refactor(cmd_ac): DRY AI autocomplete and fix trailing space API

- Replace _ai_model_autocomplete() with new _ai_provider_and_model_autocomplete()
  that handles both provider and model autocomplete in one function
- Add /ai set default-model model autocomplete support
- Remove trailing space requirement from cmd_prefix parameter (use \"/ai start\"
  instead of \"/ai start \")
- Fix const correctness in autocomplete_param_with_func() signature
- Remove unused space_at_end variable

src/command/cmd_ac.c

@@ -70,7 +70,6 @@
 
 static char* _sub_autocomplete(ProfWin* window, const char* const input, gboolean previous);
 static char* _notify_autocomplete(ProfWin* window, const char* const input, gboolean previous);
-static char* _ai_model_autocomplete(ProfWin* window, const char* const input, gboolean previous, const char* prefix);
 static char* _theme_autocomplete(ProfWin* window, const char* const input, gboolean previous);
 static char* _autoaway_autocomplete(ProfWin* window, const char* const input, gboolean previous);
 static char* _autoconnect_autocomplete(ProfWin* window, const char* const input, gboolean previous);
@@ -4488,6 +4487,10 @@ _force_encryption_autocomplete(ProfWin* window, const char* const input, gboolea
     return result;
 }
 
+/* Forward declaration */
+static char* _ai_provider_and_model_autocomplete(ProfWin* window, const char* const input,
+                                                 gboolean previous, const char* cmd);
+
 static char*
 _ai_autocomplete(ProfWin* window, const char* const input, gboolean previous)
 {
@@ -4496,7 +4499,6 @@ _ai_autocomplete(ProfWin* window, const char* const input, gboolean previous)
     /* Parse once for reuse - /ai <subcommand> [<arg1>] [<arg2>] */
     gboolean parse_result = FALSE;
     auto_gcharv gchar** args = parse_args(input, 1, 4, &parse_result);
-    gboolean space_at_end = g_str_has_suffix(input, " ");
     int num_args = g_strv_length(args);
 
     /* Top-level /ai <subcommand> autocomplete (e.g., /ai s<tab> -> /ai set) */
@@ -4552,27 +4554,20 @@ _ai_autocomplete(ProfWin* window, const char* const input, gboolean previous)
         return result;
     }
 
-    // /ai start [<provider>] [<model>] - use shared parse_args
-    if (parse_result && args && args[0] != NULL && g_strcmp0(args[0], "start") == 0) {
-        /* args[0]="start", args[1]=provider (if typed), args[2]=model (if typed) */
-        if (num_args == 1 || (num_args == 2 && !space_at_end)) {
-            result = autocomplete_param_with_func(input, "/ai start", ai_providers_find, previous, NULL);
-        } else {
-            /* /ai start <provider> <model> - model autocomplete */
-            result = _ai_model_autocomplete(window, input, previous, "/ai start ");
-        }
-        if (result) {
-            return result;
-        }
-    }
-
-    // /ai switch <provider> <model> - autocomplete model names for specified provider
-    result = _ai_model_autocomplete(window, input, previous, "/ai switch ");
+    // /ai start <provider> <model> - autocomplete provider names and model names
+    result = _ai_provider_and_model_autocomplete(window, input, previous, "/ai start");
     if (result) {
         return result;
     }
 
-    result = autocomplete_param_with_func(input, "/ai switch", ai_providers_find, previous, NULL);
+    // /ai switch <provider> <model> - autocomplete provider names and model names
+    result = _ai_provider_and_model_autocomplete(window, input, previous, "/ai switch");
+    if (result) {
+        return result;
+    }
+
+    // /ai set default-model <provider> <model> - autocomplete provider names and model names
+    result = _ai_provider_and_model_autocomplete(window, input, previous, "/ai set default-model");
     if (result) {
         return result;
     }
@@ -4592,13 +4587,26 @@ _ai_autocomplete(ProfWin* window, const char* const input, gboolean previous)
 }
 
 /**
- * Autocomplete model names for /ai <cmd> <provider> <model> patterns.
- * Extracts provider from input and provides model completions from that provider's cached models.
+ * Autocomplete provider names and model names for /ai <cmd> <provider> <model> patterns.
+ * First tries to autocomplete provider names, then model names if provider is valid.
  * Uses static ai_models_ac to preserve cycling state (last_found) across calls.
+ * The cmd_prefix should NOT include trailing space (e.g., "/ai start" not "/ai start ").
  */
 static char*
-_ai_model_autocomplete(ProfWin* window, const char* const input, gboolean previous, const char* cmd_prefix)
+_ai_provider_and_model_autocomplete(ProfWin* window, const char* const input, gboolean previous, const char* cmd)
 {
+    char* result;
+
+    /* First, try provider name autocomplete */
+    result = autocomplete_param_with_func(input, cmd, ai_providers_find, previous, NULL);
+    if (result) {
+        return result;
+    }
+
+    /* Provider was specified, try model name autocomplete */
+    /* Build the full command prefix with space */
+    auto_gchar gchar* cmd_prefix = g_strdup_printf("%s ", cmd);
+
     if (!g_str_has_prefix(input, cmd_prefix)) {
         return NULL;
     }
@@ -4634,6 +4642,6 @@ _ai_model_autocomplete(ProfWin* window, const char* const input, gboolean previo
 
     auto_gchar gchar* full_prefix = g_strdup_printf("%s%s", cmd_prefix, rest);
 
-    char* result = autocomplete_param_with_ac(input, full_prefix, ai_models_ac, TRUE, previous);
+    result = autocomplete_param_with_ac(input, full_prefix, ai_models_ac, TRUE, previous);
     return result;
 }

src/tools/autocomplete.c

@@ -305,7 +305,7 @@ autocomplete_complete(Autocomplete ac, const gchar* search_str, gboolean quote,
 // autocomplete_func func is used -> autocomplete_param_with_func
 // Autocomplete ac, gboolean quote are used -> autocomplete_param_with_ac
 static char*
-_autocomplete_param_common(const char* const input, char* command, autocomplete_func func, Autocomplete ac, gboolean quote, gboolean previous, void* context)
+_autocomplete_param_common(const char* const input, const char* command, autocomplete_func func, Autocomplete ac, gboolean quote, gboolean previous, void* context)
 {
     int len;
     auto_gchar gchar* command_cpy = g_strdup_printf("%s ", command);
@@ -344,7 +344,7 @@ _autocomplete_param_common(const char* const input, char* command, autocomplete_
 }
 
 char*
-autocomplete_param_with_func(const char* const input, char* command, autocomplete_func func, gboolean previous, void* context)
+autocomplete_param_with_func(const char* const input, const char* command, autocomplete_func func, gboolean previous, void* context)
 {
     return _autocomplete_param_common(input, command, func, NULL, FALSE, previous, context);
 }

src/tools/autocomplete.h

@@ -63,7 +63,7 @@ gchar* autocomplete_complete(Autocomplete ac, const gchar* search_str, gboolean
 GList* autocomplete_create_list(Autocomplete ac);
 gint autocomplete_length(Autocomplete ac);
 
-char* autocomplete_param_with_func(const char* const input, char* command,
+char* autocomplete_param_with_func(const char* const input, const char* command,
                                    autocomplete_func func, gboolean previous, void* context);
 
 char* autocomplete_param_with_ac(const char* const input, char* command,