fix: trailing slash mismatch causes 404 for static routes (#3721)

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: bartlomieju

packages/fresh/src/router.ts

@@ -116,9 +116,26 @@ export class UrlPatternRouter<T> implements Router<T> {
       pattern: null,
     };
 
-    const staticMatch = this.#statics.get(url.pathname);
+    let pathname = url.pathname;
+    let staticMatch = this.#statics.get(pathname);
+
+    // Try alternate trailing slash form if no exact match found.
+    // Routes may be registered with or without trailing slashes,
+    // and requests may arrive in either form (e.g. when using
+    // trailingSlashes("always")).
+    if (staticMatch === undefined && pathname !== "/") {
+      const alt = pathname.endsWith("/")
+        ? pathname.slice(0, -1)
+        : pathname + "/";
+      const altMatch = this.#statics.get(alt);
+      if (altMatch !== undefined) {
+        staticMatch = altMatch;
+        pathname = alt;
+      }
+    }
+
     if (staticMatch !== undefined) {
-      result.pattern = url.pathname;
+      result.pattern = pathname;
 
       let handlers = staticMatch.byMethod[method];
       if (method === "HEAD" && handlers.length === 0) {

packages/fresh/src/router_test.ts

@@ -70,6 +70,78 @@ Deno.test("UrlPatternRouter - wrong + correct method", () => {
   });
 });
 
+Deno.test("UrlPatternRouter - trailing slash matches route without slash", () => {
+  const router = new UrlPatternRouter();
+  const A = () => {};
+  router.add("GET", "/wissen", [A]);
+
+  const res = router.match("GET", new URL("/wissen/", "http://localhost"));
+  expect(res).toEqual({
+    params: Object.create(null),
+    handlers: [A],
+    methodMatch: true,
+    pattern: "/wissen",
+  });
+});
+
+Deno.test("UrlPatternRouter - no trailing slash matches route with slash", () => {
+  const router = new UrlPatternRouter();
+  const A = () => {};
+  router.add("GET", "/wissen/", [A]);
+
+  const res = router.match("GET", new URL("/wissen", "http://localhost"));
+  expect(res).toEqual({
+    params: Object.create(null),
+    handlers: [A],
+    methodMatch: true,
+    pattern: "/wissen/",
+  });
+});
+
+Deno.test("UrlPatternRouter - exact match takes priority over trailing slash fallback", () => {
+  const router = new UrlPatternRouter();
+  const A = () => {};
+  const B = () => {};
+  router.add("GET", "/wissen", [A]);
+  router.add("GET", "/wissen/", [B]);
+
+  const withSlash = router.match(
+    "GET",
+    new URL("/wissen/", "http://localhost"),
+  );
+  expect(withSlash).toEqual({
+    params: Object.create(null),
+    handlers: [B],
+    methodMatch: true,
+    pattern: "/wissen/",
+  });
+
+  const withoutSlash = router.match(
+    "GET",
+    new URL("/wissen", "http://localhost"),
+  );
+  expect(withoutSlash).toEqual({
+    params: Object.create(null),
+    handlers: [A],
+    methodMatch: true,
+    pattern: "/wissen",
+  });
+});
+
+Deno.test("UrlPatternRouter - root trailing slash does not double-match", () => {
+  const router = new UrlPatternRouter();
+  const A = () => {};
+  router.add("GET", "/", [A]);
+
+  const res = router.match("GET", new URL("/", "http://localhost"));
+  expect(res).toEqual({
+    params: Object.create(null),
+    handlers: [A],
+    methodMatch: true,
+    pattern: "/",
+  });
+});
+
 Deno.test("UrlPatternRouter - convert patterns automatically", () => {
   const router = new UrlPatternRouter();
   const A = () => {};