Fix trailing slash dropped when joining route segments

.chronus/changes/copilot-fix-regression-http-1-10-1-2026-2-18-18-33-49.md

@@ -0,0 +1,7 @@
+---
+changeKind: fix
+packages:
+  - "@typespec/http"
+---
+
+Fix route joining to preserve trailing `/` when it would not result in `//`

packages/http/src/route.ts

@@ -36,22 +36,25 @@ function needsSlashPrefix(fragment: string) {
   );
 }
 
-function normalizeFragment(fragment: string, trimLast = false) {
+function normalizeFragment(fragment: string) {
   if (needsSlashPrefix(fragment)) {
     // Insert the default separator
     fragment = `/${fragment}`;
   }
 
-  if (trimLast && fragment[fragment.length - 1] === "/") {
-    return fragment.slice(0, -1);
-  }
   return fragment;
 }
 
 export function joinPathSegments(rest: string[]) {
   let current = "";
-  for (const [index, segment] of rest.entries()) {
-    current += normalizeFragment(segment, index < rest.length - 1);
+  for (const segment of rest) {
+    const normalized = normalizeFragment(segment);
+    // Merge trailing and leading slashes to avoid double slashes
+    if (current.endsWith("/") && normalized.startsWith("/")) {
+      current += normalized.slice(1);
+    } else {
+      current += normalized;
+    }
   }
   return current;
 }

packages/http/test/routes.test.ts

@@ -346,6 +346,25 @@ describe("http: routes", () => {
 
       deepStrictEqual(routes, [{ verb: "get", path: `/abc${sep}restype=container`, params: [] }]);
     });
+
+    it("keeps trailing / on parent route when joining with child route", async () => {
+      const routes = await getRoutesFor(
+        `
+      @route("{blobName}/")
+      interface Container {
+        @put @route("${sep}some-query=true") op foo(blobName: string): void;
+      }
+      `,
+      );
+
+      deepStrictEqual(routes, [
+        {
+          verb: "put",
+          path: `/{blobName}/${sep}some-query=true`,
+          params: ["blobName"],
+        },
+      ]);
+    });
   });
 
   describe("joinPathSegments", () => {