diff --git a/src/web/public/constants.js b/src/web/public/constants.js
index 8a1d500f..bdf6ca0e 100644
--- a/src/web/public/constants.js
+++ b/src/web/public/constants.js
@@ -1976,6 +1976,21 @@ function previewsInFileViewer(filePath) {
   return FILE_PREVIEW_EXTENSIONS.has(ext);
 }
 
+/**
+ * Extensions the file preview renders as a PAGE, from the sandboxed
+ * `/html-view/<cap>/` route (src/web/routes/html-view-routes.ts, whose
+ * HTML_VIEW_EXTENSIONS this mirrors; test/file-preview-html-page.test.ts pins
+ * the two equal). The terminal sends a click on one to the preview, never to
+ * the log viewer, and the preview renders it with a Page pill back to source.
+ */
+const HTML_PAGE_EXTENSIONS = new Set(['html', 'htm']);
+
+/** Whether a path names a file the preview renders as a page ({@link HTML_PAGE_EXTENSIONS}). */
+function rendersAsPage(filePath) {
+  const ext = String(filePath || '').split('.').pop().toLowerCase();
+  return HTML_PAGE_EXTENSIONS.has(ext);
+}
+
 
 /**
  * The LOGICAL line a terminal row belongs to — the rows it spans, its text as one
@@ -2992,7 +3007,13 @@ function describeSessionHarness(session, catalog) {
 if (typeof window !== 'undefined') {
   window.CodemanSessionHarness = { describeSessionHarness, SESSION_MODEL_MAX_CHARS };
   window.CodemanHistoryFormat = { formatHistoryBytes, computeHistoryTruncationNotice, computeRewriteScrollLine };
-  window.CodemanFilePaths = { absoluteFilePathPattern, previewsInFileViewer, FILE_PREVIEW_EXTENSIONS };
+  window.CodemanFilePaths = {
+    absoluteFilePathPattern,
+    previewsInFileViewer,
+    FILE_PREVIEW_EXTENSIONS,
+    rendersAsPage,
+    HTML_PAGE_EXTENSIONS,
+  };
   window.CodemanTerminalLines = { terminalLogicalLine };
   window.CodemanUrlSession = { sessionIdFromFragment };
   window.CodemanSplitPane = {
diff --git a/src/web/public/panels-ui.js b/src/web/public/panels-ui.js
index 93b6cfc7..dd77afbe 100644
--- a/src/web/public/panels-ui.js
+++ b/src/web/public/panels-ui.js
@@ -24,7 +24,6 @@ const TEXT_PREVIEW_MAX_LINES = 500;
 // cap: a rendered README cut mid-way reads as the whole document.
 const MARKDOWN_PREVIEW_MAX_LINES = 10000;
 const MARKDOWN_EXTS = new Set(['md', 'markdown']);
-const HTML_PAGE_EXTS = new Set(['html', 'htm']);
 // File Viewer text-view prefs: per-device, in their own localStorage keys for
 // the same reason as FILE_BROWSER_SHOW_HIDDEN_KEY (the app-settings object is
 // rebuilt from the settings modal on save, so a key toggled from the viewer
@@ -4126,7 +4125,7 @@ Object.assign(CodemanApp.prototype, {
     // The Page pill switches to the source view below (with its Edit button), and
     // a refused mint (remote case, broad folder, hidden directory) falls through
     // to that source view on its own.
-    if (!attachmentId && HTML_PAGE_EXTS.has(ext) && this._filePreviewPref('htmlRendered')) {
+    if (!attachmentId && rendersAsPage(filePath) && this._filePreviewPref('htmlRendered')) {
       const pageUrl = await this._mintHtmlViewUrl(filePath, sessionId);
       if (pageUrl) {
         footerEl.textContent = 'HTML';
@@ -4615,7 +4614,7 @@ Object.assign(CodemanApp.prototype, {
     const htmlSource =
       view === 'text' &&
       !this.filePreviewPageRefused &&
-      HTML_PAGE_EXTS.has(text?.ext || '') &&
+      rendersAsPage(text?.filePath) &&
       (!text.attachmentId || text.filePath.startsWith('/'));
     set('filePreviewPageBtn', view === 'page' || htmlSource, view === 'page');
     set('filePreviewLinesBtn', view === 'text', this._filePreviewPref('lineNumbers'));
@@ -4642,7 +4641,7 @@ Object.assign(CodemanApp.prototype, {
     const target = this.filePreviewPage || this.filePreviewText;
     if (!target) return;
     this._setFilePreviewPref('htmlRendered', !this._filePreviewPref('htmlRendered'));
-    this.openFilePreview(target.filePath, target.sessionId);
+    return this.openFilePreview(target.filePath, target.sessionId);
   },
 
