From d02f939ccf8f53582be227232ff78df2c8860929 Mon Sep 17 00:00:00 2001
From: Team Humaki <team@humaki.com>
Date: Sun, 20 Sep 2026 07:47:41 -0700
Subject: [PATCH] group: join prefix and path with a slash

Fixes #3099. Group.GET("posts") under "/v1" concatenated to /v1posts.
Insert a slash when neither side has one; keep Static("") /prefix* wildcards.
---
 group_test.go | 15 +++++++++++++++
 route.go      | 18 +++++++++++++++++-
 route_test.go | 21 +++++++++++++++++++++
 3 files changed, 53 insertions(+), 1 deletion(-)

diff --git a/group_test.go b/group_test.go
index 56b2a465f..bc59fd433 100644
--- a/group_test.go
+++ b/group_test.go
@@ -94,6 +94,21 @@ func TestGroup_multiLevelGroup(t *testing.T) {
 	assert.Equal(t, `OK`, body)
 }
 
+func TestGroup_pathWithoutLeadingSlash(t *testing.T) {
+	e := New()
+	g := e.Group("/v1")
+	g.GET("posts", func(c *Context) error {
+		return c.String(http.StatusOK, "ok")
+	})
+
+	status, body := request(http.MethodGet, "/v1/posts", e)
+	assert.Equal(t, http.StatusOK, status)
+	assert.Equal(t, "ok", body)
+
+	status, _ = request(http.MethodGet, "/v1posts", e)
+	assert.Equal(t, http.StatusNotFound, status)
+}
+
 func TestGroupFile(t *testing.T) {
 	e := New()
 	g := e.Group("/group")
diff --git a/route.go b/route.go
index 2468a8816..a38899303 100644
--- a/route.go
+++ b/route.go
@@ -43,9 +43,25 @@ func (r Route) ToRouteInfo(params []string) RouteInfo {
 	}
 }
 
+// joinPathPrefix concatenates a group prefix and a route path.
+// A missing slash between "/v1" and "posts" used to produce "/v1posts".
+// Wildcard suffixes (`*` / `/*`) stay concatenated so Group.Static("") keeps `/prefix*`.
+func joinPathPrefix(prefix, path string) string {
+	if prefix == "" {
+		return path
+	}
+	if path == "" {
+		return prefix
+	}
+	if prefix[len(prefix)-1] != '/' && path[0] != '/' && path[0] != '*' {
+		return prefix + "/" + path
+	}
+	return prefix + path
+}
+
 // WithPrefix recreates Route with added group prefix and group middlewares it is grouped to.
 func (r Route) WithPrefix(pathPrefix string, middlewares []MiddlewareFunc) Route {
-	r.Path = pathPrefix + r.Path
+	r.Path = joinPathPrefix(pathPrefix, r.Path)
 
 	if len(middlewares) > 0 {
 		m := make([]MiddlewareFunc, 0, len(middlewares)+len(r.Middlewares))
diff --git a/route_test.go b/route_test.go
index 8e2f170ee..ba32f8ff3 100644
--- a/route_test.go
+++ b/route_test.go
@@ -145,6 +145,27 @@ func TestRoute_ForGroup(t *testing.T) {
 	assert.Equal(t, r.Name, "test route")
 }
 
+func TestJoinPathPrefix(t *testing.T) {
+	tests := []struct {
+		prefix string
+		path   string
+		want   string
+	}{
+		{prefix: "/v1", path: "/posts", want: "/v1/posts"},
+		{prefix: "/v1", path: "posts", want: "/v1/posts"},
+		{prefix: "/v1/", path: "posts", want: "/v1/posts"},
+		{prefix: "/v1/", path: "/posts", want: "/v1//posts"},
+		{prefix: "/users", path: "/test", want: "/users/test"},
+		{prefix: "/users", path: ":id", want: "/users/:id"},
+		{prefix: "/test", path: "*", want: "/test*"},
+		{prefix: "", path: "posts", want: "posts"},
+		{prefix: "/v1", path: "", want: "/v1"},
+	}
+	for _, tt := range tests {
+		assert.Equal(t, tt.want, joinPathPrefix(tt.prefix, tt.path), "prefix=%q path=%q", tt.prefix, tt.path)
+	}
+}
+
 func exampleRoutes() Routes {
 	return Routes{
 		RouteInfo{