diff --git a/pkg/config/config.go b/pkg/config/config.go
index 76a186bad..9a69f4d86 100644
--- a/pkg/config/config.go
+++ b/pkg/config/config.go
@@ -310,7 +310,16 @@ func LoadConfig(path string) (*Config, error) {
 func SaveConfig(path string, cfg *Config) error {
 	cfg.mu.RLock()
 	defer cfg.mu.RUnlock()
+	return saveConfigLocked(path, cfg)
+}
 
+// SaveConfigLocked writes cfg to path without acquiring cfg's mutex.
+// Use this when the caller manages synchronization externally.
+func SaveConfigLocked(path string, cfg *Config) error {
+	return saveConfigLocked(path, cfg)
+}
+
+func saveConfigLocked(path string, cfg *Config) error {
 	data, err := json.MarshalIndent(cfg, "", "  ")
 	if err != nil {
 		return err
@@ -324,9 +333,23 @@ func SaveConfig(path string, cfg *Config) error {
 	return os.WriteFile(path, data, 0600)
 }
 
+func (c *Config) Lock()    { c.mu.Lock() }
+func (c *Config) Unlock()  { c.mu.Unlock() }
 func (c *Config) RLock()   { c.mu.RLock() }
 func (c *Config) RUnlock() { c.mu.RUnlock() }
 
+// CopyFrom copies all configuration fields from src into c.
+// The caller must hold c's write lock. src's mutex is not acquired.
+func (c *Config) CopyFrom(src *Config) {
+	c.LLM = src.LLM
+	c.Agents = src.Agents
+	c.Channels = src.Channels
+	c.Gateway = src.Gateway
+	c.Tools = src.Tools
+	c.Heartbeat = src.Heartbeat
+	c.RateLimits = src.RateLimits
+}
+
 func (c *Config) WorkspacePath() string {
 	c.mu.RLock()
 	defer c.mu.RUnlock()
diff --git a/pkg/gateway/handlers.go b/pkg/gateway/handlers.go
index d7d260498..7c120bb8b 100644
--- a/pkg/gateway/handlers.go
+++ b/pkg/gateway/handlers.go
@@ -67,7 +67,15 @@ func (s *Server) handlePutConfig(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	if err := config.SaveConfig(s.configPath, &newCfg); err != nil {
+	// Hold write lock: save file first, then update memory only on success.
+	// Any concurrent GET /api/config will block until both are done.
+	s.cfg.Lock()
+	err = config.SaveConfigLocked(s.configPath, &newCfg)
+	if err == nil {
+		s.cfg.CopyFrom(&newCfg)
+	}
+	s.cfg.Unlock()
+	if err != nil {
 		writeJSONError(w, http.StatusInternalServerError, "failed to save config: "+err.Error())
 		return
 	}
diff --git a/pkg/gateway/server_test.go b/pkg/gateway/server_test.go
index 71eaa1eae..0f8970d91 100644
--- a/pkg/gateway/server_test.go
+++ b/pkg/gateway/server_test.go
@@ -1303,9 +1303,9 @@ func TestHandlePutConfig_MultipleSectionsSimultaneous(t *testing.T) {
 	}
 }
 
-// --- PUT /api/config: in-memory config not updated ---
+// --- PUT /api/config: in-memory config updated immediately ---
 
-func TestHandlePutConfig_InMemoryConfigUnchanged(t *testing.T) {
+func TestHandlePutConfig_InMemoryConfigUpdated(t *testing.T) {
 	cfg := config.DefaultConfig()
 	cfg.LLM.Model = "original"
 
@@ -1323,9 +1323,9 @@ func TestHandlePutConfig_InMemoryConfigUnchanged(t *testing.T) {
 		t.Fatalf("status = %d, want 200", rr.Code)
 	}
 
-	// In-memory config should still have old value (disk is updated, restart reloads)
-	if s.cfg.LLM.Model != "original" {
-		t.Errorf("in-memory model = %q, want %q (should remain unchanged until restart)", s.cfg.LLM.Model, "original")
+	// In-memory config should be updated immediately
+	if s.cfg.LLM.Model != "updated" {
+		t.Errorf("in-memory model = %q, want %q", s.cfg.LLM.Model, "updated")
 	}
 
 	// Disk should have new value
@@ -1645,7 +1645,7 @@ func TestHandlePutConfig_MCPMapUpdate(t *testing.T) {
 	}
 }
 
-func TestHandlePutConfig_MCPMapInMemoryUnchanged(t *testing.T) {
+func TestHandlePutConfig_MCPMapInMemoryUpdated(t *testing.T) {
 	cfg := config.DefaultConfig()
 	cfg.Tools.MCP = map[string]config.MCPServerConfig{
 		"original": {Command: "orig", Enabled: true},
@@ -1665,12 +1665,12 @@ func TestHandlePutConfig_MCPMapInMemoryUnchanged(t *testing.T) {
 		t.Fatalf("status = %d, want 200", rr.Code)
 	}
 
-	// In-memory config should NOT have the added server
-	if _, found := s.cfg.Tools.MCP["added"]; found {
-		t.Error("in-memory MCP should not contain 'added' (deep copy should prevent mutation)")
+	// In-memory config should now contain the added server
+	if _, found := s.cfg.Tools.MCP["added"]; !found {
+		t.Error("in-memory MCP should contain 'added' after PUT")
 	}
-	if len(s.cfg.Tools.MCP) != 1 {
-		t.Errorf("in-memory MCP count = %d, want 1", len(s.cfg.Tools.MCP))
+	if len(s.cfg.Tools.MCP) != 2 {
+		t.Errorf("in-memory MCP count = %d, want 2", len(s.cfg.Tools.MCP))
 	}
 }