fix(ai-sdk): convert message-level reasoning_content to reasoning part

Author: hannesrudolph

src/api/transform/__tests__/ai-sdk.spec.ts

@@ -354,6 +354,51 @@ describe("AI SDK conversion utilities", () => {
 				],
 			})
 		})
+
+		it("converts assistant message-level reasoning_content to reasoning part", () => {
+			const messages: Anthropic.Messages.MessageParam[] = [
+				{
+					role: "assistant",
+					content: [{ type: "text", text: "Answer" }],
+					reasoning_content: "Thinking...",
+				} as any,
+			]
+
+			const result = convertToAiSdkMessages(messages)
+
+			expect(result).toHaveLength(1)
+			expect(result[0]).toEqual({
+				role: "assistant",
+				content: [
+					{ type: "reasoning", text: "Thinking..." },
+					{ type: "text", text: "Answer" },
+				],
+			})
+		})
+
+		it("prefers message-level reasoning_content over reasoning blocks", () => {
+			const messages: Anthropic.Messages.MessageParam[] = [
+				{
+					role: "assistant",
+					content: [
+						{ type: "reasoning" as any, text: "BLOCK" },
+						{ type: "text", text: "Answer" },
+					],
+					reasoning_content: "MSG",
+				} as any,
+			]
+
+			const result = convertToAiSdkMessages(messages)
+
+			expect(result).toHaveLength(1)
+			expect(result[0]).toEqual({
+				role: "assistant",
+				content: [
+					{ type: "reasoning", text: "MSG" },
+					{ type: "text", text: "Answer" },
+				],
+			})
+		})
 	})
 
 	describe("convertToolsForAiSdk", () => {

src/api/transform/ai-sdk.ts

@@ -127,6 +127,10 @@ export function convertToAiSdkMessages(
 			} else if (message.role === "assistant") {
 				const textParts: string[] = []
 				const reasoningParts: string[] = []
+				const reasoningContent = (() => {
+					const maybe = (message as unknown as { reasoning_content?: unknown }).reasoning_content
+					return typeof maybe === "string" && maybe.length > 0 ? maybe : undefined
+				})()
 				const toolCalls: Array<{
 					type: "tool-call"
 					toolCallId: string
@@ -154,6 +158,10 @@ export function convertToAiSdkMessages(
 					// Task stores reasoning as a content block (type: "reasoning") and Anthropic extended
 					// thinking as (type: "thinking"). Convert both to AI SDK's reasoning part.
 					if ((part as unknown as { type?: string }).type === "reasoning") {
+						// If message-level reasoning_content is present, treat it as canonical and
+						// avoid mixing it with content-block reasoning (which can cause duplication).
+						if (reasoningContent) continue
+
 						const text = (part as unknown as { text?: string }).text
 						if (typeof text === "string" && text.length > 0) {
 							reasoningParts.push(text)
@@ -162,6 +170,8 @@ export function convertToAiSdkMessages(
 					}
 
 					if ((part as unknown as { type?: string }).type === "thinking") {
+						if (reasoningContent) continue
+
 						const thinking = (part as unknown as { thinking?: string }).thinking
 						if (typeof thinking === "string" && thinking.length > 0) {
 							reasoningParts.push(thinking)
@@ -176,7 +186,9 @@ export function convertToAiSdkMessages(
 					| { type: "tool-call"; toolCallId: string; toolName: string; input: unknown }
 				> = []
 
-				if (reasoningParts.length > 0) {
+				if (reasoningContent) {
+					content.push({ type: "reasoning", text: reasoningContent })
+				} else if (reasoningParts.length > 0) {
 					content.push({ type: "reasoning", text: reasoningParts.join("") })
 				}