fix: prevent write_to_file from executing with missing content parameter

When finalizeStreamingToolCall returns null (malformed/incomplete args from
LLM), the existing partial ToolUse block was promoted to non-partial while
retaining stale nativeArgs from partial parsing. The partial parser for
write_to_file used OR logic (path || content), allowing nativeArgs to be
set with only path present. This bypassed the safety check in
presentAssistantMessage that guards against missing nativeArgs.

Two fixes applied:
1. Clear nativeArgs when promoting failed finalization blocks to complete
   in Task.ts (both finalization paths). This ensures the safety check
   in presentAssistantMessage properly catches invalid tool calls.
2. Tighten partial write_to_file nativeArgs in NativeToolCallParser to
   require both path AND content (handlePartial reads from params, not
   nativeArgs, so this is safe).

Closes #12079

src/core/assistant-message/NativeToolCallParser.ts

@@ -465,7 +465,7 @@ export class NativeToolCallParser {
 				break
 
 			case "write_to_file":
-				if (partialArgs.path || partialArgs.content) {
+				if (partialArgs.path !== undefined && partialArgs.content !== undefined) {
 					nativeArgs = {
 						path: partialArgs.path,
 						content: partialArgs.content,

src/core/assistant-message/__tests__/NativeToolCallParser.spec.ts

@@ -313,6 +313,107 @@ describe("NativeToolCallParser", () => {
 		})
 	})
 
+	describe("write_to_file partial streaming", () => {
+		it("should not set nativeArgs when only path is present (content missing)", () => {
+			const id = "toolu_wtf_partial_1"
+			NativeToolCallParser.startStreamingToolCall(id, "write_to_file")
+
+			// Simulate partial chunk with only path, no content yet
+			const result = NativeToolCallParser.processStreamingChunk(id, JSON.stringify({ path: "test.txt" }))
+
+			// nativeArgs should NOT be set when content is missing
+			expect(result).not.toBeNull()
+			if (result) {
+				expect(result.nativeArgs).toBeUndefined()
+				// params should still have path for UI display
+				expect(result.params.path).toBe("test.txt")
+			}
+		})
+
+		it("should set nativeArgs when both path and content are present", () => {
+			const id = "toolu_wtf_partial_2"
+			NativeToolCallParser.startStreamingToolCall(id, "write_to_file")
+
+			const result = NativeToolCallParser.processStreamingChunk(
+				id,
+				JSON.stringify({ path: "test.txt", content: "hello world" }),
+			)
+
+			expect(result).not.toBeNull()
+			if (result) {
+				expect(result.nativeArgs).toBeDefined()
+				const nativeArgs = result.nativeArgs as { path: string; content: string }
+				expect(nativeArgs.path).toBe("test.txt")
+				expect(nativeArgs.content).toBe("hello world")
+			}
+		})
+	})
+
+	describe("write_to_file parseToolCall (complete)", () => {
+		it("should return null when content is missing from complete tool call", () => {
+			const toolCall = {
+				id: "toolu_wtf_complete_1",
+				name: "write_to_file" as const,
+				arguments: JSON.stringify({ path: "test.txt" }),
+			}
+
+			// parseToolCall throws internally and returns null for invalid args
+			const result = NativeToolCallParser.parseToolCall(toolCall)
+			expect(result).toBeNull()
+		})
+
+		it("should parse correctly when both path and content are present", () => {
+			const toolCall = {
+				id: "toolu_wtf_complete_2",
+				name: "write_to_file" as const,
+				arguments: JSON.stringify({ path: "test.txt", content: "hello world" }),
+			}
+
+			const result = NativeToolCallParser.parseToolCall(toolCall)
+			expect(result).not.toBeNull()
+			expect(result?.type).toBe("tool_use")
+			if (result?.type === "tool_use") {
+				expect(result.nativeArgs).toBeDefined()
+				const nativeArgs = result.nativeArgs as { path: string; content: string }
+				expect(nativeArgs.path).toBe("test.txt")
+				expect(nativeArgs.content).toBe("hello world")
+			}
+		})
+	})
+
+	describe("write_to_file finalization fallback", () => {
+		it("should return null when finalized with missing content", () => {
+			const id = "toolu_wtf_finalize_1"
+			NativeToolCallParser.startStreamingToolCall(id, "write_to_file")
+
+			// Stream only path, no content
+			NativeToolCallParser.processStreamingChunk(id, JSON.stringify({ path: "test.txt" }))
+
+			// Finalization should fail (return null) because content is missing
+			const result = NativeToolCallParser.finalizeStreamingToolCall(id)
+			expect(result).toBeNull()
+		})
+
+		it("should finalize successfully when both path and content are present", () => {
+			const id = "toolu_wtf_finalize_2"
+			NativeToolCallParser.startStreamingToolCall(id, "write_to_file")
+
+			NativeToolCallParser.processStreamingChunk(
+				id,
+				JSON.stringify({ path: "test.txt", content: "file content" }),
+			)
+
+			const result = NativeToolCallParser.finalizeStreamingToolCall(id)
+			expect(result).not.toBeNull()
+			expect(result?.type).toBe("tool_use")
+			if (result?.type === "tool_use") {
+				const nativeArgs = result.nativeArgs as { path: string; content: string }
+				expect(nativeArgs.path).toBe("test.txt")
+				expect(nativeArgs.content).toBe("file content")
+			}
+		})
+	})
+
 	describe("finalizeStreamingToolCall", () => {
 		describe("read_file tool", () => {
 			it("should parse read_file args on finalize", () => {

src/core/task/Task.ts

@@ -2967,6 +2967,12 @@ export class Task extends EventEmitter<TaskEvents> implements TaskLike {
 											const existingToolUse = this.assistantMessageContent[toolUseIndex]
 											if (existingToolUse && existingToolUse.type === "tool_use") {
 												existingToolUse.partial = false
+												// Clear stale nativeArgs from partial parsing so the safety check
+												// in presentAssistantMessage (!block.nativeArgs) properly catches
+												// this as an invalid tool call. Without this, incomplete partial
+												// nativeArgs (e.g., path set but content undefined) would bypass
+												// the check and cause tools to execute with missing parameters.
+												existingToolUse.nativeArgs = undefined
 												// Ensure it has the ID for native protocol
 												;(existingToolUse as any).id = event.id
 											}
@@ -3350,11 +3356,17 @@ export class Task extends EventEmitter<TaskEvents> implements TaskLike {
 							presentAssistantMessage(this)
 						} else if (toolUseIndex !== undefined) {
 							// finalizeStreamingToolCall returned null (malformed JSON or missing args)
-							// We still need to mark the tool as non-partial so it gets executed
-							// The tool's validation will catch any missing required parameters
+							// We still need to mark the tool as non-partial so it gets presented.
+							// The presentAssistantMessage safety check will catch missing nativeArgs.
 							const existingToolUse = this.assistantMessageContent[toolUseIndex]
 							if (existingToolUse && existingToolUse.type === "tool_use") {
 								existingToolUse.partial = false
+								// Clear stale nativeArgs from partial parsing so the safety check
+								// in presentAssistantMessage (!block.nativeArgs) properly catches
+								// this as an invalid tool call. Without this, incomplete partial
+								// nativeArgs (e.g., path set but content undefined) would bypass
+								// the check and cause tools to execute with missing parameters.
+								existingToolUse.nativeArgs = undefined
 								// Ensure it has the ID for native protocol
 								;(existingToolUse as any).id = event.id
 							}