diff --git a/config/scripts/mobile-web-app-session-terminal-closure.test.mjs b/config/scripts/mobile-web-app-session-terminal-closure.test.mjs
new file mode 100644
index 00000000000..e28a0c94d89
--- /dev/null
+++ b/config/scripts/mobile-web-app-session-terminal-closure.test.mjs
@@ -0,0 +1,104 @@
+import { join } from 'node:path'
+import { fileURLToPath } from 'node:url'
+import { describe, expect, it } from 'vitest'
+import { mobileWebAppRouteClosure } from './build-mobile-web-app-bundle.mjs'
+import { mobileWebAppDependenciesPresent } from './mobile-web-app-bundle-dependencies.mjs'
+import {
+  textInputFontSizeOffenders,
+  unresolvedTextInputStyles
+} from './mobile-web-app-text-input-font-size-seam.mjs'
+
+/**
+ * What putting the terminal on the page costs the session route's closure.
+ *
+ * The route is not served on the page until C7.7 — its module is still the native switch and
+ * there is no `.web.tsx` beside it — but the closure the bundler would walk is the same one, and
+ * the terminal is by far the largest thing in it. Measured here so the trade is a number rather
+ * than a claim, and so that a later change cannot quietly put the engine string back.
+ *
+ * Measured against `ota-c7-1-terminal-document` at 51ae7b1b03, which is this branch's base:
+ *
+ *   modules        4316 -> 4363   (+47)
+ *   local modules   927 ->  971   (+44)
+ *   minified bytes  3,930,787 -> 3,883,532   (-47,255)
+ *
+ * The route gets smaller. It sheds the 612 KiB engine string and the 105 KiB generated document
+ * script, both string literals it could not run, and gains xterm and the two addons as real code
+ * — 607,945 bytes minified ESM on their own — plus the document's own modules.
+ */
+
+const projectDir = fileURLToPath(new URL('../..', import.meta.url))
+const mobileDir = join(projectDir, 'mobile')
+
+const SESSION_ROUTE = 'app/h/[hostId]/session/[worktreeId].tsx'
+
+/** Gone with the WebView: string literals of a program the page has no way to run. */
+const SHED = [
+  'src/terminal/TerminalWebView.tsx',
+  'src/terminal/terminal-webview-engine.generated.ts',
+  'src/terminal/terminal-webview-document-script.generated.ts',
+  'src/terminal/terminal-webview-html.ts',
+  'src/terminal/terminal-webview-html/document-shell.ts',
+  'src/terminal/terminal-webview-html/document-close.ts'
+]
+
+/** The component, its mount, the stylesheet and the markup, and the two modules the split made. */
+const GAINED_OUTSIDE_THE_DOCUMENT = [
+  'src/terminal/TerminalWebView.web.tsx',
+  'src/terminal/terminal-web-document-mount.ts',
+  'src/terminal/terminal-webview-engine-css.generated.ts',
+  'src/terminal/terminal-webview-html.web.ts',
+  'src/terminal/terminal-webview-html/document-markup.ts',
+  'src/terminal/terminal-webview-html/document-style.ts',
+  'src/terminal/terminal-webview-ready-promises.ts',
+  'src/terminal/use-terminal-webview-controller.ts'
+]
+
+const XTERM_PACKAGES = ['@xterm/xterm', '@xterm/addon-unicode11', '@xterm/addon-webgl']
+
+/**
+ * The 16 px seam's verdict for this route, which C7.5 must leave exactly where C7.2 found it.
+ *
+ * Nine inputs under the floor is design §3's own count, and moving them is C7.2's work, not this
+ * lane's. It is asserted rather than left unmeasured because the terminal's own modules joining
+ * the closure is precisely the kind of change that could add a tenth without anyone looking.
+ */
+const EXPECTED_OFFENDERS = 9
+
+const bundles = mobileWebAppDependenciesPresent()
+const describeClosure = bundles ? describe : describe.skip
+
+describeClosure(
+  "the session route's page closure with the terminal on it",
+  () => {
+    it('gains the document, xterm and the addons, and sheds the engine string', async () => {
+      const { local, modules } = await mobileWebAppRouteClosure(SESSION_ROUTE)
+      for (const gone of SHED) {
+        expect(local, `${gone} is still in the closure`).not.toContain(gone)
+      }
+      for (const gained of GAINED_OUTSIDE_THE_DOCUMENT) {
+        expect(local, `${gained} is not in the closure`).toContain(gained)
+      }
+      for (const name of XTERM_PACKAGES) {
+        expect(
+          modules.some((module) => module.includes(`node_modules/${name}/`)),
+          `${name} is not in the closure`
+        ).toBe(true)
+      }
+      // The document, whole: every module the generator emits except the bridge, which ruling 19
+      // keeps off the page because those `message` frames belong to the shell.
+      const documentModules = local.filter((module) => module.startsWith('src/terminal/document/'))
+      expect(documentModules.length).toBeGreaterThanOrEqual(36)
+      expect(documentModules).not.toContain('src/terminal/document/message-bridge.ts')
+      expect(documentModules).toContain('src/terminal/document/page-document-modules.ts')
+    }, 300_000)
+
+    it('leaves the 16px seam census exactly where C7.2 found it', async () => {
+      const closure = await mobileWebAppRouteClosure(SESSION_ROUTE)
+      // Unresolved first: an offender count means nothing if the walk read no styles.
+      expect(unresolvedTextInputStyles(mobileDir, closure)).toEqual([])
+      expect(textInputFontSizeOffenders(mobileDir, closure)).toHaveLength(EXPECTED_OFFENDERS)
+    }, 300_000)
+  },
+  900_000
+)