contentIndex, delta, partial: output });
}

// When block completes
stream.push({ type: "text_end", contentIndex, content: block.text, partial: output });
```

### Tool Calls

Tool calls require accumulating JSON and parsing:

```typescript
// Start tool call
output.content.push({
  type: "toolCall",
  id: toolCallId,
  name: toolName,
  arguments: {}
});
stream.push({ type: "toolcall_start", contentIndex: output.content.length - 1, partial: output });

// Accumulate JSON
let partialJson = "";
partialJson += jsonDelta;
try {
  block.arguments = JSON.parse(partialJson);
} catch {}
stream.push({ type: "toolcall_delta", contentIndex, delta: jsonDelta, partial: output });

// Complete
stream.push({
  type: "toolcall_end",
  contentIndex,
  toolCall: { type: "toolCall", id, name, arguments: block.arguments },
  partial: output
});
```

### Usage and Cost

Update usage from API response and calculate cost:

```typescript
output.usage.input = response.usage.input_tokens;
output.usage.output = response.usage.output_tokens;
output.usage.cacheRead = response.usage.cache_read_tokens ?? 0;
output.usage.cacheWrite = response.usage.cache_write_tokens ?? 0;
output.usage.totalTokens = output.usage.input + output.usage.output +
                           output.usage.cacheRead + output.usage.cacheWrite;
calculateCost(model, output.usage);
```

### Context Overflow Errors

When a request exceeds the model's context window, pi can recover automatically by compacting the conversation and retrying. This recovery only kicks in if pi recognizes the failure as an overflow.

Detection runs on the finalized assistant message:

- `stopReason === "error"`
- `errorMessage` matches one of pi's known overflow patterns (see [`packages/ai/src/utils/overflow.ts`](https://github.com/earendil-works/pi-mono/blob/main/packages/ai/src/utils/overflow.ts))

If your provider returns overflow errors with a message pi does not recognize, normalize the error from the same extension that registers the provider. Use a `message_end` handler to rewrite the assistant message so its `errorMessage` starts with a phrase pi recognizes. The generic fallback `context_length_exceeded` is the safest choice.

```typescript
const MY_PROVIDER_OVERFLOW_PATTERN = /your provider's overflow phrase/i;

export default function (pi: ExtensionAPI) {
  pi.registerProvider("my-provider", { /* ... */ });

  pi.on("message_end", (event, ctx) => {
    const message = event.message;
    if (message.role !== "assistant") return;
    if (message.stopReason !== "error") return;
    if (
      message.provider !== "my-provider" &&
      ctx.model?.provider !== "my-provider"
    )
      return;

    const errorMessage = message.errorMessage ?? "";
    if (errorMessage.includes("context_length_exceeded")) return;
    if (!MY_PROVIDER_OVERFLOW_PATTERN.test(errorMessage)) return;

    return {
      message: {
        ...message,
        errorMessage: `context_length_exceeded: ${errorMessage}`,
      },
    };
  });
}
```

`message_end` runs before pi tracks the assistant message for auto-compaction, so the rewritten `errorMessage` is what pi checks. With this in place, pi will:

1. Detect the overflow from `errorMessage`.
2. Drop the failed assistant message from live context.
3. Run compaction.
4. Retry the request once.

Guard the rewrite carefully:

- Scope it to your provider (`message.provider` and `ctx.model?.provider`) so unrelated errors from other providers are untouched.
- Match a provider-specific pattern, not pi's generic overflow patterns. Rewriting rate-limit or throttling errors (`rate limit`, `too many requests`) would falsely trigger compaction instead of pi's normal retry-with-backoff path.
- Skip when `errorMessage` already includes `context_length_exceeded` so the handler is idempotent.

### Registration

Register your stream function:

```typescript
pi.registerProvider("my-provider", {
  baseUrl: "https://api.example.com",
  apiKey: "$MY_API_KEY",
  api: "my-custom-api",
  models: [...],
  streamSimple: streamMyProvider
});
```

## Testing Your Implementation

Test your provider against the same test suites used by built-in providers. Copy and adapt these test files from [packages/ai/test/](https://github.com/earendil-works/pi-mono/tree/main/packages/ai/test):

| Test | Purpose |
|------|---------|
| `stream.test.ts` | Basic streaming, text output |
| `tokens.test.ts` | Token counting and usage |
| `abort.test.ts` | AbortSignal handling |
| `empty.test.ts` | Empty/minimal responses |
| `context-overflow.test.ts` | Context window limits |
| `image-limits.test.ts` | Image input handling |
| `unicode-surrogate.test.ts` | Unicode edge cases |
| `tool-call-without-result.test.ts` | Tool call edge cases |
| `image-tool-result.test.ts` | Images in tool results |
| `total-tokens.test.ts` | Total token calculation |
| `cross-provider-handoff.test.ts` | Context handoff between providers |

Run tests with your provider/model pairs to verify compatibility.

## Config Reference

