test(ai): add failing test for orphaned function_call without reasoning item

Reproduces issue #886 where function_call is sent without its required
paired reasoning item, causing Azure/OpenAI 400 error.

packages/ai/test/openai-responses-reasoning-replay-e2e.test.ts

@@ -2,7 +2,7 @@ import { Type } from "@sinclair/typebox";
 import { describe, expect, it } from "vitest";
 import { getModel } from "../src/models.js";
 import { complete, getEnvApiKey } from "../src/stream.js";
-import type { AssistantMessage, Context, Message, Tool } from "../src/types.js";
+import type { AssistantMessage, Context, Message, ThinkingContent, Tool, ToolCall } from "../src/types.js";
 
 const testToolSchema = Type.Object({
 	value: Type.Number({ description: "A number to double" }),
@@ -78,4 +78,101 @@ describe.skipIf(!process.env.OPENAI_API_KEY)("OpenAI Responses reasoning replay
 		// Model should respond (text or tool call)
 		expect(response.content.length).toBeGreaterThan(0);
 	});
+
+	it("drops orphaned tool calls when reasoning signature is missing", { retry: 2 }, async () => {
+		// This tests the scenario where:
+		// 1. A completed turn has reasoning + function_call
+		// 2. The thinking signature gets lost (e.g., cross-provider handoff, isSameModel=false filtering)
+		// 3. The toolCall remains but reasoning is gone
+		// 4. Without the fix: Azure/OpenAI returns 400 "function_call without required reasoning item"
+		// 5. With the fix: orphaned toolCalls are dropped, conversation continues
+
+		const model = getModel("openai", "gpt-5-mini");
+
+		const apiKey = getEnvApiKey("openai");
+		if (!apiKey) {
+			throw new Error("Missing OPENAI_API_KEY");
+		}
+
+		const userMessage: Message = {
+			role: "user",
+			content: "Use the double_number tool to double 21.",
+			timestamp: Date.now(),
+		};
+
+		// Get a real response with reasoning + tool call
+		const assistantResponse = await complete(
+			model,
+			{
+				systemPrompt: "You are a helpful assistant. Always use the tool when asked.",
+				messages: [userMessage],
+				tools: [testTool],
+			},
+			{
+				apiKey,
+				reasoningEffort: "high",
+			},
+		);
+
+		const thinkingBlock = assistantResponse.content.find(
+			(block) => block.type === "thinking" && block.thinkingSignature,
+		) as ThinkingContent | undefined;
+		const toolCallBlock = assistantResponse.content.find((block) => block.type === "toolCall") as
+			| ToolCall
+			| undefined;
+
+		if (!thinkingBlock) {
+			throw new Error("Missing thinking block from OpenAI Responses");
+		}
+		if (!toolCallBlock) {
+			throw new Error("Missing tool call from OpenAI Responses - model did not use the tool");
+		}
+
+		// Simulate corruption: keep toolCall but strip thinkingSignature
+		// This mimics what happens when isSameModel=false and thinking text is empty
+		const corruptedThinking: ThinkingContent = {
+			type: "thinking",
+			thinking: thinkingBlock.thinking,
+			// thinkingSignature intentionally omitted - simulates it being lost
+		};
+
+		const corruptedAssistant: AssistantMessage = {
+			...assistantResponse,
+			content: [corruptedThinking, toolCallBlock],
+			stopReason: "toolUse", // Completed successfully, not aborted
+		};
+
+		// Provide a tool result to continue the conversation
+		const toolResult: Message = {
+			role: "toolResult",
+			toolCallId: toolCallBlock.id,
+			toolName: toolCallBlock.name,
+			content: [{ type: "text", text: "42" }],
+			isError: false,
+			timestamp: Date.now(),
+		};
+
+		const followUp: Message = {
+			role: "user",
+			content: "What was the result?",
+			timestamp: Date.now(),
+		};
+
+		const context: Context = {
+			systemPrompt: "You are a helpful assistant.",
+			messages: [userMessage, corruptedAssistant, toolResult, followUp],
+			tools: [testTool],
+		};
+
+		const response = await complete(model, context, {
+			apiKey,
+			reasoningEffort: "high",
+		});
+
+		// The key assertion: no 400 error from orphaned function_call
+		// Error would be: "function_call was provided without its required reasoning item"
+		expect(response.stopReason, `Error: ${response.errorMessage}`).not.toBe("error");
+		expect(response.errorMessage).toBeFalsy();
+		expect(response.content.length).toBeGreaterThan(0);
+	});
 });