diff --git a/src/common.cpp b/src/common.cpp
index 10e2343f8..fac1556ce 100644
--- a/src/common.cpp
+++ b/src/common.cpp
@@ -1744,8 +1744,7 @@ int common_get_width() { return get_current_winsize().ws_col; }
 int common_get_height() { return get_current_winsize().ws_row; }
 
 bool string_prefixes_string(const wchar_t *proposed_prefix, const wcstring &value) {
-    size_t prefix_size = wcslen(proposed_prefix);
-    return prefix_size <= value.size() && value.compare(0, prefix_size, proposed_prefix) == 0;
+    return string_prefixes_string(proposed_prefix, value.c_str());
 }
 
 bool string_prefixes_string(const wcstring &proposed_prefix, const wcstring &value) {
@@ -1763,6 +1762,17 @@ bool string_prefixes_string(const wchar_t *proposed_prefix, const wchar_t *value
     return true;
 }
 
+bool string_prefixes_string(const char *proposed_prefix, const std::string &value) {
+    return string_prefixes_string(proposed_prefix, value.c_str());
+}
+
+bool string_prefixes_string(const char *proposed_prefix, const char *value) {
+    for (size_t idx = 0; proposed_prefix[idx] != L'\0'; idx++) {
+        if (proposed_prefix[idx] != value[idx]) return false;
+    }
+    return true;
+}
+
 bool string_prefixes_string_case_insensitive(const wcstring &proposed_prefix,
                                              const wcstring &value) {
     size_t prefix_size = proposed_prefix.size();
diff --git a/src/common.h b/src/common.h
index d540d0996..349974f02 100644
--- a/src/common.h
+++ b/src/common.h
@@ -342,6 +342,8 @@ std::string wcs2string(const wcstring &input);
 bool string_prefixes_string(const wcstring &proposed_prefix, const wcstring &value);
 bool string_prefixes_string(const wchar_t *proposed_prefix, const wcstring &value);
 bool string_prefixes_string(const wchar_t *proposed_prefix, const wchar_t *value);
+bool string_prefixes_string(const char *proposed_prefix, const std::string &value);
+bool string_prefixes_string(const char *proposed_prefix, const char *value);
 
 /// Test if a string is a suffix of another.
 bool string_suffixes_string(const wcstring &proposed_suffix, const wcstring &value);
diff --git a/src/env_universal_common.cpp b/src/env_universal_common.cpp
index bfc5afcf9..999b084cf 100644
--- a/src/env_universal_common.cpp
+++ b/src/env_universal_common.cpp
@@ -74,6 +74,9 @@
 /// Small note about not editing ~/.fishd manually. Inserted at the top of all .fishd files.
 #define SAVE_MSG "# This file contains fish universal variable definitions.\n"
 
+/// Version for fish 3.0
+#define UVARS_VERSION_3_0 "3.0"
+
 /// The different types of messages found in the fishd file.
 enum class uvar_message_type_t { set, set_export };
 
@@ -764,7 +767,7 @@ var_table_t env_universal_t::read_message_internal(int fd) {
             // Process it if it's a newline (which is true if we are before the end of the buffer).
             if (cursor < bufflen && !line.empty()) {
                 if (utf8_to_wchar(line.data(), line.size(), &wide_line, 0)) {
-                    env_universal_t::parse_message_internal(wide_line, &result, &storage);
+                    env_universal_t::parse_message_2x_internal(wide_line, &result, &storage);
                 }
                 line.clear();
             }
@@ -778,8 +781,70 @@ var_table_t env_universal_t::read_message_internal(int fd) {
     return result;
 }
 
-/// Parse message msg/
-void env_universal_t::parse_message_internal(const wcstring &msgstr, var_table_t *vars,
+/// \return the format corresponding to file contents \p s.
+uvar_format_t env_universal_t::format_for_contents(const std::string &s) {
+    // Walk over leading comments, looking for one like '# version'
+    line_iterator_t<std::string> iter{s};
+    while (iter.next()) {
+        const std::string &line = iter.line();
+        if (line.empty()) continue;
+        if (line.front() != L'#') {
+            // Exhausted leading comments.
+            break;
+        }
+        // Note scanf %s is max characters to write; add 1 for null terminator.
+        char versionbuf[64 + 1];
+        if (sscanf(line.c_str(), "# VERSION: %64s", versionbuf) != 1) continue;
+
+        // Try reading the version.
+        if (!strcmp(versionbuf, UVARS_VERSION_3_0)) {
+            return uvar_format_t::fish_3_0;
+        } else {
+            // Unknown future version.
+            return uvar_format_t::future;
+        }
+    }
+    // No version found, assume 2.x
+    return uvar_format_t::fish_2_x;
+}
+
+void env_universal_t::populate_variables(const std::string &s, var_table_t *out_vars) {
+    // Decide on the format.
+    const uvar_format_t format = format_for_contents(s);
+
+    line_iterator_t<std::string> iter{s};
+    wcstring wide_line;
+    wcstring storage;
+    while (iter.next()) {
+        const std::string &line = iter.line();
+        // Skip empties and constants.
+        if (line.empty() || line.front() == L'#') continue;
+
+        // Convert to UTF8.
+        wide_line.clear();
+        if (!utf8_to_wchar(line.data(), line.size(), &wide_line, 0)) continue;
+
+        switch (format) {
+            case uvar_format_t::fish_2_x:
+                env_universal_t::parse_message_2x_internal(wide_line, out_vars, &storage);
+                break;
+            case uvar_format_t::fish_3_0:
+            // For future formats, just try with the most recent one.
+            case uvar_format_t::future:
+                env_universal_t::parse_message_30_internal(wide_line, out_vars, &storage);
+                break;
+        }
+    }
+}
+
+/// Parse message msg per fish 3.0 format.
+void env_universal_t::parse_message_30_internal(const wcstring &msgstr, var_table_t *vars,
+                                                wcstring *storage) {
+    // TODO.
+}
+
+/// Parse message msg per fish 2.x format.
+void env_universal_t::parse_message_2x_internal(const wcstring &msgstr, var_table_t *vars,
                                              wcstring *storage) {
     const wchar_t *msg = msgstr.c_str();
 
