diff --git a/src/main/shell-ready-marker-scanner.ts b/src/main/shell-ready-marker-scanner.ts
index 0e6250e1c4e..a8b58499852 100644
--- a/src/main/shell-ready-marker-scanner.ts
+++ b/src/main/shell-ready-marker-scanner.ts
@@ -11,6 +11,10 @@ export type ShellReadyScanResult = {
   postMarkerBytesObserved: boolean
 }
 
+type ShellReadyBoundaryScanResult = ShellReadyScanResult & {
+  postMarkerOutputIndex: number | null
+}
+
 export function createShellReadyScanState(): ShellReadyScanState {
   return { matchPos: 0, heldBytes: '' }
 }
@@ -22,7 +26,10 @@ export function drainShellReadyHeldBytes(state: ShellReadyScanState): string {
   return heldBytes
 }
 
-export function scanForShellReady(state: ShellReadyScanState, data: string): ShellReadyScanResult {
+export function scanForShellReadyBoundary(
+  state: ShellReadyScanState,
+  data: string
+): ShellReadyBoundaryScanResult {
   let output = ''
 
   for (let i = 0; i < data.length; i += 1) {
@@ -49,7 +56,8 @@ export function scanForShellReady(state: ShellReadyScanState, data: string): She
       return {
         output: output + remaining,
         matched: true,
-        postMarkerBytesObserved: remaining.length > 0
+        postMarkerBytesObserved: remaining.length > 0,
+        postMarkerOutputIndex: output.length
       }
     } else {
       output += state.heldBytes
@@ -64,5 +72,15 @@ export function scanForShellReady(state: ShellReadyScanState, data: string): She
     }
   }
 
-  return { output, matched: false, postMarkerBytesObserved: false }
+  return {
+    output,
+    matched: false,
+    postMarkerBytesObserved: false,
+    postMarkerOutputIndex: null
+  }
+}
+
+export function scanForShellReady(state: ShellReadyScanState, data: string): ShellReadyScanResult {
+  const { postMarkerOutputIndex: _, ...result } = scanForShellReadyBoundary(state, data)
+  return result
 }
diff --git a/src/main/shell-startup-output-scanner.test.ts b/src/main/shell-startup-output-scanner.test.ts
index b4ace12a456..80dbfe4f7bf 100644
--- a/src/main/shell-startup-output-scanner.test.ts
+++ b/src/main/shell-startup-output-scanner.test.ts
@@ -6,6 +6,42 @@ import {
 } from './shell-startup-output-scanner'
 
 const READY_MARKER = '\x1b]777;orca-shell-ready\x07'
+const IDENTITY_MARKER = '\x1b]777;orca-shell-start:12345\x07'
+
+function scanChunks(chunks: string[]): {
+  output: string
+  shellPid: number | null
+  ready: boolean
+  postMarkerBytesObserved: boolean
+} {
+  const state = createShellStartupOutputScanState()
+  let output = ''
+  let shellPid: number | null = null
+  let ready = false
+  let postMarkerBytesObserved = false
+
+  for (const chunk of chunks) {
+    const scanned = scanShellStartupOutput(state, chunk)
+    output += scanned.output
+    shellPid ??= scanned.shellPid
+    ready ||= scanned.ready
+    postMarkerBytesObserved ||= scanned.postMarkerBytesObserved
+  }
+  return { output, shellPid, ready, postMarkerBytesObserved }
+}
+
+function expectEverySplitToProduce(
+  input: string,
+  expected: Omit<ReturnType<typeof scanChunks>, 'postMarkerBytesObserved'>
+): void {
+  for (let split = 0; split <= input.length; split += 1) {
+    const { postMarkerBytesObserved: _, ...scanned } = scanChunks([
+      input.slice(0, split),
+      input.slice(split)
+    ])
+    expect(scanned, `split ${split}`).toEqual(expected)
+  }
+}
 
 describe('shell startup output scanner', () => {
   it.each([
@@ -22,12 +58,30 @@ describe('shell startup output scanner', () => {
     expect(output).toBe('\x1b[?2004hfish> ')
   })
 
