diff --git a/api/agent.yaml b/api/agent.yaml
index 1d3a77f..da20853 100644
--- a/api/agent.yaml
+++ b/api/agent.yaml
@@ -44,6 +44,12 @@ paths:
             application/json:
               schema:
                 $ref: "#/components/schemas/VPC"
+        "400":
+          description: Missing required field or invalid request body
+          content:
+            application/json:
+              schema:
+                $ref: "#/components/schemas/Error"
         "409":
           description: VPC already exists
           content:
@@ -179,12 +185,6 @@ paths:
                 $ref: "#/components/schemas/Subnet"
         "404":
           $ref: "#/components/responses/NotFound"
-        "409":
-          description: Subnet not in a deletable state
-          content:
-            application/json:
-              schema:
-                $ref: "#/components/schemas/Error"
         "500":
           $ref: "#/components/responses/InternalError"
 
diff --git a/internal/api/agent/subnet_test.go b/internal/api/agent/subnet_test.go
index 0fdd18b..23745a0 100644
--- a/internal/api/agent/subnet_test.go
+++ b/internal/api/agent/subnet_test.go
@@ -81,7 +81,7 @@ func TestPostSubnet_Created(t *testing.T) {
 
 func TestPostSubnet_MissingFields(t *testing.T) {
 	s, _ := newTestServer(t)
-	body, _ := json.Marshal(SubnetCreateRequest{Name: "sn-1"}) // vpc, iface_type, gateway_ip, cidr manquants
+	body, _ := json.Marshal(SubnetCreateRequest{Name: "sn-1"}) // vpc, gateway_ip, cidr manquants
 	w := httptest.NewRecorder()
 	s.SubnetsHandler(w, httptest.NewRequest(http.MethodPost, "/subnets", bytes.NewReader(body)))
 	if w.Code != http.StatusBadRequest {
@@ -89,6 +89,24 @@ func TestPostSubnet_MissingFields(t *testing.T) {
 	}
 }
 
+func TestPostSubnet_IfaceTypeOptional(t *testing.T) {
+	s, db := newTestServer(t)
+	kv.AddInDB(db, "vpc/vpc-1/state", "created")
+	req := SubnetCreateRequest{
+		Name:      "sn-opt",
+		VPC:       "vpc-1",
+		GatewayIP: "10.0.0.1",
+		CIDR:      "10.0.0.0/24",
+		// IfaceType omis — doit utiliser default_interface
+	}
+	body, _ := json.Marshal(req)
+	w := httptest.NewRecorder()
+	s.SubnetsHandler(w, httptest.NewRequest(http.MethodPost, "/subnets", bytes.NewReader(body)))
+	if w.Code != http.StatusAccepted {
+		t.Fatalf("attendu 202, obtenu %d: %s", w.Code, w.Body.String())
+	}
+}
+
 func TestPostSubnet_VPCNotFound(t *testing.T) {
 	s, _ := newTestServer(t)
 	req := SubnetCreateRequest{
@@ -101,8 +119,8 @@ func TestPostSubnet_VPCNotFound(t *testing.T) {
 	body, _ := json.Marshal(req)
 	w := httptest.NewRecorder()
 	s.SubnetsHandler(w, httptest.NewRequest(http.MethodPost, "/subnets", bytes.NewReader(body)))
-	if w.Code != http.StatusConflict {
-		t.Errorf("attendu 409, obtenu %d", w.Code)
+	if w.Code != http.StatusUnprocessableEntity {
+		t.Errorf("attendu 422, obtenu %d", w.Code)
 	}
 }
 
