From bb404f248dd8cb8d4dd2ada2a1b9a60d297242e6 Mon Sep 17 00:00:00 2001
From: Craig Gidney <craiggidney@google.com>
Date: Thu, 17 Sep 2026 16:47:51 -0700
Subject: [PATCH 1/2] Fix tag parsing failing badly when reaching unexpected
 EOF

---
 src/stim/circuit/circuit.h                | 2 +-
 src/stim/circuit/circuit.test.cc          | 6 ++++++
 src/stim/dem/detector_error_model.test.cc | 6 ++++++
 3 files changed, 13 insertions(+), 1 deletion(-)

diff --git a/src/stim/circuit/circuit.h b/src/stim/circuit/circuit.h
index d1cc0ab7a..94c6f4ba2 100644
--- a/src/stim/circuit/circuit.h
+++ b/src/stim/circuit/circuit.h
@@ -368,7 +368,7 @@ void read_tag(int &c, std::string_view name, SOURCE read_char, MonotonicBuffer<c
     c = read_char();
 
     while (c != ']') {
-        if (c == '\r' || c == '\n') {
+        if (c == '\r' || c == '\n' || c == EOF) {
             std::stringstream ss;
             ss << "A tag wasn't closed with ']' before the end of the line.\n";
             ss << "Hit a ";
diff --git a/src/stim/circuit/circuit.test.cc b/src/stim/circuit/circuit.test.cc
index bfe081a6a..3b13a3c54 100644
--- a/src/stim/circuit/circuit.test.cc
+++ b/src/stim/circuit/circuit.test.cc
@@ -2004,3 +2004,9 @@ TEST(circuit, without_tags) {
         }
     )CIRCUIT"));
 }
+
+TEST(circuit, parse_tag_eof) {
+    ASSERT_THROW({
+        Circuit("H[");
+    }, std::invalid_argument);
+}
diff --git a/src/stim/dem/detector_error_model.test.cc b/src/stim/dem/detector_error_model.test.cc
index 429642c90..00044d867 100644
--- a/src/stim/dem/detector_error_model.test.cc
+++ b/src/stim/dem/detector_error_model.test.cc
@@ -894,3 +894,9 @@ TEST(detector_error_model, parse_windows_newlines) {
         DetectorErrorModel("error(0.125) D0\r\ndetector(5) D10\r\n"),
         DetectorErrorModel("error(0.125) D0\r\ndetector(5) D10\r\n"));
 }
+
+TEST(circuit, parse_tag_eof) {
+    ASSERT_THROW({
+        DetectorErrorModel("error[");
+    }, std::invalid_argument);
+}

From 1f274088e948f94d267deb2b3c4c729ae149f98d Mon Sep 17 00:00:00 2001
From: Craig Gidney <craiggidney@google.com>
Date: Thu, 17 Sep 2026 16:52:57 -0700
Subject: [PATCH 2/2] typo

---
 src/stim/dem/detector_error_model.test.cc | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/stim/dem/detector_error_model.test.cc b/src/stim/dem/detector_error_model.test.cc
index 00044d867..f967ae565 100644
--- a/src/stim/dem/detector_error_model.test.cc
+++ b/src/stim/dem/detector_error_model.test.cc
@@ -895,7 +895,7 @@ TEST(detector_error_model, parse_windows_newlines) {
         DetectorErrorModel("error(0.125) D0\r\ndetector(5) D10\r\n"));
 }
 
-TEST(circuit, parse_tag_eof) {
+TEST(detector_error_model, parse_tag_eof) {
     ASSERT_THROW({
         DetectorErrorModel("error[");
     }, std::invalid_argument);