fix: restore Gemini thought signature round-tripping after AI SDK migration (#11237)

PR #11180 migrated Gemini/Vertex providers to the AI SDK and deleted
gemini-format.ts which contained the working thought signature round-trip
logic (originally added in PR #10590). This broke all Gemini 3 tool use
with a 400 error: 'Function call is missing a thought_signature'.

Changes:
- Gemini/Vertex handlers: capture thoughtSignature from providerMetadata
  on tool-call stream events, expose via getThoughtSignature()
- convertToAiSdkMessages(): extract thoughtSignature content blocks from
  history, attach as providerOptions on first tool-call part (per Gemini 3
  parallel call rules)
- Add 3 tests verifying thought signature round-trip behavior

src/api/providers/gemini.ts

@@ -31,6 +31,7 @@ export class GeminiHandler extends BaseProvider implements SingleCompletionHandl
 	protected options: ApiHandlerOptions
 	protected provider: GoogleGenerativeAIProvider
 	private readonly providerName = "Gemini"
+	private lastThoughtSignature: string | undefined
 
 	constructor(options: ApiHandlerOptions) {
 		super()
@@ -124,11 +125,23 @@ export class GeminiHandler extends BaseProvider implements SingleCompletionHandl
 		}
 
 		try {
+			// Reset thought signature for this request
+			this.lastThoughtSignature = undefined
+
 			// Use streamText for streaming responses
 			const result = streamText(requestOptions)
 
 			// Process the full stream to get all events including reasoning
 			for await (const part of result.fullStream) {
+				// Capture thoughtSignature from tool-call events (Gemini 3 thought signatures)
+				// The AI SDK's tool-call event includes providerMetadata with the signature
+				if (part.type === "tool-call") {
+					const googleMeta = (part as any).providerMetadata?.google
+					if (googleMeta?.thoughtSignature) {
+						this.lastThoughtSignature = googleMeta.thoughtSignature
+					}
+				}
+
 				for (const chunk of processAiSdkStreamPart(part)) {
 					yield chunk
 				}
@@ -401,4 +414,13 @@ export class GeminiHandler extends BaseProvider implements SingleCompletionHandl
 	override isAiSdkProvider(): boolean {
 		return true
 	}
+
+	/**
+	 * Returns the thought signature captured from the last Gemini response.
+	 * Gemini 3 models return thoughtSignature on function call parts,
+	 * which must be round-tripped back for tool use continuations.
+	 */
+	getThoughtSignature(): string | undefined {
+		return this.lastThoughtSignature
+	}
 }

src/api/providers/vertex.ts

@@ -35,6 +35,7 @@ export class VertexHandler extends BaseProvider implements SingleCompletionHandl
 	protected options: ApiHandlerOptions
 	protected provider: GoogleVertexProvider
 	private readonly providerName = "Vertex"
+	private lastThoughtSignature: string | undefined
 
 	constructor(options: ApiHandlerOptions) {
 		super()
@@ -138,11 +139,26 @@ export class VertexHandler extends BaseProvider implements SingleCompletionHandl
 		}
 
 		try {
+			// Reset thought signature for this request
+			this.lastThoughtSignature = undefined
+
 			// Use streamText for streaming responses
 			const result = streamText(requestOptions)
 
 			// Process the full stream to get all events including reasoning
 			for await (const part of result.fullStream) {
+				// Capture thoughtSignature from tool-call events (Gemini 3 thought signatures)
+				// The AI SDK's tool-call event includes providerMetadata with the signature
+				// Vertex AI stores it under the "vertex" key in providerMetadata
+				if (part.type === "tool-call") {
+					const vertexMeta = (part as any).providerMetadata?.vertex
+					const googleMeta = (part as any).providerMetadata?.google
+					const sig = vertexMeta?.thoughtSignature ?? googleMeta?.thoughtSignature
+					if (sig) {
+						this.lastThoughtSignature = sig
+					}
+				}
+
 				for (const chunk of processAiSdkStreamPart(part)) {
 					yield chunk
 				}
@@ -406,4 +422,13 @@ export class VertexHandler extends BaseProvider implements SingleCompletionHandl
 	override isAiSdkProvider(): boolean {
 		return true
 	}
+
+	/**
+	 * Returns the thought signature captured from the last Vertex AI response.
+	 * Gemini 3 models return thoughtSignature on function call parts,
+	 * which must be round-tripped back for tool use continuations.
+	 */
+	getThoughtSignature(): string | undefined {
+		return this.lastThoughtSignature
+	}
 }

