diff --git a/apps/sim/app/workspace/[workspaceId]/home/components/user-input/components/prompt-editor/prompt-editor.test.tsx b/apps/sim/app/workspace/[workspaceId]/home/components/user-input/components/prompt-editor/prompt-editor.test.tsx
new file mode 100644
index 00000000000..5b7fc95ee03
--- /dev/null
+++ b/apps/sim/app/workspace/[workspaceId]/home/components/user-input/components/prompt-editor/prompt-editor.test.tsx
@@ -0,0 +1,213 @@
+/**
+ * @vitest-environment jsdom
+ */
+import { act } from 'react'
+import { createRoot, type Root } from 'react-dom/client'
+import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest'
+
+vi.mock('@/hooks/queries/skills', () => ({ useSkills: () => ({ data: [] }) }))
+vi.mock('@/hooks/queries/mcp', () => ({ useMcpServers: () => ({ data: [] }) }))
+vi.mock('@/blocks/integration-matcher', () => ({
+  getIntegrationMatcher: () => ({ regex: null, byName: new Map() }),
+}))
+vi.mock(
+  '@/app/workspace/[workspaceId]/home/components/user-input/components/plus-menu-dropdown/plus-menu-dropdown',
+  () => ({ PlusMenuDropdown: () => null })
+)
+vi.mock(
+  '@/app/workspace/[workspaceId]/home/components/user-input/components/skills-menu-dropdown/skills-menu-dropdown',
+  () => ({ SkillsMenuDropdown: () => null })
+)
+
+import { PromptEditor } from '@/app/workspace/[workspaceId]/home/components/user-input/components/prompt-editor/prompt-editor'
+import { usePromptEditor } from '@/app/workspace/[workspaceId]/home/components/user-input/components/prompt-editor/use-prompt-editor'
+
+/**
+ * jsdom performs no layout, so the autosize inputs are stubbed: `editorWidth`
+ * stands for the scroller's content width and `contentHeight` for the height
+ * the text wraps to at that width. Narrowing raises the content height, exactly
+ * as rewrapping does in a browser.
+ */
+let contentHeight = 240
+let editorWidth = 700
+let autosizeCalls = 0
+
+/**
+ * `observe` only registers the target: real deliveries, including the initial
+ * one, are asynchronous, so tests drive them explicitly. Delivering inside
+ * `observe` would hide the window between the mount-time measure and the first
+ * notification — exactly where a width change can be missed.
+ */
+class FakeResizeObserver implements ResizeObserver {
+  private static instances: FakeResizeObserver[] = []
+  private readonly callback: ResizeObserverCallback
+  private targets: Element[] = []
+
+  constructor(callback: ResizeObserverCallback) {
+    this.callback = callback
+    FakeResizeObserver.instances.push(this)
+  }
+
+  observe(target: Element) {
+    this.targets.push(target)
+  }
+
+  unobserve(target: Element) {
+    this.targets = this.targets.filter((t) => t !== target)
+  }
+
+  disconnect() {
+    this.targets = []
+    FakeResizeObserver.instances = FakeResizeObserver.instances.filter((i) => i !== this)
+  }
+
+  deliver() {
+    const entries = this.targets.map(
+      (target) => ({ target, contentRect: { width: editorWidth } }) as ResizeObserverEntry
+    )
+    if (entries.length > 0) this.callback(entries, this)
+  }
+
+  static reset() {
+    FakeResizeObserver.instances = []
+  }
+
+  static observerCount() {
+    return FakeResizeObserver.instances.length
+  }
+
+  static deliverAll() {
+    for (const instance of [...FakeResizeObserver.instances]) instance.deliver()
+  }
+}
+
+function mountEditor() {
+  ;(globalThis as { IS_REACT_ACT_ENVIRONMENT?: boolean }).IS_REACT_ACT_ENVIRONMENT = true
+  const container = document.createElement('div')
+  document.body.appendChild(container)
+  const root: Root = createRoot(container)
+
+  function Probe() {
+    const editor = usePromptEditor({ workspaceId: 'ws-1', initialValue: 'a long prompt' })
+    return <PromptEditor editor={editor} className='max-h-[200px]' />
+  }
+
+  act(() => root.render(<Probe />))
+
+  const textarea = container.querySelector('textarea')
+  if (!textarea) throw new Error('textarea did not render')
+
+  return {
+    textarea,
+    unmount: () => {
+      act(() => root.unmount())
+      container.remove()
+    },
+  }
+}
+
+function resizeTo(width: number, wrappedHeight: number) {
+  editorWidth = width
+  contentHeight = wrappedHeight
+  act(() => FakeResizeObserver.deliverAll())
+}
+
+/**
+ * Delivers the observer's initial notification at the mounted width, putting the
+ * editor in the steady state a test can then resize away from.