+  it('preserves every post-ready byte at every chunk boundary', () => {
+    expectEverySplitToProduce(`${READY_MARKER}${IDENTITY_MARKER}prompt`, {
+      output: `${IDENTITY_MARKER}prompt`,
+      shellPid: null,
+      ready: true
+    })
+    expectEverySplitToProduce(`${READY_MARKER}\x1b[?2004hfish> `, {
+      output: '\x1b[?2004hfish> ',
+      shellPid: null,
+      ready: true
+    })
+  })
+
+  it('strips pre-ready markers at every chunk boundary', () => {
+    expectEverySplitToProduce(`${IDENTITY_MARKER}${READY_MARKER}prompt`, {
+      output: 'prompt',
+      shellPid: 12345,
+      ready: true
+    })
+  })
+
   it('strips identity and readiness markers from one chunk', () => {
     const state = createShellStartupOutputScanState()
-    const scanned = scanShellStartupOutput(
-      state,
-      `\x1b]777;orca-shell-start:12345\x07${READY_MARKER}prompt`
-    )
+    const scanned = scanShellStartupOutput(state, `${IDENTITY_MARKER}${READY_MARKER}prompt`)
 
     expect(scanned).toEqual({
       output: 'prompt',
diff --git a/src/main/shell-startup-output-scanner.ts b/src/main/shell-startup-output-scanner.ts
index 99606f6d047..a0b086865ba 100644
--- a/src/main/shell-startup-output-scanner.ts
+++ b/src/main/shell-startup-output-scanner.ts
@@ -1,7 +1,7 @@
 import {
   createShellReadyScanState,
   drainShellReadyHeldBytes,
-  scanForShellReady,
+  scanForShellReadyBoundary,
   type ShellReadyScanState
 } from './shell-ready-marker-scanner'
 import {
@@ -34,30 +34,30 @@ export function scanShellStartupOutput(
   state: ShellStartupOutputScanState,
   data: string
 ): ShellStartupOutputScanResult {
+  const readiness = scanForShellReadyBoundary(state.ready, data)
+  const postMarkerOutputIndex = readiness.postMarkerOutputIndex ?? readiness.output.length
+  const identityInput = readiness.output.slice(0, postMarkerOutputIndex)
   let shellPid: number | null = null
+  let output = identityInput
   if (state.identity) {
-    const identity = scanForShellStartupIdentity(state.identity, data)
-    data = identity.output
+    const identity = scanForShellStartupIdentity(state.identity, identityInput)
+    output = identity.output
     shellPid = identity.shellPid
     if (shellPid) {
       state.identity = null
     }
   }
 
-  const readiness = scanForShellReady(state.ready, data)
-  let output = readiness.output
-  let postMarkerBytesObserved = readiness.postMarkerBytesObserved
   if (readiness.matched && state.identity) {
-    const heldOutput = drainShellStartupIdentityHeldBytes(state.identity)
-    output += heldOutput
-    postMarkerBytesObserved ||= heldOutput.length > 0
+    output += drainShellStartupIdentityHeldBytes(state.identity)
     state.identity = null
   }
+  output += readiness.output.slice(postMarkerOutputIndex)
   return {
     output,
     shellPid,
     ready: readiness.matched,
-    postMarkerBytesObserved
+    postMarkerBytesObserved: readiness.postMarkerBytesObserved
   }
 }
 
@@ -65,7 +65,7 @@ export function drainShellStartupOutputScanState(state: ShellStartupOutputScanSt
   let output = state.identity ? drainShellStartupIdentityHeldBytes(state.identity) : ''
   state.identity = null
   if (output) {
-    output = scanForShellReady(state.ready, output).output
+    output = scanForShellReadyBoundary(state.ready, output).output
   }
   return output + drainShellReadyHeldBytes(state.ready)
 }