diff --git a/pkg/providers/httpapi/gemini_provider.go b/pkg/providers/httpapi/gemini_provider.go
index 9d0b44e1..338a5659 100644
--- a/pkg/providers/httpapi/gemini_provider.go
+++ b/pkg/providers/httpapi/gemini_provider.go
@@ -285,6 +285,7 @@ func (p *GeminiProvider) buildRequestBody(
 				}
 				if thoughtSignature != "" {
 					part.ThoughtSignature = thoughtSignature
+					part.ThoughtSignatureSnake = thoughtSignature
 				}
 				content.Parts = append(content.Parts, part)
 			}
diff --git a/pkg/providers/httpapi/gemini_provider_test.go b/pkg/providers/httpapi/gemini_provider_test.go
index 05080b81..bb4bea6c 100644
--- a/pkg/providers/httpapi/gemini_provider_test.go
+++ b/pkg/providers/httpapi/gemini_provider_test.go
@@ -453,7 +453,7 @@ func TestGeminiProvider_ChatStreamReturnsErrorOnInvalidDataFrame(t *testing.T) {
 	}
 }
 
-func TestGeminiProvider_BuildRequestBody_UsesCamelCaseThoughtSignatureOnly(t *testing.T) {
+func TestGeminiProvider_BuildRequestBody_SetsBothThoughtSignatureFormats(t *testing.T) {
 	provider := NewGeminiProvider("test-key", "https://example.com/v1beta", "", "", 0, nil, nil)
 
 	body := provider.buildRequestBody(
@@ -484,8 +484,8 @@ func TestGeminiProvider_BuildRequestBody_UsesCamelCaseThoughtSignatureOnly(t *te
 	if !strings.Contains(jsonBody, `"thoughtSignature":"sig-1"`) {
 		t.Fatalf("request body = %s, expected camelCase thoughtSignature", jsonBody)
 	}
-	if strings.Contains(jsonBody, `"thought_signature"`) {
-		t.Fatalf("request body = %s, unexpected snake_case thought_signature", jsonBody)
+	if !strings.Contains(jsonBody, `"thought_signature":"sig-1"`) {
+		t.Fatalf("request body = %s, expected snake_case thought_signature for Gemini 3.5 Flash Agentic compatibility", jsonBody)
 	}
 }