diff --git a/mobile/src/terminal/document/document-externals.ts b/mobile/src/terminal/document/document-externals.ts
index 58e97a4b61b..737927ea726 100644
--- a/mobile/src/terminal/document/document-externals.ts
+++ b/mobile/src/terminal/document/document-externals.ts
@@ -9,9 +9,6 @@
  * This file only shrinks. When the last group moves it is deleted.
  */
 
-/** `runtime-state-and-text-scaling`: posts one message to the native host. */
-export declare function notify(message: Record<string, unknown>): void
-
 /** `surface-touch-gestures`: binds touch, wheel and tap handlers to a surface element. */
 export declare function attachSurfaceEventHandlers(target: HTMLElement): void
 
@@ -32,9 +29,3 @@ export declare function applyXtermSelection(): void
 
 /** `selection-overlay`: moves the handles and the menu pill to the current selection. */
 export declare function repositionOverlay(): void
-
-/** `host-message-router`: routes one decoded host message. */
-export declare function handleMsg(msg: unknown): void
-
-/** `terminal-init-and-write`: reports an engine failure to the host. */
-export declare function reportEngineError(summary: string, cause: unknown, fatal: unknown): void
diff --git a/mobile/src/terminal/document/document-scope.ts b/mobile/src/terminal/document/document-scope.ts
index f546defe3d7..7e44a5c41b2 100644
--- a/mobile/src/terminal/document/document-scope.ts
+++ b/mobile/src/terminal/document/document-scope.ts
@@ -30,7 +30,7 @@ export type TerminalOscLinkService = { getLinkData?: (id: number) => { uri?: str
 
 /** The xterm internals the OSC 8 lookup walks. */
 export type TerminalDocumentCore = {
-  _renderService?: { dimensions?: { css: { cell: { height?: number; width?: number } } } }
+  _renderService?: { dimensions?: { css: { cell: { height: number; width: number } } } }
   _oscLinkService?: TerminalOscLinkService
   _inputHandler?: { _oscLinkService?: TerminalOscLinkService }
 }
@@ -62,6 +62,7 @@ export type TerminalDocumentLine = {
 
