diff --git a/packages/runtime/src/scr_fetch.c b/packages/runtime/src/scr_fetch.c
index fe1b92c0f..a7b540adb 100644
--- a/packages/runtime/src/scr_fetch.c
+++ b/packages/runtime/src/scr_fetch.c
@@ -10,6 +10,7 @@
   6665, 6666, 6667, 6668, 6669, 6679, 6697, 10080
 
 #include "scr_runtime.h"
+#include "scr_system_errors.h"
 #include "scr_url_internal.h"
 
 #include <math.h>
@@ -2888,14 +2889,21 @@ static int sf_hex_value(char c) {
   return -1;
 }
 
+/* EnvHttpProxyAgent sends Basic proxy authentication only when the proxy
+ * URL has both a username and a password. */
+static bool sf_proxy_has_credentials(const ScrStr *userinfo) {
+  const char *colon = memchr(userinfo->data, ':', userinfo->len);
+  return colon != NULL && colon != userinfo->data &&
+         colon + 1 < userinfo->data + userinfo->len;
+}
+
 /* EnvHttpProxyAgent derives Basic proxy authentication from the proxy URL's
  * decoded username/password. ScrUrl keeps the encoded userinfo as one string,
- * so decode it here and add the empty-password separator when necessary. */
+ * so decode it here. */
 static ScrStr *sf_proxy_authorization(const ScrStr *userinfo) {
   char *plain = malloc(userinfo->len + 1);
   if (!plain) sf_oom();
   size_t plain_len = 0;
-  bool has_separator = false;
   for (size_t i = 0; i < userinfo->len;) {
     char c = userinfo->data[i];
     if (c == '%' && i + 2 < userinfo->len) {
@@ -2907,11 +2915,9 @@ static ScrStr *sf_proxy_authorization(const ScrStr *userinfo) {
         continue;
       }
     }
-    if (c == ':') has_separator = true;
     plain[plain_len++] = c;
     i++;
   }
-  if (!has_separator) plain[plain_len++] = ':';
 
   static const char b64[] =
       "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
@@ -4058,15 +4064,154 @@ static void sf_on_response(ScrClosure *cb, ScrHttpReq *res) {
   sf_release(t);
 }
 
+typedef struct { int value; const char *name; } SfUvError;
+#define SF_UV_ERROR(name, message) { SCR_NODE_UV__##name, #name },
+static const SfUvError sf_uv_errors[] = { SCR_NODE_UV_ERRNO_MAP(SF_UV_ERROR) };
+#undef SF_UV_ERROR
+
+/* libuv's errno value for a code name; getaddrinfo's ENOTFOUND is
+ * EAI_NONAME. 0 when the name is not a libuv code. */
+static int sf_uv_errno(const char *name) {
+  if (strcmp(name, "ENOTFOUND") == 0) return SCR_NODE_UV__EAI_NONAME;
+  for (size_t i = 0; i < sizeof sf_uv_errors / sizeof sf_uv_errors[0]; i++) {
+    if (strcmp(sf_uv_errors[i].name, name) == 0) return sf_uv_errors[i].value;
+  }
+  return 0;
+}
+
+static const char sf_tunnel_prefix[] = "Proxy response (";
+static const char sf_not_http_prefix[] = "Response does not match the HTTP/1.1 protocol";
+
+/* The cause Node's fetch gives a transport failure, built from the
+ * runtime's failure message. "<syscall> <CODE>[ <detail>]" messages
+ * become Node's system error with code, errno and syscall (plus address
+ * and port for connect, hostname for getaddrinfo). A tunnel the proxy
+ * refused becomes undici's DOMException "Request was cancelled." whose
+ * cause is the AbortError carrying the proxy status, and a proxy reply
+ * that is not HTTP becomes the HTTPParserError. NULL (no cause) for
+ * every other message. +1. */
+static ScrDyn *sf_failure_cause(const ScrStr *message) {
+  if (message->len > sizeof sf_tunnel_prefix - 1 &&
+      memcmp(message->data, sf_tunnel_prefix, sizeof sf_tunnel_prefix - 1) == 0) {
+    ScrError *aborted = scr_error_new(SCR_ERR_ERROR, (ScrStr *)message);
+    scr_str_release(aborted->name);
+    aborted->name = scr_str_new("AbortError", 10);
+    aborted->name_present = true;
+    scr_error_set_code(aborted, "UND_ERR_ABORTED");
+    ScrDyn *inner = scr_dyn_from_error(aborted);
+    scr_error_release(aborted);
+    ScrDyn *options = scr_dyn_new_obj();
+    ScrStr *name = scr_str_new("Error", 5);
+    scr_dyn_obj_set(options, "name", 4, scr_dyn_new_str(name));
+    scr_str_release(name);
+    scr_dyn_obj_set(options, "cause", 5, inner);
+    ScrStr *text = scr_str_new("Request was cancelled.", 22);
+    ScrDyn *m = scr_dyn_new_str(text);
+    scr_str_release(text);
+    ScrError *dom = scr_domex_new(m, options);
+    scr_dyn_release(m);
+    scr_dyn_release(options);
+    ScrDyn *cause = scr_dyn_from_error(dom);
+    scr_error_release(dom);
+    return cause;
+  }
+  if (message->len >= sizeof sf_not_http_prefix - 1 &&
+      memcmp(message->data, sf_not_http_prefix, sizeof sf_not_http_prefix - 1) == 0) {
+    ScrError *parser = scr_error_new(SCR_ERR_ERROR, (ScrStr *)message);
+    scr_str_release(parser->name);
+    parser->name = scr_str_new("HTTPParserError", 15);
+    parser->name_present = true;
+    ScrDyn *cause = scr_dyn_from_error(parser);
+    scr_error_release(parser);
+    return cause;
+  }
+  const char *data = message->data;
+  const char *space = memchr(data, ' ', message->len);
+  if (space == NULL || space == data) return NULL;
+  size_t syscall_len = (size_t)(space - data);
+  const char *code = space + 1;
+  const char *end = data + message->len;
+  const char *code_end = code;
+  while (code_end < end && *code_end != ' ') code_end++;
+  char code_buf[32];
+  size_t code_len = (size_t)(code_end - code);
+  if (code_len < 2 || code_len >= sizeof code_buf || code[0] != 'E') return NULL;
+  for (size_t i = 0; i < code_len; i++) {
+    char c = code[i];
+    if (!((c >= 'A' && c <= 'Z') || (c >= '0' && c <= '9') || c == '_')) return NULL;
+  }
+  memcpy(code_buf, code, code_len);
+  code_buf[code_len] = '\0';
+  int uv = sf_uv_errno(code_buf);
+  if (uv == 0) return NULL;
+  ScrError *e = scr_error_new(SCR_ERR_ERROR, (ScrStr *)message);
+  scr_error_set_code(e, code_buf);
+  e->system_errno = uv;
+  e->system_call = scr_str_new(data, syscall_len);
+  ScrDyn *cause = scr_dyn_from_error(e);
+  scr_error_release(e);
+  const char *detail = code_end < end ? code_end + 1 : end;
+  size_t detail_len = (size_t)(end - detail);
+  if (syscall_len == 7 && memcmp(data, "connect", 7) == 0 && detail_len > 0) {
+    const char *colon = NULL;
+    for (const char *q = detail; q < end; q++) {
+      if (*q == ':') colon = q;
+    }
+    if (colon != NULL && colon > detail && colon + 1 < end) {
+      ScrStr *address = scr_str_new(detail, (size_t)(colon - detail));
+      scr_dyn_obj_set(cause, "address", 7, scr_dyn_new_str(address));
+      scr_str_release(address);
+      scr_dyn_obj_set(cause, "port", 4, scr_dyn_new_num(strtod(colon + 1, NULL)));
+    }
+  } else if (syscall_len == 11 && memcmp(data, "getaddrinfo", 11) == 0 && detail_len > 0) {
+    ScrStr *hostname = scr_str_new(detail, detail_len);
+    scr_dyn_obj_set(cause, "hostname", 8, scr_dyn_new_str(hostname));
+    scr_str_release(hostname);
+  }
+  return cause;
+}
+
+/* A plain Error cause. +1. */
+static ScrDyn *sf_plain_cause(const char *message) {
+  ScrStr *text = scr_str_new(message, strlen(message));
+  ScrError *e = scr_error_new(SCR_ERR_ERROR, text);
+  scr_str_release(text);
+  ScrDyn *cause = scr_dyn_from_error(e);
+  scr_error_release(e);
+  return cause;
+}
+
+/* Rejects with TypeError "fetch failed" whose cause is `cause` (moves;
+ * NULL rejects without a cause). */
+static void sf_reject_cause(SfTransfer *t, ScrDyn *cause) {
+  if (t->done || cause == NULL) {
+    scr_dyn_release(cause);
+    sf_reject(t, "fetch failed");
+    return;
+  }
+  ScrError *e = NULL;
+  ScrDyn *reason = sf_type_reason("fetch failed", &e);
+  scr_error_set_cause(e, cause);
+  scr_dyn_release(cause);
+  scr_error_release(e);
+  sf_reject_reason(t, reason);
+  scr_dyn_release(reason);
+}
+
+/* Rejects with TypeError "fetch failed", with Node's cause when the
+ * failure message has one. */
+static void sf_reject_failure(SfTransfer *t, const ScrStr *message) {
+  sf_reject_cause(t, message != NULL ? sf_failure_cause(message) : NULL);
+}
+
 static void sf_on_client_error(ScrClosure *cb, ScrStr *message) {
-  (void)message;
   SfTransfer *t = sf_from(cb);
   if (!t) return;
   if (!t->done) {
     if (t->response_sent && t->response_stream) {
       sf_response_terminate(t);
     } else {
-      sf_reject(t, "fetch failed");
+      sf_reject_failure(t, message);
     }
   }
   sf_release(t);
@@ -4150,25 +4295,77 @@ static bool sf_redirect(SfTransfer *t, int status,
   return true;
 }
 
