[Fix] DeepSeek 400 errors when mixed-model tasks continue long conversations

src/api/transform/__tests__/openai-format.spec.ts

@@ -112,6 +112,26 @@ describe("convertToOpenAiMessages", () => {
 		})
 	})
 
+	it("preserves assistant reasoning_content for OpenAI-compatible replay", () => {
+		const anthropicMessages = [
+			{
+				role: "assistant",
+				content: "First answer",
+				reasoning_content: "First reasoning",
+			},
+			{
+				role: "assistant",
+				content: [{ type: "text", text: "Second answer" }],
+				reasoning_content: "Second reasoning",
+			},
+		] as unknown as Anthropic.Messages.MessageParam[]
+
+		const openAiMessages = convertToOpenAiMessages(anthropicMessages)
+
+		expect((openAiMessages[0] as any).reasoning_content).toBe("First reasoning")
+		expect((openAiMessages[1] as any).reasoning_content).toBe("Second reasoning")
+	})
+
 	it("should handle user messages with tool results (no normalization without normalizeToolCallId)", () => {
 		const anthropicMessages: Anthropic.Messages.MessageParam[] = [
 			{

src/api/transform/openai-format.ts

@@ -307,7 +307,10 @@ export function convertToOpenAiMessages(
 			// If a message also contains reasoning_details (Gemini 3 / xAI / o-series, etc.),
 			// we must preserve it here as well.
 			const messageWithDetails = anthropicMessage as any
-			const baseMessage: OpenAI.Chat.ChatCompletionMessageParam & { reasoning_details?: any[] } = {
+			const baseMessage: OpenAI.Chat.ChatCompletionMessageParam & {
+				reasoning_details?: any[]
+				reasoning_content?: string
+			} = {
 				role: anthropicMessage.role,
 				content: anthropicMessage.content,
 			}
@@ -317,6 +320,13 @@ export function convertToOpenAiMessages(
 				if (mapped) {
 					;(baseMessage as any).reasoning_details = mapped
 				}
+
+				if (
+					typeof messageWithDetails.reasoning_content === "string" &&
+					messageWithDetails.reasoning_content.trim().length > 0
+				) {
+					baseMessage.reasoning_content = messageWithDetails.reasoning_content
+				}
 			}
 
 			openAiMessages.push(baseMessage)
@@ -480,6 +490,7 @@ export function convertToOpenAiMessages(
 				// when sending messages back to some APIs.
 				const baseMessage: OpenAI.Chat.ChatCompletionAssistantMessageParam & {
 					reasoning_details?: any[]
+					reasoning_content?: string
 				} = {
 					role: "assistant",
 					// Use empty string instead of undefined for providers like Gemini (via OpenRouter)
@@ -494,6 +505,13 @@ export function convertToOpenAiMessages(
 					baseMessage.reasoning_details = mapped
 				}
 
+				if (
+					typeof messageWithDetails.reasoning_content === "string" &&
+					messageWithDetails.reasoning_content.trim().length > 0
+				) {
+					baseMessage.reasoning_content = messageWithDetails.reasoning_content
+				}
+
 				// Add tool_calls after reasoning_details
 				// Cannot be an empty array. API expects an array with minimum length 1, and will respond with an error if it's empty
 				if (tool_calls.length > 0) {

src/core/task/Task.ts

@@ -4355,6 +4355,17 @@ export class Task extends EventEmitter<TaskEvents> implements TaskLike {
 		const cleanConversationHistory: (Anthropic.Messages.MessageParam | ReasoningItemForRequest)[] = []
 
