diff --git a/src/api/providers/openai.ts b/src/api/providers/openai.ts
index 2d56e20c89..1008ecc5c4 100644
--- a/src/api/providers/openai.ts
+++ b/src/api/providers/openai.ts
@@ -99,7 +99,11 @@ export class OpenAiHandler extends BaseProvider implements SingleCompletionHandl
 		const modelId = this.options.openAiModelId ?? ""
 		const enabledR1Format = this.options.openAiR1FormatEnabled ?? false
 		const isAzureAiInference = this._isAzureAiInference(modelUrl)
-		const deepseekReasoner = modelId.includes("deepseek-reasoner") || enabledR1Format
+		const deepseekReasoner =
+			modelId.includes("deepseek-reasoner") ||
+			modelId.includes("deepseek-v4-pro") ||
+			modelId.includes("deepseek-v4-flash") ||
+			enabledR1Format
 
 		if (modelId.includes("o1") || modelId.includes("o3") || modelId.includes("o4")) {
 			yield* this.handleO3FamilyMessage(modelId, systemPrompt, messages, metadata)
@@ -340,7 +344,11 @@ export class OpenAiHandler extends BaseProvider implements SingleCompletionHandl
 		})
 
 		const enabledR1Format = this.options.openAiR1FormatEnabled ?? false
-		const deepseekReasoner = id.includes("deepseek-reasoner") || enabledR1Format
+		const deepseekReasoner =
+			id.includes("deepseek-reasoner") ||
+			id.includes("deepseek-v4-pro") ||
+			id.includes("deepseek-v4-flash") ||
+			enabledR1Format
 
 		return {
 			id,
diff --git a/src/api/transform/openai-format.ts b/src/api/transform/openai-format.ts
index 8974dd599b..3596343335 100644
--- a/src/api/transform/openai-format.ts
+++ b/src/api/transform/openai-format.ts
@@ -317,6 +317,9 @@ export function convertToOpenAiMessages(
 				if (mapped) {
 					;(baseMessage as any).reasoning_details = mapped
 				}
+				if (messageWithDetails.reasoning_content !== undefined) {
+					;(baseMessage as any).reasoning_content = messageWithDetails.reasoning_content
+				}
 			}
 
 			openAiMessages.push(baseMessage)
@@ -494,6 +497,11 @@ export function convertToOpenAiMessages(
 					baseMessage.reasoning_details = mapped
 				}
 
+				// Pass through reasoning_content for DeepSeek V4/R1 thinking mode
+				if (messageWithDetails.reasoning_content !== undefined) {
+					;(baseMessage as any).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) {
diff --git a/src/core/task/Task.ts b/src/core/task/Task.ts
index 13da6f23b2..b68c5dcaf0 100644
--- a/src/core/task/Task.ts
+++ b/src/core/task/Task.ts
@@ -4469,6 +4469,12 @@ export class Task extends EventEmitter<TaskEvents> implements TaskLike {
 
 		const cleanConversationHistory: (Anthropic.Messages.MessageParam | ReasoningItemForRequest)[] = []
 
+		// Check if the current model requires reasoning to be preserved (e.g., DeepSeek V4/R1).
+		// When preserveReasoning is true, ALL assistant messages must include reasoning_content
+		// to satisfy the API protocol. Messages from other models (e.g., Gemini) that lack
+		// reasoning_content need to be backfilled with an empty string.
+		const shouldPreserveForApi = this.api.getModel().info.preserveReasoning === true
+
 		for (const msg of messages) {
 			// Standalone reasoning: send encrypted, skip plain text
 			if (msg.type === "reasoning") {
@@ -4595,10 +4601,22 @@ export class Task extends EventEmitter<TaskEvents> implements TaskLike {
 
 			// Default path for regular messages (no embedded reasoning)
 			if (msg.role) {
-				cleanConversationHistory.push({
+				const baseMessage: any = {
 					role: msg.role,
 					content: msg.content as Anthropic.Messages.ContentBlockParam[] | string,
-				})
+				}
+
+				// DeepSeek thinking mode requires ALL assistant messages to have reasoning_content.
+				// If a message from another model (e.g., Gemini) lacks it, add an empty placeholder
+				// to prevent 400 errors when switching back to DeepSeek mid-conversation.
+				if (msg.role === "assistant" && shouldPreserveForApi) {
+					// Use a space instead of an empty string. Some proxies/gateways or
+					// internal SDK logic may silently strip empty string fields, causing
+					// DeepSeek to reject the request with 400.
+					baseMessage.reasoning_content = " "
+				}
+
+				cleanConversationHistory.push(baseMessage)
 			}
 		}