fix: avoid unsafe array index access in AnthropicVertex completePrompt (#789)

The completePrompt method accessed response.content[0] directly, which
could throw a TypeError when the content array is empty, and would
return an empty string when the first block is a non-text block (e.g.
thinking or tool_use) even when a text block is present later in the
array.

Use Array.prototype.find() to locate the first text block, matching
the pattern already used in anthropic.ts. This safely returns undefined
for empty arrays and surfaces the actual text response regardless of
its position among content blocks.

Add regression tests covering an empty content array and a mixed
content array where a thinking block precedes the text block.

Signed-off-by: daewoongoh <dw.oh@samsung.com>

Author: daewoongoh

src/api/providers/__tests__/anthropic-vertex.spec.ts

@@ -895,6 +895,41 @@ describe("VertexHandler", () => {
 			const result = await handler.completePrompt("Test prompt")
 			expect(result).toBe("")
 		})
+
+		it("should handle empty content array for Claude", async () => {
+			handler = new AnthropicVertexHandler({
+				apiModelId: "claude-3-5-sonnet-v2@20241022",
+				vertexProjectId: "test-project",
+				vertexRegion: "us-central1",
+			})
+
+			const mockCreate = vitest.fn().mockResolvedValue({
+				content: [],
+			})
+			;(handler["client"].messages as any).create = mockCreate
+
+			const result = await handler.completePrompt("Test prompt")
+			expect(result).toBe("")
+		})
+
+		it("should return text from first text block when mixed content for Claude", async () => {
+			handler = new AnthropicVertexHandler({
+				apiModelId: "claude-3-5-sonnet-v2@20241022",
+				vertexProjectId: "test-project",
+				vertexRegion: "us-central1",
+			})
+
+			const mockCreate = vitest.fn().mockResolvedValue({
+				content: [
+					{ type: "thinking", thinking: "internal reasoning" },
+					{ type: "text", text: "visible response" },
+				],
+			})
+			;(handler["client"].messages as any).create = mockCreate
+
+			const result = await handler.completePrompt("Test prompt")
+			expect(result).toBe("visible response")
+		})
 	})
 
 	describe("getModel", () => {
@@ -1342,7 +1377,9 @@ describe("VertexHandler", () => {
 			}))
 			;(sonnetHandler["client"].messages as any).create = mockCreate
 
-			await sonnetHandler.createMessage("You are a helpful assistant", [{ role: "user", content: "Hello" }]).next()
+			await sonnetHandler
+				.createMessage("You are a helpful assistant", [{ role: "user", content: "Hello" }])
+				.next()
 
 			expect(mockCreate).toHaveBeenCalledWith(
 				expect.objectContaining({

src/api/providers/anthropic-vertex.ts

@@ -297,13 +297,9 @@ export class AnthropicVertexHandler extends BaseProvider implements SingleComple
 			} as Anthropic.Messages.MessageCreateParamsNonStreaming
 
 			const response = await this.client.messages.create(params)
-			const content = response.content[0]
+			const content = response.content.find(({ type }) => type === "text")
 
-			if (content.type === "text") {
-				return content.text
-			}
-
-			return ""
+			return content?.type === "text" ? content.text : ""
 		} catch (error) {
 			if (error instanceof Error) {
 				throw new Error(`Vertex completion error: ${error.message}`)