+/* A CONNECT tunnel for one request: the head sent to the proxy, and the
+ * target's TLS client context when the target is https. */
+typedef struct {
+  char *head;
+  size_t head_len;
+  void *tls_ctx;
+} SfTunnel;
+
+/* undici's CONNECT head: the target as host:port, its Host value, and
+ * the proxy credentials from the proxy URL. Returns malloc'd storage. */
+static char *sf_tunnel_head(const ScrUrl *url, int port, const char *authority,
+                            size_t authority_len, const ScrUrl *proxy, size_t *len_out) {
+  ScrStr *authorization = sf_proxy_has_credentials(proxy->userinfo)
+                              ? sf_proxy_authorization(proxy->userinfo)
+                              : NULL;
+  size_t cap = url->host->len + authority_len + 160 + (authorization ? authorization->len : 0);
+  char *head = malloc(cap);
+  if (!head) sf_oom();
+  int n = snprintf(head, cap, "CONNECT %.*s:%d HTTP/1.1\r\nhost: %.*s\r\nconnection: close\r\n",
+                   (int)url->host->len, url->host->data, port, (int)authority_len, authority);
+  if (authorization) {
+    n += snprintf(head + n, cap - (size_t)n, "proxy-authorization: %.*s\r\n",
+                  (int)authorization->len, authorization->data);
+    scr_str_release(authorization);
+  }
+  n += snprintf(head + n, cap - (size_t)n, "proxy-connection: keep-alive\r\n\r\n");
+  *len_out = (size_t)n;
+  return head;
+}
+
+static SfTunnel *sf_tunnel_new(const ScrUrl *url, int port, const char *authority,
+                               size_t authority_len, const ScrUrl *proxy, void *tls_ctx) {
+  SfTunnel *tunnel = malloc(sizeof *tunnel);
+  if (!tunnel) sf_oom();
+  tunnel->head = sf_tunnel_head(url, port, authority, authority_len, proxy, &tunnel->head_len);
+  tunnel->tls_ctx = tls_ctx;
+  return tunnel;
+}
+
+/* The client's wrap hook for a tunneled request: the CONNECT exchange
+ * runs on the proxy socket, then the target's TLS session (if any) runs
+ * inside the tunnel. */
+static void sf_tunnel_wrap(ScrNetSocket *sock, void *ctx) {
+  SfTunnel *tunnel = ctx;
+  scr_net_sock_set_tunnel(sock, tunnel->head, tunnel->head_len);
+  if (tunnel->tls_ctx) scr_tls_fetch_client_wrap(sock, tunnel->tls_ctx);
+  free(tunnel->head);
+  free(tunnel);
+}
+
 static bool sf_start_hop(SfTransfer *t) {
   ScrUrl *url = t->url;
   bool https = sf_eq_ci(url->scheme, "https");
   int default_port = https ? 443 : 80;
   int port = sf_port(url, default_port);
   if (sf_bad_port(port)) {
-    sf_reject(t, "fetch failed");
+    sf_reject_cause(t, sf_plain_cause("bad port"));
     return false;
   }
   bool invalid_proxy = false;
   ScrUrl *proxy = sf_proxy_for(url, https, port, &invalid_proxy);
   bool proxy_http = proxy && sf_eq_ci(proxy->scheme, "http");
   bool proxy_https = proxy && sf_eq_ci(proxy->scheme, "https");
-  if (invalid_proxy || (proxy && !proxy_http && !proxy_https) ||
-      (https && proxy)) {
+  if (invalid_proxy || (proxy && !proxy_http && !proxy_https)) {
     scr_url_release(proxy);
     sf_reject(t, "fetch failed");
     return false;
   }
+  /* A proxy carries every request through a CONNECT tunnel, as undici's
+   * ProxyAgent does; an https:// proxy runs the tunnel inside a TLS
+   * session to the proxy. */
   size_t user_len = (size_t)scr_arr_len(t->headers);
   ScrArr *headers = scr_arr_new(SCR_ELEM_STR, user_len + 16);
 