 		for (const msg of messages) {
+			const preservedReasoningContent =
+				msg.role === "assistant" &&
+				typeof (msg as ApiMessage).reasoning_content === "string" &&
+				(msg as ApiMessage).reasoning_content!.trim().length > 0
+					? (msg as ApiMessage).reasoning_content
+					: undefined
+			const shouldReplayReasoningContent =
+				msg.role === "assistant" &&
+				this.api.getModel().info.preserveReasoning === true &&
+				!!preservedReasoningContent
+
 			// Standalone reasoning: send encrypted, skip plain text
 			if (msg.type === "reasoning") {
 				if (msg.encrypted_content) {
@@ -4442,40 +4453,42 @@ export class Task extends EventEmitter<TaskEvents> implements TaskLike {
 					continue
 				} else if (hasPlainTextReasoning) {
 					// Check if the model's preserveReasoning flag is set
-					// If true, include the reasoning block in API requests
-					// If false/undefined, strip it out (stored for history only, not sent back to API)
-					const shouldPreserveForApi = this.api.getModel().info.preserveReasoning === true
+					// Replay preserved reasoning_content only when this exact message was
+					// stored with an explicit reasoning_content payload. This avoids
+					// converting unrelated reasoning blocks from other models into
+					// DeepSeek/Z.ai/MiMo continuation history during mixed-model tasks.
 					let assistantContent: Anthropic.Messages.MessageParam["content"]
 
-					if (shouldPreserveForApi) {
-						// Include reasoning block in the content sent to API
-						assistantContent = contentArray
+					if (rest.length === 0) {
+						assistantContent = ""
+					} else if (rest.length === 1 && rest[0].type === "text") {
+						assistantContent = (rest[0] as Anthropic.Messages.TextBlockParam).text
 					} else {
-						// Strip reasoning out - stored for history only, not sent back to API
-						if (rest.length === 0) {
-							assistantContent = ""
-						} else if (rest.length === 1 && rest[0].type === "text") {
-							assistantContent = (rest[0] as Anthropic.Messages.TextBlockParam).text
-						} else {
-							assistantContent = rest
-						}
+						assistantContent = rest
 					}
 
-					cleanConversationHistory.push({
+					const assistantMessage: Anthropic.Messages.MessageParam & { reasoning_content?: string } = {
 						role: "assistant",
 						content: assistantContent,
-					} satisfies Anthropic.Messages.MessageParam)
+						...(shouldReplayReasoningContent ? { reasoning_content: preservedReasoningContent } : {}),
+					}
+
+					cleanConversationHistory.push(assistantMessage)
 
 					continue
 				}
 			}
 
 			// Default path for regular messages (no embedded reasoning)
 			if (msg.role) {
-				cleanConversationHistory.push({
+				const messageForRequest: Anthropic.Messages.MessageParam & { reasoning_content?: string } = {
 					role: msg.role,
 					content: msg.content as Anthropic.Messages.ContentBlockParam[] | string,
-				})
+					...(msg.role === "assistant" && shouldReplayReasoningContent
+						? { reasoning_content: preservedReasoningContent }
+						: {}),
+				}
+				cleanConversationHistory.push(messageForRequest)
 			}
 		}
 

src/core/task/__tests__/Task.persistence.spec.ts

@@ -416,6 +416,54 @@ describe("Task persistence", () => {
 		})
 	})
 
+	describe("buildCleanConversationHistory", () => {
+		it("replays reasoning_content only for assistant turns that were explicitly marked for it", () => {
+			const task = new Task({
+				provider: mockProvider,
+				apiConfiguration: mockApiConfig,
+				task: "test task",
+				startTask: false,
+			})
+
+			task.api = {
+				getModel: vi.fn().mockReturnValue({
+					id: "deepseek-v4-pro",
+					info: { preserveReasoning: true },
+				}),
+			} as any
+
+			const result = (task as any).buildCleanConversationHistory([
+				{
+					role: "assistant",
+					content: [
+						{ type: "reasoning", text: "Codex reasoning", summary: [] },
+						{ type: "text", text: "Codex answer" },
+					],
+				},
+				{
+					role: "assistant",
+					content: [
+						{ type: "reasoning", text: "DeepSeek reasoning", summary: [] },
+						{ type: "text", text: "DeepSeek answer" },
+					],
+					reasoning_content: "DeepSeek reasoning",
+				},
+			])
+
+			expect(result).toEqual([
+				{
+					role: "assistant",
+					content: "Codex answer",
+				},
+				{
+					role: "assistant",
+					content: "DeepSeek answer",
+					reasoning_content: "DeepSeek reasoning",
+				},
+			])
+		})
+	})
+
 	// ── flushPendingToolResultsToHistory — save failure/success ───────────
 