 /** One side of xterm's buffer, as the document reads it. */
 export type TerminalDocumentBuffer = {
+  readonly length: number
   readonly viewportY: number
   readonly baseY: number
   readonly cursorY: number
@@ -89,6 +90,9 @@ export type TerminalDocumentTerminal = {
   write: (data: string, callback?: () => void) => void
   open: (element: HTMLElement) => void
   scrollToLine: (line: number) => void
+  clear: () => void
+  reset: () => void
+  selectAll: () => void
   readonly unicode: { activeVersion: string }
   attachCustomKeyEventHandler: (handler: () => boolean) => void
   onData: (listener: (data: string) => void) => TerminalDocumentDisposable
@@ -170,7 +174,7 @@ export type TerminalDocumentScope = {
   /** `text-scaling`: the largest text-scale preset. */
   MAX_TEXT_SCALE: number
   /** `viewport-transform`: host message ids already handled, to drop repeats. */
-  handledMessageIds: string[]
+  handledMessageIds: number[]
   /** `runtime-state`: the text scale the user picked, as a preset index. */
   currentTextScale: number
   /** `runtime-state`: the font stack xterm renders with. */
diff --git a/mobile/src/terminal/document/host-message-router.test.ts b/mobile/src/terminal/document/host-message-router.test.ts
new file mode 100644
index 00000000000..3916290526d
--- /dev/null
+++ b/mobile/src/terminal/document/host-message-router.test.ts
@@ -0,0 +1,30 @@
+import { fileURLToPath } from 'node:url'
+import { describe, expect, it } from 'vitest'
+import { emitTerminalDocumentModule } from '../../../scripts/build-terminal-document-script.mjs'
+import { TERMINAL_HTML_HOST_MESSAGE_ROUTER } from '../terminal-webview-html/host-message-router'
+import { compareTerminalDocumentScripts } from './terminal-document-equivalence.test-support'
+
+/** The slice opens with the already-extracted reflow group. */
+const modulePaths = ['./reflow.ts', './host-notify.ts', './host-message-router.ts'].map(
+  (relative) => fileURLToPath(new URL(relative, import.meta.url))
+)
+
+describe('the host-message-router slice', () => {
+  it('emits the script the document carries, modulo the six normalisations', async () => {
+    const emitted = (await Promise.all(modulePaths.map(emitTerminalDocumentModule))).join('\n')
+    expect(
+      compareTerminalDocumentScripts(TERMINAL_HTML_HOST_MESSAGE_ROUTER, emitted, 'scope')
+    ).toEqual({
+      equivalent: true,
+      normalisations: {
+        qualifiedReferences: 48,
+        scopeFieldDeclarations: 0,
+        rebindings: 20,
+        bracedBodies: 12,
+        unboundCatches: 2,
+        numberProperties: 0,
+        shorthandProperties: 0
+      }
+    })
+  })
+})
diff --git a/mobile/src/terminal/document/host-message-router.ts b/mobile/src/terminal/document/host-message-router.ts
new file mode 100644
index 00000000000..f9caa4bae8e
--- /dev/null
+++ b/mobile/src/terminal/document/host-message-router.ts
@@ -0,0 +1,195 @@
+import { scope } from './document-scope'
+import { applyFitScale } from './fit-scale'
+import { notify } from './host-notify'
+import { emitKeyboardAvoidanceMetrics } from './keyboard-avoidance-metrics'
+import { emitModesIfChanged } from './mode-mirroring'
+import { reflow } from './reflow'
+import { resumeTerminalDataReplyAuthority } from './query-reply'
+import { cancelSelect, repositionOverlay } from './document-externals'
+import { resetEvictionCounter } from './selection-state-and-eviction'
+import { applyTerminalTheme } from './terminal-theme'
+import { init, resize, write } from './terminal-init'
+import { applyTextScale } from './text-scaling'
+import { flog } from './viewport-transform'
+import { resetWriteQueue } from './write-queue'
+
+/** One message from the host. Every field is optional because the router reads them by type. */
+export type TerminalHostMessage = {
+  id?: number
+  type?: string
+  cols?: number
+  rows?: number
+  initialData?: unknown
+  terminalTheme?: Parameters<typeof applyTerminalTheme>[0]
+  fontScale?: number
+  preserveScroll?: boolean
+  oscLinks?: unknown
+  data?: string
+  containerHeight?: number
+}
+
+export function measureFitDimensions(containerHeightPx: unknown, retriesLeft?: number) {
+  if (typeof retriesLeft !== 'number') {
+    retriesLeft = 30
+  }
+  // Why: init and measure are posted back-to-back from React, but
+  // init has an async rAF chain. A measure that runs synchronously
+  // after init can find term null, disposed, lacking element, or
