fix(ai): preserve function arguments with empty custom payloads

Prefer function payloads when malformed OpenAI-compatible tool-call deltas also include an empty custom object, preserving parsed arguments.

Fixes #7160

Co-authored-by: Sunny Young <isunnyyoung@gmail.com>

packages/ai/src/api/openai-completions.ts

@@ -383,9 +383,8 @@ export const stream: StreamFunction<"openai-completions", OpenAICompletionsOptio
 				if (!block) {
 					// Note: the "input" fallback here should/must not be taken.  in case the LLM makes up
 					// a tool we don't knwo about, we at least have a place to stash our stuff.
-					const customInputProperty = toolCall.custom
-						? (grammarToolInputProperties.get(name) ?? "input")
-						: undefined;
+					const customInputProperty =
+						toolCall.custom && !toolCall.function ? (grammarToolInputProperties.get(name) ?? "input") : undefined;
 					const hasCustomInput = customInputProperty !== undefined;
 					block = {
 						type: "toolCall",
@@ -421,7 +420,7 @@ export const stream: StreamFunction<"openai-completions", OpenAICompletionsOptio
 				if (!block.name && name) {
 					block.name = name;
 				}
-				if (toolCall.custom && !block.customInput) {
+				if (toolCall.custom && !toolCall.function && !block.customInput) {
 					const customInputProperty = grammarToolInputProperties.get(block.name) ?? "input";
 					block.arguments = { [customInputProperty]: "" };
 					block.customInput = {

packages/ai/test/openai-completions-tool-choice.test.ts

@@ -653,6 +653,55 @@ describe("openai-completions tool_choice", () => {
 		expect(response.errorMessage).toBe("Stream ended without finish_reason");
 	});
 
+	it("ignores empty custom objects on function tool call deltas", async () => {
+		mockState.chunks = [
+			{
+				id: "chatcmpl-empty-custom",
+				choices: [
+					{
+						delta: {
+							tool_calls: [
+								{
+									index: 0,
+									id: "call_1",
+									type: "function",
+									function: { name: "read", arguments: '{"path":"README.md"}' },
+									custom: {},
+								},
+							],
+						},
+						finish_reason: "tool_calls",
+					},
+				],
+			},
+		];
+
+		const { compat: _compat, ...baseModel } = getModel("openai", "gpt-4o-mini")!;
+		const model = { ...baseModel, api: "openai-completions" } as const;
+		const tool: Tool = {
+			name: "read",
+			description: "Read a file",
+			parameters: Type.Object({ path: Type.String() }),
+		};
+		const response = await streamSimple(
+			model,
+			{
+				messages: [{ role: "user", content: "Read README.md", timestamp: Date.now() }],
+				tools: [tool],
+			},
+			{ apiKey: "test" },
+		).result();
+
+		expect(response.content).toEqual([
+			{
+				type: "toolCall",
+				id: "call_1",
+				name: "read",
+				arguments: { path: "README.md" },
+			},
+		]);
+	});
+
 	it("coalesces tool call deltas by stable index when provider mutates ids mid-stream", async () => {
 		mockState.chunks = [
 			{