fix: coerce write_to_file content to string when model sends object

Some models (e.g., GLM 4.7 via OpenAI Compatible API) incorrectly pass
the content parameter as a JSON object instead of a string when calling
the write_to_file tool. This causes processing to break silently.

This fix adds defensive type coercion in NativeToolCallParser to:
1. Detect when content is not a string
2. Convert it to a formatted JSON string using JSON.stringify
3. Log a clear warning message so users understand what happened

This follows the existing pattern for type coercion (e.g.,
coerceOptionalBoolean for booleans).

Fixes #11040

src/core/assistant-message/NativeToolCallParser.ts

@@ -89,6 +89,25 @@ export class NativeToolCallParser {
 		return undefined
 	}
 
+	/**
+	 * Coerce content to a string for write_to_file tool.
+	 * Some models (e.g., GLM 4.7 via OpenAI Compatible API) incorrectly pass content
+	 * as a JSON object instead of a string. This handles that case gracefully.
+	 */
+	private static coerceContentToString(value: unknown, toolName: string): string {
+		if (typeof value === "string") {
+			return value
+		}
+
+		// Value is not a string - convert it and log a warning
+		console.warn(
+			`[NativeToolCallParser] Model sent non-string content for '${toolName}' tool. ` +
+				`Expected string, received ${typeof value}. Converting to JSON string. ` +
+				`This may indicate the model is not following the tool schema correctly.`,
+		)
+		return JSON.stringify(value, null, 2)
+	}
+
 	/**
 	 * Process a raw tool call chunk from the API stream.
 	 * Handles tracking, buffering, and emits start/delta/end events.
@@ -401,10 +420,13 @@ export class NativeToolCallParser {
 				break
 
 			case "write_to_file":
-				if (partialArgs.path || partialArgs.content) {
+				if (partialArgs.path || partialArgs.content !== undefined) {
 					nativeArgs = {
 						path: partialArgs.path,
-						content: partialArgs.content,
+						content:
+							partialArgs.content !== undefined
+								? this.coerceContentToString(partialArgs.content, "write_to_file")
+								: undefined,
 					}
 				}
 				break
@@ -805,7 +827,7 @@ export class NativeToolCallParser {
 					if (args.path !== undefined && args.content !== undefined) {
 						nativeArgs = {
 							path: args.path,
-							content: args.content,
+							content: this.coerceContentToString(args.content, "write_to_file"),
 						} as NativeArgsFor<TName>
 					}
 					break

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

@@ -238,4 +238,167 @@ describe("NativeToolCallParser", () => {
 			})
 		})
 	})
+
+	describe("write_to_file tool - content type coercion", () => {
+		describe("parseToolCall", () => {
+			it("should handle content as a string (normal case)", () => {
+				const toolCall = {
+					id: "toolu_123",
+					name: "write_to_file" as const,
+					arguments: JSON.stringify({
+						path: "package.json",
+						content: '{\n  "name": "test"\n}',
+					}),
+				}
+
+				const result = NativeToolCallParser.parseToolCall(toolCall)
+
+				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("package.json")
+					expect(nativeArgs.content).toBe('{\n  "name": "test"\n}')
+				}
+			})
+
+			it("should coerce content from object to JSON string", () => {
+				// This simulates the bug where models like GLM 4.7 pass content as an object
+				const toolCall = {
+					id: "toolu_456",
+					name: "write_to_file" as const,
+					arguments: JSON.stringify({
+						path: "package.json",
+						content: { name: "sample-project", version: "1.0.0" },
+					}),
+				}
+
+				const consoleSpy = vi.spyOn(console, "warn").mockImplementation(() => {})
+
+				const result = NativeToolCallParser.parseToolCall(toolCall)
+
+				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("package.json")
+					// Content should be converted to a formatted JSON string
+					expect(nativeArgs.content).toBe(
+						JSON.stringify({ name: "sample-project", version: "1.0.0" }, null, 2),
+					)
+				}
+
+				// Should log a warning about the type coercion
+				expect(consoleSpy).toHaveBeenCalledWith(
+					expect.stringContaining("Model sent non-string content for 'write_to_file' tool"),
+				)
+
+				consoleSpy.mockRestore()
+			})
+
+			it("should coerce content from array to JSON string", () => {
+				const toolCall = {
+					id: "toolu_789",
+					name: "write_to_file" as const,
+					arguments: JSON.stringify({
+						path: "data.json",
+						content: [1, 2, 3, { key: "value" }],
+					}),
+				}
+
+				const consoleSpy = vi.spyOn(console, "warn").mockImplementation(() => {})
+
+				const result = NativeToolCallParser.parseToolCall(toolCall)
+
+				expect(result).not.toBeNull()
+				if (result?.type === "tool_use") {
+					const nativeArgs = result.nativeArgs as { path: string; content: string }
+					expect(nativeArgs.content).toBe(JSON.stringify([1, 2, 3, { key: "value" }], null, 2))
+				}
+
+				expect(consoleSpy).toHaveBeenCalled()
+				consoleSpy.mockRestore()
+			})
+
+			it("should coerce content from number to string", () => {
+				const toolCall = {
+					id: "toolu_num",
+					name: "write_to_file" as const,
+					arguments: JSON.stringify({
+						path: "number.txt",
+						content: 42,
+					}),
+				}
+
+				const consoleSpy = vi.spyOn(console, "warn").mockImplementation(() => {})
+
+				const result = NativeToolCallParser.parseToolCall(toolCall)
+
+				expect(result).not.toBeNull()
+				if (result?.type === "tool_use") {
+					const nativeArgs = result.nativeArgs as { path: string; content: string }
+					expect(nativeArgs.content).toBe("42")
+				}
+
+				expect(consoleSpy).toHaveBeenCalled()
+				consoleSpy.mockRestore()
+			})
+		})
+
+		describe("processStreamingChunk", () => {
+			it("should coerce content from object to JSON string during streaming", () => {
+				const id = "toolu_streaming_write"
+				NativeToolCallParser.startStreamingToolCall(id, "write_to_file")
+
+				const consoleSpy = vi.spyOn(console, "warn").mockImplementation(() => {})
+
+				const fullArgs = JSON.stringify({
+					path: "config.json",
+					content: { setting: true, value: 123 },
+				})
+
+				const result = NativeToolCallParser.processStreamingChunk(id, fullArgs)
+
+				expect(result).not.toBeNull()
+				if (result?.nativeArgs) {
+					const nativeArgs = result.nativeArgs as { path: string; content: string }
+					expect(nativeArgs.path).toBe("config.json")
+					expect(nativeArgs.content).toBe(JSON.stringify({ setting: true, value: 123 }, null, 2))
+				}
+
+				expect(consoleSpy).toHaveBeenCalled()
+				consoleSpy.mockRestore()
+			})
+		})
+
+		describe("finalizeStreamingToolCall", () => {
+			it("should coerce content from object to JSON string on finalize", () => {
+				const id = "toolu_finalize_write"
+				NativeToolCallParser.startStreamingToolCall(id, "write_to_file")
+
+				const consoleSpy = vi.spyOn(console, "warn").mockImplementation(() => {})
+
+				NativeToolCallParser.processStreamingChunk(
+					id,
+					JSON.stringify({
+						path: "tsconfig.json",
+						content: { compilerOptions: { strict: true } },
+					}),
+				)
+
+				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("tsconfig.json")
+					expect(nativeArgs.content).toBe(JSON.stringify({ compilerOptions: { strict: true } }, null, 2))
+				}
+
+				expect(consoleSpy).toHaveBeenCalled()
+				consoleSpy.mockRestore()
+			})
+		})
+	})
 })