@@ -4214,13 +4411,6 @@ static bool sf_start_hop(SfTransfer *t) {
     scr_arr_push_ref(headers, name);
     scr_arr_push_ref(headers, value);
   }
-  if (proxy && proxy->userinfo->len > 0 &&
-      !sf_pairs_have(t->headers, "proxy-authorization")) {
-    ScrStr *authorization = sf_proxy_authorization(proxy->userinfo);
-    sf_push_header_text(headers, "proxy-authorization",
-                        authorization->data, authorization->len);
-    scr_str_release(authorization);
-  }
   if (!sf_pairs_have(t->headers, "accept")) {
     sf_push_header_text(headers, "accept", "*/*", 3);
   }
@@ -4240,17 +4430,29 @@ static bool sf_start_hop(SfTransfer *t) {
   ScrStr *bare =
       sf_bare_host(proxy ? proxy->host : url->host);
   ScrStr *dial = scr_net_blocking_lookup(bare);
-  ScrStr *path = proxy ? sf_url_serialize(url) : sf_path(url);
+  ScrStr *path = sf_path(url);
   int dial_port =
       proxy ? sf_port(proxy, proxy_https ? 443 : 80) : port;
-  ScrStr *sni =
-      https ? sf_bare_host(url->host)
-            : proxy_https ? sf_bare_host(proxy->host) : NULL;
-  void *tls_ctx =
-      sni ? scr_tls_fetch_client_ctx(sni, true) : NULL;
+  ScrStr *sni = https ? sf_bare_host(url->host) : NULL;
+  void (*wrap)(ScrNetSocket *, void *) = NULL;
+  void *wrap_ctx = NULL;
+  if (proxy_https) {
+    size_t head_len = 0;
+    char *head = sf_tunnel_head(url, port, authority, (size_t)authority_len, proxy, &head_len);
+    wrap_ctx = scr_tls_fetch_tunnel_ctx(bare, sni, head, head_len);
+    wrap = &scr_tls_fetch_tunnel_wrap;
+    free(head);
+  } else if (proxy_http) {
+    void *tls_ctx = sni ? scr_tls_fetch_client_ctx(sni, true) : NULL;
+    wrap_ctx = sf_tunnel_new(url, port, authority, (size_t)authority_len, proxy, tls_ctx);
+    wrap = &sf_tunnel_wrap;
+  } else if (sni) {
+    wrap_ctx = scr_tls_fetch_client_ctx(sni, true);
+    wrap = &scr_tls_fetch_client_wrap;
+  }
   ScrHttpClientReq *client = scr_http_request_ex(
       dial, dial_port, path, t->method, 0, headers, false, NULL, NULL,
-      default_port, sni ? &scr_tls_fetch_client_wrap : NULL, tls_ctx);
+      default_port, wrap, wrap_ctx);
   scr_arr_release(headers);
   scr_str_release(path);
   scr_str_release(dial);
diff --git a/packages/runtime/src/scr_http2.h b/packages/runtime/src/scr_http2.h
index d5b1cae2d..e37e32c08 100644
--- a/packages/runtime/src/scr_http2.h
+++ b/packages/runtime/src/scr_http2.h
@@ -196,6 +196,13 @@ ScrHttpClientReq *scr_https_request_agent(ScrStr *host /*borrowed*/, double port
  * the fresh socket. The ctx moves into the transport at wrap time. */
 void *scr_tls_fetch_client_ctx(ScrStr *host /*borrowed*/, bool reject_unauthorized);
 void scr_tls_fetch_client_wrap(ScrNetSocket *sock, void *ctx);
+/* The fetch tunnel through an https:// proxy: TLS to the proxy, the
+ * CONNECT head (copied) inside it, then TLS to target_host inside the
+ * tunnel (NULL target_host: an http:// target's plaintext). The ctx moves
+ * into the socket's transport at wrap time. */
+void *scr_tls_fetch_tunnel_ctx(ScrStr *proxy_host /*borrowed*/, ScrStr *target_host /*borrowed, nullable*/,
+                               const char *request, size_t len);
+void scr_tls_fetch_tunnel_wrap(ScrNetSocket *sock, void *ctx);
 /* The loop-side registration (scr_async.c, always linked): `pending`
  * joins the exhaustion test (a live handle keeps the process alive, like
  * Node's active TCP handles), `dispatch` runs at every turn top like the
diff --git a/packages/runtime/src/scr_net.c b/packages/runtime/src/scr_net.c
index 64f5dee02..9aba35b96 100644
--- a/packages/runtime/src/scr_net.c
+++ b/packages/runtime/src/scr_net.c
@@ -593,8 +593,28 @@ struct ScrNetSocket {
   bool t_want_write; /* a handshake send hit EAGAIN: arm the write filter */
   bool in_registry;
   struct ScrNetSocket *next;
+  /* an HTTP CONNECT tunnel (scr_net_sock_set_tunnel), set from the dial
+   * until the proxy answers 200: writes buffer and the transport
+   * handshake waits until the tunnel is up */
+  struct ScrNetTunnel *tunnel;
 };
 
+/* The CONNECT head still being sent from out_off, and the proxy's
+ * response head as it arrives. */
+typedef struct ScrNetTunnel {
+  char *out;
+  size_t out_len, out_off;
+  char *in;
+  size_t in_len;
+} ScrNetTunnel;
+
+static void scr_net_tunnel_free(ScrNetTunnel *t) {
+  if (t == NULL) return;
+  free(t->out);
+  free(t->in);
+  free(t);
+}
+
 #ifdef SCR_RC_AUDIT
 static SCR_TL long scr_net_live = 0;
 long scr_net_live_count(void) { return scr_net_live; }
@@ -665,6 +685,7 @@ void scr_net_sock_release(ScrNetSocket *s) {
     free(s->dial_ips);
     scr_str_release(s->dial_err);
     scr_str_release(s->dial_host);
+    scr_net_tunnel_free(s->tunnel);
     free(s->wbuf);
     free(s->rbuf);
     if (s->pipe_dst) scr_net_sock_release(s->pipe_dst);
@@ -910,6 +931,7 @@ static ScrNetSocket *scr_net_sock_new(void) {
 static void scr_net_sock_update_read(ScrNetSocket *s) {
   bool want = s->fd >= 0 && !s->connecting && !s->rd_eof && !s->user_paused &&
               (s->data_ls.n > 0 || s->pipe_dst != NULL || s->native_data != NULL ||
+               s->tunnel != NULL /* the proxy's CONNECT answer */ ||
                s->flowing /* resume(): flow (and discard sans listeners) */ ||
                s->readable_ls.n > 0 /* paused-mode consumer: bytes buffer */ ||
                s->tops != NULL /* a transport is ALWAYS a consumer: the TLS
@@ -935,7 +957,9 @@ static void scr_net_sock_update_read(ScrNetSocket *s) {
 }
 
 static void scr_net_sock_update_write(ScrNetSocket *s) {
-  bool want = s->fd >= 0 && (s->connecting || s->wlen > s->whead || s->t_want_write);
+  bool want = s->fd >= 0 &&
+              (s->connecting || (s->tunnel != NULL ? s->tunnel->out_off < s->tunnel->out_len
+                                              : s->wlen > s->whead || s->t_want_write));
   if (want && !s->write_armed) {
     scr_net_watch_write(s->fd, s, true);
     s->write_armed = true;
@@ -960,7 +984,7 @@ static void scr_net_sock_close_fd(ScrNetSocket *s) {
 /* The FIN half: called when the write half is ENDING and the buffer just
  * drained (or was empty). Full close once the read half is done too. */
 static void scr_net_sock_maybe_finish_write(ScrNetSocket *s) {
-  if (s->fd < 0 || !s->wr_ending || s->wr_done || s->wlen > s->whead) return;
+  if (s->fd < 0 || !s->wr_ending || s->wr_done || s->wlen > s->whead || s->tunnel != NULL) return;
   /* a transport says goodbye first (TLS close_notify), best-effort */
   if (s->tops && s->t_est) s->tops->shutdown_write(s->tctx);
   shutdown(s->fd, SHUT_WR);
@@ -1009,7 +1033,8 @@ static void scr_net_sock_write_raw(ScrNetSocket *s, const char *data, size_t len
   if (!pre_dial && (s->fd < 0 || s->wr_ending || s->close_emitted)) return;
   if (len == 0) return;
   s->bytes_written += len; /* accepted bytes (Node counts buffered ones too) */
-  if (pre_dial || s->connecting || s->wlen > s->whead || (s->tops && !s->t_est)) {
+  if (pre_dial || s->connecting || s->tunnel != NULL || s->wlen > s->whead ||
+      (s->tops && !s->t_est)) {
     /* mid-connect and mid-handshake bytes buffer; the flush after
      * 'connect'/establishment sends them */
     scr_net_sock_buffer(s, data, len);
@@ -1053,7 +1078,7 @@ void scr_net_sock_writev_native(ScrNetSocket *s, const ScrNetSlice *slices, size
   int cnt = 0;
   size_t total = 0;
   bool immediate = n <= 8 && s->fd >= 0 && !s->wr_ending && !s->close_emitted &&
-                   !s->connecting && s->wlen == s->whead && !s->tops;
+                   !s->connecting && s->tunnel == NULL && s->wlen == s->whead && !s->tops;
   if (immediate) {
     for (size_t i = 0; i < n; i++) {
       if (slices[i].len == 0) continue;
@@ -1106,6 +1131,7 @@ void scr_net_sock_writev_native(ScrNetSocket *s, const ScrNetSlice *slices, size
 