+  // with cells size 0. Retry the whole gate for ~500ms.
+  const notReady = !scope.term || !scope.term.element
+  let cellWidth = 0
+  let cellHeight = 0
+  if (!notReady) {
+    // oxlint-disable-next-line typescript/no-non-null-assertion -- SAFETY: notReady is exactly the absence check above.
+    const core = scope.term!._core
+    if (core && core._renderService && core._renderService.dimensions) {
+      cellWidth = core._renderService.dimensions.css.cell.width
+      cellHeight = core._renderService.dimensions.css.cell.height
+    }
+  }
+  if (notReady || cellWidth <= 0 || cellHeight <= 0) {
+    if (retriesLeft > 0) {
+      requestAnimationFrame(function () {
+        measureFitDimensions(containerHeightPx, retriesLeft - 1)
+      })
+      return
+    }
+    flog('measure-fail', {
+      notReady: notReady,
+      cellWidth: cellWidth,
+      cellHeight: cellHeight,
+      retriesLeft: retriesLeft
+    })
+    notify({ type: 'measure-result', cols: null, rows: null })
+    return
+  }
+  const vpWidth = window.innerWidth
+  // Why: prefer the container height passed from React Native over
+  // window.innerHeight. The RN layout system knows the exact pixel
+  // height of the terminal frame after the accessory/input bars are
+  // subtracted, whereas innerHeight can overstate the visible area
+  // due to layout timing or safe-area insets.
+  const vpHeight =
+    typeof containerHeightPx === 'number' && containerHeightPx > 0
+      ? containerHeightPx
+      : window.innerHeight
+  const cols = Math.floor(vpWidth / cellWidth)
+  if (cols < scope.MIN_FIT_COLS) {
+    flog('measure-skip-small-width', {
+      vpWidth: vpWidth,
+      cellWidth: cellWidth,
+      cols: cols
+    })
+    notify({ type: 'measure-result', cols: null, rows: null })
+    return
+  }
+  // Why: the rows we report become the PTY's actual row count after the
+  // server fits to viewport, and xterm renders exactly that many lines
+  // anchored top-left of the WebView. Subtracting rows here would leave
+  // dead xterm-background space at the bottom of the container and make
+  // the last PTY rows visually appear above an "invisible line." Any
+  // safety margin between the prompt and the accessory bar must come
+  // from RN layout (terminalFrame's flex bounds), not from undersizing
+  // the PTY.
+  const rows = Math.max(8, Math.floor(vpHeight / cellHeight))
+  notify({ type: 'measure-result', cols: cols, rows: rows })
+}
+
+export function handleMsg(msg: TerminalHostMessage) {
+  if (typeof msg.id === 'number') {
+    // oxlint-disable-next-line unicorn/prefer-includes -- the document's text is pinned token for token; rewriting this changes the native program
+    if (scope.handledMessageIds.indexOf(msg.id) !== -1) {
+      return
+    }
+    scope.handledMessageIds.push(msg.id)
+    if (scope.handledMessageIds.length > 256) {
+      scope.handledMessageIds.shift()
+    }
+  }
+  if (msg.type === 'ping') {
+    notify({ type: 'pong', pingId: msg.id })
+  } else if (msg.type === 'init') {
+    init(
+      msg.cols!,
+      msg.rows!,
+      msg.initialData,
+      msg.terminalTheme,
+      msg.fontScale,
+      msg.preserveScroll!,
+      msg.oscLinks
+    )
+  } else if (msg.type === 'set-font-scale') {
+    // Why: ignore RN echoing back the value a pinch just set (msg.fontScale ===
+    // currentTextScale) so the post-pinch state isn't reset; only apply changes.
+    if (
+      typeof msg.fontScale === 'number' &&
+      msg.fontScale > 0 &&
+      msg.fontScale !== scope.currentTextScale
+    ) {
+      scope.userScale = 1
+      scope.panX = 0
+      scope.panY = 0
+      applyTextScale(msg.fontScale)
+    }
+  } else if (msg.type === 'resize') {
+    resize(msg.cols!, msg.rows!)
+  } else if (msg.type === 'reflow') {
+    reflow(msg.cols!, msg.rows!)
+  } else if (msg.type === 'write') {
+    write(msg.data!)
+  } else if (msg.type === 'clear') {
+    scope.terminalGeneration++
+    resetWriteQueue()
+    resumeTerminalDataReplyAuthority() // Why: clear drops the replay boundary.
+    scope.statusDotPendingSelector = false
+    scope.afterDrainCallbacks = []
+    scope.writesDraining = false
+    scope.mouseModeScanTail = ''
+    scope.trackedMouseTrackingMode = 'none'
+    scope.sgrMouseMode = false
+    scope.sgrMousePixelsMode = false
+    scope.initialOscLinks = []
+    scope.initialOscLinkRowOffset = 0
+    scope.initialOscLinkEvictionReady = false
