diff --git a/frontend-modern/browser-tests/README.md b/frontend-modern/browser-tests/README.md
index 31fb421a88..6474961e41 100644
--- a/frontend-modern/browser-tests/README.md
+++ b/frontend-modern/browser-tests/README.md
@@ -81,6 +81,13 @@ own-parent and final-content receipts still require a real browser pass. No
 native guest-agent, thaw, filesystem-write, installed or publication acceptance
 is established by compiling these synthetic fixtures.
 
+The isolated browser copy has no Git store. When preparing its preview there,
+pass `--source-sha <full SHA>` with the source owner's externally checked HEAD.
+The binding marks this as `supplied-for-isolated-copy`, not a Git observation.
+The default remains an actual Git HEAD read, with no silent fallback. Verify
+the binding's source and artifact hashes against the assigned tree before using
+the preview as evidence; a supplied SHA alone establishes no source identity.
+
 ## CI and release evidence
 
 These standalone runners are manual qualification commands: the current GitHub
diff --git a/frontend-modern/browser-tests/build-publication-fixtures.mjs b/frontend-modern/browser-tests/build-publication-fixtures.mjs
index 1da4c13c85..550461acc9 100644
--- a/frontend-modern/browser-tests/build-publication-fixtures.mjs
+++ b/frontend-modern/browser-tests/build-publication-fixtures.mjs
@@ -8,6 +8,7 @@ import fs from 'node:fs';
 import path from 'node:path';
 import { execFileSync } from 'node:child_process';
 import { fileURLToPath } from 'node:url';
+import sourceIdentity from './publication-fixture-source.cjs';
 import {
   entryPaths,
   harnessPaths,
@@ -21,7 +22,9 @@ const outDir = path.resolve(root, '../tmp/publication-final-static');
 const started = Date.now();
 const binding = {
   version: 1,
-  source_sha: execFileSync('git', ['rev-parse', 'HEAD'], { cwd: root, encoding: 'utf8' }).trim(),
+  ...sourceIdentity.publicationSourceIdentity(process.argv.slice(2), () =>
+    execFileSync('git', ['rev-parse', 'HEAD'], { cwd: root, encoding: 'utf8' }),
+  ),
   graph: hash(fs.readFileSync(path.join(root, 'package-lock.json'))),
   entries: entryPaths,
   sources: {},
diff --git a/frontend-modern/browser-tests/publication-fixture-server.cjs b/frontend-modern/browser-tests/publication-fixture-server.cjs
index e43d60598a..748180cffa 100644
--- a/frontend-modern/browser-tests/publication-fixture-server.cjs
+++ b/frontend-modern/browser-tests/publication-fixture-server.cjs
@@ -20,6 +20,7 @@ const entryPaths = [
   'browser-tests/patrol-rule-removal.html',
 ];
 const harnessPaths = [
+  'browser-tests/publication-fixture-source.cjs',
   'browser-tests/build-publication-fixtures.mjs',
   'browser-tests/publication-fixture-server.cjs',
   'browser-tests/guest-disk-provenance.cjs',
diff --git a/frontend-modern/browser-tests/publication-fixture-source.cjs b/frontend-modern/browser-tests/publication-fixture-source.cjs
new file mode 100644
index 0000000000..7ac71f0b05
--- /dev/null
+++ b/frontend-modern/browser-tests/publication-fixture-source.cjs
@@ -0,0 +1,18 @@
+// The browser's isolated source copy does not mount the worktree's Git store.
+// A supplied SHA is an attribution supplied by the source owner, not a Git read
+// or execution authority. Imported source and served bytes remain hash-bound.
+const assert = require('node:assert/strict');
+
+function publicationSourceIdentity(args, readHead) {
+  if (args.length === 0) {
+    const source_sha = readHead().trim();
+    assert.match(source_sha, /^[a-f0-9]{40}$/, 'invalid Git source identity');
+    return { source_sha, source_identity: 'git-head' };
+  }
+  assert.equal(args.length, 2, 'expected only --source-sha <full SHA>');
+  assert.equal(args[0], '--source-sha', 'unknown fixture preparation argument');
+  assert.match(args[1], /^[a-f0-9]{40}$/, 'expected a full lowercase source SHA');
+  return { source_sha: args[1], source_identity: 'supplied-for-isolated-copy' };
+}
+
+module.exports = { publicationSourceIdentity };
diff --git a/frontend-modern/browser-tests/publication-fixture-source.test.mjs b/frontend-modern/browser-tests/publication-fixture-source.test.mjs
new file mode 100644
index 0000000000..255e70de39
--- /dev/null
+++ b/frontend-modern/browser-tests/publication-fixture-source.test.mjs
@@ -0,0 +1,37 @@
+import { describe, expect, it } from 'vitest';
+import source from './publication-fixture-source.cjs';
+
+const sha = 'a'.repeat(40);
+describe('publication fixture source attribution', () => {
+  it('reads the actual Git head by default', () => {
+    expect(source.publicationSourceIdentity([], () => sha + '\n')).toEqual({
+      source_sha: sha,
+      source_identity: 'git-head',
+    });
+  });
+  it('never silently substitutes a supplied identity after a failed Git read', () => {
+    expect(() =>
+      source.publicationSourceIdentity([], () => {
+        throw new Error('Git unavailable');
+      }),
+    ).toThrow('Git unavailable');
+  });
+  it('explicitly attributes an isolated copy without trying to access an unmounted Git store', () => {
+    expect(
+      source.publicationSourceIdentity(['--source-sha', sha], () => {
+        throw new Error('must not read outside the isolated copy');
+      }),
+    ).toEqual({ source_sha: sha, source_identity: 'supplied-for-isolated-copy' });
+  });
+  it.each([
+    [],
+    ['--source-sha'],
+    ['--source-sha', 'a'.repeat(39)],
+    ['--source-sha', 'A'.repeat(40)],
+    ['--source-sha', 'main'],
+    ['--source-sha', sha, '--extra'],
+    ['--unknown', sha],
+  ])('rejects malformed identity or arguments %j', (...args) => {
+    expect(() => source.publicationSourceIdentity(args, () => 'main')).toThrow();
+  });
+});