 /* Flush on write-readiness; drives the FIN when ENDING. */
 static void scr_net_sock_flush(ScrNetSocket *s) {
+  if (s->tunnel != NULL) return; /* nothing moves until the proxy answers */
   if (s->tops && !s->t_est) return; /* nothing moves until the handshake ends */
   while (s->fd >= 0 && s->wlen > s->whead) {
     ssize_t sent = s->tops ? s->tops->xwrite(s->tctx, s->wbuf + s->whead, s->wlen - s->whead)
@@ -1206,6 +1232,7 @@ static void scr_net_sock_transport_pump(ScrNetSocket *s);
 
 static void scr_net_sock_read(ScrNetSocket *s) {
   size_t reads_left = 32; /* at most 2 MiB with the 64 KiB read buffer */
+  if (s->tunnel != NULL) return; /* readiness feeds the tunnel pump instead */
   if (s->tops && !s->t_est) return; /* readiness feeds the handshake pump instead */
   for (;;) {
     if (s->fd < 0 || s->user_paused) return;
@@ -1298,6 +1325,8 @@ static void scr_net_sock_read(ScrNetSocket *s) {
 static void scr_net_sock_transport_pump(ScrNetSocket *s);
 
 static void scr_net_sock_dial_next(ScrNetSocket *s);
+static void scr_net_sock_connected(ScrNetSocket *s);
+static void scr_net_sock_tunnel_pump(ScrNetSocket *s);
 
 /* Connect completion (write-readiness while connecting). */
 static void scr_net_sock_connect_done(ScrNetSocket *s) {
@@ -1342,6 +1371,19 @@ static void scr_net_sock_connect_done(ScrNetSocket *s) {
     scr_str_release(s->dial_err);
     s->dial_err = NULL;
   }
+  if (s->tunnel != NULL) {
+    /* TCP to the proxy is up: the CONNECT exchange runs first */
+    scr_net_sock_touch(s);
+    scr_net_sock_tunnel_pump(s);
+    return;
+  }
+  scr_net_sock_connected(s);
+}
+
+/* The connection is usable (TCP up, and the proxy tunnel when there is
+ * one): the transport handshake starts, or 'connect' fires and buffered
+ * writes flush. */
+static void scr_net_sock_connected(ScrNetSocket *s) {
   scr_net_sock_update_write(s); /* keeps the filter iff bytes are buffered */
   scr_net_sock_update_read(s);
   scr_net_sock_touch(s);
@@ -1363,6 +1405,106 @@ static void scr_net_sock_connect_done(ScrNetSocket *s) {
   scr_net_sock_flush(s);
 }
 
+/* Fails a tunnel being set up: the message is the socket's 'error'. */
+static void scr_net_sock_tunnel_fail(ScrNetSocket *s, const char *msg) {
+  if (!s->pending_err) s->pending_err = scr_str_new(msg, strlen(msg));
+  s->had_error = true;
+  scr_net_sock_close_fd(s);
+}
+
+int scr_net_tunnel_status(const char *head, size_t len) {
+  if (len >= 12 && memcmp(head, "HTTP/1.", 7) == 0 && head[8] == ' ' &&
+      isdigit((unsigned char)head[9]) && isdigit((unsigned char)head[10]) &&
+      isdigit((unsigned char)head[11])) {
+    return (head[9] - '0') * 100 + (head[10] - '0') * 10 + (head[11] - '0');
+  }
+  return -1;
+}
+
+void scr_net_tunnel_failure(int status, char *msg, size_t cap) {
+  if (status < 0) {
+    /* llhttp's message for a response that does not start with a status line */
+    snprintf(msg, cap, "Response does not match the HTTP/1.1 protocol (Expected HTTP/, RTSP/ or ICE/)");
+  } else {
+    snprintf(msg, cap, "Proxy response (%d) !== 200 when HTTP Tunneling", status);
+  }
+}
+
+/* The proxy's response head is at most this long. */
+#define SCR_NET_TUNNEL_HEAD_MAX 16384u
+
+/* Drives the CONNECT exchange off readiness: sends the request head,
+ * then reads the proxy's response head. A 200 status makes the
+ * connection usable (bytes after the head belong to the tunnel); any
+ * other status fails the socket with undici's message. */
+static void scr_net_sock_tunnel_pump(ScrNetSocket *s) {
+  ScrNetTunnel *t = s->tunnel;
+  while (s->fd >= 0 && t->out_off < t->out_len) {
+    ssize_t sent =
+        scr_net_send(s->fd, t->out + t->out_off, t->out_len - t->out_off);
+    if (sent < 0) {
+      if (errno == EINTR) continue;
+      if (errno == EAGAIN || errno == EWOULDBLOCK) break;
+      char msg[96];
+      snprintf(msg, sizeof msg, "write %s", scr_net_errname(errno));
+      scr_net_sock_tunnel_fail(s, msg);
+      return;
+    }
+    t->out_off += (size_t)sent;
+  }
+  scr_net_sock_update_write(s);
+  if (s->fd < 0 || t->out_off < t->out_len) return;
+  for (;;) {
+    char buf[4096];
+    ssize_t n = read(s->fd, buf, sizeof buf);
+    if (n < 0) {
+      if (errno == EINTR) continue;
+      if (errno == EAGAIN || errno == EWOULDBLOCK) {
+        scr_net_sock_update_read(s);
+        return;
+      }
+      char msg[96];
+      snprintf(msg, sizeof msg, "read %s", scr_net_errname(errno));
+      scr_net_sock_tunnel_fail(s, msg);
+      return;
+    }
+    if (n == 0) {
+      scr_net_sock_tunnel_fail(s, "socket hang up");
+      return;
+    }
+    if (t->in_len + (size_t)n > SCR_NET_TUNNEL_HEAD_MAX) {
+      scr_net_sock_tunnel_fail(s, "Proxy response head is too large");
+      return;
+    }
+    char *grown = realloc(t->in, t->in_len + (size_t)n);
+    if (!grown) scr_net_oom();
+    t->in = grown;
+    memcpy(t->in + t->in_len, buf, (size_t)n);
+    t->in_len += (size_t)n;
+    size_t end = 0;
+    for (size_t i = 3; i < t->in_len; i++) {
+      if (memcmp(t->in + i - 3, "\r\n\r\n", 4) == 0) {
+        end = i + 1;
+        break;
+      }
+    }
+    if (end == 0) continue;
+    int status = scr_net_tunnel_status(t->in, end);
+    if (status != 200) {
+      char msg[128];
+      scr_net_tunnel_failure(status, msg, sizeof msg);
+      scr_net_sock_tunnel_fail(s, msg);
+      return;
+    }
+    scr_net_sock_append_rbuf(s, t->in + end, t->in_len - end);
+    scr_net_tunnel_free(t);
+    s->tunnel = NULL;
+    scr_net_sock_touch(s);
+    scr_net_sock_connected(s);
+    return;
+  }
+}
+
 /* Drive the transport handshake off readiness until it settles. On
  * establishment: the protocol layer attaches (https' parser), a
  * deferred server 'connection' fires ('secureConnection' timing), the
@@ -1371,7 +1513,7 @@ static void scr_net_sock_connect_done(ScrNetSocket *s) {
  * handshake flight delivers. Failure already tore the socket down via
  * scr_net_sock_transport_error. */
 static void scr_net_sock_transport_pump(ScrNetSocket *s) {
-  if (!s->tops || s->t_est || s->fd < 0 || s->connecting) return;
+  if (!s->tops || s->t_est || s->fd < 0 || s->connecting || s->tunnel != NULL) return;
   int r = s->tops->handshake(s->tctx);
   if (r == 0) {
     scr_net_sock_update_read(s);
@@ -3907,9 +4049,9 @@ ScrDyn *scr_net_sock_addr_field(ScrNetSocket *s, double field) {
 double scr_net_sock_bytes_read(ScrNetSocket *s) { return (double)s->bytes_read; }
 
 /* socket.connecting: a dial in progress (deferred, looking up, or
- * awaiting the TCP handshake). */
+ * awaiting the TCP handshake or the proxy tunnel). */
 bool scr_net_sock_connecting(ScrNetSocket *s) {
-  return s->fd >= 0 ? s->connecting : (s->dial_deferred || s->lookup_wait);
+  return s->fd >= 0 ? s->connecting || s->tunnel != NULL : (s->dial_deferred || s->lookup_wait);
 }
 
