fix(ai): Use API type instead of model for message compatibility checks

- Add getApi() method to all providers to identify the API type
- Add api field to AssistantMessage to track which API generated it
- Update transformMessages to check API compatibility instead of model
- Fixes issue where OpenAI Responses API failed when switching models
- Preserves thinking blocks and signatures when staying within same API

packages/ai/src/providers/anthropic.ts

@@ -77,10 +77,15 @@ export class AnthropicLLM implements LLM<AnthropicLLMOptions> {
 		return this.modelInfo;
 	}
 
+	getApi(): string {
+		return "anthropic-messages";
+	}
+
 	async generate(context: Context, options?: AnthropicLLMOptions): Promise<AssistantMessage> {
 		const output: AssistantMessage = {
 			role: "assistant",
 			content: [],
+			api: this.getApi(),
 			provider: this.modelInfo.provider,
 			model: this.modelInfo.id,
 			usage: {
@@ -260,7 +265,7 @@ export class AnthropicLLM implements LLM<AnthropicLLMOptions> {
 		const params: MessageParam[] = [];
 
 		// Transform messages for cross-provider compatibility
-		const transformedMessages = transformMessages(messages, this.modelInfo);
+		const transformedMessages = transformMessages(messages, this.modelInfo, this.getApi());
 
 		for (const msg of transformedMessages) {
 			if (msg.role === "user") {
@@ -290,9 +295,12 @@ export class AnthropicLLM implements LLM<AnthropicLLMOptions> {
 							};
 						}
 					});
+					const filteredBlocks = !this.modelInfo?.input.includes("image")
+						? blocks.filter((b) => b.type !== "image")
+						: blocks;
 					params.push({
 						role: "user",
-						content: blocks,
+						content: filteredBlocks,
 					});
 				}
 			} else if (msg.role === "assistant") {

packages/ai/src/providers/google.ts

@@ -6,6 +6,7 @@ import {
 	type GenerateContentParameters,
 	GoogleGenAI,
 	type Part,
+	setDefaultBaseUrls,
 } from "@google/genai";
 import { calculateCost } from "../models.js";
 import type {
@@ -52,10 +53,15 @@ export class GoogleLLM implements LLM<GoogleLLMOptions> {
 		return this.modelInfo;
 	}
 
+	getApi(): string {
+		return "google-generative-ai";
+	}
+
 	async generate(context: Context, options?: GoogleLLMOptions): Promise<AssistantMessage> {
 		const output: AssistantMessage = {
 			role: "assistant",
 			content: [],
+			api: this.getApi(),
 			provider: this.modelInfo.provider,
 			model: this.modelInfo.id,
 			usage: {
@@ -247,7 +253,7 @@ export class GoogleLLM implements LLM<GoogleLLMOptions> {
 		const contents: Content[] = [];
 
 		// Transform messages for cross-provider compatibility
-		const transformedMessages = transformMessages(messages, this.modelInfo);
+		const transformedMessages = transformMessages(messages, this.modelInfo, this.getApi());
 
 		for (const msg of transformedMessages) {
 			if (msg.role === "user") {
@@ -272,9 +278,12 @@ export class GoogleLLM implements LLM<GoogleLLMOptions> {
 							};
 						}
 					});
+					const filteredParts = !this.modelInfo?.input.includes("image")
+						? parts.filter((p) => p.text !== undefined)
+						: parts;
 					contents.push({
 						role: "user",
-						parts,
+						parts: filteredParts,
 					});
 				}
 			} else if (msg.role === "assistant") {

packages/ai/src/providers/openai-completions.ts

@@ -48,10 +48,15 @@ export class OpenAICompletionsLLM implements LLM<OpenAICompletionsLLMOptions> {
 		return this.modelInfo;
 	}
 
+	getApi(): string {
+		return "openai-completions";
+	}
+
 	async generate(request: Context, options?: OpenAICompletionsLLMOptions): Promise<AssistantMessage> {
 		const output: AssistantMessage = {
 			role: "assistant",
 			content: [],
+			api: this.getApi(),
 			provider: this.modelInfo.provider,
 			model: this.modelInfo.id,
 			usage: {
@@ -313,7 +318,7 @@ export class OpenAICompletionsLLM implements LLM<OpenAICompletionsLLMOptions> {
 		const params: ChatCompletionMessageParam[] = [];
 
