diff --git a/client/src/components/Artifacts/ArtifactCodeEditor.test.tsx b/client/src/components/Artifacts/ArtifactCodeEditor.test.tsx
new file mode 100644
index 0000000000..cfb6bce9cb
--- /dev/null
+++ b/client/src/components/Artifacts/ArtifactCodeEditor.test.tsx
@@ -0,0 +1,244 @@
+import React from 'react';
+import { render, act } from '@testing-library/react';
+import type { editor } from 'monaco-editor';
+import type { Artifact } from '~/common';
+import { ArtifactCodeEditor } from './ArtifactCodeEditor';
+
+interface MutationVars {
+  updated: string;
+  messageId: string;
+  index: number;
+  original?: string;
+}
+
+interface MutationHandlers {
+  onMutate?: (vars: MutationVars) => void;
+  onSuccess?: (data: unknown, vars: MutationVars) => void;
+  onError?: (error?: unknown) => void;
+}
+
+const mockEditorProps: { onChange?: (value: string | undefined) => void } = {};
+const mockMutationHandlers: MutationHandlers = {};
+
+// Calling mutate replays onMutate synchronously so currentUpdateRef reflects the
+// in-flight content, matching how react-query drives the real mutation lifecycle.
+const mockMutate = jest.fn((vars: MutationVars) => {
+  mockMutationHandlers.onMutate?.(vars);
+});
+
+jest.mock('@monaco-editor/react', () => ({
+  __esModule: true,
+  default: (props: { onChange?: (value: string | undefined) => void }) => {
+    mockEditorProps.onChange = props.onChange;
+    return null;
+  },
+}));
+
+jest.mock('~/Providers/EditorContext', () => {
+  const ReactModule = jest.requireActual<typeof import('react')>('react');
+  return {
+    useMutationState: () => {
+      const [isMutating, setIsMutating] = ReactModule.useState(false);
+      return { isMutating, setIsMutating };
+    },
+    useCodeState: () => {
+      const [currentCode, setCurrentCode] = ReactModule.useState('');
+      return { currentCode, setCurrentCode };
+    },
+  };
+});
+
+jest.mock('~/Providers', () => ({
+  useArtifactsContext: () => ({ isSubmitting: false }),
+}));
+
+jest.mock('~/data-provider', () => ({
+  useEditArtifact: (handlers: MutationHandlers) => {
+    mockMutationHandlers.onMutate = handlers.onMutate;
+    mockMutationHandlers.onSuccess = handlers.onSuccess;
+    mockMutationHandlers.onError = handlers.onError;
+    return { mutate: mockMutate };
+  },
+}));
+
+const ORIGINAL = 'ORIGINAL';
+
+const artifact: Artifact = {
+  id: 'artifact-1',
+  lastUpdateTime: 0,
+  index: 0,
+  messageId: 'msg-1',
+  content: ORIGINAL,
+  type: 'text/plain',
+};
+
+const otherArtifact: Artifact = {
+  id: 'artifact-2',
+  lastUpdateTime: 0,
+  index: 0,
+  messageId: 'msg-2',
+  content: 'ORIGINAL-B',
+  type: 'text/plain',
+};
+
+const renderEditor = (initial: Artifact = artifact) => {
+  const monacoRef: React.MutableRefObject<editor.IStandaloneCodeEditor | null> = { current: null };
+  const utils = render(<ArtifactCodeEditor artifact={initial} monacoRef={monacoRef} />);
+  const rerenderWith = (next: Artifact) =>
+    utils.rerender(<ArtifactCodeEditor artifact={next} monacoRef={monacoRef} />);
+  return { ...utils, rerenderWith };
+};
+
+const fireEdit = (value: string) => {
+  act(() => {
+    mockEditorProps.onChange?.(value);
+    jest.advanceTimersByTime(500);
+  });
+};
+
+describe('ArtifactCodeEditor retry guard', () => {
+  beforeEach(() => {
+    jest.useFakeTimers();
+    mockEditorProps.onChange = undefined;
+    mockMutationHandlers.onMutate = undefined;
+    mockMutationHandlers.onSuccess = undefined;
+    mockMutationHandlers.onError = undefined;
+  });
+
+  afterEach(() => {
+    jest.runOnlyPendingTimers();
+    jest.useRealTimers();
+  });
+
+  it('does not re-run a mutation for content that just failed', () => {
+    renderEditor();
+
+    fireEdit('EDITED');
+    expect(mockMutate).toHaveBeenCalledTimes(1);
+    expect(mockMutate).toHaveBeenLastCalledWith(expect.objectContaining({ updated: 'EDITED' }));
+
+    act(() => {
+      mockMutationHandlers.onError?.({ status: 400 });
+    });
+
+    fireEdit('EDITED');
+    expect(mockMutate).toHaveBeenCalledTimes(1);
