serve UI HTML for wildcard or missing Accept header

bgentry · bgentry · commit 70b986d1fe63 · 2026-01-22T20:20:53.000-06:00
Fixes #485.
diff --git a/CHANGELOG.md b/CHANGELOG.md
@@ -10,6 +10,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Fixed
 
 - Prevent double slash in URLs for root path prefix. Thanks [Jan Kott](https://github.com/boostvolt)! [PR #487](https://github.com/riverqueue/riverui/pull/487)
+- Serve UI HTML for wildcard or missing Accept headers and return 406 for explicit non-HTML requests. Fixes #485. [PR #XXX](https://github.com/riverqueue/riverui/pull/XXX).
 
 ## [v0.14.0] - 2026-01-02
 
diff --git a/spa_response_writer.go b/spa_response_writer.go
@@ -6,7 +6,9 @@ import (
 	"fmt"
 	"html/template"
 	"io"
+	"mime"
 	"net/http"
+	"strconv"
 	"strings"
 )
 
@@ -30,9 +32,9 @@ func intercept404(handler, on404 http.Handler) http.Handler {
 func serveIndexHTML(devMode bool, manifest map[string]any, pathPrefix string, files http.FileSystem) http.HandlerFunc {
 	return func(rw http.ResponseWriter, req *http.Request) {
 		// Restrict only to instances where the browser is looking for an HTML file
-		if !strings.Contains(req.Header.Get("Accept"), "text/html") {
-			rw.WriteHeader(http.StatusNotFound)
-			fmt.Fprint(rw, "404 not found")
+		if !acceptsHTML(req) {
+			rw.WriteHeader(http.StatusNotAcceptable)
+			fmt.Fprint(rw, "not acceptable: only text/html is available")
 
 			return
 		}
@@ -94,6 +96,46 @@ func serveIndexHTML(devMode bool, manifest map[string]any, pathPrefix string, fi
 	}
 }
 
+func acceptsHTML(req *http.Request) bool {
+	accept := strings.TrimSpace(req.Header.Get("Accept"))
+	if accept == "" {
+		return true
+	}
+
+	for part := range strings.SplitSeq(accept, ",") {
+		part = strings.TrimSpace(part)
+		if part == "" {
+			continue
+		}
+
+		mediaType, params, err := mime.ParseMediaType(part)
+		if err != nil {
+			mediaType = strings.TrimSpace(strings.SplitN(part, ";", 2)[0])
+			params = nil
+		}
+
+		quality := 1.0
+		if params != nil {
+			if qRaw, ok := params["q"]; ok {
+				if parsed, err := strconv.ParseFloat(qRaw, 64); err == nil {
+					quality = parsed
+				}
+			}
+		}
+
+		if quality <= 0 {
+			continue
+		}
+
+		switch mediaType {
+		case "text/html", "text/*", "*/*":
+			return true
+		}
+	}
+
+	return false
+}
+
 type spaResponseWriter struct {
 	http.ResponseWriter
 
diff --git a/spa_response_writer_test.go b/spa_response_writer_test.go
@@ -0,0 +1,73 @@
+package riverui
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"testing"
+	"testing/fstest"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestServeIndexHTMLAcceptNegotiation(t *testing.T) {
+	t.Parallel()
+
+	files := fstest.MapFS{
+		"index.html": &fstest.MapFile{Data: []byte("<html>ok</html>")},
+	}
+
+	handler := serveIndexHTML(false, map[string]any{}, "/riverui", http.FS(files))
+
+	tests := []struct {
+		name         string
+		acceptHeader string
+		setAccept    bool
+		wantStatus   int
+	}{
+		{
+			name:         "AcceptHTML",
+			acceptHeader: "text/html",
+			setAccept:    true,
+			wantStatus:   http.StatusOK,
+		},
+		{
+			name:         "AcceptWildcard",
+			acceptHeader: "*/*",
+			setAccept:    true,
+			wantStatus:   http.StatusOK,
+		},
+		{
+			name:         "AcceptTextWildcard",
+			acceptHeader: "text/*",
+			setAccept:    true,
+			wantStatus:   http.StatusOK,
+		},
+		{
+			name:       "AcceptMissing",
+			setAccept:  false,
+			wantStatus: http.StatusOK,
+		},
+		{
+			name:         "AcceptJSON",
+			acceptHeader: "application/json",
+			setAccept:    true,
+			wantStatus:   http.StatusNotAcceptable,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			req := httptest.NewRequest(http.MethodGet, "/", nil)
+			if tt.setAccept {
+				req.Header.Set("Accept", tt.acceptHeader)
+			}
+
+			recorder := httptest.NewRecorder()
+			handler.ServeHTTP(recorder, req)
+
+			require.Equal(t, tt.wantStatus, recorder.Result().StatusCode)
+		})
+	}
+}
