diff --git a/server/src/main/java/io/kroki/server/error/ErrorHandler.java b/server/src/main/java/io/kroki/server/error/ErrorHandler.java
index 53d964f4f..f34e64cd6 100644
--- a/server/src/main/java/io/kroki/server/error/ErrorHandler.java
+++ b/server/src/main/java/io/kroki/server/error/ErrorHandler.java
@@ -95,7 +95,9 @@ public void handle(RoutingContext context) {
     if (errorCode == 404) {
       statusMessage = "Not Found";
       errorMessage = statusMessage;
-    } else if (failure instanceof BadRequestException || failure instanceof IllegalStateException) {
+    } else if (failure instanceof BadRequestException
+            || failure instanceof IllegalStateException
+            || failure instanceof MethodNotAllowedException) {
       if (failure instanceof BadRequestException) {
         int statusCode = ((BadRequestException) failure).getStatusCode();
         if (statusCode != -1) {
@@ -108,6 +110,9 @@ public void handle(RoutingContext context) {
       } else {
         statusMessage = statusMessageMap.getOrDefault(errorCode, "Bad Request");
       }
+      if (failure instanceof MethodNotAllowedException) {
+        context.response().putHeader("Allow", ((MethodNotAllowedException) failure).getAllowMethods());
+      }
       errorMessage = failure.getMessage();
       if (failure instanceof BadRequestException) {
         htmlErrorMessage = ((BadRequestException) failure).getMessageHTML();
diff --git a/server/src/main/java/io/kroki/server/error/MethodNotAllowedException.java b/server/src/main/java/io/kroki/server/error/MethodNotAllowedException.java
new file mode 100644
index 000000000..3af03c541
--- /dev/null
+++ b/server/src/main/java/io/kroki/server/error/MethodNotAllowedException.java
@@ -0,0 +1,22 @@
+package io.kroki.server.error;
+
+import java.util.List;
+
+public class MethodNotAllowedException extends RuntimeException {
+
+    private final List<String> allowMethods;
+
+    public MethodNotAllowedException(List<String> allowMethods) {
+        super();
+        this.allowMethods = allowMethods;
+    }
+
+    @Override
+    public String getMessage() {
+        return "Method not allowed. Please use a " + String.join(" or ", this.allowMethods) + " request.";
+    }
+
+    public List<String> getAllowMethods() {
+        return allowMethods;
+    }
+}
diff --git a/server/src/main/java/io/kroki/server/service/DiagramRegistry.java b/server/src/main/java/io/kroki/server/service/DiagramRegistry.java
index 5f4afe7ff..cb70dab4e 100644
--- a/server/src/main/java/io/kroki/server/service/DiagramRegistry.java
+++ b/server/src/main/java/io/kroki/server/service/DiagramRegistry.java
@@ -1,17 +1,21 @@
 package io.kroki.server.service;
 
+import io.kroki.server.error.MethodNotAllowedException;
+import io.kroki.server.error.UnsupportedFormatException;
 import io.kroki.server.response.Caching;
+import io.vertx.core.http.HttpMethod;
 import io.vertx.ext.web.Router;
 import io.vertx.ext.web.handler.BodyHandler;
 
 import java.util.HashMap;
+import java.util.List;
 import java.util.Map;
 import java.util.Set;
 import java.util.stream.Collectors;
 
 public class DiagramRegistry {
 
-  private Map<String, DiagramHandler> registry = new HashMap<>();
+  private final Map<String, DiagramHandler> registry = new HashMap<>();
   private final Router router;
   private final BodyHandler bodyHandler;
 
@@ -27,10 +31,33 @@ public void register(DiagramService diagramService, String... names) {
       router.get("/" + name + "/:output_format/:source_encoded")
         .handler(diagramHandler.createRequestReceived(name))
         .handler(diagramHandler.createGet(name));
+      router.route("/" + name)
+        .handler(event -> {
+          if (HttpMethod.POST.equals(event.request().method())) {
+            event.next();
+            return;
+          }
+          event.fail(405, new MethodNotAllowedException(List.of("POST")));
+        });
       router.post("/" + name)
         .handler(bodyHandler)
         .handler(diagramHandler.createRequestReceived(name))
         .handler(diagramHandler.createPost(name));
+      router.route("/" + name + "/:output_format")
+        .handler(event -> {
+          if (HttpMethod.POST.equals(event.request().method())) {
+            event.next();
+            return;
+          }
+          String outputFormat = event.pathParam("output_format");
+          try {
+            diagramHandler.validate(name, outputFormat);
+          } catch (UnsupportedFormatException e) {
+            event.fail(e);
+            return;
+          }
+          event.fail(405, new MethodNotAllowedException(List.of("POST")));
+        });
       router.post("/" + name + "/:output_format")
         .handler(bodyHandler)
         .handler(diagramHandler.createRequestReceived(name))