test(translator, runtime): ensure empty text parts are skipped without null values

- Added tests for `ConvertOpenAIRequestToAntigravity` and `ConvertOpenAIRequestToGemini` to verify skipping of empty text parts while maintaining non-null structure.
- Refactored part index logic to correctly handle placement of replay parts in `antigravity_reasoning_replay`.
- Introduced helper functions `antigravityExistingReplayPartPath` and `antigravityReplayPartWritePath` for consistent replay part path management.

internal/runtime/executor/antigravity_reasoning_replay.go

@@ -368,10 +368,41 @@ func insertAntigravityModelFunctionCallBeforeContent(payload []byte, beforeIndex
 func antigravityRequestHasThoughtSignatureAt(payload []byte, itemResult gjson.Result) bool {
 	ci := int(itemResult.Get("contentIndex").Int())
 	pi := int(itemResult.Get("partIndex").Int())
-	path := fmt.Sprintf("request.contents.%d.parts.%d.thoughtSignature", ci, pi)
+	partPath, ok := antigravityExistingReplayPartPath(payload, ci, pi)
+	if !ok {
+		return false
+	}
+	path := partPath + ".thoughtSignature"
 	return strings.TrimSpace(gjson.GetBytes(payload, path).String()) != ""
 }
 
+func antigravityExistingReplayPartPath(payload []byte, contentIndex int, partIndex int) (string, bool) {
+	if contentIndex < 0 || partIndex < 0 {
+		return "", false
+	}
+	partsPath := fmt.Sprintf("request.contents.%d.parts", contentIndex)
+	parts := gjson.GetBytes(payload, partsPath)
+	if !parts.IsArray() {
+		return "", false
+	}
+	arr := parts.Array()
+	if partIndex >= len(arr) || arr[partIndex].Type == gjson.Null {
+		return "", false
+	}
+	return fmt.Sprintf("%s.%d", partsPath, partIndex), true
+}
+
+func antigravityReplayPartWritePath(payload []byte, contentIndex int, partIndex int) string {
+	if path, ok := antigravityExistingReplayPartPath(payload, contentIndex, partIndex); ok {
+		return path
+	}
+	partsPath := fmt.Sprintf("request.contents.%d.parts", contentIndex)
+	if gjson.GetBytes(payload, partsPath).IsArray() {
+		return partsPath + ".-1"
+	}
+	return partsPath + ".0"
+}
+
 func insertAntigravityReasoningReplayItems(payload []byte, items [][]byte) ([]byte, bool) {
 	out := payload
 	changed := false
@@ -385,10 +416,14 @@ func insertAntigravityReasoningReplayItems(payload []byte, items [][]byte) ([]by
 			if sig == "" {
 				continue
 			}
-			path := fmt.Sprintf("request.contents.%d.parts.%d.thoughtSignature", ci, pi)
-			if strings.TrimSpace(gjson.GetBytes(out, path).String()) != "" {
-				continue
+			partPath, exists := antigravityExistingReplayPartPath(out, ci, pi)
+			if exists {
+				path := partPath + ".thoughtSignature"
+				if strings.TrimSpace(gjson.GetBytes(out, path).String()) != "" {
+					continue
+				}
 			}
+			path := antigravityReplayPartWritePath(out, ci, pi) + ".thoughtSignature"
 			updated, err := sjson.SetBytes(out, path, sig)
 			if err != nil {
 				continue
@@ -433,16 +468,41 @@ func mergeAntigravityFunctionCallPartReplay(payload []byte, itemResult gjson.Res
 
 	ci := antigravityReasoningReplayResolveContentIndex(payload, int(itemResult.Get("contentIndex").Int()))
 	pi := int(itemResult.Get("partIndex").Int())
-	pathSig := fmt.Sprintf("request.contents.%d.parts.%d.thoughtSignature", ci, pi)
 	out := payload
 	changed := false
+
+	partPath, exists := antigravityExistingReplayPartPath(out, ci, pi)
+	if !exists {
+		fc := map[string]any{"name": name}
+		if callID != "" {
+			fc["id"] = callID
+		}
+		if args.Type == gjson.String {
+			fc["args"] = args.String()
+		} else {
+			var parsed any
+			if json.Unmarshal([]byte(args.Raw), &parsed) == nil {
+				fc["args"] = parsed
+			}
+		}
+		part := map[string]any{"functionCall": fc}
+		if sig != "" {
+			part["thoughtSignature"] = sig
+		}
+		if updated, err := sjson.SetBytes(out, antigravityReplayPartWritePath(out, ci, pi), part); err == nil {
+			return updated, true
+		}
+		return payload, false
+	}
+
+	pathSig := partPath + ".thoughtSignature"
 	if sig != "" && strings.TrimSpace(gjson.GetBytes(out, pathSig).String()) == "" {
 		if updated, err := sjson.SetBytes(out, pathSig, sig); err == nil {
 			out = updated
 			changed = true
 		}
 	}
-	pathFC := fmt.Sprintf("request.contents.%d.parts.%d.functionCall", ci, pi)
+	pathFC := partPath + ".functionCall"
 	if !gjson.GetBytes(out, pathFC).Exists() {
 		fc := map[string]any{"name": name}
 		if callID != "" {

internal/runtime/executor/antigravity_reasoning_replay_test.go

@@ -114,6 +114,36 @@ func TestMergeAntigravityFunctionCallPartReplayMergesSignatureIntoExistingFuncti
 	}
 }
 