 /* socket.pending: no open handle yet (or anymore), or still connecting. */
@@ -4253,7 +4395,7 @@ void scr_net_sock_on_connect(ScrNetSocket *s, ScrClosure *cb /*moves*/, bool onc
     scr_closure_release(cb);
     return;
   }
-  if (!s->connecting) {
+  if (!s->connecting && s->tunnel == NULL) {
     /* already connected: Node fires a late once('connect')… never — the
      * event is past. Release, like listeners after 'exit'. */
     scr_closure_release(cb);
@@ -4365,7 +4507,7 @@ bool scr_net_sock_writable(ScrNetSocket *s) {
 }
 
 bool scr_net_sock_established(ScrNetSocket *s) {
-  return s->fd >= 0 && !s->connecting && (!s->tops || s->t_est);
+  return s->fd >= 0 && !s->connecting && s->tunnel == NULL && (!s->tops || s->t_est);
 }
 
 /* The protocol layer's timeout/error hooks (scr_http.c's client and the
@@ -4393,6 +4535,17 @@ void scr_net_sock_write_native(ScrNetSocket *s, const char *buf, size_t n) {
 
 /* ── the transport hooks (scr_tls.c) ─────────────────────────────────── */
 
+void scr_net_sock_set_tunnel(ScrNetSocket *s, const char *request, size_t len) {
+  if (s->fd < 0 || !s->connecting || s->tunnel != NULL) return;
+  ScrNetTunnel *t = calloc(1, sizeof *t);
+  if (!t) scr_net_oom();
+  t->out = malloc(len);
+  if (!t->out) scr_net_oom();
+  memcpy(t->out, request, len);
+  t->out_len = len;
+  s->tunnel = t;
+}
+
 void scr_net_sock_set_transport(ScrNetSocket *s, const ScrNetTransportOps *ops, void *tctx) {
   s->tops = ops;
   s->tctx = tctx;
@@ -4811,6 +4964,7 @@ static void scr_net_dispatch(void) {
          * failure may have closed. */
         if (evs[i].events & SCRP_WRITABLE) {
           if (s->connecting) scr_net_sock_connect_done(s);
+          else if (s->tunnel != NULL) scr_net_sock_tunnel_pump(s);
           else if (s->tops && !s->t_est) {
             s->t_want_write = false;
             scr_net_sock_transport_pump(s);
@@ -4821,7 +4975,8 @@ static void scr_net_dispatch(void) {
           }
         }
         if ((evs[i].events & SCRP_READABLE) && !scr_exc_pending() && s->fd >= 0) {
-          if (s->tops && !s->t_est) scr_net_sock_transport_pump(s);
+          if (s->tunnel != NULL) scr_net_sock_tunnel_pump(s);
+          else if (s->tops && !s->t_est) scr_net_sock_transport_pump(s);
           else scr_net_sock_read(s);
         }
       }
diff --git a/packages/runtime/src/scr_net.h b/packages/runtime/src/scr_net.h
index 539c14cdb..12e30acb4 100644
--- a/packages/runtime/src/scr_net.h
+++ b/packages/runtime/src/scr_net.h
@@ -341,6 +341,18 @@ typedef struct ScrNetTransportOps {
   void (*free)(void *tctx);
 } ScrNetTransportOps;
 void scr_net_sock_set_transport(ScrNetSocket *s, const ScrNetTransportOps *ops, void *tctx);
+/* Routes a client socket through an HTTP CONNECT proxy. Call it right
+ * after the dial (scr_net_connect to the proxy) and before any transport
+ * is set: once TCP is up, `request` (the CONNECT head, copied) is sent
+ * and the proxy's response head is read. A 200 status starts the
+ * transport handshake, or fires 'connect'; any other status fails the
+ * socket with "Proxy response (<status>) !== 200 when HTTP Tunneling". */
+void scr_net_sock_set_tunnel(ScrNetSocket *s, const char *request, size_t len);
+/* The status code of a CONNECT response head (head[0..len) ends with the
+ * blank line), or -1 when it does not start with an HTTP/1.x status
+ * line; and undici's message for a status other than 200 (or -1). */
+int scr_net_tunnel_status(const char *head, size_t len);
+void scr_net_tunnel_failure(int status, char *msg, size_t cap);
 int scr_net_sock_fd(ScrNetSocket *s);
 /* Drain up to n bytes of the socket's receive buffer (peeked/unshifted
  * bytes — the demux path); consumers call it before the fd. */
diff --git a/packages/runtime/src/scr_tls.c b/packages/runtime/src/scr_tls.c
index 1f481c4fd..9cca0b051 100644
--- a/packages/runtime/src/scr_tls.c
+++ b/packages/runtime/src/scr_tls.c
@@ -1089,6 +1089,234 @@ void scr_tls_fetch_client_wrap(ScrNetSocket *sock, void *ctx) {
   scr_tls_cli_wrap(sock, ctx);
 }
 
