fix(ai-sdk): preserve reasoning parts in message conversion #11217

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Merged

hannesrudolph merged 5 commits into main from reapply/pr-11196

Feb 5, 2026

src/api/index.ts

-Original file line number
+Diff line change
@@ Expand Up / @@ -117,6 +117,15 @@ export interface ApiHandler { @@
     	 * @returns A promise resolving to the token count
     	 */
     	countTokens(content: Array<Anthropic.Messages.ContentBlockParam>): Promise<number>
+    	/**
+    	 * Indicates whether this provider uses the Vercel AI SDK for streaming.
+    	 * AI SDK providers handle reasoning blocks differently and need to preserve
+    	 * them in conversation history for proper round-tripping.
+    	 *
+    	 * @returns true if the provider uses AI SDK, false otherwise
+    	 */
+    	isAiSdkProvider(): boolean
     }
     export function buildApiHandler(configuration: ProviderSettings): ApiHandler {
@@ Expand Down @@

src/api/providers/base-provider.ts

-Original file line number
+Diff line change
@@ Expand Up / @@ -119,4 +119,12 @@ export abstract class BaseProvider implements ApiHandler { @@
     		return countTokens(content, { useWorker: true })
     	}
+    	/**
+    	 * Default implementation returns false.
+    	 * AI SDK providers should override this to return true.
+    	 */
+    	isAiSdkProvider(): boolean {
+    		return false
+    	}
     }

src/api/providers/cerebras.ts

-Original file line number
+Diff line change
@@ Expand Up @@
     		return text
     	}
+    	override isAiSdkProvider(): boolean {
+    		return true
+    	}
     }

src/api/providers/deepseek.ts

-Original file line number
+Diff line change
@@ Expand Up @@
     		return text
     	}
+    	override isAiSdkProvider(): boolean {
+    		return true
+    	}
     }

src/api/providers/fake-ai.ts

-Original file line number
+Diff line change
@@ Expand Up @@
     	completePrompt(prompt: string): Promise<string> {
     		return this.ai.completePrompt(prompt)
     	}
+    	isAiSdkProvider(): boolean {
+    		return false
+    	}
     }

src/api/providers/fireworks.ts

-Original file line number
+Diff line change
@@ Expand Up @@
     		return text
     	}
+    	override isAiSdkProvider(): boolean {
+    		return true
+    	}
     }

src/api/providers/gemini.ts

-Original file line number
+Diff line change
@@ Expand Up @@
     		return totalCost
     	}
+    	override isAiSdkProvider(): boolean {
+    		return true
+    	}
     }

src/api/providers/groq.ts

-Original file line number
+Diff line change
@@ Expand Up @@
     		return text
     	}
+    	override isAiSdkProvider(): boolean {
+    		return true
+    	}
     }

src/api/providers/huggingface.ts

-Original file line number
+Diff line change
@@ Expand Up @@
     		return text
     	}
+    	override isAiSdkProvider(): boolean {
+    		return true
+    	}
     }

src/api/providers/mistral.ts

-Original file line number
+Diff line change
@@ Expand Up @@
     		return text
     	}
+    	override isAiSdkProvider(): boolean {
+    		return true
+    	}
     }

src/api/providers/openai-compatible.ts

-Original file line number
+Diff line change
@@ Expand Up @@
     		return text
     	}
+    	override isAiSdkProvider(): boolean {
+    		return true
+    	}
     }

src/api/providers/sambanova.ts

-Original file line number
+Diff line change
@@ Expand Up @@
     		return text
     	}
+    	override isAiSdkProvider(): boolean {
+    		return true
+    	}
     }

src/api/providers/vertex.ts

-Original file line number
+Diff line change
@@ Expand Up @@
     		return totalCost
     	}
+    	override isAiSdkProvider(): boolean {
+    		return true
+    	}
     }

src/api/providers/xai.ts

-Original file line number
+Diff line change
@@ Expand Up @@
     			throw handleAiSdkError(error, "xAI")
     		}
     	}
+    	override isAiSdkProvider(): boolean {
+    		return true
+    	}
     }