   toggleFilePreviewMd() {
diff --git a/src/web/public/terminal-ui.js b/src/web/public/terminal-ui.js
index fc3bc94f..b83d6f0f 100644
--- a/src/web/public/terminal-ui.js
+++ b/src/web/public/terminal-ui.js
@@ -2091,7 +2091,7 @@ Object.assign(CodemanApp.prototype, {
               const sessionId = getSessionId();
               // An HTML file renders as a page in the preview, never as source
               // in the log viewer.
-              if (previewsInFileViewer(text) || /\.html?$/i.test(text) || self._isExternalPreviewPath(text, sessionId)) {
+              if (previewsInFileViewer(text) || rendersAsPage(text) || self._isExternalPreviewPath(text, sessionId)) {
                 self.openFilePreview(text, sessionId);
                 return;
               }
diff --git a/test/file-preview-html-page.test.ts b/test/file-preview-html-page.test.ts
new file mode 100644
index 00000000..c50c7574
--- /dev/null
+++ b/test/file-preview-html-page.test.ts
@@ -0,0 +1,41 @@
+/**
+ * @fileoverview File Viewer: an `.html` file renders as a sandboxed page.
+ *
+ * The server half (`/html-view/<cap>/...`, html-view-routes.ts) is pinned by
+ * test/routes/html-view-routes.test.ts. This file pins the frontend half:
+ *
+ *  1. `rendersAsPage()` (constants.js) and the server's HTML_VIEW_EXTENSIONS
+ *     name the same extensions, so the terminal, the preview and the route
+ *     agree on what a page is.
+ *
+ * Loaded via `vm` with a jsdom document injected, like
+ * file-preview-markdown.test.ts: constants.js + panels-ui.js only.
+ */
+
+import { readFileSync } from 'node:fs';
+import { resolve } from 'node:path';
+import vm from 'node:vm';
+import { describe, expect, it } from 'vitest';
+import { HTML_VIEW_EXTENSIONS } from '../src/web/routes/html-view-routes.js';
+
+const PUBLIC = resolve(import.meta.dirname, '../src/web/public');
+const constantsJs = readFileSync(resolve(PUBLIC, 'constants.js'), 'utf8');
+
+describe('rendersAsPage', () => {
+  it('names exactly the extensions the html-view route renders', () => {
+    const context = vm.createContext({});
+    vm.runInContext(`${constantsJs}\nglobalThis.__page = { rendersAsPage, HTML_PAGE_EXTENSIONS };`, context, {
+      filename: 'constants.js',
+    });
+    const { rendersAsPage, HTML_PAGE_EXTENSIONS } = (
+      context as { __page: { rendersAsPage: (p: string) => boolean; HTML_PAGE_EXTENSIONS: Set<string> } }
+    ).__page;
+
+    expect([...HTML_PAGE_EXTENSIONS].sort()).toEqual([...HTML_VIEW_EXTENSIONS].sort());
+    expect(rendersAsPage('/tmp/out/report.html')).toBe(true);
+    expect(rendersAsPage('docs/INDEX.HTM')).toBe(true);
+    expect(rendersAsPage('notes.md')).toBe(false);
+    expect(rendersAsPage('page.html.txt')).toBe(false);
+    expect(rendersAsPage(undefined as unknown as string)).toBe(false);
+  });
+});