fix(ai): correct Gemini tool result format and improve type safety

- Fix tool result format for Gemini 3 Flash Preview compatibility
  - Use 'output' key for successful results (not 'result')
  - Use 'error' key for error results (not 'isError')
  - Per Google SDK documentation for FunctionResponse.response

- Improve type safety in google.ts provider
  - Add ImageContent import and use proper type guards
  - Replace 'as any' casts with proper typing
  - Import and use Schema type for tool parameters
  - Add proper typing for index deletion in error handler

- Add comprehensive test for Gemini 3 Flash tool calling
  - Tests successful tool call and result handling
  - Tests error tool result handling
  - Verifies fix for issue #213

Fixes #213

packages/ai/src/providers/google.ts

@@ -6,6 +6,7 @@ import {
 	type GenerateContentParameters,
 	GoogleGenAI,
 	type Part,
+	type Schema,
 	type ThinkingConfig,
 	type ThinkingLevel,
 } from "@google/genai";
@@ -14,6 +15,7 @@ import type {
 	Api,
 	AssistantMessage,
 	Context,
+	ImageContent,
 	Model,
 	StopReason,
 	StreamFunction,
@@ -238,7 +240,12 @@ export const streamGoogle: StreamFunction<"google-generative-ai"> = (
 			stream.push({ type: "done", reason: output.stopReason, message: output });
 			stream.end();
 		} catch (error) {
-			for (const block of output.content) delete (block as any).index;
+			// Remove internal index property used during streaming
+			for (const block of output.content) {
+				if ("index" in block) {
+					delete (block as { index?: number }).index;
+				}
+			}
 			output.stopReason = options?.signal?.aborted ? "aborted" : "error";
 			output.errorMessage = error instanceof Error ? error.message : JSON.stringify(error);
 			stream.push({ type: "error", reason: output.stopReason, error: output });
@@ -389,33 +396,33 @@ function convertMessages(model: Model<"google-generative-ai">, context: Context)
 			const parts: Part[] = [];
 
 			// Extract text and image content
-			const textResult = msg.content
-				.filter((c) => c.type === "text")
-				.map((c) => (c as any).text)
-				.join("\n");
-			const imageBlocks = model.input.includes("image") ? msg.content.filter((c) => c.type === "image") : [];
+			const textContent = msg.content.filter((c): c is TextContent => c.type === "text");
+			const textResult = textContent.map((c) => c.text).join("\n");
+			const imageContent = model.input.includes("image")
+				? msg.content.filter((c): c is ImageContent => c.type === "image")
+				: [];
 
 			// Always add functionResponse with text result (or placeholder if only images)
 			const hasText = textResult.length > 0;
-			const hasImages = imageBlocks.length > 0;
+			const hasImages = imageContent.length > 0;
+
+			// Use "output" key for success, "error" key for errors as per SDK documentation
+			const responseValue = hasText ? sanitizeSurrogates(textResult) : hasImages ? "(see attached image)" : "";
 
 			parts.push({
 				functionResponse: {
 					id: msg.toolCallId,
 					name: msg.toolName,
-					response: {
-						result: hasText ? sanitizeSurrogates(textResult) : hasImages ? "(see attached image)" : "",
-						isError: msg.isError,
-					},
+					response: msg.isError ? { error: responseValue } : { output: responseValue },
 				},
 			});
 
 			// Add any images as inlineData parts
-			for (const imageBlock of imageBlocks) {
+			for (const imageBlock of imageContent) {
 				parts.push({
 					inlineData: {
-						mimeType: (imageBlock as any).mimeType,
-						data: (imageBlock as any).data,
+						mimeType: imageBlock.mimeType,
+						data: imageBlock.data,
 					},
 				});
 			}
@@ -430,14 +437,16 @@ function convertMessages(model: Model<"google-generative-ai">, context: Context)
 	return contents;
 }
 
-function convertTools(tools: Tool[]): any[] | undefined {
+function convertTools(
+	tools: Tool[],
+): { functionDeclarations: { name: string; description?: string; parameters: Schema }[] }[] | undefined {
 	if (tools.length === 0) return undefined;
 	return [
 		{
 			functionDeclarations: tools.map((tool) => ({
 				name: tool.name,
 				description: tool.description,
-				parameters: tool.parameters as any, // TypeBox already generates JSON Schema
+				parameters: tool.parameters as Schema, // TypeBox generates JSON Schema compatible with SDK Schema type
 			})),
 		},
 	];