+  });
+
+  it('blocks the retry for a real AxiosError 400 (production error shape)', () => {
+    renderEditor();
+
+    fireEdit('EDITED');
+    expect(mockMutate).toHaveBeenCalledTimes(1);
+
+    act(() => {
+      mockMutationHandlers.onError?.({ isAxiosError: true, response: { status: 400 } });
+    });
+
+    fireEdit('EDITED');
+    expect(mockMutate).toHaveBeenCalledTimes(1);
+  });
+
+  it('blocks the synchronous pending re-run of content that just failed', () => {
+    renderEditor();
+
+    fireEdit('EDITED');
+    expect(mockMutate).toHaveBeenCalledTimes(1);
+
+    fireEdit('EDITED');
+    expect(mockMutate).toHaveBeenCalledTimes(1);
+
+    act(() => {
+      mockMutationHandlers.onError?.({ status: 400 });
+    });
+
+    expect(mockMutate).toHaveBeenCalledTimes(1);
+  });
+
+  it('clears the guard on success so the same content can run again', () => {
+    renderEditor();
+
+    fireEdit('EDITED');
+    act(() => {
+      mockMutationHandlers.onError?.({ status: 400 });
+    });
+    fireEdit('EDITED');
+    expect(mockMutate).toHaveBeenCalledTimes(1);
+
+    fireEdit('DIFFERENT');
+    expect(mockMutate).toHaveBeenCalledTimes(2);
+
+    act(() => {
+      mockMutationHandlers.onSuccess?.(undefined, {
+        updated: 'DIFFERENT',
+        messageId: 'msg-1',
+        index: 0,
+      });
+    });
+    fireEdit('EDITED');
+    expect(mockMutate).toHaveBeenCalledTimes(3);
+    expect(mockMutate).toHaveBeenLastCalledWith(expect.objectContaining({ updated: 'EDITED' }));
+  });
+
+  it('clears the guard when the artifact changes', () => {
+    const { rerenderWith } = renderEditor();
+
+    fireEdit('EDITED');
+    act(() => {
+      mockMutationHandlers.onError?.({ status: 400 });
+    });
+    fireEdit('EDITED');
+    expect(mockMutate).toHaveBeenCalledTimes(1);
+
+    act(() => {
+      rerenderWith(otherArtifact);
+    });
+    fireEdit('EDITED');
+    expect(mockMutate).toHaveBeenCalledTimes(2);
+    expect(mockMutate).toHaveBeenLastCalledWith(expect.objectContaining({ updated: 'EDITED' }));
+  });
+
+  it('re-runs identical content after a non-400 client error (transient, not deterministic)', () => {
+    renderEditor();
+
+    fireEdit('EDITED');
+    expect(mockMutate).toHaveBeenCalledTimes(1);
+
+    act(() => {
+      mockMutationHandlers.onError?.({ status: 429 });
+    });
+
+    fireEdit('EDITED');
+    expect(mockMutate).toHaveBeenCalledTimes(2);
+    expect(mockMutate).toHaveBeenLastCalledWith(expect.objectContaining({ updated: 'EDITED' }));
+  });
+
+  it('re-runs identical content after a 5xx error (transient, not deterministic)', () => {
+    renderEditor();
+
+    fireEdit('EDITED');
+    expect(mockMutate).toHaveBeenCalledTimes(1);
+
+    act(() => {
+      mockMutationHandlers.onError?.({ status: 503 });
+    });
+
+    fireEdit('EDITED');
+    expect(mockMutate).toHaveBeenCalledTimes(2);
+    expect(mockMutate).toHaveBeenLastCalledWith(expect.objectContaining({ updated: 'EDITED' }));
+  });
+
+  it('re-runs identical content after an error with no status (network blip)', () => {
+    renderEditor();
+
+    fireEdit('EDITED');
+    expect(mockMutate).toHaveBeenCalledTimes(1);
+
+    act(() => {
+      mockMutationHandlers.onError?.(new Error('Network Error'));
+    });
+
+    fireEdit('EDITED');
+    expect(mockMutate).toHaveBeenCalledTimes(2);
+  });
+});
diff --git a/client/src/components/Artifacts/ArtifactCodeEditor.tsx b/client/src/components/Artifacts/ArtifactCodeEditor.tsx
index a349f30eda..c8963c2dcc 100644
--- a/client/src/components/Artifacts/ArtifactCodeEditor.tsx
+++ b/client/src/components/Artifacts/ArtifactCodeEditor.tsx
@@ -5,6 +5,7 @@ import type { Monaco } from '@monaco-editor/react';
 import type { editor } from 'monaco-editor';
 import type { Artifact } from '~/common';
 import { useMutationState, useCodeState } from '~/Providers/EditorContext';
+import { getResponseStatus } from '~/utils/errors';
 import { useArtifactsContext } from '~/Providers';
 import { useEditArtifact } from '~/data-provider';
 