+/* ── the fetch tunnel through an https:// proxy ───────────────────────
+ * One transport carries three stages over the proxy socket: the TLS
+ * session to the proxy (verified against the proxy host), the CONNECT
+ * exchange inside it, then the target's TLS session (verified against
+ * the target host) inside the tunnel. An http:// target uses the
+ * tunnel's plaintext directly. */
+
+enum { SCR_TUN_OUTER = 0, SCR_TUN_SEND, SCR_TUN_HEAD, SCR_TUN_INNER, SCR_TUN_UP };
+
+typedef struct {
+  ScrTlsSock *outer; /* the session to the proxy */
+  ScrTlsSock *inner; /* the session to the target; NULL for http:// */
+  int stage;
+  char *req; /* the CONNECT head */
+  size_t req_len, req_off;
+  char *head; /* the proxy's response head as it arrives */
+  size_t head_len;
+  char *extra; /* tunnel bytes that arrived with the response head */
+  size_t extra_len, extra_off;
+} ScrTlsTunnel;
+
+static ScrTlsSock *scr_tls_cli_sock(ScrNetSocket *sock, ScrTlsCli *cli) {
+  ScrTlsSock *t = calloc(1, sizeof *t);
+  if (!t) scr_tls_oom();
+  mbedtls_ssl_init(&t->ssl);
+  if (mbedtls_ssl_setup(&t->ssl, &cli->conf) != 0) scr_tls_oom();
+  (void)mbedtls_ssl_set_hostname(&t->ssl, cli->hostname);
+  mbedtls_ssl_set_verify(&t->ssl, &scr_tls_vrfy_cb, t);
+  t->sock = sock;
+  t->cli = cli;
+  return t;
+}
+
+/* Reads tunnel plaintext: bytes that arrived with the proxy's head
+ * first, then the proxy session. mbedTLS return conventions. */
+static int scr_tls_tunnel_read(ScrTlsTunnel *tun, unsigned char *buf, size_t len) {
+  if (tun->extra_off < tun->extra_len) {
+    size_t n = tun->extra_len - tun->extra_off;
+    if (n > len) n = len;
+    memcpy(buf, tun->extra + tun->extra_off, n);
+    tun->extra_off += n;
+    return (int)n;
+  }
+  int r = mbedtls_ssl_read(&tun->outer->ssl, buf, len);
+  if (r == MBEDTLS_ERR_SSL_PEER_CLOSE_NOTIFY) return 0;
+#if defined(MBEDTLS_ERR_SSL_RECEIVED_NEW_SESSION_TICKET)
+  if (r == MBEDTLS_ERR_SSL_RECEIVED_NEW_SESSION_TICKET) return MBEDTLS_ERR_SSL_WANT_READ;
+#endif
+  return r;
+}
+
+/* The target session's BIOs: records travel inside the proxy session. */
+static int scr_tls_tunnel_bio_send(void *ctx, const unsigned char *buf, size_t len) {
+  ScrTlsTunnel *tun = (ScrTlsTunnel *)ctx;
+  return mbedtls_ssl_write(&tun->outer->ssl, buf, len);
+}
+
+static int scr_tls_tunnel_bio_recv(void *ctx, unsigned char *buf, size_t len) {
+  return scr_tls_tunnel_read((ScrTlsTunnel *)ctx, buf, len);
+}
+
+static int scr_tls_tunnel_handshake(void *tctx) {
+  ScrTlsTunnel *tun = (ScrTlsTunnel *)tctx;
+  ScrNetSocket *sock = tun->outer->sock;
+  if (tun->stage == SCR_TUN_OUTER) {
+    int r = scr_tls_handshake(tun->outer);
+    if (r <= 0) return r;
+    tun->outer->established = true;
+    tun->stage = SCR_TUN_SEND;
+  }
+  if (tun->stage == SCR_TUN_SEND) {
+    while (tun->req_off < tun->req_len) {
+      int r = mbedtls_ssl_write(&tun->outer->ssl, (const unsigned char *)tun->req + tun->req_off,
+                                tun->req_len - tun->req_off);
+      if (r == MBEDTLS_ERR_SSL_WANT_READ || r == MBEDTLS_ERR_SSL_WANT_WRITE) return 0;
+      if (r < 0) {
+        scr_net_sock_transport_error(sock, "write EPIPE");
+        return -1;
+      }
+      tun->req_off += (size_t)r;
+    }
+    tun->stage = SCR_TUN_HEAD;
+  }
+  if (tun->stage == SCR_TUN_HEAD) {
+    for (;;) {
+      unsigned char buf[4096];
+      int r = scr_tls_tunnel_read(tun, buf, sizeof buf);
+      if (r == MBEDTLS_ERR_SSL_WANT_READ || r == MBEDTLS_ERR_SSL_WANT_WRITE) return 0;
+      if (r < 0 && r != MBEDTLS_ERR_SSL_CONN_EOF) {
+        scr_net_sock_transport_error(sock, "read ECONNRESET");
+        return -1;
+      }
+      if (r <= 0) {
+        scr_net_sock_transport_error(sock, "socket hang up");
+        return -1;
+      }
+      if (tun->head_len + (size_t)r > 16384) {
+        scr_net_sock_transport_error(sock, "Proxy response head is too large");
+        return -1;
+      }
+      char *grown = realloc(tun->head, tun->head_len + (size_t)r);
+      if (!grown) scr_tls_oom();
+      tun->head = grown;
+      memcpy(tun->head + tun->head_len, buf, (size_t)r);
+      tun->head_len += (size_t)r;
+      size_t end = 0;
+      for (size_t i = 3; i < tun->head_len; i++) {
+        if (memcmp(tun->head + i - 3, "\r\n\r\n", 4) == 0) {
+          end = i + 1;
+          break;
+        }
+      }
+      if (end == 0) continue;
+      int status = scr_net_tunnel_status(tun->head, end);
+      if (status != 200) {
+        char msg[128];
+        scr_net_tunnel_failure(status, msg, sizeof msg);
+        scr_net_sock_transport_error(sock, msg);
+        return -1;
+      }
+      tun->extra = malloc(tun->head_len - end + 1);
+      if (!tun->extra) scr_tls_oom();
+      memcpy(tun->extra, tun->head + end, tun->head_len - end);
+      tun->extra_len = tun->head_len - end;
+      tun->extra_off = 0;
+      free(tun->head);
+      tun->head = NULL;
+      tun->head_len = 0;
+      tun->stage = tun->inner != NULL ? SCR_TUN_INNER : SCR_TUN_UP;
+      break;
+    }
+  }
+  if (tun->stage == SCR_TUN_INNER) {
+    int r = scr_tls_handshake(tun->inner);
+    if (r <= 0) return r;
+    tun->stage = SCR_TUN_UP;
+  }
+  return 1;
+}
+
+static ssize_t scr_tls_tunnel_xread(void *tctx, char *buf, size_t n) {
+  ScrTlsTunnel *tun = (ScrTlsTunnel *)tctx;
+  if (tun->inner != NULL) return scr_tls_xread(tun->inner, buf, n);
+  if (tun->extra_off < tun->extra_len) {
+    return scr_tls_tunnel_read(tun, (unsigned char *)buf, n);
+  }
+  return scr_tls_xread(tun->outer, buf, n);
+}
+
+static ssize_t scr_tls_tunnel_xwrite(void *tctx, const char *buf, size_t n) {
+  ScrTlsTunnel *tun = (ScrTlsTunnel *)tctx;
+  return scr_tls_xwrite(tun->inner != NULL ? tun->inner : tun->outer, buf, n);
+}
+
+static void scr_tls_tunnel_on_established(void *tctx) {
+  ScrTlsTunnel *tun = (ScrTlsTunnel *)tctx;
+  if (tun->inner != NULL) tun->inner->established = true;
+}
+
+static bool scr_tls_tunnel_pending(void *tctx) {
+  ScrTlsTunnel *tun = (ScrTlsTunnel *)tctx;
+  return tun->extra_off < tun->extra_len || (tun->inner != NULL && scr_tls_pending(tun->inner)) ||
+         scr_tls_pending(tun->outer);
+}
+
+static void scr_tls_tunnel_shutdown_write(void *tctx) {
+  ScrTlsTunnel *tun = (ScrTlsTunnel *)tctx;
+  if (tun->inner != NULL) scr_tls_shutdown_write(tun->inner);
+  scr_tls_shutdown_write(tun->outer);
+}
+
+static void scr_tls_tunnel_free(void *tctx) {
+  ScrTlsTunnel *tun = (ScrTlsTunnel *)tctx;
+  if (tun->inner != NULL) scr_tls_free(tun->inner);
+  scr_tls_free(tun->outer);
+  free(tun->req);
+  free(tun->head);
+  free(tun->extra);
+  free(tun);
+}
+
+static const ScrNetTransportOps SCR_TLS_TUNNEL_OPS = {
+  .xread = scr_tls_tunnel_xread,
+  .xwrite = scr_tls_tunnel_xwrite,
+  .handshake = scr_tls_tunnel_handshake,
+  .on_established = scr_tls_tunnel_on_established,
+  .pending = scr_tls_tunnel_pending,