+func TestPrepareAntigravityGeminiReasoningReplayPayloadAppendsStaleThoughtSignatureWithoutNullParts(t *testing.T) {
+	internalcache.ClearAntigravityReasoningReplayCache()
+	t.Cleanup(internalcache.ClearAntigravityReasoningReplayCache)
+
+	item := []byte(`{"type":"thought_signature","contentIndex":8,"partIndex":3,"thoughtSignature":"sig-text"}`)
+	internalcache.CacheAntigravityReasoningReplayItems("gemini-3-flash-agent", "session:sess-stale-text", [][]byte{item})
+
+	payload := []byte(`{"sessionId":"sess-stale-text","request":{"contents":[{"role":"user","parts":[{"text":"hi"}]},{"role":"model","parts":[{"text":"visible answer"}]},{"role":"user","parts":[{"text":"next"}]}]}}`)
+	out, _, err := prepareAntigravityGeminiReasoningReplayPayload(context.Background(), "gemini-3-flash-agent", cliproxyexecutor.Request{}, cliproxyexecutor.Options{}, payload)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	parts := gjson.GetBytes(out, "request.contents.1.parts").Array()
+	if len(parts) != 2 {
+		t.Fatalf("parts length = %d, want 2; body=%s", len(parts), out)
+	}
+	for i, part := range parts {
+		if part.Type == gjson.Null {
+			t.Fatalf("parts.%d is null; body=%s", i, out)
+		}
+	}
+	if got := parts[0].Get("text").String(); got != "visible answer" {
+		t.Fatalf("text part = %q, want visible answer; body=%s", got, out)
+	}
+	if got := parts[1].Get("thoughtSignature").String(); got != "sig-text" {
+		t.Fatalf("thoughtSignature = %q, want sig-text; body=%s", got, out)
+	}
+}
+
 func TestAntigravityReasoningReplayScopeUsesStableSessionWithoutSessionId(t *testing.T) {
 	payload := []byte(`{"request":{"contents":[{"role":"user","parts":[{"text":"stable-user-text"}]}]}}`)
 	scope := antigravityReasoningReplayScopeFromPayload("gemini-3-flash-agent", payload)

internal/translator/antigravity/openai/chat-completions/antigravity_openai_request.go

@@ -183,8 +183,8 @@ func ConvertOpenAIRequestToAntigravity(modelName string, inputRawJSON []byte, _
 							text := item.Get("text").String()
 							if text != "" {
 								node, _ = sjson.SetBytes(node, "parts."+itoa(p)+".text", text)
+								p++
 							}
-							p++
 						case "image_url":
 							imageURL := item.Get("image_url.url").String()
 							if len(imageURL) > 5 {
@@ -257,8 +257,8 @@ func ConvertOpenAIRequestToAntigravity(modelName string, inputRawJSON []byte, _
 							text := item.Get("text").String()
 							if text != "" {
 								node, _ = sjson.SetBytes(node, "parts."+itoa(p)+".text", text)
+								p++
 							}
-							p++
 						case "image_url":
 							// If the assistant returned an inline data URL, preserve it for history fidelity.
 							imageURL := item.Get("image_url.url").String()

internal/translator/antigravity/openai/chat-completions/antigravity_openai_request_test.go

