struct alternates_request {
        struct walker *walker;
        const char *base;
-       char *url;
+       struct strbuf *url;
        struct strbuf *buffer;
        struct active_request_slot *slot;
        int http_specific;
 
                        /* Try reusing the slot to get non-http alternates */
                        alt_req->http_specific = 0;
-                       sprintf(alt_req->url, "%s/objects/info/alternates",
-                               base);
+                       strbuf_reset(alt_req->url);
+                       strbuf_addf(alt_req->url, "%s/objects/info/alternates",
+                                   base);
                        curl_easy_setopt(slot->curl, CURLOPT_URL,
-                                        alt_req->url);
+                                        alt_req->url->buf);
                        active_requests++;
                        slot->in_use = 1;
                        if (slot->finished != NULL)
 static void fetch_alternates(struct walker *walker, const char *base)
 {
        struct strbuf buffer = STRBUF_INIT;
-       char *url;
+       struct strbuf url = STRBUF_INIT;
        struct active_request_slot *slot;
        struct alternates_request alt_req;
        struct walker_data *cdata = walker->data;
        if (walker->get_verbosely)
                fprintf(stderr, "Getting alternates list for %s\n", base);
 
-       url = xstrfmt("%s/objects/info/http-alternates", base);
+       strbuf_addf(&url, "%s/objects/info/http-alternates", base);
 
        /*
         * Use a callback to process the result, since another request
 
        curl_easy_setopt(slot->curl, CURLOPT_FILE, &buffer);
        curl_easy_setopt(slot->curl, CURLOPT_WRITEFUNCTION, fwrite_buffer);
-       curl_easy_setopt(slot->curl, CURLOPT_URL, url);
+       curl_easy_setopt(slot->curl, CURLOPT_URL, url.buf);
 
        alt_req.base = base;
-       alt_req.url = url;
+       alt_req.url = &url;
        alt_req.buffer = &buffer;
        alt_req.http_specific = 1;
        alt_req.slot = slot;
                cdata->got_alternates = -1;
 
        strbuf_release(&buffer);
-       free(url);
+       strbuf_release(&url);
 }
 
 static int fetch_indices(struct walker *walker, struct alt_base *repo)