+  .shutdown_write = scr_tls_tunnel_shutdown_write,
+  .free = scr_tls_tunnel_free,
+};
+
+typedef struct {
+  ScrTlsCli *proxy;  /* moves into the transport */
+  ScrTlsCli *target; /* NULL for an http:// target */
+  char *req;
+  size_t req_len;
+} ScrTlsTunnelCtx;
+
+void *scr_tls_fetch_tunnel_ctx(ScrStr *proxy_host /*borrowed*/, ScrStr *target_host /*borrowed, nullable*/,
+                               const char *request, size_t len) {
+  ScrTlsTunnelCtx *ctx = calloc(1, sizeof *ctx);
+  if (!ctx) scr_tls_oom();
+  ctx->proxy = scr_tls_cli_new(proxy_host, true, NULL, 0);
+  ctx->target = target_host != NULL ? scr_tls_cli_new(target_host, true, NULL, 0) : NULL;
+  ctx->req = malloc(len);
+  if (!ctx->req) scr_tls_oom();
+  memcpy(ctx->req, request, len);
+  ctx->req_len = len;
+  return ctx;
+}
+
+void scr_tls_fetch_tunnel_wrap(ScrNetSocket *sock, void *vctx) {
+  ScrTlsTunnelCtx *ctx = (ScrTlsTunnelCtx *)vctx;
+  ScrTlsTunnel *tun = calloc(1, sizeof *tun);
+  if (!tun) scr_tls_oom();
+  tun->outer = scr_tls_cli_sock(sock, ctx->proxy);
+  mbedtls_ssl_set_bio(&tun->outer->ssl, tun->outer, scr_tls_bio_send, scr_tls_bio_recv, NULL);
+  if (ctx->target != NULL) {
+    tun->inner = scr_tls_cli_sock(sock, ctx->target);
+    mbedtls_ssl_set_bio(&tun->inner->ssl, tun, scr_tls_tunnel_bio_send, scr_tls_tunnel_bio_recv,
+                        NULL);
+  }
+  tun->req = ctx->req;
+  tun->req_len = ctx->req_len;
+  free(ctx);
+  scr_net_sock_set_transport(sock, &SCR_TLS_TUNNEL_OPS, tun);
+}
+
 ScrHttpClientReq *scr_https_request(ScrStr *host /*borrowed*/, double port,
                                      ScrStr *path /*borrowed*/, ScrStr *method /*borrowed*/,
                                      double timeout_ms, ScrArr *header_pairs /*borrowed*/,
diff --git a/tests/fixtures/fetch/servers.mjs b/tests/fixtures/fetch/servers.mjs
index c0fe8a0d5..d3f04b153 100644
--- a/tests/fixtures/fetch/servers.mjs
+++ b/tests/fixtures/fetch/servers.mjs
@@ -370,6 +370,7 @@ export async function startFetchServers() {
   // CONNECT tunnels (undici's ProxyAgent shape — it tunnels even plain
   // http) splice raw sockets. One count per proxied request either way.
   let proxiedRequests = 0;
+  let tunneledRequests = 0;
   const proxyAuthorizations = [];
   const proxyRequest = (req, res) => {
     // Relative-path requests are never proxy traffic — the standalone
@@ -410,6 +411,7 @@ export async function startFetchServers() {
   };
   const proxyConnect = (req, clientSocket, head) => {
     proxiedRequests++;
+    tunneledRequests++;
     proxyAuthorizations.push(req.headers["proxy-authorization"] ?? "");
     const [host, portStr] = (req.url ?? "").split(":");
     const upstream = connect(Number(portStr ?? "80"), host ?? "127.0.0.1", () => {
@@ -450,6 +452,7 @@ export async function startFetchServers() {
     proxyUrl,
     secureProxyUrl,
     proxiedRequests: () => proxiedRequests,
+    tunneledRequests: () => tunneledRequests,
     proxyAuthorizations: () => proxyAuthorizations.slice(),
     close: async () => {
       await new Promise((resolve) => server.close(() => resolve()));
diff --git a/tests/fixtures/fetch/static-proxy-error/main.mts b/tests/fixtures/fetch/static-proxy-error/main.mts
new file mode 100644
index 000000000..26b57c579
--- /dev/null
+++ b/tests/fixtures/fetch/static-proxy-error/main.mts
@@ -0,0 +1,25 @@
+// Prints a failed fetch's error and its cause chain: name, message and
+// code at each level, plus the system error fields Node sets.
+function describe(value: unknown): string {
+  const error = value as {
+    name?: unknown;
+    message?: unknown;
+    code?: unknown;
+    errno?: unknown;
+    syscall?: unknown;
+    address?: unknown;
+    port?: unknown;
+  };
+  return `${String(error.name)} | ${String(error.message)} | ${String(error.code)} | ${String(error.errno)} | ${String(error.syscall)} | ${String(error.address)} | ${String(error.port)}`;
+}
+
+try {
+  const response = await fetch(process.argv[2]);
+  console.log(response.status, await response.text());
+} catch (error) {
+  let level: unknown = error;
+  for (let depth = 0; level !== undefined && depth < 4; depth++) {
+    console.log(depth, describe(level));
+    level = (level as { cause?: unknown }).cause;
+  }
+}
diff --git a/tests/harness/fetch.test.ts b/tests/harness/fetch.test.ts
index 4cc3bf86f..6b04acf59 100644
--- a/tests/harness/fetch.test.ts
+++ b/tests/harness/fetch.test.ts
@@ -3,6 +3,7 @@ import { createHash } from "node:crypto";
 import { globSync, mkdirSync, readFileSync } from "node:fs";
 import { createServer as createHttpServer } from "node:http";
 import { createServer as createHttpsServer } from "node:https";
+import { createServer as createNetServer, type AddressInfo } from "node:net";
 import { join } from "node:path";
 import { promisify } from "node:util";
 import { afterAll, beforeAll, describe, expect, test } from "vitest";
@@ -461,6 +462,7 @@ describe(`proxy env opt-in (NODE_USE_ENV_PROXY${sanitize ? ", sanitized" : ""})`
       NO_PROXY: "",
     };
     const before = servers.proxiedRequests();
+    const tunneledBefore = servers.tunneledRequests();
     const authBefore = servers.proxyAuthorizations().length;
     const [nodeRes, nativeRes] = await Promise.all([
       runBinary("node", [entry, ...argv], env),
@@ -470,6 +472,8 @@ describe(`proxy env opt-in (NODE_USE_ENV_PROXY${sanitize ? ", sanitized" : ""})`
     expect(nativeRes.exitCode, nativeRes.stderr.toString("utf8")).toBe(nodeRes.exitCode);
     expectCleanStderr(nativeRes, nodeRes);
     expect(servers.proxiedRequests() - before).toBe(2);
+    // Both lanes reach the http:// target through a CONNECT tunnel.
+    expect(servers.tunneledRequests() - tunneledBefore).toBe(2);
     expect(servers.proxyAuthorizations().slice(authBefore)).toEqual([
       expectedProxyAuthorization,
       expectedProxyAuthorization,
@@ -522,6 +526,7 @@ describe(`proxy env opt-in (NODE_USE_ENV_PROXY${sanitize ? ", sanitized" : ""})`
         NO_PROXY: "",
       };
       const before = servers.proxiedRequests();
+      const tunneledBefore = servers.tunneledRequests();
       const authBefore = servers.proxyAuthorizations().length;
       const [nodeRes, nativeRes] = await Promise.all([
         runBinary("node", [entry, baseUrl], env),
@@ -531,6 +536,7 @@ describe(`proxy env opt-in (NODE_USE_ENV_PROXY${sanitize ? ", sanitized" : ""})`
       expect(nativeRes.exitCode, nativeRes.stderr.toString("utf8")).toBe(nodeRes.exitCode);
       expectCleanStderr(nativeRes, nodeRes);
       expect(servers.proxiedRequests() - before).toBe(2);
+      expect(servers.tunneledRequests() - tunneledBefore).toBe(2);
       expect(servers.proxyAuthorizations().slice(authBefore)).toEqual([
         expectedProxyAuthorization,
         expectedProxyAuthorization,
@@ -593,6 +599,244 @@ describe(`proxy env opt-in (NODE_USE_ENV_PROXY${sanitize ? ", sanitized" : ""})`
     }
   }, 120_000);
 