```typescript
interface ProviderConfig {
  /** Display name for the provider in UI such as /login. */
  name?: string;

  /** API endpoint URL. Required when defining models. */
  baseUrl?: string;

  /** API key literal, env interpolation ($ENV_VAR or ${ENV_VAR}), or !command. Required when defining models (unless oauth). */
  apiKey?: string;

  /** API type for streaming. Required at provider or model level when defining models. */
  api?: Api;

  /** Custom streaming implementation for non-standard APIs. */
  streamSimple?: (
    model: Model<Api>,
    context: Context,
    options?: SimpleStreamOptions
  ) => AssistantMessageEventStream;

  /** Custom headers to include in requests. Values use the same resolution syntax as apiKey. */
  headers?: Record<string, string>;

  /** If true, adds Authorization: Bearer header with the resolved API key. */
  authHeader?: boolean;

  /** Models to register. If provided, replaces all existing models for this provider. */
  models?: ProviderModelConfig[];

  /** OAuth provider for /login support. */
  oauth?: {
    name: string;
    login(callbacks: OAuthLoginCallbacks): Promise<OAuthCredentials>;
    refreshToken(credentials: OAuthCredentials): Promise<OAuthCredentials>;
    getApiKey(credentials: OAuthCredentials): string;
  };
}
```

## Model Definition Reference

```typescript
interface ProviderModelConfig {
  /** Model ID (e.g., "claude-sonnet-4-20250514"). */
  id: string;

  /** Display name (e.g., "Claude 4 Sonnet"). */
  name: string;

  /** API type override for this specific model. */
  api?: Api;

  /** API endpoint URL override for this specific model. */
  baseUrl?: string;

  /** Whether the model supports extended thinking. */
  reasoning: boolean;

  /** Maps pi thinking levels to provider/model-specific values; null marks a level unsupported. */
  thinkingLevelMap?: Partial<Record<"off" | "minimal" | "low" | "medium" | "high" | "xhigh" | "max", string | null>>;

  /** Supported input types. */
  input: ("text" | "image")[];

  /** Cost per million tokens (for usage tracking). */
  cost: {
    input: number;
    output: number;
    cacheRead: number;
    cacheWrite: number;
  };

  /** Maximum context window size in tokens. */
  contextWindow: number;

  /** Maximum output tokens. */
  maxTokens: number;

  /** Custom headers for this specific model. */
  headers?: Record<string, string>;

  /** Compatibility settings for the selected API. */
  compat?: {
    // openai-completions
    supportsStore?: boolean;
    supportsDeveloperRole?: boolean;
    supportsReasoningEffort?: boolean;
    supportsUsageInStreaming?: boolean;
    supportsStrictMode?: boolean;
    supportsOpenAIGrammarTools?: boolean; // openai-completions/openai-responses; false falls back to normal function tools
    maxTokensField?: "max_completion_tokens" | "max_tokens";
    requiresToolResultName?: boolean;
    requiresAssistantAfterToolResult?: boolean;
    requiresThinkingAsText?: boolean;
    requiresReasoningContentOnAssistantMessages?: boolean;
    thinkingFormat?: "openai" | "openrouter" | "deepseek" | "together" | "zai" | "qwen" | "chat-template" | "qwen-chat-template" | "string-thinking" | "ant-ling";
    chatTemplateKwargs?: Record<string, string | number | boolean | null | { "$var": "thinking.enabled" | "thinking.effort"; omitWhenOff?: boolean }>;
    cacheControlFormat?: "anthropic";
    sessionAffinityFormat?: "openai" | "openai-nosession" | "openrouter";
    sendSessionAffinityHeaders?: boolean;

    // anthropic-messages
    supportsEagerToolInputStreaming?: boolean;
    supportsLongCacheRetention?: boolean;
    sendSessionAffinityHeaders?: boolean;
    supportsCacheControlOnTools?: boolean;
    forceAdaptiveThinking?: boolean;
    allowEmptySignature?: boolean;
    supportsStrictTools?: boolean;
  };
}
```

`openrouter` sends `reasoning: { effort }`. `deepseek` sends `thinking: { type: "enabled" | "disabled" }` and `reasoning_effort` when enabled. `together` sends `reasoning: { enabled }` and also `reasoning_effort` when `supportsReasoningEffort` is enabled. `qwen` is for DashScope-style top-level `enable_thinking`. Use `qwen-chat-template` for local Qwen-compatible servers that read `chat_template_kwargs.enable_thinking` and need `preserve_thinking`. Use `chat-template` for configurable `chat_template_kwargs`, for example DeepSeek V3.x behind vLLM with `chatTemplateKwargs: { "thinking": { "$var": "thinking.enabled" } }`.
`cacheControlFormat: "anthropic"` applies Anthropic-style `cache_control` markers to the system prompt, last tool definition, and last user, assistant, or tool-result text content.