@@ -0,0 +1,56 @@
+package chat_completions
+
+import (
+	"testing"
+
+	"github.com/tidwall/gjson"
+)
+
+func TestConvertOpenAIRequestToAntigravitySkipsEmptyTextPartsWithoutNulls(t *testing.T) {
+	inputJSON := `{
+		"model": "gemini-3-flash",
+		"messages": [
+			{
+				"role": "user",
+				"content": [
+					{"type": "text", "text": ""},
+					{"type": "input_audio", "input_audio": {"data": "SUQzBA==", "format": "mp3"}}
+				]
+			},
+			{
+				"role": "assistant",
+				"content": [{"type": "text", "text": ""}],
+				"tool_calls": [{
+					"id": "call_1",
+					"type": "function",
+					"function": {"name": "read_file", "arguments": "{\"path\":\"a.txt\"}"}
+				}]
+			},
+			{"role": "tool", "tool_call_id": "call_1", "content": "{\"output\":\"ok\"}"},
+			{"role": "user", "content": "done"}
+		]
+	}`
+
+	result := ConvertOpenAIRequestToAntigravity("gemini-3-flash", []byte(inputJSON), false)
+	userParts := gjson.GetBytes(result, "request.contents.0.parts").Array()
+	if len(userParts) != 1 {
+		t.Fatalf("user parts length = %d, want 1. Output: %s", len(userParts), result)
+	}
+	if userParts[0].Type == gjson.Null {
+		t.Fatalf("user parts.0 is null. Output: %s", result)
+	}
+	if got := userParts[0].Get("inlineData.mime_type").String(); got != "audio/mpeg" {
+		t.Fatalf("audio mime_type = %q, want audio/mpeg. Output: %s", got, result)
+	}
+
+	assistantParts := gjson.GetBytes(result, "request.contents.1.parts").Array()
+	if len(assistantParts) != 1 {
+		t.Fatalf("assistant parts length = %d, want 1. Output: %s", len(assistantParts), result)
+	}
+	if assistantParts[0].Type == gjson.Null {
+		t.Fatalf("assistant parts.0 is null. Output: %s", result)
+	}
+	if !assistantParts[0].Get("functionCall").Exists() {
+		t.Fatalf("functionCall missing. Output: %s", result)
+	}
+}

internal/translator/gemini/openai/chat-completions/gemini_openai_request.go

@@ -181,8 +181,8 @@ func ConvertOpenAIRequestToGemini(modelName string, inputRawJSON []byte, _ bool)
 							text := item.Get("text").String()
 							if text != "" {
 								node, _ = sjson.SetBytes(node, "parts."+itoa(p)+".text", text)
+								p++
 							}
-							p++
 						case "image_url":
 							imageURL := item.Get("image_url.url").String()
 							if len(imageURL) > 5 {
@@ -249,8 +249,8 @@ func ConvertOpenAIRequestToGemini(modelName string, inputRawJSON []byte, _ bool)
 							text := item.Get("text").String()
 							if text != "" {
 								node, _ = sjson.SetBytes(node, "parts."+itoa(p)+".text", text)
+								p++
 							}
-							p++
 						case "image_url":
 							// If the assistant returned an inline data URL, preserve it for history fidelity.
 							imageURL := item.Get("image_url.url").String()

internal/translator/gemini/openai/chat-completions/gemini_openai_request_test.go

@@ -91,6 +91,55 @@ func TestConvertOpenAIRequestToGeminiPreservesVideoURL(t *testing.T) {
 	}
 }
 
+func TestConvertOpenAIRequestToGeminiSkipsEmptyTextPartsWithoutNulls(t *testing.T) {
+	inputJSON := `{
+		"model": "gemini-3-flash",
+		"messages": [
+			{
+				"role": "user",
+				"content": [
+					{"type": "text", "text": ""},
+					{"type": "input_audio", "input_audio": {"data": "SUQzBA==", "format": "mp3"}}
+				]
+			},
+			{
+				"role": "assistant",
+				"content": [{"type": "text", "text": ""}],
+				"tool_calls": [{
+					"id": "call_1",
+					"type": "function",
+					"function": {"name": "read_file", "arguments": "{\"path\":\"a.txt\"}"}
+				}]
+			},
+			{"role": "tool", "tool_call_id": "call_1", "content": "{\"output\":\"ok\"}"},
+			{"role": "user", "content": "done"}
+		]
+	}`
+
+	result := ConvertOpenAIRequestToGemini("gemini-3-flash", []byte(inputJSON), false)
+	userParts := gjson.GetBytes(result, "contents.0.parts").Array()
+	if len(userParts) != 1 {
+		t.Fatalf("user parts length = %d, want 1. Output: %s", len(userParts), result)
+	}
+	if userParts[0].Type == gjson.Null {
+		t.Fatalf("user parts.0 is null. Output: %s", result)
+	}
+	if got := userParts[0].Get("inlineData.mime_type").String(); got != "audio/mpeg" {
+		t.Fatalf("audio mime_type = %q, want audio/mpeg. Output: %s", got, result)
+	}
+
+	assistantParts := gjson.GetBytes(result, "contents.1.parts").Array()
+	if len(assistantParts) != 1 {
+		t.Fatalf("assistant parts length = %d, want 1. Output: %s", len(assistantParts), result)
+	}
+	if assistantParts[0].Type == gjson.Null {
+		t.Fatalf("assistant parts.0 is null. Output: %s", result)
+	}
+	if !assistantParts[0].Get("functionCall").Exists() {
+		t.Fatalf("functionCall missing. Output: %s", result)
+	}
+}
+
 func TestConvertOpenAIRequestToGeminiCleansToolSchemaRequiredFields(t *testing.T) {
 	inputJSON := `{
 		"model": "gemini-2.0-flash",