diff --git a/internal/runtime/executor/codex_websockets_executor.go b/internal/runtime/executor/codex_websockets_executor.go
index 7e74d953d..5c11a3fbc 100644
--- a/internal/runtime/executor/codex_websockets_executor.go
+++ b/internal/runtime/executor/codex_websockets_executor.go
@@ -5,6 +5,7 @@ package executor
 import (
 	"bytes"
 	"context"
+	"errors"
 	"fmt"
 	"io"
 	"net"
@@ -347,8 +348,9 @@ func (e *CodexWebsocketsExecutor) Execute(ctx context.Context, auth *cliproxyaut
 		}
 		msgType, payload, errRead := readCodexWebsocketMessage(ctx, sess, conn, readCh)
 		if errRead != nil {
-			helps.RecordAPIWebsocketError(ctx, e.cfg, "read", errRead)
-			return resp, errRead
+			mappedErr := mapCodexWebsocketReadError(errRead)
+			helps.RecordAPIWebsocketError(ctx, e.cfg, "read", mappedErr)
+			return resp, mappedErr
 		}
 		if msgType != websocket.TextMessage {
 			if msgType == websocket.BinaryMessage {
@@ -608,11 +610,12 @@ func (e *CodexWebsocketsExecutor) ExecuteStream(ctx context.Context, auth *clipr
 					_ = send(cliproxyexecutor.StreamChunk{Err: ctx.Err()})
 					return
 				}
+				mappedErr := mapCodexWebsocketReadError(errRead)
 				terminateReason = "read_error"
-				terminateErr = errRead
-				helps.RecordAPIWebsocketError(ctx, e.cfg, "read", errRead)
-				reporter.PublishFailure(ctx, errRead)
-				_ = send(cliproxyexecutor.StreamChunk{Err: errRead})
+				terminateErr = mappedErr
+				helps.RecordAPIWebsocketError(ctx, e.cfg, "read", mappedErr)
+				reporter.PublishFailure(ctx, mappedErr)
+				_ = send(cliproxyexecutor.StreamChunk{Err: mappedErr})
 				return
 			}
 			if msgType != websocket.TextMessage {
@@ -724,6 +727,17 @@ func writeCodexWebsocketMessage(sess *codexWebsocketSession, conn *websocket.Con
 	return conn.WriteMessage(websocket.TextMessage, payload)
 }
 
+func mapCodexWebsocketReadError(err error) error {
+	if err == nil {
+		return nil
+	}
+	var closeErr *websocket.CloseError
+	if errors.As(err, &closeErr) && closeErr.Code == websocket.CloseMessageTooBig {
+		return statusErr{code: http.StatusRequestEntityTooLarge, msg: `{"error":{"message":"upstream websocket message too big","type":"invalid_request_error","code":"message_too_big"}}`}
+	}
+	return err
+}
+
 func buildCodexWebsocketRequestBody(body []byte) []byte {
 	if len(body) == 0 {
 		return nil
diff --git a/internal/runtime/executor/codex_websockets_executor_test.go b/internal/runtime/executor/codex_websockets_executor_test.go
index db76f0621..be1ae149d 100644
--- a/internal/runtime/executor/codex_websockets_executor_test.go
+++ b/internal/runtime/executor/codex_websockets_executor_test.go
@@ -227,6 +227,68 @@ func TestCodexWebsocketsExecuteStreamPropagatesUpstreamErrorForDownstreamWebsock
 	}
 }
 
+func TestCodexWebsocketsExecuteStreamMapsMessageTooBigClose(t *testing.T) {
+	upgrader := websocket.Upgrader{CheckOrigin: func(*http.Request) bool { return true }}
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		conn, err := upgrader.Upgrade(w, r, nil)
+		if err != nil {
+			t.Errorf("upgrade websocket: %v", err)
+			return
+		}
+		defer func() { _ = conn.Close() }()
+
+		if _, _, errRead := conn.ReadMessage(); errRead != nil {
+			t.Errorf("read upstream websocket message: %v", errRead)
+			return
+		}
+		deadline := time.Now().Add(time.Second)
+		closeMessage := websocket.FormatCloseMessage(websocket.CloseMessageTooBig, "message too big")
+		if errWrite := conn.WriteControl(websocket.CloseMessage, closeMessage, deadline); errWrite != nil {
+			t.Errorf("write close websocket message: %v", errWrite)
+			return
+		}
+	}))
+	defer server.Close()
+
+	exec := NewCodexWebsocketsExecutor(&config.Config{SDKConfig: config.SDKConfig{DisableImageGeneration: config.DisableImageGenerationAll}})
+	auth := &cliproxyauth.Auth{Attributes: map[string]string{"api_key": "sk-test", "base_url": server.URL}}
+	req := cliproxyexecutor.Request{
+		Model:   "gpt-5-codex",
+		Payload: []byte(`{"model":"gpt-5-codex","input":[{"type":"message","role":"user","content":"hello"}]}`),
+	}
+	opts := cliproxyexecutor.Options{
+		SourceFormat:   sdktranslator.FromString("openai-response"),
+		ResponseFormat: sdktranslator.FromString("openai-response"),
+	}
+
+	result, err := exec.ExecuteStream(context.Background(), auth, req, opts)
+	if err != nil {
+		t.Fatalf("ExecuteStream() error = %v", err)
+	}
+
+	select {
+	case chunk, ok := <-result.Chunks:
+		if !ok {
+			t.Fatal("stream closed before error chunk")
+		}
+		if chunk.Err == nil {
+			t.Fatal("error chunk Err = nil, want message-too-big error")
+		}
+		statusErr, ok := chunk.Err.(interface{ StatusCode() int })
+		if !ok {
+			t.Fatalf("error type %T does not expose StatusCode", chunk.Err)
+		}
+		if got := statusErr.StatusCode(); got != http.StatusRequestEntityTooLarge {
+			t.Fatalf("status = %d, want %d", got, http.StatusRequestEntityTooLarge)
+		}
+		if got := gjson.Get(chunk.Err.Error(), "error.code").String(); got != "message_too_big" {
+			t.Fatalf("error code = %q, want message_too_big; err=%v", got, chunk.Err)
+		}
+	case <-time.After(5 * time.Second):
+		t.Fatal("timed out waiting for error stream chunk")
+	}
+}
+
 func TestCodexWebsocketsUpstreamDisconnectChanSignalsOnInvalidate(t *testing.T) {
 	upgrader := websocket.Upgrader{CheckOrigin: func(*http.Request) bool { return true }}
 	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {