diff --git a/java/src/main/java/org/softwareheritage/graph/server/App.java b/java/src/main/java/org/softwareheritage/graph/server/App.java
--- a/java/src/main/java/org/softwareheritage/graph/server/App.java
+++ b/java/src/main/java/org/softwareheritage/graph/server/App.java
@@ -77,6 +77,38 @@
 
         Javalin app = Javalin.create().start(port);
 
+        // default endpoints when swhid missing
+
+        app.get("/leaves/", ctx -> {
+            ctx.status(400);
+            ctx.json("src (SWHID) argument missing");
+        });
+
+        app.get("/neighbors/", ctx -> {
+            ctx.status(400);
+            ctx.json("src (SWHID) argument missing");
+        });
+
+        app.get("/walk/", ctx -> {
+            ctx.status(400);
+            ctx.json("src (SWHID) and dst (SWHID or node type) arguments missing");
+        });
+
+        app.get("/walk/:dst", ctx -> {
+            ctx.status(400);
+            ctx.json("dst (SWHID or node type) argument missing");
+        });
+
+        app.get("/visit/nodes/", ctx -> {
+            ctx.status(400);
+            ctx.json("src (SWHID) argument missing");
+        });
+
+        app.get("/visit/paths/", ctx -> {
+            ctx.status(400);
+            ctx.json("src (SWHID) argument missing");
+        });
+
         app.before("/stats/*", ctx -> {
             checkQueryStrings(ctx, "");
         });