@@ -138,8 +156,8 @@ func TestPostSubnet_VPCDeleting(t *testing.T) {
 	body, _ := json.Marshal(req)
 	w := httptest.NewRecorder()
 	s.SubnetsHandler(w, httptest.NewRequest(http.MethodPost, "/subnets", bytes.NewReader(body)))
-	if w.Code != http.StatusConflict {
-		t.Errorf("attendu 409, obtenu %d", w.Code)
+	if w.Code != http.StatusUnprocessableEntity {
+		t.Errorf("attendu 422, obtenu %d", w.Code)
 	}
 }
 
diff --git a/internal/api/agent/subnets.go b/internal/api/agent/subnets.go
index b0fb2fb..60467aa 100644
--- a/internal/api/agent/subnets.go
+++ b/internal/api/agent/subnets.go
@@ -69,9 +69,9 @@ func (s *Server) postSubnet(w http.ResponseWriter, r *http.Request) {
 		json.NewEncoder(w).Encode(ErrorResponse{Error: "invalid request body"})
 		return
 	}
-	if req.Name == "" || req.VPC == "" || req.IfaceType == "" || req.GatewayIP == "" || req.CIDR == "" {
+	if req.Name == "" || req.VPC == "" || req.GatewayIP == "" || req.CIDR == "" {
 		w.WriteHeader(http.StatusBadRequest)
-		json.NewEncoder(w).Encode(ErrorResponse{Error: "name, vpc, iface_type, gateway_ip and cidr are required"})
+		json.NewEncoder(w).Encode(ErrorResponse{Error: "name, vpc, gateway_ip and cidr are required"})
 		return
 	}
 	cmd := dispatcher.CreateSubnetCommand{
@@ -83,7 +83,11 @@ func (s *Server) postSubnet(w http.ResponseWriter, r *http.Request) {
 		CIDR:      req.CIDR,
 	}
 	if err := s.dispatcher.Prepare(cmd); err != nil {
-		w.WriteHeader(http.StatusConflict)
+		if _, dbErr := kv.GetFromDB(s.db, "subnet/"+req.Name+"/state"); dbErr == nil {
+			w.WriteHeader(http.StatusConflict)
+		} else {
+			w.WriteHeader(http.StatusUnprocessableEntity)
+		}
 		json.NewEncoder(w).Encode(ErrorResponse{Error: err.Error()})
 		return
 	}
diff --git a/internal/api/agent/vpc.go b/internal/api/agent/vpc.go
index df2d47c..43cc33c 100644
--- a/internal/api/agent/vpc.go
+++ b/internal/api/agent/vpc.go
@@ -42,7 +42,11 @@ func (s *Server) getVpc(w http.ResponseWriter, _ *http.Request, name string) {
 func (s *Server) deleteVpc(w http.ResponseWriter, _ *http.Request, name string) {
 	cmd := dispatcher.DeleteVPCCommand{Name: name}
 	if err := s.dispatcher.Prepare(cmd); err != nil {
-		w.WriteHeader(http.StatusNotFound)
+		if _, dbErr := kv.GetFromDB(s.db, "vpc/"+name+"/state"); dbErr != nil {
+			w.WriteHeader(http.StatusNotFound)
+		} else {
+			w.WriteHeader(http.StatusConflict)
+		}
 		json.NewEncoder(w).Encode(ErrorResponse{Error: err.Error()})
 		return
 	}
diff --git a/internal/api/agent/vpc_test.go b/internal/api/agent/vpc_test.go
index 0edcd5b..01308a5 100644
--- a/internal/api/agent/vpc_test.go
+++ b/internal/api/agent/vpc_test.go
@@ -171,8 +171,8 @@ func TestDeleteVpc_BlockedByActiveSubnet(t *testing.T) {
 	req := httptest.NewRequest(http.MethodDelete, "/vpcs/vpc-busy", nil)
 	w := httptest.NewRecorder()
 	s.VpcByNameHandler(w, req)
-	if w.Code != http.StatusNotFound {
-		t.Errorf("attendu 404 (Prepare échoue), obtenu %d: %s", w.Code, w.Body.String())
+	if w.Code != http.StatusConflict {
+		t.Errorf("attendu 409, obtenu %d: %s", w.Code, w.Body.String())
 	}
 }