+    if (scope.term) {
+      scope.term.clear()
+      scope.term.reset()
+    }
+    emitModesIfChanged()
+    emitKeyboardAvoidanceMetrics()
+    resetEvictionCounter()
+    if (scope.selMode === 'select') {
+      notify({ type: 'selection-evicted' })
+      cancelSelect()
+    }
+  } else if (msg.type === 'measure') {
+    measureFitDimensions(msg.containerHeight)
+  } else if (msg.type === 'reset-zoom') {
+    applyFitScale('reset-zoom-msg')
+  } else if (msg.type === 'set-theme') {
+    applyTerminalTheme(msg.terminalTheme)
+  } else if (msg.type === 'cancel-select') {
+    if (scope.selMode === 'select') {
+      cancelSelect()
+    }
+  } else if (msg.type === 'do-select-all') {
+    if (scope.term) {
+      try {
+        scope.term.selectAll()
+        const b = scope.term.buffer.active
+        if (scope.selMode !== 'select') {
+          scope.selMode = 'select'
+          // oxlint-disable-next-line typescript/no-non-null-assertion -- SAFETY: the document shell ships the overlay element.
+          scope.selectionOverlay!.classList.add('active')
+          notify({ type: 'set-select-mode', enabled: true })
+        }
+        scope.sel = {
+          anchor: { col: 0, row: 0 },
+          focus: { col: scope.term.cols - 1, row: b.length - 1 },
+          activeHandle: null
+        }
+        repositionOverlay()
+      } catch {}
+    }
+  }
+}
diff --git a/mobile/src/terminal/document/host-notify.ts b/mobile/src/terminal/document/host-notify.ts
new file mode 100644
index 00000000000..310a80e2b39
--- /dev/null
+++ b/mobile/src/terminal/document/host-notify.ts
@@ -0,0 +1,86 @@
+import { scope } from './document-scope'
+
+/**
+ * The postMessage bridge to the host, and the engine error reporting that rides on it.
+ *
+ * They are one module because the document declares them together, ahead of the message router
+ * that both serve.
+ */
+
+declare global {
+  interface Window {
+    __engineErrors: string[]
+  }
+}
+
+export function notify(msg: Record<string, unknown>) {
+  if (window.ReactNativeWebView) {
+    window.ReactNativeWebView.postMessage(JSON.stringify(msg))
+  }
+}
+
+/** What a thrown value can be here: an Error-shaped object, a string, or nothing. */
+export type TerminalEngineError = string | null | undefined | { message?: unknown }
+
+export function engineErrorText(err: TerminalEngineError) {
+  if (!err) {
+    return ''
+  }
+  if (typeof err === 'string') {
+    return err
+  }
+  if (err && typeof err.message === 'string') {
+    return err.message
+  }
+  try {
+    return String(err)
+  } catch {
+    return ''
+  }
+}
+
+export function chromeVersionText() {
+  const match = String(navigator.userAgent || '').match(/(?:Chrome|Chromium)\/([0-9.]+)/)
+  return match ? 'Chrome ' + match[1] : 'Chrome version unknown'
+}
+
+let nonFatalErrorNotifies = 0
+
+export function reportEngineError(context: string, err: TerminalEngineError, fatal?: unknown) {
+  const isFatal = fatal === undefined ? !scope.everReady : !!fatal
+  if (!isFatal) {
+    // Why: a constructed-but-degraded engine can throw per frame; cap
+    // non-fatal notifies so RN isn't flooded. Fatal reports always emit.
+    nonFatalErrorNotifies++
+    if (nonFatalErrorNotifies > 5) {
+      return
+    }
+  }
+  const parts = [context]
+  const errText = engineErrorText(err)
+  if (errText) {
+    parts.push(errText)
+  }
+  if (window.__engineErrors && window.__engineErrors.length) {
+    parts.push('captured: ' + window.__engineErrors.join(' | '))
+  }
+  parts.push(chromeVersionText())
+  notify({
+    type: 'error',
+    fatal: isFatal,
+    message: parts.join(' - ')
+  })
+}
+
+window.onerror = function (
+  msg: string | (Event & { message?: unknown }),
+  source,
+  line,
+  column,
+  err?: TerminalEngineError
+) {
+  if (window.__engineErrors.length < 20) {
+    window.__engineErrors.push(String(msg))
+  }
+  reportEngineError('terminal runtime error', err || msg)
+}
diff --git a/mobile/src/terminal/document/keyboard-avoidance-metrics.ts b/mobile/src/terminal/document/keyboard-avoidance-metrics.ts
index 6e28992fcca..93cb9866d59 100644
--- a/mobile/src/terminal/document/keyboard-avoidance-metrics.ts
+++ b/mobile/src/terminal/document/keyboard-avoidance-metrics.ts
@@ -1,4 +1,4 @@
-import { notify } from './document-externals'
+import { notify } from './host-notify'
 import { scope, type TerminalDocumentCell, type TerminalDocumentLine } from './document-scope'
 