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

-Original file line number
+Diff line change
@@ Expand Up / @@ -308,6 +308,97 @@ describe("AI SDK conversion utilities", () => { @@
     				content: [{ type: "text", text: "" }],
     			})
     		})
+    		it("converts assistant reasoning blocks", () => {
+    			const messages: Anthropic.Messages.MessageParam[] = [
+    				{
+    					role: "assistant",
+    					content: [
+    						{ type: "reasoning" as any, text: "Thinking..." },
+    						{ type: "text", text: "Answer" },
+    					],
+    				},
+    			]
+    			const result = convertToAiSdkMessages(messages)
+    			expect(result).toHaveLength(1)
+    			expect(result[0]).toEqual({
+    				role: "assistant",
+    				content: [
+    					{ type: "reasoning", text: "Thinking..." },
+    					{ type: "text", text: "Answer" },
+    				],
+    			})
+    		})
+    		it("converts assistant thinking blocks to reasoning", () => {
+    			const messages: Anthropic.Messages.MessageParam[] = [
+    				{
+    					role: "assistant",
+    					content: [
+    						{ type: "thinking" as any, thinking: "Deep thought", signature: "sig" },
+    						{ type: "text", text: "OK" },
+    					],
+    				},
+    			]
+    			const result = convertToAiSdkMessages(messages)
+    			expect(result).toHaveLength(1)
+    			expect(result[0]).toEqual({
+    				role: "assistant",
+    				content: [
+    					{ type: "reasoning", text: "Deep thought" },
+    					{ type: "text", text: "OK" },
+    				],
+    			})
+    		})
+    		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", () => {
@@ Expand Down Expand Up / @@ -817,5 +908,54 @@ describe("AI SDK conversion utilities", () => { @@
     			expect(result[0].content).toBe("\nHello")
     		})
+    		it("should strip reasoning parts and flatten text for string-only models", () => {
+    			const messages = [
+    				{
+    					role: "assistant" as const,
+    					content: [
+    						{ type: "reasoning" as const, text: "I am thinking about this..." },
+    						{ type: "text" as const, text: "Here is my answer" },
+    					],
+    				},
+    			]
+    			const result = flattenAiSdkMessagesToStringContent(messages)
+    			// Reasoning should be stripped, only text should remain
+    			expect(result[0].content).toBe("Here is my answer")
+    		})
+    		it("should handle messages with only reasoning parts", () => {
+    			const messages = [
+    				{
+    					role: "assistant" as const,
+    					content: [{ type: "reasoning" as const, text: "Only reasoning, no text" }],
+    				},
+    			]
+    			const result = flattenAiSdkMessagesToStringContent(messages)
+    			// Should flatten to empty string when only reasoning is present
+    			expect(result[0].content).toBe("")
+    		})
+    		it("should not flatten if tool calls are present with reasoning", () => {
+    			const messages = [
+    				{
+    					role: "assistant" as const,
+    					content: [
+    						{ type: "reasoning" as const, text: "Thinking..." },
+    						{ type: "text" as const, text: "Using tool" },
+    						{ type: "tool-call" as const, toolCallId: "abc", toolName: "test", input: {} },
+    					],
+    				},
+    			]
+    			const result = flattenAiSdkMessagesToStringContent(messages)
+    			// Should not flatten because there's a tool call
+    			expect(result[0]).toEqual(messages[0])
+    		})
     	})
     })

src/api/transform/__tests__/image-cleaning.spec.ts

-Original file line number
+Diff line change
@@ Expand Up / @@ -18,6 +18,7 @@ describe("maybeRemoveImageBlocks", () => { @@
     			}),
     			createMessage: vitest.fn(),
     			countTokens: vitest.fn(),
+    			isAiSdkProvider: vitest.fn().mockReturnValue(false),
     		}
     	}
@@ Expand Down @@

src/api/transform/ai-sdk.ts

-Original file line number
+Diff line change
@@ Expand Up / @@ -126,6 +126,11 @@ 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
@@ Expand All / @@ -136,21 +141,57 @@ export function convertToAiSdkMessages( @@
     				for (const part of message.content) {
     					if (part.type === "text") {
     						textParts.push(part.text)
-    					} else if (part.type === "tool_use") {
+    						continue
+    					}
+    					if (part.type === "tool_use") {
     						toolCalls.push({
     							type: "tool-call",
     							toolCallId: part.id,
     							toolName: part.name,
     							input: part.input,
     						})
+    						continue
+    					}
+    					// Some providers (DeepSeek, Gemini, etc.) require reasoning to be round-tripped.
+    					// 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)
+    						}
+    						continue
+    					}
+    					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)
+    						}
+    						continue
     					}
     				}
     				const content: Array<
+    					| { type: "reasoning"; text: string }
     					| { type: "text"; text: string }
     					| { type: "tool-call"; toolCallId: string; toolName: string; input: unknown }
     				> = []
+    				if (reasoningContent) {
+    					content.push({ type: "reasoning", text: reasoningContent })
+    				} else if (reasoningParts.length > 0) {
+    					content.push({ type: "reasoning", text: reasoningParts.join("") })
+    				}
     				if (textParts.length > 0) {
     					content.push({ type: "text", text: textParts.join("\n") })
     				}
@@ Expand Down Expand Up / @@ -226,10 +267,13 @@ export function flattenAiSdkMessagesToStringContent( @@
     		// Handle assistant messages
     		if (message.role === "assistant" && flattenAssistantMessages && Array.isArray(message.content)) {
     			const parts = message.content as Array<{ type: string; text?: string }>
-    			// Only flatten if all parts are text (no tool calls)
-    			const allText = parts.every((part) => part.type === "text")
-    			if (allText && parts.length > 0) {
-    				const textContent = parts.map((part) => part.text || "").join("\n")
+    			// Only flatten if all parts are text or reasoning (no tool calls)
+    			// Reasoning parts are included in text to avoid sending multipart content to string-only models
+    			const allTextOrReasoning = parts.every((part) => part.type === "text" || part.type === "reasoning")
+    			if (allTextOrReasoning && parts.length > 0) {
+    				// Extract only text parts for the flattened content (reasoning is stripped for string-only models)
+    				const textParts = parts.filter((part) => part.type === "text")
+    				const textContent = textParts.map((part) => part.text || "").join("\n")
     				return {
     					...message,
     					content: textContent,
@@ Expand Down @@

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fix(ai-sdk): preserve reasoning parts in message conversion #11217

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