diff --git a/src/server/services/agent/__tests__/fileChanges.test.ts b/src/server/services/agent/__tests__/fileChanges.test.ts
new file mode 100644
index 0000000..194ba93
--- /dev/null
+++ b/src/server/services/agent/__tests__/fileChanges.test.ts
@@ -0,0 +1,66 @@
+/**
+ * Copyright 2026 GoodRx, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import { buildProposedFileChanges } from '../fileChanges';
+
+describe('buildProposedFileChanges', () => {
+  it('keeps workspace edit approvals as before-and-after previews instead of fake diffs', () => {
+    const [change] = buildProposedFileChanges({
+      toolCallId: 'tool-1',
+      sourceTool: 'workspace.edit_file',
+      input: {
+        path: '/workspace/sample-service/app.js',
+        oldText: 'before',
+        newText: 'after',
+      },
+    });
+
+    expect(change).toMatchObject({
+      id: 'tool-1:sample-service/app.js',
+      toolCallId: 'tool-1',
+      sourceTool: 'workspace.edit_file',
+      path: '/workspace/sample-service/app.js',
+      displayPath: 'sample-service/app.js',
+      stage: 'awaiting-approval',
+      unifiedDiff: null,
+      beforeTextPreview: 'before',
+      afterTextPreview: 'after',
+    });
+  });
+
+  it('keeps workspace writes as preview-only changes', () => {
+    const [change] = buildProposedFileChanges({
+      toolCallId: 'tool-2',
+      sourceTool: 'workspace.write_file',
+      input: {
+        path: '/workspace/sample-service/README.md',
+        content: '# Sample service',
+      },
+    });
+
+    expect(change).toMatchObject({
+      id: 'tool-2:sample-service/README.md',
+      toolCallId: 'tool-2',
+      sourceTool: 'workspace.write_file',
+      path: '/workspace/sample-service/README.md',
+      displayPath: 'sample-service/README.md',
+      stage: 'awaiting-approval',
+      unifiedDiff: null,
+      beforeTextPreview: null,
+      afterTextPreview: '# Sample service',
+    });
+  });
+});
diff --git a/src/server/services/agent/fileChanges.ts b/src/server/services/agent/fileChanges.ts
index 8ed7064..f84ec41 100644
--- a/src/server/services/agent/fileChanges.ts
+++ b/src/server/services/agent/fileChanges.ts
@@ -92,25 +92,6 @@ function countChangedLines(value: string): number {
   return value.split('\n').length;
 }
 
-function buildUnifiedDiff(path: string, before: string, after: string): string | null {
-  if (before === after) {
-    return null;
-  }
-
-  const displayPath = trimWorkspacePrefix(path);
-  const beforeLines = before.split('\n');
-  const afterLines = after.split('\n');
-
-  return [
-    `diff --git a/${displayPath} b/${displayPath}`,
-    `--- a/${displayPath}`,
-    `+++ b/${displayPath}`,
-    `@@ -1,${beforeLines.length} +1,${afterLines.length} @@`,
-    ...beforeLines.map((line) => `-${line}`),
-    ...afterLines.map((line) => `+${line}`),
-  ].join('\n');
-}
-
 function unwrapToolPayload(value: unknown): unknown {
   if (!isRecord(value) || !Array.isArray(value.content)) {
     return value;
@@ -270,7 +251,7 @@ export function buildProposedFileChanges({
       additions: countChangedLines(args.newText),
       deletions: countChangedLines(args.oldText),
       truncated: false,
-      unifiedDiff: buildUnifiedDiff(args.path, args.oldText, args.newText),
+      unifiedDiff: null,
       beforeTextPreview: args.oldText,
       afterTextPreview: args.newText,
       summary: `Proposed update to ${trimWorkspacePrefix(args.path)}`,