packages/ai/test/gemini-3-flash-tool-calling.test.ts

@@ -0,0 +1,166 @@
+import { Type } from "@sinclair/typebox";
+import { describe, expect, it } from "vitest";
+import { getModel } from "../src/models.js";
+import { complete } from "../src/stream.js";
+import type { Context, Tool, ToolResultMessage } from "../src/types.js";
+import { StringEnum } from "../src/utils/typebox-helpers.js";
+
+/**
+ * Test for Gemini 3 Flash Preview tool calling compatibility.
+ *
+ * Issue #213: The model works and tool calling works, but the problem is how pi-ai
+ * formats the tool result message when sending it back to Gemini 3 Flash Preview.
+ *
+ * The SDK documentation states:
+ * "Use 'output' key to specify function output and 'error' key to specify error details"
+ *
+ * But the code was using `result` and `isError` keys, which Gemini 3 Flash Preview
+ * rejects (older models were more lenient).
+ */
+
+// Calculator tool definition
+const calculatorSchema = Type.Object({
+	a: Type.Number({ description: "First number" }),
+	b: Type.Number({ description: "Second number" }),
+	operation: StringEnum(["add", "subtract", "multiply", "divide"], {
+		description: "The operation to perform. One of 'add', 'subtract', 'multiply', 'divide'.",
+	}),
+});
+
+const calculatorTool: Tool<typeof calculatorSchema> = {
+	name: "calculator",
+	description: "Perform basic arithmetic operations",
+	parameters: calculatorSchema,
+};
+
+describe("Gemini 3 Flash Preview Tool Calling", () => {
+	it("should handle tool calls and tool results with correct format", async () => {
+		if (!process.env.GEMINI_API_KEY) {
+			console.log("Skipping test - GEMINI_API_KEY not set");
+			return;
+		}
+
+		const model = getModel("google", "gemini-3-flash-preview");
+
+		const context: Context = {
+			systemPrompt: "You are a helpful assistant that uses tools when asked.",
+			messages: [
+				{
+					role: "user",
+					content: "Calculate 15 + 27 using the calculator tool.",
+					timestamp: Date.now(),
+				},
+			],
+			tools: [calculatorTool],
+		};
+
+		// First call - model should request tool call
+		const firstResponse = await complete(model, context);
+
+		expect(firstResponse.role).toBe("assistant");
+		expect(firstResponse.stopReason).toBe("toolUse");
+		expect(firstResponse.errorMessage).toBeFalsy();
+
+		const toolCall = firstResponse.content.find((b) => b.type === "toolCall");
+		expect(toolCall).toBeTruthy();
+		expect(toolCall?.type).toBe("toolCall");
+
+		if (toolCall?.type === "toolCall") {
+			expect(toolCall.name).toBe("calculator");
+			expect(toolCall.id).toBeTruthy();
+			expect(toolCall.arguments).toBeTruthy();
+
+			const { a, b, operation } = toolCall.arguments;
+			expect(a).toBe(15);
+			expect(b).toBe(27);
+			expect(operation).toBe("add");
+
+			// Execute the tool
+			const result = 15 + 27;
+
+			// Add tool result to context - this is where the bug was
+			// The SDK expects { output: value } for success, not { result: value, isError: false }
+			context.messages.push(firstResponse);
+			const toolResult: ToolResultMessage = {
+				role: "toolResult",
+				toolCallId: toolCall.id,
+				toolName: toolCall.name,
+				content: [{ type: "text", text: `${result}` }],
+				isError: false,
+				timestamp: Date.now(),
+			};
+			context.messages.push(toolResult);
+
+			// Second call - model should process the tool result and respond
+			// This is where Gemini 3 Flash Preview would fail with the old format
+			const secondResponse = await complete(model, context);
+
+			expect(secondResponse.role).toBe("assistant");
+			expect(secondResponse.stopReason).toBe("stop");
+			expect(secondResponse.errorMessage).toBeFalsy();
+
+			const textContent = secondResponse.content
+				.filter((b) => b.type === "text")
+				.map((b) => (b.type === "text" ? b.text : ""))
+				.join("");
+
+			expect(textContent).toBeTruthy();
+			// Should mention the result 42
+			expect(textContent.toLowerCase()).toMatch(/42/);
+		}
+	}, 30000); // 30 second timeout
+
+	it("should handle tool errors with correct format", async () => {
+		if (!process.env.GEMINI_API_KEY) {
+			console.log("Skipping test - GEMINI_API_KEY not set");
+			return;
+		}
+
+		const model = getModel("google", "gemini-3-flash-preview");
+
+		const context: Context = {
+			systemPrompt: "You are a helpful assistant that uses tools when asked.",
+			messages: [
+				{
+					role: "user",
+					content: "Calculate 10 divided by 0 using the calculator tool.",
+					timestamp: Date.now(),
+				},
+			],
+			tools: [calculatorTool],
+		};
+
+		const firstResponse = await complete(model, context);
+		expect(firstResponse.stopReason).toBe("toolUse");
+
+		const toolCall = firstResponse.content.find((b) => b.type === "toolCall");
+		if (toolCall?.type === "toolCall") {
+			// Add error result - should use { error: message } format
+			context.messages.push(firstResponse);
+			const errorResult: ToolResultMessage = {
+				role: "toolResult",
+				toolCallId: toolCall.id,
+				toolName: toolCall.name,
+				content: [{ type: "text", text: "Error: Division by zero" }],
+				isError: true,
+				timestamp: Date.now(),
+			};
+			context.messages.push(errorResult);
+
+			// Model should handle the error response
+			const secondResponse = await complete(model, context);
+
+			expect(secondResponse.role).toBe("assistant");
+			expect(secondResponse.errorMessage).toBeFalsy();
+
+			const textContent = secondResponse.content
+				.filter((b) => b.type === "text")
+				.map((b) => (b.type === "text" ? b.text : ""))
+				.join("");
+
+			expect(textContent).toBeTruthy();
+			// Should acknowledge the error
+			expect(textContent.toLowerCase()).toMatch(/error|cannot|division|zero/);
+		}
+	}, 30000);
+});