diff --git a/internal/translator/antigravity/openai/chat-completions/antigravity_openai_response.go b/internal/translator/antigravity/openai/chat-completions/antigravity_openai_response.go
index 60fe4895..54458e6b 100644
--- a/internal/translator/antigravity/openai/chat-completions/antigravity_openai_response.go
+++ b/internal/translator/antigravity/openai/chat-completions/antigravity_openai_response.go
@@ -95,9 +95,7 @@ func ConvertAntigravityResponseToOpenAI(_ context.Context, _ string, originalReq
 	// Extract and set usage metadata (token counts).
 	if usageResult := gjson.GetBytes(rawJSON, "response.usageMetadata"); usageResult.Exists() {
 		cachedTokenCount := usageResult.Get("cachedContentTokenCount").Int()
-		if candidatesTokenCountResult := usageResult.Get("candidatesTokenCount"); candidatesTokenCountResult.Exists() {
-			template, _ = sjson.SetBytes(template, "usage.completion_tokens", candidatesTokenCountResult.Int())
-		}
+		template, _ = sjson.SetBytes(template, "usage.completion_tokens", usageResult.Get("candidatesTokenCount").Int())
 		if totalTokenCountResult := usageResult.Get("totalTokenCount"); totalTokenCountResult.Exists() {
 			template, _ = sjson.SetBytes(template, "usage.total_tokens", totalTokenCountResult.Int())
 		}
diff --git a/internal/translator/antigravity/openai/chat-completions/antigravity_openai_response_test.go b/internal/translator/antigravity/openai/chat-completions/antigravity_openai_response_test.go
index 18f0f58a..39429e06 100644
--- a/internal/translator/antigravity/openai/chat-completions/antigravity_openai_response_test.go
+++ b/internal/translator/antigravity/openai/chat-completions/antigravity_openai_response_test.go
@@ -128,6 +128,20 @@ func TestNoFinishReasonOnIntermediateChunks(t *testing.T) {
 	}
 }
 
+func TestConvertAntigravityResponseToOpenAIIncludesZeroCompletionTokensWhenMissing(t *testing.T) {
+	var param any
+	chunk := []byte(`{"response":{"usageMetadata":{"promptTokenCount":16,"thoughtsTokenCount":42,"totalTokenCount":58}}}`)
+
+	result := ConvertAntigravityResponseToOpenAI(context.Background(), "model", nil, nil, chunk, &param)
+	if len(result) != 1 {
+		t.Fatalf("expected 1 result, got %d", len(result))
+	}
+	completionTokens := gjson.GetBytes(result[0], "usage.completion_tokens")
+	if !completionTokens.Exists() || completionTokens.Int() != 0 {
+		t.Fatalf("completion_tokens = %s, want present with value 0. Output: %s", completionTokens.Raw, result[0])
+	}
+}
+
 func TestConvertAntigravityResponseToOpenAINonStreamRestoresDisambiguatedName(t *testing.T) {
 	first := "mcp__plugin_cloudflare_cloudflare-builds__workers_builds_get_build"
 	second := "mcp__plugin_cloudflare_cloudflare-builds__workers_builds_get_build_logs"
diff --git a/internal/translator/gemini/openai/chat-completions/gemini_openai_response.go b/internal/translator/gemini/openai/chat-completions/gemini_openai_response.go
index cb44c258..b9c68aa0 100644
--- a/internal/translator/gemini/openai/chat-completions/gemini_openai_response.go
+++ b/internal/translator/gemini/openai/chat-completions/gemini_openai_response.go
@@ -110,9 +110,7 @@ func ConvertGeminiResponseToOpenAI(_ context.Context, _ string, originalRequestR
 	// Usage is applied to the base template so it appears in the chunks.
 	if usageResult := gjson.GetBytes(rawJSON, "usageMetadata"); usageResult.Exists() {
 		cachedTokenCount := usageResult.Get("cachedContentTokenCount").Int()
-		if candidatesTokenCountResult := usageResult.Get("candidatesTokenCount"); candidatesTokenCountResult.Exists() {
-			baseTemplate, _ = sjson.SetBytes(baseTemplate, "usage.completion_tokens", candidatesTokenCountResult.Int())
-		}
+		baseTemplate, _ = sjson.SetBytes(baseTemplate, "usage.completion_tokens", usageResult.Get("candidatesTokenCount").Int())
 		if totalTokenCountResult := usageResult.Get("totalTokenCount"); totalTokenCountResult.Exists() {
 			baseTemplate, _ = sjson.SetBytes(baseTemplate, "usage.total_tokens", totalTokenCountResult.Int())
 		}
@@ -312,9 +310,7 @@ func ConvertGeminiResponseToOpenAINonStream(_ context.Context, _ string, origina
 	}
 
 	if usageResult := gjson.GetBytes(rawJSON, "usageMetadata"); usageResult.Exists() {
-		if candidatesTokenCountResult := usageResult.Get("candidatesTokenCount"); candidatesTokenCountResult.Exists() {
-			template, _ = sjson.SetBytes(template, "usage.completion_tokens", candidatesTokenCountResult.Int())
-		}
+		template, _ = sjson.SetBytes(template, "usage.completion_tokens", usageResult.Get("candidatesTokenCount").Int())
 		if totalTokenCountResult := usageResult.Get("totalTokenCount"); totalTokenCountResult.Exists() {
 			template, _ = sjson.SetBytes(template, "usage.total_tokens", totalTokenCountResult.Int())
 		}
diff --git a/internal/translator/gemini/openai/chat-completions/gemini_openai_response_test.go b/internal/translator/gemini/openai/chat-completions/gemini_openai_response_test.go
index 177f4082..c2714c49 100644
--- a/internal/translator/gemini/openai/chat-completions/gemini_openai_response_test.go
+++ b/internal/translator/gemini/openai/chat-completions/gemini_openai_response_test.go
@@ -7,6 +7,30 @@ import (
 	"github.com/tidwall/gjson"
 )
 
+func TestConvertGeminiResponseToOpenAIIncludesZeroCompletionTokensWhenMissing(t *testing.T) {
+	var param any
+	chunk := []byte(`{"usageMetadata":{"promptTokenCount":16,"thoughtsTokenCount":42,"totalTokenCount":58}}`)
+
+	result := ConvertGeminiResponseToOpenAI(context.Background(), "model", nil, nil, chunk, &param)
+	if len(result) != 1 {
+		t.Fatalf("expected 1 result, got %d", len(result))
+	}
+	completionTokens := gjson.GetBytes(result[0], "usage.completion_tokens")
+	if !completionTokens.Exists() || completionTokens.Int() != 0 {
+		t.Fatalf("completion_tokens = %s, want present with value 0. Output: %s", completionTokens.Raw, result[0])
+	}
+}
+
+func TestConvertGeminiResponseToOpenAINonStreamIncludesZeroCompletionTokensWhenMissing(t *testing.T) {
+	response := []byte(`{"usageMetadata":{"promptTokenCount":16,"thoughtsTokenCount":42,"totalTokenCount":58}}`)
+
+	result := ConvertGeminiResponseToOpenAINonStream(context.Background(), "model", nil, nil, response, nil)
+	completionTokens := gjson.GetBytes(result, "usage.completion_tokens")
+	if !completionTokens.Exists() || completionTokens.Int() != 0 {
+		t.Fatalf("completion_tokens = %s, want present with value 0. Output: %s", completionTokens.Raw, result)
+	}
+}
+
 func TestGeminiFinishReasonOnlyOnFinalChunk(t *testing.T) {
 	ctx := context.Background()
 	var param any