@@ -74,6 +74,21 @@ describe('native computer use transport', () => {
     expect(reset).not.toHaveBeenCalled()
   })
 
+  it.each(['\u0000', '漢'])(
+    'rejects requests above the native byte limit without disrupting the helper (%j)',
+    async (character) => {
+      const { client, reset } = helper(`let count = 0; ${reader}
+        count++;
+        process.stdout.write(JSON.stringify({id:request.id,result:{...${status},accessibility:count===2}})+'\\n');
+      });`)
+      await client.request('status', {})
+      const params = { text: character.repeat(character === '\u0000' ? 32_000 : 48_000) }
+      await expect(client.request('type_text', params)).rejects.toThrow('exceeds 128 KiB')
+      await expect(client.request('status', {})).resolves.toMatchObject({ accessibility: true })
+      expect(reset).not.toHaveBeenCalled()
+    }
+  )
+
   it('rejects malformed output and can start a fresh helper afterward', async () => {
     const { client, reset } = helper(`${reader}
       process.stdout.write(request.method==='bad'?'not json\\n':JSON.stringify({id:request.id,result:${status}})+'\\n');
 
@@ -6,6 +6,8 @@ import {
 } from '@sim/desktop-bridge/computer-use'
 import { generateId } from '@sim/utils/id'
 
+// ComputerUse.swift bounds each request line independently of the decoded argument schema.
+const MAX_REQUEST_BYTES = 128 * 1024
 const MAX_FRAME_BYTES = 16 * 1024 * 1024
 const REQUEST_TIMEOUT_MS = 30_000
 
@@ -38,14 +40,21 @@ export class NativeComputerUseClient implements ComputerUseNativeClient {
     await this.stopping
     if (generation !== this.generation) throw new Error('Computer Use stopped.')
     if (this.pending.size >= 16) return Promise.reject(new Error('Computer Use is busy.'))
-    const child = this.start()
     const id = generateId()
+    const request = JSON.stringify({ id, method, params })
+    if (Buffer.byteLength(request, 'utf8') > MAX_REQUEST_BYTES)
+      throw new ComputerUseError({
+        code: 'invalid_request',
+        message: 'Computer Use request exceeds 128 KiB.',
+        dispatchState: 'not_started',
+      })
+    const child = this.start()
     return new Promise((resolve, reject) => {
       const timer = setTimeout(() => {
         this.stopWithError(new Error('Computer Use timed out; inspect the app before retrying.'))
       }, REQUEST_TIMEOUT_MS)
       this.pending.set(id, { resolve, reject, timer })
-      child.stdin.write(`${JSON.stringify({ id, method, params })}\n`, (error) => {
+      child.stdin.write(`${request}\n`, (error) => {
         if (error && this.child === child)
           this.stopWithError(new Error('Computer Use helper disconnected.'))
       })
 
@@ -134,6 +134,40 @@ describe('native computer model projection', () => {
     }
   )
 
+  it.each([false, true])(
+    'omits an oversized screenshot while retaining usable state and dispatch certainty (nested=%s)',
+    (nested) => {
+      const state = snapshot()
+      state.screenshot = {
+        base64: Buffer.alloc(8 * 1024 * 1024).toString('base64'),
+        mimeType: 'image/png',
+        width: 3840,
+        height: 2160,
+      }
+      const result = computerToolResultForModel(nested ? sequence(state) : state)
+      const body = JSON.stringify({
+        toolCallId: 'large-screenshot',
+        status: 'success',
+        message: 'Observed',
+        data: result,
+      })
+      expect(Buffer.byteLength(body)).toBeLessThan(10 * 1024 * 1024)
+      expect(result).not.toHaveProperty('observations')
+      const projected = 'observation' in result ? result.observation : result
+      expect(projected).toMatchObject({
+        screenshotError: expect.stringContaining('omitted'),
+        accessibilityTree: expect.stringContaining('editor AXTextArea'),
+      })
+      expect(projected).not.toHaveProperty('screenshotSize')
+      if (nested)
+        expect(result).toMatchObject({
+          dispatched: true,
+          verified: false,
+          sequence: { completedSteps: 1 },
+        })
+    }
+  )
+
   it('does not split an emoji when bounding text for the model', () => {
     const state = snapshot()
     state.nodes[0] = {
 