 	describe("flushPendingToolResultsToHistory persistence", () => {

src/core/task/__tests__/apiConversationHistory.spec.ts

@@ -39,7 +39,9 @@ describe("prepareApiConversationMessage", () => {
 		const result = prepareApiConversationMessage({
 			message: { role: "assistant", content: "answer" },
 			reasoning: "visible reasoning",
-			api: {} as any,
+			api: {
+				getModel: () => ({ info: {} }),
+			} as any,
 			apiConfiguration: { apiProvider: "openrouter", openRouterModelId: "openai/gpt-4" } as any,
 			apiConversationHistory: [],
 		}) as any
@@ -50,11 +52,31 @@ describe("prepareApiConversationMessage", () => {
 		])
 	})
 
+	it("stores reasoning_content for OpenAI-format models that need reasoning replay", () => {
+		const result = prepareApiConversationMessage({
+			message: { role: "assistant", content: "answer" },
+			reasoning: "visible reasoning",
+			api: {
+				getModel: () => ({ info: { preserveReasoning: true } }),
+			} as any,
+			apiConfiguration: { apiProvider: "deepseek", apiModelId: "deepseek-v4-pro" } as any,
+			apiConversationHistory: [],
+		}) as any
+
+		expect(result.reasoning_content).toBe("visible reasoning")
+		expect(result.content).toEqual([
+			{ type: "reasoning", text: "visible reasoning", summary: [] },
+			{ type: "text", text: "answer" },
+		])
+	})
+
 	it("falls back to generic reasoning blocks for Anthropic messages without thought signatures", () => {
 		const result = prepareApiConversationMessage({
 			message: { role: "assistant", content: "answer" },
 			reasoning: "private reasoning",
-			api: {} as any,
+			api: {
+				getModel: () => ({ info: {} }),
+			} as any,
 			apiConfiguration: { apiProvider: "anthropic", apiModelId: "claude-3-5-sonnet" } as any,
 			apiConversationHistory: [],
 		}) as any
@@ -70,6 +92,7 @@ describe("prepareApiConversationMessage", () => {
 		const result = prepareApiConversationMessage({
 			message: { role: "assistant", content: [{ type: "text", text: "answer" }] },
 			api: {
+				getModel: () => ({ info: {} }),
 				getEncryptedContent: () => ({ encrypted_content: "encrypted", id: "reasoning-1" }),
 			} as any,
 			apiConfiguration: { apiProvider: "openrouter", openRouterModelId: "openai/gpt-4" } as any,
@@ -86,6 +109,7 @@ describe("prepareApiConversationMessage", () => {
 		const result = prepareApiConversationMessage({
 			message: { role: "assistant", content: "answer" },
 			api: {
+				getModel: () => ({ info: {} }),
 				getThoughtSignature: () => "signature-1",
 				getReasoningDetails: () => [{ type: "reasoning", text: "detail" }],
 			} as any,

src/core/task/apiConversationHistory.ts

@@ -54,10 +54,13 @@ function prepareAssistantMessage(
 		modelId,
 	)
 	const isAnthropicProtocol = apiProtocol === "anthropic"
+	const shouldPersistReasoningContent =
+		apiProtocol === "openai" && reasoning && !reasoningDetails && handler.getModel().info.preserveReasoning === true
 
 	const messageWithTs: any = {
 		...message,
 		...(responseId ? { id: responseId } : {}),
+		...(shouldPersistReasoningContent ? { reasoning_content: reasoning } : {}),
 		ts: Date.now(),
 	}