diff --git a/.github/bench/history.test.cjs b/.github/bench/history.test.cjs
new file mode 100644
index 000000000..a662511ff
--- /dev/null
+++ b/.github/bench/history.test.cjs
@@ -0,0 +1,49 @@
+// Execute the workflow's upload shell with a capture-only Bencher function.
+// This checks the actual arguments without uploading or accessing credentials.
+const assert = require('node:assert/strict');
+const { test } = require('node:test');
+const fs = require('node:fs');
+const os = require('node:os');
+const path = require('node:path');
+const { spawnSync } = require('node:child_process');
+
+const workflow = fs.readFileSync(path.join(__dirname, '../workflows/bench.yml'), 'utf8');
+const step = workflow.split('      - name: Upload to Bencher\n')[1];
+assert.ok(step, 'native upload step must exist');
+const shell = step.split('        run: |\n')[1].split('\n\n')[0]
+  .replace(/^          /gm, '')
+  .replace(/\$\{\{ secrets\.BENCHER_API_TOKEN \}\}/g, 'unused-test-placeholder');
+
+function uploadArgs(ref) {
+  const cwd = fs.mkdtempSync(path.join(os.tmpdir(), 'minigraf-history-'));
+  try {
+    const result = spawnSync('bash', ['-c', 'bencher() { printf "%s\\0" "$@"; }\n' + shell], {
+      cwd, encoding: 'utf8',
+      env: { ...process.env, BENCH_REF: ref, BENCH_SHA: 'a'.repeat(40),
+        BENCH_THRESHOLD: '0.999', BENCH_FILE: 'unused.txt' },
+    });
+    assert.equal(result.status, 0, result.stderr);
+    return result.stdout.split('\0').filter(Boolean);
+  } finally {
+    fs.rmSync(cwd, { recursive: true, force: true });
+  }
+}
+
+for (const ref of ['main', 'v3']) {
+  test(`${ref} preserves its own history when the other branch advances`, () => {
+    const args = uploadArgs(ref);
+    assert.equal(args[args.indexOf('--branch') + 1], ref);
+    assert.ok(!args.some(arg => arg.startsWith('--start-point')),
+      'a persistent release branch must not rebase its Bencher head');
+    assert.equal(args[args.indexOf('--threshold-test') + 1], 't_test');
+    assert.equal(args[args.indexOf('--threshold-max-sample-size') + 1], '30');
+    assert.equal(args[args.indexOf('--threshold-upper-boundary') + 1], '0.999');
+  });
+}
+
+test('ad hoc branches retain their explicit main comparison history', () => {
+  const args = uploadArgs('experiment/840');
+  assert.equal(args[args.indexOf('--branch') + 1], 'experiment/840');
+  assert.equal(args[args.indexOf('--start-point') + 1], 'main');
+  assert.ok(args.includes('--start-point-clone-thresholds'));
+});
diff --git a/.github/workflows/bench.yml b/.github/workflows/bench.yml
index 031b2eea6..558a63d6c 100644
--- a/.github/workflows/bench.yml
+++ b/.github/workflows/bench.yml
@@ -363,9 +363,12 @@ jobs:
           # alerting for weeks; with 30, a lasting change alerts on about five
           # runs (enough for the two-in-a-row issue below), then becomes the
           # baseline.
-          # A branch other than main starts from main's history and thresholds.
+          # Long-lived main/v3 histories must survive the other branch advancing.
+          # --start-point main rebases the entire v3 head (including bindings)
+          # when main changes, discarding independent v3 comparisons (#840).
+          # Only ad hoc comparison branches inherit main's current history.
           start_point=()
-          if [ "$BENCH_REF" != main ]; then
+          if [ "$BENCH_REF" != main ] && [ "$BENCH_REF" != v3 ]; then
             start_point=(--start-point main --start-point-clone-thresholds)
           fi
           bencher run \