diff --git a/Makefile b/Makefile
index 22e01d57..957d2004 100644
--- a/Makefile
+++ b/Makefile
@@ -11,3 +11,10 @@ build/StrfryTemplates.h: $(shell find src/tmpls/ -type f -name '*.tmpl')
 	PERL5LIB=golpe/vendor/ perl golpe/external/templar/templar.pl src/tmpls/ strfrytmpl $@
 
 src/apps/relay/RelayWebsocket.o: build/StrfryTemplates.h
+
+.PHONY: test-subid
+test-subid: build/subid_tests
+	build/subid_tests
+
+build/subid_tests: test/SubIdTests.cpp build/golpe.h
+	$(CXX) $(CXXFLAGS) $(INCS) $< -o $@
diff --git a/src/Subscription.h b/src/Subscription.h
index 4ffe786a..6d440cf0 100644
--- a/src/Subscription.h
+++ b/src/Subscription.h
@@ -6,12 +6,11 @@
 
 
 struct SubId {
-    char buf[72];
+    char buf[MAX_SUBID_SIZE + 1];
 
     SubId(std::string_view val) {
-        static_assert(MAX_SUBID_SIZE == 71, "MAX_SUBID_SIZE mismatch");
-        if (val.size() > 71) throw herr("subscription id too long");
-        if (val.size() == 0) throw herr("subscription id too short");
+        static_assert(MAX_SUBID_SIZE <= 255, "MAX_SUBID_SIZE must fit in a byte");
+        if (val.empty() || val.size() > MAX_SUBID_SIZE) throw herr("invalid subscription id length");
 
         auto badChar = [](char c){
             return c < 0x20 || c == '\\' || c == '"' || c >= 0x7F;
diff --git a/src/constants.h b/src/constants.h
index cc3b0041..79ae5374 100644
--- a/src/constants.h
+++ b/src/constants.h
@@ -1,5 +1,5 @@
 #pragma once
 
 const uint64_t CURR_DB_VERSION = 3;
-const size_t MAX_SUBID_SIZE = 71; // Statically allocated size in SubId
+const size_t MAX_SUBID_SIZE = 64; // NIP-01: REQ subscription ids must be non-empty and <=64 bytes
 const size_t MAX_INDEXED_TAG_VAL_SIZE = 255;
diff --git a/test/SubIdTests.cpp b/test/SubIdTests.cpp
new file mode 100644
index 00000000..0f1445a5
--- /dev/null
+++ b/test/SubIdTests.cpp
@@ -0,0 +1,45 @@
+#include "Subscription.h"
+
+#include <cstdlib>
+#include <exception>
+#include <iostream>
+#include <string>
+#include <string_view>
+
+namespace {
+
+void expectSuccess(std::string_view name, std::string subId) {
+    try {
+        SubId s(subId);
+        if (s.sv() != std::string_view(subId)) {
+            std::cerr << name << ": round-trip mismatch\n";
+            std::exit(EXIT_FAILURE);
+        }
+    } catch (const std::exception &e) {
+        std::cerr << name << ": expected success but threw: " << e.what() << "\n";
+        std::exit(EXIT_FAILURE);
+    }
+}
+
+void expectFailure(std::string_view name, std::string subId) {
+    try {
+        SubId s(subId);
+        std::cerr << name << ": expected failure but constructed successfully\n";
+        std::exit(EXIT_FAILURE);
+    } catch (const std::exception &) {
+    }
+}
+
+} // namespace
+
+int main() {
+    expectSuccess("max length", std::string(64, 'a'));
+    expectFailure("too long", std::string(65, 'a'));
+    expectFailure("empty", std::string());
+    expectFailure("control char", std::string(1, '\x1F'));
+    expectFailure("quote char", std::string("\""));
+    expectFailure("backslash", std::string("\\"));
+
+    std::cout << "SubId tests passed\n";
+    return EXIT_SUCCESS;
+}