+ */
+function settle() {
+  act(() => FakeResizeObserver.deliverAll())
+}
+
+describe('PromptEditor autosize', () => {
+  let originalScrollHeight: PropertyDescriptor | undefined
+
+  beforeEach(() => {
+    contentHeight = 240
+    editorWidth = 700
+    autosizeCalls = 0
+    FakeResizeObserver.reset()
+
+    originalScrollHeight = Object.getOwnPropertyDescriptor(Element.prototype, 'scrollHeight')
+    Object.defineProperty(Element.prototype, 'scrollHeight', {
+      configurable: true,
+      get() {
+        if (!(this instanceof HTMLTextAreaElement)) return 0
+        autosizeCalls++
+        return contentHeight
+      },
+    })
+    vi.stubGlobal('ResizeObserver', FakeResizeObserver)
+  })
+
+  afterEach(() => {
+    if (originalScrollHeight) {
+      Object.defineProperty(Element.prototype, 'scrollHeight', originalScrollHeight)
+    }
+    vi.unstubAllGlobals()
+  })
+
+  it('sizes the textarea to its content height on mount', () => {
+    const { textarea, unmount } = mountEditor()
+
+    expect(textarea.style.height).toBe('240px')
+    unmount()
+  })
+
+  it('re-measures when the editor width changes so no text falls outside the textarea', () => {
+    const { textarea, unmount } = mountEditor()
+    settle()
+    expect(textarea.style.height).toBe('240px')
+
+    resizeTo(340, 500)
+
+    expect(textarea.style.height).toBe('500px')
+    unmount()
+  })
+
+  it('re-measures again when the editor widens back', () => {
+    const { textarea, unmount } = mountEditor()
+    settle()
+
+    resizeTo(340, 500)
+    resizeTo(700, 240)
+
+    expect(textarea.style.height).toBe('240px')
+    unmount()
+  })
+
+  /** Distinct from the case above: here the width moves before any delivery lands. */
+  it('re-measures on the first delivery when the width changed before it arrived', () => {
+    const { textarea, unmount } = mountEditor()
+    expect(textarea.style.height).toBe('240px')
+
+    resizeTo(340, 500)
+
+    expect(textarea.style.height).toBe('500px')
+    unmount()
+  })
+
+  /** The height `autosize` writes re-notifies this observer, so this guard breaks the loop. */
+  it('ignores resize notifications that do not change the width', () => {
+    const { textarea, unmount } = mountEditor()
+    settle()
+    const callsAfterSettle = autosizeCalls
+
+    contentHeight = 500
+    act(() => FakeResizeObserver.deliverAll())
+
+    expect(textarea.style.height).toBe('240px')
+    expect(autosizeCalls).toBe(callsAfterSettle)
+    unmount()
+  })
+
+  it('stops observing after unmount', () => {
+    const { unmount } = mountEditor()
+    expect(FakeResizeObserver.observerCount()).toBe(1)
+
+    unmount()
+
+    expect(FakeResizeObserver.observerCount()).toBe(0)
+  })
+})
diff --git a/apps/sim/app/workspace/[workspaceId]/home/components/user-input/components/prompt-editor/prompt-editor.tsx b/apps/sim/app/workspace/[workspaceId]/home/components/user-input/components/prompt-editor/prompt-editor.tsx
index db58cdcf461..af7013cb84b 100644
--- a/apps/sim/app/workspace/[workspaceId]/home/components/user-input/components/prompt-editor/prompt-editor.tsx
+++ b/apps/sim/app/workspace/[workspaceId]/home/components/user-input/components/prompt-editor/prompt-editor.tsx
@@ -88,7 +88,7 @@ export function PromptEditor({
    * container, letting the browser clamp a bottom-pinned transcript upward by
    * the input's grown height on every multi-line edit.
    */
-  useLayoutEffect(() => {
+  const autosize = useCallback(() => {
     const textarea = textareaRef.current
     if (!textarea) return
     const scroller = scrollerRef.current
@@ -96,7 +96,38 @@ export function PromptEditor({
     textarea.style.height = 'auto'
     textarea.style.height = `${textarea.scrollHeight}px`
     if (scroller) scroller.style.height = ''
-  }, [value, textareaRef])
+  }, [textareaRef])
+
+  useLayoutEffect(() => {
+    autosize()
+  }, [value, autosize])
+
+  /**
+   * The textarea carries an inline pixel height, so a width change (window
+   * resize, sidebar toggle, chat column reflow) rewraps the text taller while
+   * the box stays at its old height. The mirror overlay paints the full text
+   * regardless, so the spilled lines render over the scroller with no textarea
+   * beneath them — visible, scrollable text that swallows clicks instead of
+   * placing the caret.
+   *
+   * Only width is compared: `autosize` writes the textarea's height, which
+   * re-notifies this observer, so reacting to height would feed itself. The
+   * first delivery is measured like any other — the width can change between
+   * the mount-time measure and `observe()`.
+   */
+  useEffect(() => {
+    const scroller = scrollerRef.current
+    if (!scroller) return
+    let lastWidth: number | null = null
+    const observer = new ResizeObserver(([entry]) => {
+      const width = entry.contentRect.width
+      if (width === lastWidth) return
+      lastWidth = width
+      autosize()
+    })
+    observer.observe(scroller)
+    return () => observer.disconnect()
+  }, [autosize])
 
   useEffect(() => {
     if (autoFocus && !readOnly) editor.focusAtEnd()