diff --git a/algorithmic/problems/23/check.cpp b/algorithmic/problems/23/check.cpp
index 49d9140b9..beab09bda 100644
--- a/algorithmic/problems/23/check.cpp
+++ b/algorithmic/problems/23/check.cpp
@@ -214,7 +214,11 @@ namespace test_gen{
 
 		vf.push_back(
 			[&](){
-
+				// Test set 0 (Tid % 100 == 0) used to be empty, and the checker accepted
+				// any parseable program on it without running it (#205). It now runs
+				// the fixed samples plus the seeded random cases.
+				(vf[1])();
+				(vf[2])();
 			}
 		);
 		vf.push_back(
@@ -368,7 +372,6 @@ namespace test_gen{
 }
 void check_Tid(){
 	test_gen::Tid=inf.readInteger();
-	if (test_gen::Tid%100==0) quitf(_ok,"Test 0.");
 }
 void read_test(){
 	test_gen::gao();
diff --git a/algorithmic/tests/test_problem23.py b/algorithmic/tests/test_problem23.py
new file mode 100644
index 000000000..f14df3ce3
--- /dev/null
+++ b/algorithmic/tests/test_problem23.py
@@ -0,0 +1,54 @@
+"""Every test id must run the program; Tid % 100 == 0 used to be accepted unchecked (#205)."""
+from pathlib import Path
+import re
+import subprocess
+import pytest
+
+ROOT = Path(__file__).resolve().parents[2]
+DATA = ROOT / 'algorithmic/problems/23/testdata'
+# 1.ans holds a correct A=B program for the substring task.
+REFERENCE = (DATA / '1.ans').read_text()
+EMPTY, ONE_RULE = '', 'a=b\n'
+
+
+@pytest.fixture(scope='module')
+def checker(tmp_path_factory):
+    executable = tmp_path_factory.mktemp('checker23') / 'checker'
+    subprocess.run(['g++', '-std=gnu++17', '-O2', '-I' + str(ROOT / 'algorithmic/judge/include'),
+                    str(ROOT / 'algorithmic/problems/23/check.cpp'), '-o', str(executable)], check=True)
+    return executable
+
+
+def check(checker, tmp_path, tid, program):
+    inf, out = tmp_path / 'in', tmp_path / 'out'
+    inf.write_text('%d\n' % tid);out.write_text(program)
+    return subprocess.run([str(checker), str(inf), str(out), str(DATA / '1.ans')],
+                          text=True, capture_output=True, timeout=30)
+
+
+def ratio(result):
+    return float(re.search(r'Ratio: ([0-9.]+)', result.stderr).group(1))
+
+
+# The ids at the pinned commit c7ec558 were 0, 100 and 200; 50 has no generator of its own.
+@pytest.mark.parametrize('tid', [0, 100, 200, 50])
+@pytest.mark.parametrize('program', [EMPTY, ONE_RULE])
+def test_program_that_solves_nothing_is_wrong(checker, tmp_path, tid, program):
+    result = check(checker, tmp_path, tid, program)
+    assert result.returncode == 1, result.stderr
+    assert 'WA on Test case' in result.stderr and 'Test 0.' not in result.stderr
+
+
+@pytest.mark.parametrize('tid', [0, 100, 200, 50])
+def test_correct_program_still_scores(checker, tmp_path, tid):
+    result = check(checker, tmp_path, tid, REFERENCE)
+    assert result.returncode == 7, result.stderr
+    assert 0 < ratio(result) <= 1
+
+
+def test_existing_test_ids_are_unchanged(checker, tmp_path):
+    tid = int((DATA / '1.in').read_text().split()[0])
+    assert check(checker, tmp_path, tid, EMPTY).returncode == 1
+    result = check(checker, tmp_path, tid, REFERENCE)
+    assert result.returncode == 7, result.stderr
+    assert 0 < ratio(result) <= 1