diff --git a/src/common.cpp b/src/common.cpp
index ccfc0764f..bd4fa8e3c 100644
--- a/src/common.cpp
+++ b/src/common.cpp
@@ -55,6 +55,7 @@
 #include "flog.h"
 #include "future_feature_flags.h"
 #include "global_safety.h"
+#include "iothread.h"
 #include "proc.h"
 #include "signal.h"
 #include "wildcard.h"
@@ -64,8 +65,6 @@ constexpr wint_t NOT_A_WCHAR = static_cast<wint_t>(WEOF);
 
 struct termios shell_modes;
 
-/// This allows us to determine if we're running on the main thread
-static std::atomic<size_t> thread_id{0};
 /// This allows us to notice when we've forked.
 static relaxed_atomic_bool_t is_forked_proc{false};
 /// This allows us to bypass the main thread checks
@@ -2279,10 +2278,10 @@ extern "C" {
 }
 
 void set_main_thread() {
-    // Just call is_main_thread() once to force increment of thread_id.
-    bool x = is_main_thread();
-    assert(x && "set_main_thread should be main thread");
-    (void)x;
+    // Just call thread_id() once to force increment of thread_id.
+    uint64_t tid = thread_id();
+    assert(tid == 1 && "main thread should have thread ID 1");
+    (void)tid;
 }
 
 void configure_thread_assertions_for_testing() { thread_asserts_cfg_for_testing = true; }
@@ -2310,10 +2309,7 @@ void restore_term_foreground_process_group() {
     }
 }
 
-bool is_main_thread() {
-    static thread_local int local_thread_id = thread_id++;
-    return local_thread_id == 0;
-}
+bool is_main_thread() { return thread_id() == 1; }
 
 void assert_is_main_thread(const char *who) {
     if (!is_main_thread() && !thread_asserts_cfg_for_testing) {
diff --git a/src/iothread.cpp b/src/iothread.cpp
index f206e776a..43854dcc8 100644
--- a/src/iothread.cpp
+++ b/src/iothread.cpp
@@ -357,6 +357,8 @@ bool make_pthread(pthread_t *result, void *(*func)(void *), void *param) {
 using void_func_t = std::function<void(void)>;
 
 static void *func_invoker(void *param) {
+    // Acquire a thread id for this thread.
+    (void)thread_id();
     void_func_t *vf = static_cast<void_func_t *>(param);
     (*vf)();
     delete vf;
@@ -373,3 +375,15 @@ bool make_pthread(pthread_t *result, void_func_t &&func) {
     delete vf;
     return false;
 }
+
+static uint64_t next_thread_id() {
+    // Note 0 is an invalid thread id.
+    static owning_lock<uint64_t> s_last_thread_id{};
+    auto tid = s_last_thread_id.acquire();
+    return ++*tid;
+}
+
+uint64_t thread_id() {
+    static thread_local uint64_t tl_tid = next_thread_id();
+    return tl_tid;
+}
diff --git a/src/iothread.h b/src/iothread.h
index 39d99e539..70a5a2c58 100644
--- a/src/iothread.h
+++ b/src/iothread.h
@@ -82,4 +82,8 @@ void iothread_perform_on_main(std::function<void(void)> &&func);
 bool make_pthread(pthread_t *result, void *(*func)(void *), void *param);
 bool make_pthread(pthread_t *result, std::function<void(void)> &&func);
 
+/// \returns a thread ID for this thread.
+/// Thread IDs are never repeated.
+uint64_t thread_id();
+
 #endif