 export function lineHasVisibleContent(
diff --git a/mobile/src/terminal/document/message-bridge.ts b/mobile/src/terminal/document/message-bridge.ts
index 34997a0bc62..7f64fe0fb76 100644
--- a/mobile/src/terminal/document/message-bridge.ts
+++ b/mobile/src/terminal/document/message-bridge.ts
@@ -1,5 +1,7 @@
 import { adjustRowsForViewport, applyFitScale, clampPan } from './fit-scale'
-import { handleMsg, notify, reportEngineError, repositionOverlay } from './document-externals'
+import { repositionOverlay } from './document-externals'
+import { handleMsg, type TerminalHostMessage } from './host-message-router'
+import { notify, reportEngineError, type TerminalEngineError } from './host-notify'
 import { updateTransform } from './viewport-transform'
 import { scope } from './document-scope'
 
@@ -9,9 +11,6 @@ declare global {
   }
 }
 
-/** The decoded host message; only its type is read here, by the error reporter. */
-type TerminalHostMessage = { type?: unknown } | undefined
-
 export function handleIncomingMessage(e: Event & { data?: TerminalHostMessage | string }) {
   let msg: TerminalHostMessage
   try {
@@ -20,11 +19,12 @@ export function handleIncomingMessage(e: Event & { data?: TerminalHostMessage |
     return
   }
   try {
-    handleMsg(msg)
+    handleMsg(msg!)
   } catch (ex) {
     reportEngineError(
       msg && msg.type === 'init' ? 'terminal init failed' : 'terminal message failed',
-      ex,
+      // oxlint-disable-next-line typescript/consistent-type-assertions -- SAFETY: a catch binding is `unknown`; the reporter reads only `message` and falls back to String().
+      ex as TerminalEngineError,
       msg && msg.type === 'init' && !scope.everReady
     )
   }
diff --git a/mobile/src/terminal/document/mode-mirroring.ts b/mobile/src/terminal/document/mode-mirroring.ts
index a9f6f740808..c5471bb8db4 100644
--- a/mobile/src/terminal/document/mode-mirroring.ts
+++ b/mobile/src/terminal/document/mode-mirroring.ts
@@ -1,4 +1,4 @@
-import { notify } from './document-externals'
+import { notify } from './host-notify'
 import { getMouseTrackingMode } from './mouse-input-encoding'
 import { scope } from './document-scope'
 
diff --git a/mobile/src/terminal/document/mouse-click-drag.ts b/mobile/src/terminal/document/mouse-click-drag.ts
index 6b0a3d5016c..f9a4e3c80d7 100644
--- a/mobile/src/terminal/document/mouse-click-drag.ts
+++ b/mobile/src/terminal/document/mouse-click-drag.ts
@@ -2,10 +2,10 @@ import {
   applyXtermSelection,
   cancelSelect,
   handleDragMove,
-  notify,
   repositionOverlay,
   stopEdgeScroll
 } from './document-externals'
+import { notify } from './host-notify'
 import { getMouseTrackingMode, isSafeSgrMouseCoordinate } from './mouse-input-encoding'
 import { viewportToCell } from './viewport-cell'
 import { scope } from './document-scope'
diff --git a/mobile/src/terminal/document/mouse-input-encoding.ts b/mobile/src/terminal/document/mouse-input-encoding.ts
index 9c314c6eaf5..1ea8360cca3 100644
--- a/mobile/src/terminal/document/mouse-input-encoding.ts
+++ b/mobile/src/terminal/document/mouse-input-encoding.ts
@@ -1,4 +1,4 @@
-import { notify } from './document-externals'
+import { notify } from './host-notify'
 import { scope } from './document-scope'
 import { viewportToMouseReportCell } from './mouse-report-cell'
 
diff --git a/mobile/src/terminal/document/query-reply.ts b/mobile/src/terminal/document/query-reply.ts
index 824604430ca..70509735b2c 100644
--- a/mobile/src/terminal/document/query-reply.ts
+++ b/mobile/src/terminal/document/query-reply.ts
@@ -1,5 +1,5 @@
 import { enqueueWriteBoundary } from './write-queue'
-import { notify } from './document-externals'
+import { notify } from './host-notify'
 import { scope, type TerminalDocumentDisposable } from './document-scope'
 