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

@@ -399,6 +399,101 @@ describe("AI SDK conversion utilities", () => {
 				],
 			})
 		})
+
+		it("attaches thoughtSignature to first tool-call part for Gemini 3 round-tripping", () => {
+			const messages: Anthropic.Messages.MessageParam[] = [
+				{
+					role: "assistant",
+					content: [
+						{ type: "text", text: "Let me check that." },
+						{
+							type: "tool_use",
+							id: "tool-1",
+							name: "read_file",
+							input: { path: "test.txt" },
+						},
+						{ type: "thoughtSignature", thoughtSignature: "encrypted-sig-abc" } as any,
+					],
+				},
+			]
+
+			const result = convertToAiSdkMessages(messages)
+
+			expect(result).toHaveLength(1)
+			const assistantMsg = result[0]
+			expect(assistantMsg.role).toBe("assistant")
+
+			const content = assistantMsg.content as any[]
+			expect(content).toHaveLength(2) // text + tool-call (thoughtSignature block is consumed, not passed through)
+
+			const toolCallPart = content.find((p: any) => p.type === "tool-call")
+			expect(toolCallPart).toBeDefined()
+			expect(toolCallPart.providerOptions).toEqual({
+				google: { thoughtSignature: "encrypted-sig-abc" },
+				vertex: { thoughtSignature: "encrypted-sig-abc" },
+			})
+		})
+
+		it("attaches thoughtSignature only to the first tool-call in parallel calls", () => {
+			const messages: Anthropic.Messages.MessageParam[] = [
+				{
+					role: "assistant",
+					content: [
+						{
+							type: "tool_use",
+							id: "tool-1",
+							name: "get_weather",
+							input: { city: "Paris" },
+						},
+						{
+							type: "tool_use",
+							id: "tool-2",
+							name: "get_weather",
+							input: { city: "London" },
+						},
+						{ type: "thoughtSignature", thoughtSignature: "sig-parallel" } as any,
+					],
+				},
+			]
+
+			const result = convertToAiSdkMessages(messages)
+			const content = (result[0] as any).content as any[]
+
+			const toolCalls = content.filter((p: any) => p.type === "tool-call")
+			expect(toolCalls).toHaveLength(2)
+
+			// Only the first tool call should have the signature
+			expect(toolCalls[0].providerOptions).toEqual({
+				google: { thoughtSignature: "sig-parallel" },
+				vertex: { thoughtSignature: "sig-parallel" },
+			})
+			// Second tool call should NOT have the signature
+			expect(toolCalls[1].providerOptions).toBeUndefined()
+		})
+
+		it("does not attach providerOptions when no thoughtSignature block is present", () => {
+			const messages: Anthropic.Messages.MessageParam[] = [
+				{
+					role: "assistant",
+					content: [
+						{ type: "text", text: "Using tool" },
+						{
+							type: "tool_use",
+							id: "tool-1",
+							name: "read_file",
+							input: { path: "test.txt" },
+						},
+					],
+				},
+			]
+
+			const result = convertToAiSdkMessages(messages)
+			const content = (result[0] as any).content as any[]
+			const toolCallPart = content.find((p: any) => p.type === "tool-call")
+
+			expect(toolCallPart).toBeDefined()
+			expect(toolCallPart.providerOptions).toBeUndefined()
+		})
 	})
 
 	describe("convertToolsForAiSdk", () => {

src/api/transform/ai-sdk.ts

@@ -136,8 +136,19 @@ export function convertToAiSdkMessages(
 					toolCallId: string
 					toolName: string
 					input: unknown
+					providerOptions?: Record<string, Record<string, unknown>>
 				}> = []
 
+				// Extract thoughtSignature from content blocks (Gemini 3 thought signature round-tripping).
+				// Task.ts stores these as { type: "thoughtSignature", thoughtSignature: "..." } blocks.
+				let thoughtSignature: string | undefined
+				for (const part of message.content) {
+					const partAny = part as unknown as { type?: string; thoughtSignature?: string }
+					if (partAny.type === "thoughtSignature" && partAny.thoughtSignature) {
+						thoughtSignature = partAny.thoughtSignature
+					}
+				}
+
 				for (const part of message.content) {
 					if (part.type === "text") {
 						textParts.push(part.text)
@@ -145,12 +156,25 @@ export function convertToAiSdkMessages(
 					}
 
 					if (part.type === "tool_use") {
-						toolCalls.push({
+						const toolCall: (typeof toolCalls)[number] = {
 							type: "tool-call",
 							toolCallId: part.id,
 							toolName: part.name,
 							input: part.input,
-						})
+						}
+
+						// Attach thoughtSignature as providerOptions on tool-call parts.
+						// The AI SDK's @ai-sdk/google provider reads providerOptions.google.thoughtSignature
+						// and attaches it to the Gemini functionCall part.
+						// Per Gemini 3 rules: only the FIRST functionCall in a parallel batch gets the signature.
+						if (thoughtSignature && toolCalls.length === 0) {
+							toolCall.providerOptions = {
+								google: { thoughtSignature },
+								vertex: { thoughtSignature },
+							}
+						}
+
+						toolCalls.push(toolCall)
 						continue
 					}
 
@@ -183,7 +207,13 @@ export function convertToAiSdkMessages(
 				const content: Array<
 					| { type: "reasoning"; text: string }
 					| { type: "text"; text: string }
-					| { type: "tool-call"; toolCallId: string; toolName: string; input: unknown }
+					| {
+							type: "tool-call"
+							toolCallId: string
+							toolName: string
+							input: unknown
+							providerOptions?: Record<string, Record<string, unknown>>
+					  }
 				> = []
 
 				if (reasoningContent) {