diff --git a/src/env_universal_common.h b/src/env_universal_common.h
index c597a031a..daebe7a2c 100644
--- a/src/env_universal_common.h
+++ b/src/env_universal_common.h
@@ -30,6 +30,10 @@ struct callback_data_t {
 
 typedef std::vector<struct callback_data_t> callback_data_list_t;
 
+// List of fish universal variable formats.
+// This is exposed for testing.
+enum class uvar_format_t { fish_2_x, fish_3_0, future };
+
 bool get_hostname_identifier(wcstring &result);
 /// Class representing universal variables.
 class env_universal_t {
@@ -68,7 +72,10 @@ class env_universal_t {
     // vars_to_acquire.
     void acquire_variables(var_table_t &vars_to_acquire);
 
-    static void parse_message_internal(const wcstring &msg, var_table_t *vars, wcstring *storage);
+    static void parse_message_2x_internal(const wcstring &msg, var_table_t *vars,
+                                          wcstring *storage);
+    static void parse_message_30_internal(const wcstring &msg, var_table_t *vars,
+                                          wcstring *storage);
     static var_table_t read_message_internal(int fd);
 
    public:
@@ -95,6 +102,13 @@ class env_universal_t {
     /// Reads and writes variables at the correct path. Returns true if modified variables were
     /// written.
     bool sync(callback_data_list_t &callbacks);
+
+    /// Populate a variable table \p out_vars from a \p s string.
+    /// This is exposed for testing only.
+    static void populate_variables(const std::string &s, var_table_t *out_vars);
+
+    /// Guess a file format. Exposed for testing only.
+    static uvar_format_t format_for_contents(const std::string &s);
 };
 
 /// The "universal notifier" is an object responsible for broadcasting and receiving universal
diff --git a/src/fish_tests.cpp b/src/fish_tests.cpp
index cc2dbeef7..ce1282a9a 100644
--- a/src/fish_tests.cpp
+++ b/src/fish_tests.cpp
@@ -2940,6 +2940,27 @@ static void test_universal_callbacks() {
     (void)system("rm -Rf test/fish_uvars_test/");
 }
 
+static void test_universal_formats() {
+    say(L"Testing universal format detection");
+    const struct {
+        const char *str;
+        uvar_format_t format;
+    } tests[] = {
+        {"# VERSION: 3.0", uvar_format_t::fish_3_0},
+        {"# version: 3.0", uvar_format_t::fish_2_x},
+        {"# blah blahVERSION: 3.0", uvar_format_t::fish_2_x},
+        {"stuff\n# blah blahVERSION: 3.0", uvar_format_t::fish_2_x},
+        {"# blah\n# VERSION: 3.0", uvar_format_t::fish_3_0},
+        {"# blah\n#VERSION: 3.0", uvar_format_t::fish_3_0},
+        {"# blah\n#VERSION:3.0", uvar_format_t::fish_3_0},
+        {"# blah\n#VERSION:3.1", uvar_format_t::future},
+    };
+    for (const auto &test : tests) {
+        uvar_format_t format = env_universal_t::format_for_contents(test.str);
+        do_test(format == test.format);
+    }
+}
+
 bool poll_notifier(const std::unique_ptr<universal_notifier_t> &note) {
     bool result = false;
     if (note->usec_delay_between_polls() > 0) {
@@ -4824,6 +4845,7 @@ int main(int argc, char **argv) {
     if (should_test_function("line_iterator")) test_line_iterator();
     if (should_test_function("universal")) test_universal();
     if (should_test_function("universal")) test_universal_callbacks();
+    if (should_test_function("universal")) test_universal_formats();
     if (should_test_function("notifiers")) test_universal_notifiers();
     if (should_test_function("completion_insertions")) test_completion_insertions();
     if (should_test_function("autosuggestion_ignores")) test_autosuggestion_ignores();