diff --git a/src/common.h b/src/common.h
index eadf02372..ae9950ac6 100644
--- a/src/common.h
+++ b/src/common.h
@@ -495,14 +495,16 @@ inline wcstring to_string(long x) {
     return wcstring(buff);
 }
 
-inline wcstring to_string(int x) { return to_string(static_cast<long>(x)); }
-
-inline wcstring to_string(size_t x) {
+inline wcstring to_string(unsigned long long x) {
     wchar_t buff[64];
     format_ullong_safe(buff, x);
     return wcstring(buff);
 }
 
+inline wcstring to_string(int x) { return to_string(static_cast<long>(x)); }
+
+inline wcstring to_string(size_t x) { return to_string(static_cast<unsigned long long>(x)); }
+
 inline bool bool_from_string(const std::string &x) {
     if (x.empty()) return false;
     switch (x.front()) {
diff --git a/src/exec.cpp b/src/exec.cpp
index c988ea52c..a43b24829 100644
--- a/src/exec.cpp
+++ b/src/exec.cpp
@@ -369,6 +369,9 @@ static bool run_internal_process(process_t *p, std::string outdata, std::string
     p->internal_proc_ = std::make_shared<internal_proc_t>();
     f->internal_proc = p->internal_proc_;
 
+    FLOGF(proc_internal_proc, "Created internal proc %llu to write output for proc '%ls'",
+          p->internal_proc_->get_id(), p->argv0());
+
     // Resolve the IO chain.
     // Note it's important we do this even if we have no out or err data, because we may have been
     // asked to truncate a file (e.g. `echo -n '' > /tmp/truncateme.txt'). The open() in the dup2
diff --git a/src/flog.h b/src/flog.h
index 48b5728ba..3e7838d65 100644
--- a/src/flog.h
+++ b/src/flog.h
@@ -59,6 +59,8 @@ class category_list_t {
 
     category_t proc_termowner{L"proc-termowner", L"Terminal ownership events"};
 
+    category_t proc_internal_proc{L"proc-internal-proc", L"Internal (non-forked) process events"};
+
     category_t env_locale{L"env-locale", L"Changes to locale variables"};
 };
 
diff --git a/src/proc.cpp b/src/proc.cpp
index a9615f93a..b41c3247f 100644
--- a/src/proc.cpp
+++ b/src/proc.cpp
@@ -214,8 +214,17 @@ void internal_proc_t::mark_exited(proc_status_t status) {
     status_.store(status, std::memory_order_relaxed);
     exited_.store(true, std::memory_order_release);
     topic_monitor_t::principal().post(topic_t::internal_exit);
+    FLOG(proc_internal_proc, "Internal proc", internal_proc_id_, "exited with status",
+         status.status_value());
 }
 
+static int64_t next_proc_id() {
+    static std::atomic<uint64_t> s_next{};
+    return ++s_next;
+}
+
+internal_proc_t::internal_proc_t() : internal_proc_id_(next_proc_id()) {}
+
 static void mark_job_complete(const job_t *j) {
     for (auto &p : j->processes) {
         p->completed = 1;
diff --git a/src/proc.h b/src/proc.h
index eb12e5b92..cc52fc845 100644
--- a/src/proc.h
+++ b/src/proc.h
@@ -113,6 +113,10 @@ class proc_status_t {
 /// A structure representing a "process" internal to fish. This is backed by a pthread instead of a
 /// separate process.
 class internal_proc_t {
+    /// An identifier for internal processes.
+    /// This is used for logging purposes only.
+    const uint64_t internal_proc_id_;
+
     /// Whether the process has exited.
     std::atomic<bool> exited_{};
 
@@ -130,6 +134,10 @@ class internal_proc_t {
         assert(exited() && "Process is not exited");
         return status_.load(std::memory_order_relaxed);
     }
+
+    uint64_t get_id() const { return internal_proc_id_; }
+
+    internal_proc_t();
 };
 
 /// A structure representing a single fish process. Contains variables for tracking process state