+  /** An HTTPS target on localhost that counts the requests it serves,
+   * with the localhost certificate or another fixture certificate. */
+  async function secureTarget(cert = "localhost"): Promise<{
+    url: string;
+    requests: () => number;
+    close: () => Promise<void>;
+  }> {
+    const certs = join(fixturesRoot, "../server/certs");
+    let requests = 0;
+    const server = createHttpsServer(
+      {
+        key: readFileSync(join(certs, `${cert}-key.pem`)),
+        cert: readFileSync(join(certs, `${cert}.pem`)),
+      },
+      (request, response) => {
+        requests++;
+        response.end(`secure ${request.url} ${request.headers.host}`);
+      },
+    );
+    await new Promise<void>((resolve) => server.listen(0, "127.0.0.1", resolve));
+    const { port } = server.address() as AddressInfo;
+    return {
+      url: `https://localhost:${port}`,
+      requests: () => requests,
+      close: () =>
+        new Promise<void>((resolve, reject) => {
+          server.close((error) => (error ? reject(error) : resolve()));
+        }),
+    };
+  }
+
+  test.for(["llvm"] as const)(
+    "%s fetch tunnels HTTPS through an http_proxy with CONNECT",
+    async () => {
+      const target = await secureTarget();
+      try {
+        const entry = join(fixturesRoot, "static-proxy/main.mts");
+        const binary = await build(entry, "proxy-connect");
+        const env = {
+          ...process.env,
+          NODE_EXTRA_CA_CERTS: join(fixturesRoot, "../server/certs/ca.pem"),
+          NODE_USE_ENV_PROXY: "1",
+          https_proxy: authenticatedProxyUrl,
+          HTTPS_PROXY: authenticatedProxyUrl,
+          no_proxy: "",
+          NO_PROXY: "",
+        };
+        const before = servers.proxiedRequests();
+        const authBefore = servers.proxyAuthorizations().length;
+        const [nodeRes, nativeRes] = await Promise.all([
+          runBinary("node", [entry, target.url], env),
+          runBinary(binary, [target.url], env),
+        ]);
+        expect(nativeRes.stdout.toString("utf8")).toBe(nodeRes.stdout.toString("utf8"));
+        expect(nativeRes.stdout.toString("utf8")).toContain("200 secure /text localhost:");
+        expect(nativeRes.exitCode, nativeRes.stderr.toString("utf8")).toBe(nodeRes.exitCode);
+        expectCleanStderr(nativeRes, nodeRes);
+        expect(servers.proxiedRequests() - before).toBe(2);
+        expect(target.requests()).toBe(2);
+        expect(servers.proxyAuthorizations().slice(authBefore)).toEqual([
+          expectedProxyAuthorization,
+          expectedProxyAuthorization,
+        ]);
+      } finally {
+        await target.close();
+      }
+    },
+    120_000,
+  );
+
+  test.for(["llvm"] as const)(
+    "%s fetch verifies the target host through the CONNECT tunnel",
+    async () => {
+      // The alt certificate names alt.localhost only.
+      const target = await secureTarget("alt");
+      try {
+        const entry = join(fixturesRoot, "static-network-error/main.mts");
+        const binary = await build(entry, "proxy-connect-verify");
+        const url = target.url;
+        const env = {
+          ...process.env,
+          NODE_EXTRA_CA_CERTS: join(fixturesRoot, "../server/certs/ca.pem"),
+          NODE_USE_ENV_PROXY: "1",
+          https_proxy: proxyUrl,
+          HTTPS_PROXY: proxyUrl,
+          no_proxy: "",
+          NO_PROXY: "",
+        };
+        const [nodeRes, nativeRes] = await Promise.all([
+          runBinary("node", [entry, url], env),
+          runBinary(binary, [url], env),
+        ]);
+        expect(nativeRes.stdout.toString("utf8")).toBe(nodeRes.stdout.toString("utf8"));
+        expect(nativeRes.stdout.toString("utf8")).toBe("TypeError fetch failed true\n");
+        expect(target.requests()).toBe(0);
+      } finally {
+        await target.close();
+      }
+    },
+    120_000,
+  );
+
+  test.for(["llvm"] as const)(
+    "%s fetch sends proxy credentials only with a username and a password",
+    async () => {
+      const entry = join(fixturesRoot, "static-proxy/main.mts");
+      const binary = await build(entry, "proxy-user-only");
+      const userOnly = proxyUrl.replace("http://", "http://proxy-user@");
+      const env = {
+        ...process.env,
+        NODE_USE_ENV_PROXY: "1",
+        http_proxy: userOnly,
+        HTTP_PROXY: userOnly,
+        no_proxy: "",
+        NO_PROXY: "",
+      };
+      const authBefore = servers.proxyAuthorizations().length;
+      const [nodeRes, nativeRes] = await Promise.all([
+        runBinary("node", [entry, baseUrl], env),
+        runBinary(binary, [baseUrl], env),
+      ]);
+      expect(nativeRes.stdout.equals(nodeRes.stdout)).toBe(true);
+      expect(nativeRes.exitCode, nativeRes.stderr.toString("utf8")).toBe(nodeRes.exitCode);
+      expect(servers.proxyAuthorizations().slice(authBefore)).toEqual(["", ""]);
+    },
+    120_000,
+  );
+
+  test.for(["llvm"] as const)(
+    "%s fetch failures through a proxy carry Node's cause",
+    async () => {
+      // Answers every CONNECT with one fixed status line.
+      const statusProxy = createNetServer((socket) => {
+        socket.once("data", () => {
+          socket.end("HTTP/1.1 502 Bad Gateway\r\nContent-Length: 0\r\nConnection: close\r\n\r\n");
+        });
+        socket.on("error", () => {});
+      });
+      await new Promise<void>((resolve) => statusProxy.listen(0, "127.0.0.1", resolve));
+      const statusProxyUrl = `http://127.0.0.1:${(statusProxy.address() as AddressInfo).port}`;
+      try {
+        const entry = join(fixturesRoot, "static-proxy-error/main.mts");
+        const binary = await build(entry, "proxy-error-cause");
+        for (const proxy of [statusProxyUrl, refusedUrl]) {
+          const env = {
+            ...process.env,
+            NODE_USE_ENV_PROXY: "1",
+            https_proxy: proxy,
+            HTTPS_PROXY: proxy,
+            no_proxy: "",
+            NO_PROXY: "",
+          };
+          const [nodeRes, nativeRes] = await Promise.all([
+            runBinary("node", [entry, "https://localhost/"], env),
+            runBinary(binary, ["https://localhost/"], env),
+          ]);
+          expect(nativeRes.stdout.toString("utf8")).toBe(nodeRes.stdout.toString("utf8"));
+          expect(nativeRes.exitCode, nativeRes.stderr.toString("utf8")).toBe(nodeRes.exitCode);
+        }
+      } finally {
+        await new Promise<void>((resolve) => statusProxy.close(() => resolve()));
+      }
+    },
+    120_000,
+  );
+
+  test.for(["llvm"] as const)(
+    "%s fetch tunnels HTTPS through an HTTPS proxy with CONNECT",
+    async () => {
+      const target = await secureTarget();
+      try {
+        const entry = join(fixturesRoot, "static-proxy/main.mts");
+        const binary = await build(entry, "proxy-https-https");
+        const env = {
+          ...process.env,
+          NODE_EXTRA_CA_CERTS: join(fixturesRoot, "../server/certs/ca.pem"),
+          NODE_USE_ENV_PROXY: "1",
+          https_proxy: secureAuthenticatedProxyUrl,
+          HTTPS_PROXY: secureAuthenticatedProxyUrl,
+          no_proxy: "",
+          NO_PROXY: "",
+        };
+        const before = servers.proxiedRequests();
+        const tunneledBefore = servers.tunneledRequests();
+        const authBefore = servers.proxyAuthorizations().length;
+        const [nodeRes, nativeRes] = await Promise.all([
+          runBinary("node", [entry, target.url], env),
+          runBinary(binary, [target.url], env),
+        ]);
+        expect(nativeRes.stdout.toString("utf8")).toBe(nodeRes.stdout.toString("utf8"));
+        expect(nativeRes.stdout.toString("utf8")).toContain("200 secure /text localhost:");
+        expect(nativeRes.exitCode, nativeRes.stderr.toString("utf8")).toBe(nodeRes.exitCode);
+        expectCleanStderr(nativeRes, nodeRes);
+        expect(servers.proxiedRequests() - before).toBe(2);
+        expect(servers.tunneledRequests() - tunneledBefore).toBe(2);
+        expect(target.requests()).toBe(2);
+        expect(servers.proxyAuthorizations().slice(authBefore)).toEqual([
+          expectedProxyAuthorization,
+          expectedProxyAuthorization,
+        ]);
+      } finally {
+        await target.close();
+      }
+    },
+    120_000,
+  );
+
+  test.for(["llvm"] as const)(
+    "%s fetch verifies the target host through an HTTPS proxy tunnel",
+    async () => {
+      // The alt certificate names alt.localhost only.
+      const target = await secureTarget("alt");
+      try {
+        const entry = join(fixturesRoot, "static-network-error/main.mts");
+        const binary = await build(entry, "proxy-https-verify");
+        const env = {
+          ...process.env,
+          NODE_EXTRA_CA_CERTS: join(fixturesRoot, "../server/certs/ca.pem"),
+          NODE_USE_ENV_PROXY: "1",
+          https_proxy: secureProxyUrl,
+          HTTPS_PROXY: secureProxyUrl,
+          no_proxy: "",
+          NO_PROXY: "",
+        };
+        const [nodeRes, nativeRes] = await Promise.all([
+          runBinary("node", [entry, target.url], env),
+          runBinary(binary, [target.url], env),
+        ]);
+        expect(nativeRes.stdout.toString("utf8")).toBe(nodeRes.stdout.toString("utf8"));
+        expect(nativeRes.stdout.toString("utf8")).toBe("TypeError fetch failed true\n");
+        expect(target.requests()).toBe(0);
+      } finally {
+        await target.close();
+      }
+    },
+    120_000,
+  );
+
   test.for(["llvm"] as const)(
     "%s fetch rejects an invalid opted-in proxy instead of dialing directly",
     async () => {