@@ -111,10 +112,12 @@ export const ArtifactCodeEditor = function ArtifactCodeEditor({
   const { setCurrentCode } = useCodeState();
   const [currentUpdate, setCurrentUpdate] = useState<string | null>(null);
   const { isMutating, setIsMutating } = useMutationState();
+  const [failedContent, setFailedContent] = useState<string | null>(null);
   const artifactRef = useRef(artifact);
   const isMutatingRef = useRef(isMutating);
   const currentUpdateRef = useRef(currentUpdate);
   const setCurrentCodeRef = useRef(setCurrentCode);
+  const failedContentRef = useRef(failedContent);
   const pendingUpdateRef = useRef<PendingUpdate | null>(null);
   const runMutationRef = useRef<(code: string, original?: string) => void>(() => {});
 
@@ -130,6 +133,7 @@ export const ArtifactCodeEditor = function ArtifactCodeEditor({
       currentUpdateRef.current = null;
       setIsMutating(false);
       setCurrentUpdate(null);
+      setFailedContent(null);
 
       const pending = pendingUpdateRef.current;
       pendingUpdateRef.current = null;
@@ -148,7 +152,12 @@ export const ArtifactCodeEditor = function ArtifactCodeEditor({
         runMutationRef.current(pending.code, original);
       }
     },
-    onError: () => {
+    onError: (error) => {
+      const status = getResponseStatus(error);
+      if (status === 400 && currentUpdateRef.current != null) {
+        setFailedContent(currentUpdateRef.current);
+        failedContentRef.current = currentUpdateRef.current;
+      }
       const pending = pendingUpdateRef.current;
       pendingUpdateRef.current = null;
       isMutatingRef.current = false;
@@ -182,6 +191,7 @@ export const ArtifactCodeEditor = function ArtifactCodeEditor({
   currentUpdateRef.current = currentUpdate;
   editArtifactRef.current = editArtifact;
   setCurrentCodeRef.current = setCurrentCode;
+  failedContentRef.current = failedContent;
 
   const runMutation = useCallback(
     (code: string, originalOverride?: string) => {
@@ -209,6 +219,10 @@ export const ArtifactCodeEditor = function ArtifactCodeEditor({
         return;
       }
 
+      if (failedContentRef.current != null && code.trim() === failedContentRef.current.trim()) {
+        return;
+      }
+
       setCurrentCodeRef.current(code);
       editArtifactRef.current.mutate({
         index: target.index,
@@ -284,6 +298,7 @@ export const ArtifactCodeEditor = function ArtifactCodeEditor({
     }
     prevArtifactId.current = artifact.id;
     pendingUpdateRef.current = null;
+    setFailedContent(null);
     prevContentRef.current = artifact.content ?? '';
     const ed = monacoRef.current;
     if (ed && artifact.content != null) {