🤖 Collapse WRITE DENIED errors by default (#403)

## Problem

File edit tool calls that fail with `WRITE DENIED` errors are displayed
expanded by default. These errors happen frequently during normal
operation (when a file is modified between read and write) and don't
require immediate attention, making the expanded state noisy.

## Solution

Modified `FileEditToolCall` to detect `WRITE DENIED` errors via prefix
match and start them in collapsed state. Other errors and successful
edits remain expanded as before.

Users can still manually expand WRITE DENIED errors if they want to see
the details.

## Testing

- Type checking passes
- Logic is straightforward: check error prefix and set initial expansion
state

_Generated with `cmux`_

Author: ammar-agent

docs/AGENTS.md

@@ -481,6 +481,8 @@ The IPC layer is the boundary between backend and frontend. Follow these rules t
 Notice when you've made the same change many times, refactor to create a shared function
 or component, update all the duplicated code, and then continue on with the original work.
 When repeating string literals (especially in error messages, UI text, or system instructions), extract them to named constants in a relevant constants/utils file - never define the same string literal multiple times across files.
+Before defining a constant, search the codebase to see if it already exists and import it instead.
+If a constant exists in a layer-specific location (`services/`, `utils/main/`) but is needed across layers, move it to a shared location (`src/constants/`, `src/types/`) rather than duplicating it.
 
 **Avoid unnecessary callback indirection**: If a hook detects a condition and has access to all data needed to handle it, let it handle the action directly rather than passing callbacks up to parent components. Keep hooks self-contained when possible.
 

src/components/tools/FileEditToolCall.tsx

@@ -22,6 +22,7 @@ import { useToolExpansion, getStatusDisplay, type ToolStatus } from "./shared/to
 import { TooltipWrapper, Tooltip } from "../Tooltip";
 import { DiffContainer, DiffRenderer, SelectableDiffRenderer } from "../shared/DiffRenderer";
 import { KebabMenu, type KebabMenuItem } from "../KebabMenu";
+import { WRITE_DENIED_PREFIX } from "@/types/tools";
 
 type FileEditOperationArgs =
   | FileEditReplaceStringToolArgs
@@ -97,7 +98,11 @@ export const FileEditToolCall: React.FC<FileEditToolCallProps> = ({
   status = "pending",
   onReviewNote,
 }) => {
-  const { expanded, toggleExpanded } = useToolExpansion(true);
+  // Collapse WRITE DENIED errors by default since they're common and expected
+  const isWriteDenied = result && !result.success && result.error?.startsWith(WRITE_DENIED_PREFIX);
+  const initialExpanded = !isWriteDenied;
+
+  const { expanded, toggleExpanded } = useToolExpansion(initialExpanded);
   const [showRaw, setShowRaw] = React.useState(false);
   const [copied, setCopied] = React.useState(false);
 

src/services/tools/fileCommon.ts

@@ -2,11 +2,7 @@ import type * as fs from "fs";
 import * as path from "path";
 import { createPatch } from "diff";
 
-/**
- * Prefix for all file write error messages.
- * This consistent prefix helps models detect when writes fail and need to retry.
- */
-export const WRITE_DENIED_PREFIX = "WRITE DENIED, FILE UNMODIFIED:";
+// WRITE_DENIED_PREFIX moved to @/types/tools for frontend/backend sharing
 
 /**
  * Maximum file size for file operations (1MB)

src/services/tools/file_edit_insert.ts

@@ -4,7 +4,8 @@ import * as path from "path";
 import type { FileEditInsertToolResult } from "@/types/tools";
 import type { ToolConfiguration, ToolFactory } from "@/utils/tools/tools";
 import { TOOL_DEFINITIONS } from "@/utils/tools/toolDefinitions";
-import { validatePathInCwd, WRITE_DENIED_PREFIX } from "./fileCommon";
+import { validatePathInCwd } from "./fileCommon";
+import { WRITE_DENIED_PREFIX } from "@/types/tools";
 import { executeFileEditOperation } from "./file_edit_operation";
 
 /**

src/services/tools/file_edit_operation.test.ts

@@ -1,6 +1,6 @@
 import { describe, it, expect } from "bun:test";
 import { executeFileEditOperation } from "./file_edit_operation";
-import { WRITE_DENIED_PREFIX } from "./fileCommon";
+import { WRITE_DENIED_PREFIX } from "@/types/tools";
 
 const TEST_CWD = "/tmp";
 

src/services/tools/file_edit_operation.ts

@@ -2,13 +2,9 @@ import * as fs from "fs/promises";
 import * as path from "path";
 import writeFileAtomic from "write-file-atomic";
 import type { FileEditDiffSuccessBase, FileEditErrorResult } from "@/types/tools";
+import { WRITE_DENIED_PREFIX } from "@/types/tools";
 import type { ToolConfiguration } from "@/utils/tools/tools";
-import {
-  generateDiff,
-  validateFileSize,
-  validatePathInCwd,
-  WRITE_DENIED_PREFIX,
-} from "./fileCommon";
+import { generateDiff, validateFileSize, validatePathInCwd } from "./fileCommon";
 
 type FileEditOperationResult<TMetadata> =
   | {

src/types/tools.ts

@@ -117,6 +117,12 @@ export interface FileEditInsertToolArgs {
 
 export type FileEditInsertToolResult = FileEditDiffSuccessBase | FileEditErrorResult;
 
+/**
+ * Prefix for file write denial error messages.
+ * This consistent prefix helps both the UI and models detect when writes fail.
+ */
+export const WRITE_DENIED_PREFIX = "WRITE DENIED, FILE UNMODIFIED:";
+
 export type FileEditToolArgs =
   | FileEditReplaceStringToolArgs
   | FileEditReplaceLinesToolArgs