diff --git a/backend/internal/service/openai_codex_transform.go b/backend/internal/service/openai_codex_transform.go
index e765d7e9..59fb7a33 100644
--- a/backend/internal/service/openai_codex_transform.go
+++ b/backend/internal/service/openai_codex_transform.go
@@ -853,6 +853,14 @@ func filterCodexInput(input []any, preserveReferences bool) []any {
 		}
 		typ, _ := m["type"].(string)
 
+		// chatgpt.com codex backend (OAuth path) does not persist reasoning
+		// items because applyCodexOAuthTransform forces store=false. Any rs_*
+		// reference replayed in input is guaranteed to 404 upstream
+		// ("Item with id 'rs_...' not found"). Drop reasoning items entirely.
+		if typ == "reasoning" {
+			continue
+		}
+
 		// 仅修正真正的 tool/function call 标识，避免误改普通 message/reasoning id；
 		// 若 item_reference 指向 legacy call_* 标识，则仅修正该引用本身。
 		fixCallIDPrefix := func(id string) string {
diff --git a/backend/internal/service/openai_codex_transform_test.go b/backend/internal/service/openai_codex_transform_test.go
index 75f5c55c..b392cf96 100644
--- a/backend/internal/service/openai_codex_transform_test.go
+++ b/backend/internal/service/openai_codex_transform_test.go
@@ -1,6 +1,8 @@
 package service
 
 import (
+	"fmt"
+	"strings"
 	"testing"
 
 	"github.com/stretchr/testify/require"
@@ -1094,3 +1096,54 @@ func TestIsInstructionsEmpty(t *testing.T) {
 		})
 	}
 }
+
+func TestFilterCodexInput_DropsReasoningItemsRegardlessOfPreserveReferences(t *testing.T) {
+	// Reasoning items in input[] reference rs_* IDs that were emitted by
+	// chatgpt.com under store=false (forced by applyCodexOAuthTransform).
+	// They are never persisted upstream, so forwarding them produces a
+	// guaranteed 404 ("Item with id 'rs_...' not found"). Drop them
+	// regardless of preserveReferences. See: Wei-Shaw/sub2api issue #1957.
+
+	build := func() []any {
+		return []any{
+			map[string]any{"type": "message", "id": "msg_0", "role": "user", "content": "hi"},
+			map[string]any{
+				"type":    "reasoning",
+				"id":      "rs_0672f12450da0b9c0169f07220a6c08198b68c2455ced99344",
+				"summary": []any{},
+			},
+			map[string]any{"type": "function_call", "id": "fc_1", "call_id": "call_1", "name": "tool"},
+			map[string]any{"type": "function_call_output", "call_id": "call_1", "output": "{}"},
+		}
+	}
+
+	for _, preserve := range []bool{true, false} {
+		preserve := preserve
+		t.Run(fmt.Sprintf("preserveReferences=%v", preserve), func(t *testing.T) {
+			filtered := filterCodexInput(build(), preserve)
+
+			for _, raw := range filtered {
+				item, ok := raw.(map[string]any)
+				require.True(t, ok)
+				require.NotEqual(t, "reasoning", item["type"],
+					"reasoning items must be dropped from input on the OAuth path")
+				if id, ok := item["id"].(string); ok {
+					require.False(t, strings.HasPrefix(id, "rs_"),
+						"no item carrying an rs_* id should survive the filter")
+				}
+			}
+
+			// Sanity check: the non-reasoning items should still be present.
+			gotTypes := make(map[string]int)
+			for _, raw := range filtered {
+				item, ok := raw.(map[string]any)
+				require.True(t, ok)
+				gotTypes[item["type"].(string)]++
+			}
+			require.Equal(t, 1, gotTypes["message"])
+			require.Equal(t, 1, gotTypes["function_call"])
+			require.Equal(t, 1, gotTypes["function_call_output"])
+			require.Equal(t, 0, gotTypes["reasoning"])
+		})
+	}
+}