 		// Transform messages for cross-provider compatibility
-		const transformedMessages = transformMessages(messages, this.modelInfo);
+		const transformedMessages = transformMessages(messages, this.modelInfo, this.getApi());
 
 		// Add system prompt if provided
 		if (systemPrompt) {
@@ -353,9 +358,12 @@ export class OpenAICompletionsLLM implements LLM<OpenAICompletionsLLMOptions> {
 							} satisfies ChatCompletionContentPartImage;
 						}
 					});
+					const filteredContent = !this.modelInfo?.input.includes("image")
+						? content.filter((c) => c.type !== "image_url")
+						: content;
 					params.push({
 						role: "user",
-						content,
+						content: filteredContent,
 					});
 				}
 			} else if (msg.role === "assistant") {

packages/ai/src/providers/openai-responses.ts

@@ -51,10 +51,15 @@ export class OpenAIResponsesLLM implements LLM<OpenAIResponsesLLMOptions> {
 		return this.modelInfo;
 	}
 
+	getApi(): string {
+		return "openai-responses";
+	}
+
 	async generate(request: Context, options?: OpenAIResponsesLLMOptions): Promise<AssistantMessage> {
 		const output: AssistantMessage = {
 			role: "assistant",
 			content: [],
+			api: this.getApi(),
 			provider: this.modelInfo.provider,
 			model: this.modelInfo.id,
 			usage: {
@@ -287,7 +292,7 @@ export class OpenAIResponsesLLM implements LLM<OpenAIResponsesLLMOptions> {
 		const input: ResponseInput = [];
 
 		// Transform messages for cross-provider compatibility
-		const transformedMessages = transformMessages(messages, this.modelInfo);
+		const transformedMessages = transformMessages(messages, this.modelInfo, this.getApi());
 
 		// Add system prompt if provided
 		if (systemPrompt) {
@@ -324,9 +329,12 @@ export class OpenAIResponsesLLM implements LLM<OpenAIResponsesLLMOptions> {
 							} satisfies ResponseInputImage;
 						}
 					});
+					const filteredContent = !this.modelInfo?.input.includes("image")
+						? content.filter((c) => c.type !== "input_image")
+						: content;
 					input.push({
 						role: "user",
-						content,
+						content: filteredContent,
 					});
 				}
 			} else if (msg.role === "assistant") {

packages/ai/src/providers/utils.ts

@@ -12,7 +12,7 @@ import type { AssistantMessage, Message, Model } from "../types.js";
  * @param model The target model that will process these messages
  * @returns A copy of the messages array with transformations applied
  */
-export function transformMessages(messages: Message[], model: Model): Message[] {
+export function transformMessages(messages: Message[], model: Model, api: string): Message[] {
 	return messages.map((msg) => {
 		// User and toolResult messages pass through unchanged
 		if (msg.role === "user" || msg.role === "toolResult") {
@@ -23,8 +23,8 @@ export function transformMessages(messages: Message[], model: Model): Message[]
 		if (msg.role === "assistant") {
 			const assistantMsg = msg as AssistantMessage;
 
-			// If message is from the same provider and model, keep as-is
+			// If message is from the same provider and API, keep as is
-			if (assistantMsg.provider === model.provider && assistantMsg.model === model.id) {
+			if (assistantMsg.provider === model.provider && assistantMsg.api === api) {
 				return msg;
 			}
 

packages/ai/src/types.ts

@@ -8,6 +8,7 @@ export interface LLMOptions {
 export interface LLM<T extends LLMOptions> {
 	generate(request: Context, options?: T): Promise<AssistantMessage>;
 	getModel(): Model;
+	getApi(): string;
 }
 
 export interface TextContent {
@@ -59,6 +60,7 @@ export interface UserMessage {
 export interface AssistantMessage {
 	role: "assistant";
 	content: (TextContent | ThinkingContent | ToolCall)[];
+	api: string;
 	provider: string;
 	model: string;
 	usage: Usage;