diff --git a/src/api/transform/__tests__/r1-format.spec.ts b/src/api/transform/__tests__/r1-format.spec.ts
index 3d875e9392..7fab8bae84 100644
--- a/src/api/transform/__tests__/r1-format.spec.ts
+++ b/src/api/transform/__tests__/r1-format.spec.ts
@@ -614,6 +614,86 @@ describe("convertToR1Format", () => {
 				// Most importantly: NO user message after tool message
 				expect(result.filter((m) => m.role === "user")).toHaveLength(1)
 			})
+
+			it("should always include empty reasoning_content on assistant messages when mergeToolResultText is true", () => {
+				// This simulates a condensed summary message that has no reasoning_content
+				const input: Anthropic.Messages.MessageParam[] = [
+					{ role: "user", content: "Start" },
+					{
+						role: "assistant",
+						content: "This is a condensed summary of the previous conversation.",
+					},
+				]
+
+				const result = convertToR1Format(input, { mergeToolResultText: true })
+
+				expect(result).toHaveLength(2)
+				expect(result[0]).toEqual({ role: "user", content: "Start" })
+				// Should have empty reasoning_content for thinking models
+				expect((result[1] as any).reasoning_content).toBe("")
+				expect(result[1]).toEqual({
+					role: "assistant",
+					content: "This is a condensed summary of the previous conversation.",
+					reasoning_content: "",
+				})
+			})
+
+			it("should NOT add reasoning_content on assistant messages when mergeToolResultText is false", () => {
+				// For non-thinking models, reasoning_content should not be added
+				const input: Anthropic.Messages.MessageParam[] = [
+					{ role: "user", content: "Start" },
+					{
+						role: "assistant",
+						content: "This is a regular response.",
+					},
+				]
+
+				// Without mergeToolResultText option (default behavior)
+				const result = convertToR1Format(input)
+
+				expect(result).toHaveLength(2)
+				// Should NOT have reasoning_content for non-thinking models
+				expect((result[1] as any).reasoning_content).toBeUndefined()
+				expect(result[1]).toEqual({
+					role: "assistant",
+					content: "This is a regular response.",
+				})
+			})
+
+			it("should include empty reasoning_content on assistant messages with array content when mergeToolResultText is true", () => {
+				const input: Anthropic.Messages.MessageParam[] = [
+					{ role: "user", content: "Start" },
+					{
+						role: "assistant",
+						content: [{ type: "text", text: "Array content response without reasoning" }],
+					},
+				]
+
+				const result = convertToR1Format(input, { mergeToolResultText: true })
+
+				expect(result).toHaveLength(2)
+				// Should have empty reasoning_content for thinking models even with array content
+				expect((result[1] as any).reasoning_content).toBe("")
+			})
+
+			it("should preserve existing reasoning_content when present even with mergeToolResultText", () => {
+				const input = [
+					{ role: "user" as const, content: "Start" },
+					{
+						role: "assistant" as const,
+						content: "Response with reasoning",
+						reasoning_content: "My actual reasoning process",
+					},
+				]
+
+				const result = convertToR1Format(input as Anthropic.Messages.MessageParam[], {
+					mergeToolResultText: true,
+				})
+
+				expect(result).toHaveLength(2)
+				// Should preserve the actual reasoning_content, not replace with empty
+				expect((result[1] as any).reasoning_content).toBe("My actual reasoning process")
+			})
 		})
 	})
 })
diff --git a/src/api/transform/r1-format.ts b/src/api/transform/r1-format.ts
index 8231e24f76..839678ea60 100644
--- a/src/api/transform/r1-format.ts
+++ b/src/api/transform/r1-format.ts
@@ -190,12 +190,21 @@ export function convertToR1Format(
 				// Use reasoning from content blocks if not provided at top level
 				const finalReasoning = reasoningContent || extractedReasoning
 
+				// For thinking models (indicated by mergeToolResultText), always include reasoning_content
+				// DeepSeek's reasoner API requires this field to be present on all assistant messages
+				const shouldAlwaysIncludeReasoningContent = options?.mergeToolResultText === true
+
 				const assistantMessage: DeepSeekAssistantMessage = {
 					role: "assistant",
 					content: textParts.length > 0 ? textParts.join("\n") : null,
 					...(toolCalls.length > 0 && { tool_calls: toolCalls }),
 					// Preserve reasoning_content for DeepSeek interleaved thinking
-					...(finalReasoning && { reasoning_content: finalReasoning }),
+					// Always include for thinking models (even if empty) to satisfy DeepSeek's API requirement
+					...(finalReasoning
+						? { reasoning_content: finalReasoning }
+						: shouldAlwaysIncludeReasoningContent
+							? { reasoning_content: "" }
+							: {}),
 				}
 
 				// Check if we can merge with the last message (only if no tool calls)
@@ -229,10 +238,17 @@ export function convertToR1Format(
 						;(lastMessage as DeepSeekAssistantMessage).reasoning_content = reasoningContent
 					}
 				} else {
+					// For thinking models (indicated by mergeToolResultText), always include reasoning_content
+					const shouldAlwaysIncludeReasoningContent = options?.mergeToolResultText === true
 					const assistantMessage: DeepSeekAssistantMessage = {
 						role: "assistant",
 						content: message.content,
-						...(reasoningContent && { reasoning_content: reasoningContent }),
+						// Always include for thinking models (even if empty) to satisfy DeepSeek's API requirement
+						...(reasoningContent
+							? { reasoning_content: reasoningContent }
+							: shouldAlwaysIncludeReasoningContent
+								? { reasoning_content: "" }
+								: {}),
 					}
 					result.push(assistantMessage)
 				}