From d899c9623952299042507cbbfbf6372d0a52d741 Mon Sep 17 00:00:00 2001
From: Luis Pater <webmaster@idotorg.org>
Date: Thu, 9 Jul 2026 04:17:28 +0800
Subject: [PATCH] feat(translator): map OpenAI max_tokens to Gemini's
 maxOutputTokens

- Added logic to convert OpenAI `max_tokens` and `max_completion_tokens` to `generationConfig.maxOutputTokens`.
- Implemented unit tests to validate proper mapping behavior, including precedence of `max_tokens` over `max_completion_tokens`.

Closes: #4108
---
 .../chat-completions/gemini_openai_request.go |  7 ++++
 .../gemini_openai_request_test.go             | 33 +++++++++++++++++++
 2 files changed, 40 insertions(+)

diff --git a/internal/translator/gemini/openai/chat-completions/gemini_openai_request.go b/internal/translator/gemini/openai/chat-completions/gemini_openai_request.go
index bfe2ca13..d7b5e178 100644
--- a/internal/translator/gemini/openai/chat-completions/gemini_openai_request.go
+++ b/internal/translator/gemini/openai/chat-completions/gemini_openai_request.go
@@ -68,6 +68,13 @@ func ConvertOpenAIRequestToGemini(modelName string, inputRawJSON []byte, _ bool)
 		out, _ = sjson.SetBytes(out, "generationConfig.topK", tkr.Num)
 	}
 
+	// OpenAI max_tokens / max_completion_tokens -> Gemini generationConfig.maxOutputTokens
+	if mt := gjson.GetBytes(rawJSON, "max_tokens"); mt.Exists() && mt.Type == gjson.Number {
+		out, _ = sjson.SetBytes(out, "generationConfig.maxOutputTokens", mt.Num)
+	} else if mct := gjson.GetBytes(rawJSON, "max_completion_tokens"); mct.Exists() && mct.Type == gjson.Number {
+		out, _ = sjson.SetBytes(out, "generationConfig.maxOutputTokens", mct.Num)
+	}
+
 	// Candidate count (OpenAI 'n' parameter)
 	if n := gjson.GetBytes(rawJSON, "n"); n.Exists() && n.Type == gjson.Number {
 		if val := n.Int(); val > 1 {
diff --git a/internal/translator/gemini/openai/chat-completions/gemini_openai_request_test.go b/internal/translator/gemini/openai/chat-completions/gemini_openai_request_test.go
index f29221f2..bbeaae7c 100644
--- a/internal/translator/gemini/openai/chat-completions/gemini_openai_request_test.go
+++ b/internal/translator/gemini/openai/chat-completions/gemini_openai_request_test.go
@@ -140,6 +140,39 @@ func TestConvertOpenAIRequestToGeminiSkipsEmptyTextPartsWithoutNulls(t *testing.
 	}
 }
 
+func TestConvertOpenAIRequestToGeminiMapsMaxTokens(t *testing.T) {
+	tests := []struct {
+		name string
+		body string
+		want int64
+	}{
+		{
+			name: "max_tokens",
+			body: `{"model":"gemini-2.0-flash","messages":[{"role":"user","content":"hi"}],"max_tokens":30}`,
+			want: 30,
+		},
+		{
+			name: "max_completion_tokens",
+			body: `{"model":"gemini-2.0-flash","messages":[{"role":"user","content":"hi"}],"max_completion_tokens":40}`,
+			want: 40,
+		},
+		{
+			name: "max_tokens preferred over max_completion_tokens",
+			body: `{"model":"gemini-2.0-flash","messages":[{"role":"user","content":"hi"}],"max_tokens":30,"max_completion_tokens":40}`,
+			want: 30,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			out := ConvertOpenAIRequestToGemini("gemini-2.0-flash", []byte(tt.body), false)
+			if got := gjson.GetBytes(out, "generationConfig.maxOutputTokens").Int(); got != tt.want {
+				t.Fatalf("generationConfig.maxOutputTokens = %d, want %d. Output: %s", got, tt.want, out)
+			}
+		})
+	}
+}
+
 func TestConvertOpenAIRequestToGeminiCleansToolSchemaRequiredFields(t *testing.T) {
 	inputJSON := `{
 		"model": "gemini-2.0-flash",