diff --git a/internal/runtime/executor/antigravity_reasoning_replay.go b/internal/runtime/executor/antigravity_reasoning_replay.go
index 3b21c99e..68bd45d6 100644
--- a/internal/runtime/executor/antigravity_reasoning_replay.go
+++ b/internal/runtime/executor/antigravity_reasoning_replay.go
@@ -1236,16 +1236,19 @@ func degradeAntigravityClaudeToolProvenanceIDs(payload []byte) ([]byte, int) {
 	if !contents.IsArray() {
 		return payload, 0
 	}
-	out := payload
-	degraded := 0
-	for ci, content := range contents.Array() {
+	type partReplacement struct {
+		start int
+		end   int
+		data  []byte
+	}
+	replacements := make([]partReplacement, 0)
+	for _, content := range contents.Array() {
 		parts := content.Get("parts")
 		if !parts.IsArray() {
 			continue
 		}
 		seenFunctionCallInTurn := false
-		for pi, part := range parts.Array() {
-			partPath := fmt.Sprintf("request.contents.%d.parts.%d", ci, pi)
+		for _, part := range parts.Array() {
 			if fc := part.Get("functionCall"); fc.Exists() {
 				isFirstFC := !seenFunctionCallInTurn
 				seenFunctionCallInTurn = true
@@ -1253,15 +1256,19 @@ func degradeAntigravityClaudeToolProvenanceIDs(payload []byte) ([]byte, int) {
 				if !util.IsGeminiClaudeToolUseID(id) {
 					continue
 				}
-				out, _ = sjson.SetBytes(out, partPath+".functionCall.id", antigravitySyntheticToolCallID(id))
+				updatedPart, _ := sjson.SetBytes([]byte(part.Raw), "functionCall.id", antigravitySyntheticToolCallID(id))
 				if part.Get("thoughtSignature").Exists() && part.Get("thoughtSignature").String() != "" {
 					if isFirstFC {
-						out, _ = sjson.SetBytes(out, partPath+".thoughtSignature", internalsignature.GeminiSkipThoughtSignatureValidator)
+						updatedPart, _ = sjson.SetBytes(updatedPart, "thoughtSignature", internalsignature.GeminiSkipThoughtSignatureValidator)
 					} else {
-						out, _ = sjson.DeleteBytes(out, partPath+".thoughtSignature")
+						updatedPart, _ = sjson.DeleteBytes(updatedPart, "thoughtSignature")
 					}
 				}
-				degraded++
+				replacements = append(replacements, partReplacement{
+					start: part.Index,
+					end:   part.Index + len(part.Raw),
+					data:  updatedPart,
+				})
 				continue
 			}
 			if fr := part.Get("functionResponse"); fr.Exists() {
@@ -1269,12 +1276,36 @@ func degradeAntigravityClaudeToolProvenanceIDs(payload []byte) ([]byte, int) {
 				if !util.IsGeminiClaudeToolUseID(id) {
 					continue
 				}
-				out, _ = sjson.SetBytes(out, partPath+".functionResponse.id", antigravitySyntheticToolCallID(id))
-				degraded++
+				updatedPart, _ := sjson.SetBytes([]byte(part.Raw), "functionResponse.id", antigravitySyntheticToolCallID(id))
+				replacements = append(replacements, partReplacement{
+					start: part.Index,
+					end:   part.Index + len(part.Raw),
+					data:  updatedPart,
+				})
 			}
 		}
 	}
-	return out, degraded
+	if len(replacements) == 0 {
+		return payload, 0
+	}
+
+	// Mutate each small part independently, then copy the full request only once.
+	outputSize := len(payload)
+	for _, replacement := range replacements {
+		outputSize += len(replacement.data) - (replacement.end - replacement.start)
+	}
+	out := make([]byte, 0, outputSize)
+	last := 0
+	for _, replacement := range replacements {
+		if replacement.start < last || replacement.end > len(payload) {
+			return payload, 0
+		}
+		out = append(out, payload[last:replacement.start]...)
+		out = append(out, replacement.data...)
+		last = replacement.end
+	}
+	out = append(out, payload[last:]...)
+	return out, len(replacements)
 }
 
 // antigravityRepairUnsignedFirstFunctionCalls restores Gemini's bypass sentinel on
diff --git a/internal/runtime/executor/antigravity_reasoning_replay_test.go b/internal/runtime/executor/antigravity_reasoning_replay_test.go
index 94d1fc52..18db2f91 100644
--- a/internal/runtime/executor/antigravity_reasoning_replay_test.go
+++ b/internal/runtime/executor/antigravity_reasoning_replay_test.go
@@ -1820,6 +1820,38 @@ func TestDegradeAntigravityClaudeToolProvenanceIDs_AllCallsSigned_OnlyFirstGetsB
 	}
 }
 
+var antigravityDegradeBenchmarkOutput []byte
+
+func BenchmarkDegradeAntigravityClaudeToolProvenanceIDs(b *testing.B) {
+	const calls = 217
+	var payload strings.Builder
+	payload.Grow(1<<20 + calls*512)
+	payload.WriteString(`{"request":{"contents":[{"role":"user","parts":[{"text":"`)
+	payload.WriteString(strings.Repeat("x", 1<<20))
+	payload.WriteString(`"}]}`)
+	for i := range calls {
+		id := util.GeminiClaudeToolUseID(fmt.Sprintf("native-%d", i), "Read", `{"file_path":"/tmp/a"}`)
+		fmt.Fprintf(&payload, `,{"role":"model","parts":[{"thoughtSignature":"signature-%d","functionCall":{"id":"%s","name":"Read","args":{"file_path":"/tmp/a"}}}]}`, i, id)
+		fmt.Fprintf(&payload, `,{"role":"user","parts":[{"functionResponse":{"id":"%s","name":"Read","response":{"result":"ok"}}}]}`, id)
+	}
+	payload.WriteString(`]}}`)
+	input := []byte(payload.String())
+	if _, degraded := degradeAntigravityClaudeToolProvenanceIDs(input); degraded != calls*2 {
+		b.Fatalf("degraded = %d, want %d", degraded, calls*2)
+	}
+
+	b.SetBytes(int64(len(input)))
+	b.ReportAllocs()
+	b.ResetTimer()
+	for b.Loop() {
+		var degraded int
+		antigravityDegradeBenchmarkOutput, degraded = degradeAntigravityClaudeToolProvenanceIDs(input)
+		if degraded != calls*2 {
+			b.Fatalf("degraded = %d, want %d", degraded, calls*2)
+		}
+	}
+}
+
 func TestPrepareAntigravityGeminiReasoningReplay_ReordersPermutedParallelToolResponsesWithDegradedIDs(t *testing.T) {
 	internalcache.ClearAntigravityReasoningReplayCache()
 	t.Cleanup(internalcache.ClearAntigravityReasoningReplayCache)