diff --git a/internal/translator/codex/claude/codex_claude_response.go b/internal/translator/codex/claude/codex_claude_response.go
index 798089d0..4557606f 100644
--- a/internal/translator/codex/claude/codex_claude_response.go
+++ b/internal/translator/codex/claude/codex_claude_response.go
@@ -84,7 +84,7 @@ func ConvertCodexResponseToClaude(_ context.Context, _ string, originalRequestRa
 		if params.ThinkingBlockOpen && params.ThinkingStopPending {
 			output = append(output, finalizeCodexThinkingBlock(params)...)
 		}
-		template = []byte(`{"type":"content_block_start","index":0,"content_block":{"type":"thinking","thinking":"","signature":""}}`)
+		template = []byte(`{"type":"content_block_start","index":0,"content_block":{"type":"thinking","thinking":""}}`)
 		template, _ = sjson.SetBytes(template, "index", params.BlockIndex)
 		params.ThinkingBlockOpen = true
 		params.ThinkingStopPending = false
@@ -270,9 +270,11 @@ func ConvertCodexResponseToClaudeNonStream(_ context.Context, _ string, original
 					}
 				}
 				if thinkingBuilder.Len() > 0 || signature != "" {
-					block := []byte(`{"type":"thinking","thinking":"","signature":""}`)
+					block := []byte(`{"type":"thinking","thinking":""}`)
 					block, _ = sjson.SetBytes(block, "thinking", thinkingBuilder.String())
-					block, _ = sjson.SetBytes(block, "signature", signature)
+					if signature != "" {
+						block, _ = sjson.SetBytes(block, "signature", signature)
+					}
 					out, _ = sjson.SetRawBytes(out, "content.-1", block)
 				}
 			case "message":
diff --git a/internal/translator/codex/claude/codex_claude_response_test.go b/internal/translator/codex/claude/codex_claude_response_test.go
index 5a25057c..f436711e 100644
--- a/internal/translator/codex/claude/codex_claude_response_test.go
+++ b/internal/translator/codex/claude/codex_claude_response_test.go
@@ -40,8 +40,8 @@ func TestConvertCodexResponseToClaude_StreamThinkingIncludesSignature(t *testing
 			case "content_block_start":
 				if data.Get("content_block.type").String() == "thinking" {
 					startFound = true
-					if !data.Get("content_block.signature").Exists() {
-						t.Fatalf("thinking start block missing signature field: %s", line)
+					if data.Get("content_block.signature").Exists() {
+						t.Fatalf("thinking start block should NOT have signature field when signature is unknown: %s", line)
 					}
 				}
 			case "content_block_delta":
@@ -97,8 +97,8 @@ func TestConvertCodexResponseToClaude_StreamThinkingWithoutReasoningItemStillInc
 			data := gjson.Parse(strings.TrimPrefix(line, "data: "))
 			if data.Get("type").String() == "content_block_start" && data.Get("content_block.type").String() == "thinking" {
 				thinkingStartFound = true
-				if !data.Get("content_block.signature").Exists() {
-					t.Fatalf("thinking start block missing signature field: %s", line)
+				if data.Get("content_block.signature").Exists() {
+					t.Fatalf("thinking start block should NOT have signature field without encrypted_content: %s", line)
 				}
 			}
 			if data.Get("type").String() == "content_block_stop" && data.Get("index").Int() == 0 {