@@ -1,6 +1,8 @@
 import type { ComputerUseResult, ComputerUseSnapshot } from '@sim/desktop-bridge/computer-use'
 import { truncateAtCodePoint } from '@sim/utils/string'
 
+// Keep encoded pixels below the 10 MiB completion body cap, with room for state and its envelope.
+const MAX_SCREENSHOT_BASE64_BYTES = 9 * 1024 * 1024
 const MAX_STATE_BYTES = 44 * 1024
 const encoder = new TextEncoder()
 
@@ -44,23 +46,32 @@ function nodeRow(node: ComputerUseSnapshot['nodes'][number]): string {
 
 function snapshotForModel(result: ComputerUseSnapshot, envelopeBytes = 0) {
   const { screenshot, nodes, windows: allWindows, ...state } = result
+  const image =
+    screenshot && screenshot.base64.length <= MAX_SCREENSHOT_BASE64_BYTES ? screenshot : undefined
+  const screenshotOmitted = Boolean(screenshot && !image)
   const selectedWindow = allWindows.find((candidate) => candidate.windowId === state.windowId)
   const windows = [
     ...(selectedWindow ? [selectedWindow] : []),
     ...allWindows.filter((candidate) => candidate.windowId !== state.windowId).slice(0, 19),
   ].map((window) => ({ ...window, title: truncateAtCodePoint(window.title, 256, '') }))
   const model = {
     ...state,
+    ...(screenshotOmitted
+      ? {
+          screenshotError:
+            'Screenshot omitted because its encoded size exceeds completion limits. Accessibility state is still available.',
+        }
+      : {}),
     windows,
     omittedWindowCount: allWindows.length - windows.length,
     accessibilityTree: '',
     omittedNodeCount: nodes.length,
     nodeTextLimit: 512,
-    ...(screenshot
+    ...(image
       ? {
-          screenshotSize: { width: screenshot.width, height: screenshot.height },
+          screenshotSize: { width: image.width, height: image.height },
           content: selectedWindow
-            ? `Screenshot of window ${state.windowId}. Coordinate actions use window-local macOS points. Convert encoded image coordinates: x = imageX * ${selectedWindow.width} / ${screenshot.width}; y = imageY * ${selectedWindow.height} / ${screenshot.height}. Accessibility screenRect coordinates are global points; subtract window origin (${selectedWindow.x}, ${selectedWindow.y}). Prefer element IDs.`
+            ? `Screenshot of window ${state.windowId}. Coordinate actions use window-local macOS points. Convert encoded image coordinates: x = imageX * ${selectedWindow.width} / ${image.width}; y = imageY * ${selectedWindow.height} / ${image.height}. Accessibility screenRect coordinates are global points; subtract window origin (${selectedWindow.x}, ${selectedWindow.y}). Prefer element IDs.`
             : 'Screenshot coordinate mapping is unavailable. Use accessibility element IDs or take a fresh state before acting.',
         }
       : {}),
@@ -145,9 +156,14 @@ function snapshotForModel(result: ComputerUseSnapshot, envelopeBytes = 0) {
       state.truncated ||
       selected.size < nodes.length ||
       windows.length < allWindows.length ||
-      clipped,
-    observations: screenshot
-      ? [{ name: 'Computer screenshot', mediaType: screenshot.mimeType, data: screenshot.base64 }]
-      : undefined,
+      clipped ||
+      screenshotOmitted,
+    ...(image
+      ? {
+          observations: [
+            { name: 'Computer screenshot', mediaType: image.mimeType, data: image.base64 },
+          ],
+        }
+      : {}),
   }
 }