diff --git a/internal/gateway/codex_backend.go b/internal/gateway/codex_backend.go
index 34adf663..d8e184d6 100644
--- a/internal/gateway/codex_backend.go
+++ b/internal/gateway/codex_backend.go
@@ -320,6 +320,21 @@ func (s *Server) codexUpstream(w http.ResponseWriter, r *http.Request, rest stri
 		end(res.StatusCode, res.Status, 0, 0)
 		return
 	}
+	if rest == "/responses" && res.StatusCode >= 400 {
+		msg, _ := io.ReadAll(io.LimitReader(res.Body, 1<<20))
+		res.Body = io.NopCloser(bytes.NewReader(msg))
+		if len(bytes.TrimSpace(msg)) == 0 {
+			// a failure with nothing said of it, which Codex shows as
+			// "Unknown error" alone (#409): said where the turn went
+			said := codexFailedEmpty(modelOf(body), res.Status)
+			writeError(w, provider.Responses, res.StatusCode, said)
+			s.record(Call{Time: start, From: provider.Responses, To: provider.Responses, Model: modelOf(body),
+				Provider: "openai", Agent: agentOf(r), Kind: callKind(r.Header), Status: res.StatusCode,
+				Millis: time.Since(start).Milliseconds(), Error: said})
+			end(res.StatusCode, said, 0, 0)
+			return
+		}
+	}
 	for k, vs := range res.Header {
 		if !hopHeader(k) {
 			w.Header()[k] = vs
@@ -453,6 +468,16 @@ func codexUnreached(model string, err error) string {
 		"(provider/model, or set it in magpie's Agents view), or let Codex reach OpenAI"
 }
 
+// codexFailedEmpty says why a model of Codex's own failed when OpenAI gave
+// an error status and nothing else: where it went, and what to do — a
+// Codex left on its own model while magpie's Agents view picked another
+// for it sends the turn to OpenAI, not to that one (#409).
+func codexFailedEmpty(model, status string) string {
+	return "OpenAI answered " + status + " and said nothing more. " + model + " is one of Codex's own models, " +
+		"which goes to OpenAI with Codex's own sign-in, not to a provider in magpie: pick one of magpie's models in Codex " +
+		"(provider/model, or set it in magpie's Agents view), or try again later"
+}
+
 // apiKey reports whether Codex signed in with an API key rather than a
 // ChatGPT account.
 func apiKey(h http.Header) bool {
diff --git a/internal/gateway/codex_empty_error_test.go b/internal/gateway/codex_empty_error_test.go
new file mode 100644
index 00000000..fefc1192
--- /dev/null
+++ b/internal/gateway/codex_empty_error_test.go
@@ -0,0 +1,67 @@
+package gateway
+
+import (
+	"encoding/json"
+	"io"
+	"net/http"
+	"strings"
+	"testing"
+
+	"github.com/yetone/magpie/internal/provider"
+)
+
+// One of Codex's own models, failed by the ChatGPT backend with an error
+// status and nothing in its body, fails saying where it went and what to do:
+// relayed as it came, Codex said only "unexpected status 502 Bad Gateway:
+// Unknown error" (#409, Codex's app left on GPT-5.6 Terra while magpie's
+// Agents view had DeepSeek picked for it).
+func TestCodexOwnModelEmptyError(t *testing.T) {
+	setup(t, provider.Chat, &fake{t: t})
+	chatgpt(t, func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusBadGateway)
+	})
+	code, body := codexPost(t, `{"model":"gpt-5.6-terra","stream":true,"input":"今天几号"}`)
+	var e struct {
+		Error struct {
+			Message string `json:"message"`
+		} `json:"error"`
+	}
+	if code != 502 || json.Unmarshal([]byte(body), &e) != nil {
+		t.Fatalf("%d %q", code, body)
+	}
+	for _, want := range []string{"502 Bad Gateway", "gpt-5.6-terra is one of Codex's own models", "pick one of magpie's models"} {
+		if !strings.Contains(e.Error.Message, want) {
+			t.Errorf("no %q in %q", want, e.Error.Message)
+		}
+	}
+}
+
+// What the ChatGPT backend said of a failure still goes to Codex as it said
+// it.
+func TestCodexOwnModelErrorBodyKept(t *testing.T) {
+	setup(t, provider.Chat, &fake{t: t})
+	said := `{"error":{"message":"The model is overloaded","type":"server_error"}}`
+	chatgpt(t, func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Content-Type", "application/json")
+		w.WriteHeader(http.StatusBadGateway)
+		io.WriteString(w, said)
+	})
+	if code, body := codexPost(t, `{"model":"gpt-5.6-terra","stream":true,"input":"hi"}`); code != 502 || body != said {
+		t.Fatalf("%d %s", code, body)
+	}
+}
+
+// A magpie model whose vendor fails with an empty body is never relayed
+// empty: the agent is told the vendor and its status.
+func TestCodexMagpieModelEmptyError(t *testing.T) {
+	setup(t, provider.Responses, &fake{t: t, refuse: func([]byte) (int, string) { return http.StatusBadGateway, " " }})
+	code, body := codexPost(t, `{"model":"fake/m1","stream":true,"input":"hi"}`)
+	var e struct {
+		Error struct {
+			Message string `json:"message"`
+		} `json:"error"`
+	}
+	if code != 502 || json.Unmarshal([]byte(body), &e) != nil || !strings.Contains(e.Error.Message, "502") {
+		t.Fatalf("%d %q", code, body)
+	}
+}