diff --git a/options.go b/options.go
index 5f67acf..6b576c6 100644
--- a/options.go
+++ b/options.go
@@ -4,6 +4,7 @@ import (
 	"fmt"
 	"net/http"
 	"net/url"
+	"strings"
 	"time"
 )
 
@@ -11,6 +12,10 @@ import (
 type Option func(*Client)
 
 // WithBaseURL overrides the API base URL (default https://api.flashcat.cloud).
+// A path prefix is preserved: with "https://gateway.example.com/api", requests
+// go to /api/<endpoint>. The path is normalized to end with "/" so that
+// relative resolution appends endpoint paths instead of replacing the last
+// path segment.
 func WithBaseURL(raw string) Option {
 	parsed, err := url.Parse(raw)
 	return func(c *Client) {
@@ -18,6 +23,9 @@ func WithBaseURL(raw string) Option {
 			c.optionErr = fmt.Errorf("flashduty: invalid base URL %q: %w", raw, err)
 			return
 		}
+		if !strings.HasSuffix(parsed.Path, "/") {
+			parsed.Path += "/"
+		}
 		c.BaseURL = parsed
 	}
 }
diff --git a/options_test.go b/options_test.go
index 12cdab4..1c3af01 100644
--- a/options_test.go
+++ b/options_test.go
@@ -18,7 +18,7 @@ func TestNewClientDefaultsAndOptions(t *testing.T) {
 	if err != nil {
 		t.Fatal(err)
 	}
-	if c.BaseURL.String() != "https://example.test" {
+	if c.BaseURL.String() != "https://example.test/" {
 		t.Fatalf("BaseURL = %s", c.BaseURL)
 	}
 	if c.UserAgent != "ua/1" {
@@ -35,6 +35,34 @@ func TestWithBaseURLInvalidReturnsError(t *testing.T) {
 	}
 }
 
+func TestWithBaseURLPreservesPathPrefix(t *testing.T) {
+	for _, tc := range []struct {
+		base string
+		want string
+	}{
+		{"https://example.test", "https://example.test/rum/data/query"},
+		{"https://example.test/", "https://example.test/rum/data/query"},
+		{"https://example.test/api", "https://example.test/api/rum/data/query"},
+		{"https://example.test/api/", "https://example.test/api/rum/data/query"},
+		{"https://example.test/a/b", "https://example.test/a/b/rum/data/query"},
+		{"http://192.0.2.10:12345", "http://192.0.2.10:12345/rum/data/query"},
+		{"http://192.0.2.10:12345/api", "http://192.0.2.10:12345/api/rum/data/query"},
+	} {
+		c, err := NewClient("KEY", WithBaseURL(tc.base))
+		if err != nil {
+			t.Fatalf("NewClient(%q): %v", tc.base, err)
+		}
+		req, err := c.newRequest(t.Context(), http.MethodPost, "/rum/data/query", nil)
+		if err != nil {
+			t.Fatalf("newRequest with base %q: %v", tc.base, err)
+		}
+		req.URL.RawQuery = ""
+		if got := req.URL.String(); got != tc.want {
+			t.Errorf("base %q: request URL = %q, want %q", tc.base, got, tc.want)
+		}
+	}
+}
+
 func TestWithHTTPClientNilIgnored(t *testing.T) {
 	c, err := NewClient("KEY", WithHTTPClient(nil))
 	if err != nil || c.client == nil {