 /**
diff --git a/mobile/src/terminal/document/selection-state-and-eviction.ts b/mobile/src/terminal/document/selection-state-and-eviction.ts
index 426a2c9ef6b..d2ac2f63580 100644
--- a/mobile/src/terminal/document/selection-state-and-eviction.ts
+++ b/mobile/src/terminal/document/selection-state-and-eviction.ts
@@ -1,4 +1,5 @@
-import { cancelSelect, notify, repositionOverlay } from './document-externals'
+import { cancelSelect, repositionOverlay } from './document-externals'
+import { notify } from './host-notify'
 import { scope } from './document-scope'
 
 // ============================================================
diff --git a/mobile/src/terminal/document/surface-tap.ts b/mobile/src/terminal/document/surface-tap.ts
index efb7bbaa9b3..475e3992648 100644
--- a/mobile/src/terminal/document/surface-tap.ts
+++ b/mobile/src/terminal/document/surface-tap.ts
@@ -1,4 +1,4 @@
-import { notify } from './document-externals'
+import { notify } from './host-notify'
 import {
   buildMouseClickInput,
   getMouseTrackingMode,
diff --git a/mobile/src/terminal/document/tap-dispatch.ts b/mobile/src/terminal/document/tap-dispatch.ts
index f1aec1efe1d..3c8d1e5819d 100644
--- a/mobile/src/terminal/document/tap-dispatch.ts
+++ b/mobile/src/terminal/document/tap-dispatch.ts
@@ -1,10 +1,5 @@
-import {
-  cancelSelect,
-  enterSelect,
-  handleDragMove,
-  notify,
-  stopEdgeScroll
-} from './document-externals'
+import { cancelSelect, enterSelect, handleDragMove, stopEdgeScroll } from './document-externals'
+import { notify } from './host-notify'
 import { viewportToCell } from './viewport-cell'
 import { scope } from './document-scope'
 import { notifyTerminalSurfaceTap } from './surface-tap'
diff --git a/mobile/src/terminal/document/terminal-init.ts b/mobile/src/terminal/document/terminal-init.ts
index 8afc1227c25..c744918bcdf 100644
--- a/mobile/src/terminal/document/terminal-init.ts
+++ b/mobile/src/terminal/document/terminal-init.ts
@@ -5,7 +5,7 @@ import {
   terminalCursorStyle,
   terminalShowCursorImmediately
 } from './document-constants'
-import { notify } from './document-externals'
+import { notify } from './host-notify'
 import { fontPxForScale } from './text-scaling'
 import {
   scope,
diff --git a/mobile/src/terminal/document/terminal-theme.ts b/mobile/src/terminal/document/terminal-theme.ts
index 7f7f2666690..cd4eb5743a1 100644
--- a/mobile/src/terminal/document/terminal-theme.ts
+++ b/mobile/src/terminal/document/terminal-theme.ts
@@ -8,10 +8,10 @@ export type TerminalDocumentRgb = { r: number; g: number; b: number }
 export type TerminalDocumentRgba = TerminalDocumentRgb & { a: number }
 
 /** The theme payload the host publishes; an older host omits the contrast floor. */
-export type TerminalDocumentThemeMessage = {
-  theme?: Record<string, string>
-  minimumContrastRatio?: number
-} | null
+export type TerminalDocumentThemeMessage =
+  | { theme?: Record<string, string>; minimumContrastRatio?: number }
+  | null
+  | undefined
 
 const DARK_BG_MIN_CONTRAST = 3
 const LIGHT_BG_MIN_CONTRAST = 4.5