fix(coding-agent): report edit access failures correctly (#3955)

* fix(coding-agent): report edit access failures correctly closes #3894

- classify edit and edit-preview access errors by errno
- add regressions for missing, permission, and fallback cases
- document the fix in the coding-agent changelog

* chore: get rid of CHANGELOG.md entry

* refactor(coding-agent): apply review suggestions - use single error msg

* refactor(coding-agent): clean up test cases

packages/coding-agent/src/core/tools/edit-diff.ts

@@ -412,8 +412,9 @@ export async function computeEditsDiff(
 		// Check if file exists and is readable
 		try {
 			await access(absolutePath, constants.R_OK);
-		} catch {
-			return { error: `File not found: ${path}` };
+		} catch (error: unknown) {
+			const errorMessage = error instanceof Error && "code" in error ? `Error code: ${error.code}` : String(error);
+			return { error: `Could not write file: ${path}. ${errorMessage}.` };
 		}
 
 		// Read the file

packages/coding-agent/src/core/tools/edit.ts

@@ -341,11 +341,13 @@ export function createEditToolDefinition(
 								// Check if file exists.
 								try {
 									await ops.access(absolutePath);
-								} catch {
+								} catch (error: unknown) {
+									const errorMessage =
+										error instanceof Error && "code" in error ? `Error code: ${error.code}` : String(error);
 									if (signal) {
 										signal.removeEventListener("abort", onAbort);
 									}
-									reject(new Error(`File not found: ${path}`));
+									reject(new Error(`Could not write file: ${path}. ${errorMessage}.`));
 									return;
 								}
 

packages/coding-agent/test/tools.test.ts

@@ -1,9 +1,10 @@
-import { existsSync, mkdirSync, readFileSync, rmSync, writeFileSync } from "fs";
+import { chmodSync, existsSync, mkdirSync, readFileSync, rmSync, writeFileSync } from "fs";
 import { tmpdir } from "os";
 import { join } from "path";
 import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
 import { executeBashWithOperations } from "../src/core/bash-executor.js";
 import { createBashTool, createLocalBashOperations } from "../src/core/tools/bash.js";
+import { computeEditsDiff } from "../src/core/tools/edit-diff.js";
 import {
 	createEditTool,
 	createFindTool,
@@ -253,6 +254,17 @@ describe("Coding Agent Tools", () => {
 			).rejects.toThrow(/Could not find the exact text/);
 		});
 
+		it("should include ENOENT when the edit target does not exist", async () => {
+			const missingFile = join(testDir, "missing.txt");
+
+			await expect(
+				editTool.execute("test-call-6b", {
+					path: missingFile,
+					edits: [{ oldText: "hello", newText: "world" }],
+				}),
+			).rejects.toThrow(`Could not write file: ${missingFile}. Error code: ENOENT.`);
+		});
+
 		it("should fail if text appears multiple times", async () => {
 			const testFile = join(testDir, "edit-test.txt");
 			const originalContent = "foo foo foo";
@@ -366,6 +378,55 @@ describe("Coding Agent Tools", () => {
 
 			expect(readFileSync(testFile, "utf-8")).toBe(originalContent);
 		});
+
+		it("should include EACCES for read-only files", async () => {
+			const testFile = join(testDir, "edit-readonly.txt");
+			writeFileSync(testFile, "hello\n");
+			chmodSync(testFile, 0o444);
+
+			await expect(
+				editTool.execute("test-call-14", {
+					path: testFile,
+					edits: [{ oldText: "hello", newText: "world" }],
+				}),
+			).rejects.toThrow(`Could not write file: ${testFile}. Error code: EACCES.`);
+		});
+
+		it("should include the original error message for unknown edit access errors", async () => {
+			const genericFailureTool = createEditTool(testDir, {
+				operations: {
+					access: async () => {
+						throw new Error("disk offline");
+					},
+					readFile: async () => Buffer.from("hello\n", "utf-8"),
+					writeFile: async () => {},
+				},
+			});
+
+			await expect(
+				genericFailureTool.execute("test-call-16", {
+					path: "broken.txt",
+					edits: [{ oldText: "hello", newText: "world" }],
+				}),
+			).rejects.toThrow("Could not write file: broken.txt. Error: disk offline.");
+		});
+
+		it("should include ENOENT in diff preview for missing files", async () => {
+			const missingFile = join(testDir, "missing-preview.txt");
+			const result = await computeEditsDiff(missingFile, [{ oldText: "hello", newText: "world" }], testDir);
+
+			expect(result).toEqual({ error: `Could not write file: ${missingFile}. Error code: ENOENT.` });
+		});
+
+		it("should include EACCES in diff preview for unreadable files", async () => {
+			const unreadableFile = join(testDir, "unreadable-preview.txt");
+			writeFileSync(unreadableFile, "hello\n");
+			chmodSync(unreadableFile, 0o222);
+
+			const result = await computeEditsDiff(unreadableFile, [{ oldText: "hello", newText: "world" }], testDir);
+
+			expect(result).toEqual({ error: `Could not write file: ${unreadableFile}. Error code: EACCES.` });
+		});
 	});
 
 	describe("bash tool", () => {