feat(translator): add response_format conversion in Chat Completions to Gemini

Map OpenAI Chat Completions response_format (json_object / json_schema) to
Gemini generationConfig.responseMimeType and responseJsonSchema, mirroring
the Responses-side translator (bc652c7). The schema is passed through raw
to preserve additionalProperties:false and $defs/$ref that Gemini response
structured output requires; util.CleanJSONSchemaForGemini is deliberately
not applied (it is tuned for tool-calling schemas).

Co-Authored-By: Claude <noreply@anthropic.com>

Author: xyonium

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

@@ -75,6 +75,9 @@ func ConvertOpenAIRequestToGemini(modelName string, inputRawJSON []byte, _ bool)
 		}
 	}
 
+	// JSON structured output (OpenAI response_format -> Gemini responseMimeType/responseJsonSchema)
+	out = applyOpenAIResponseFormatToGemini(out, rawJSON)
+
 	// Map OpenAI modalities -> Gemini generationConfig.responseModalities
 	// e.g. "modalities": ["image", "text"] -> ["IMAGE", "TEXT"]
 	if mods := gjson.GetBytes(rawJSON, "modalities"); mods.Exists() && mods.IsArray() {
@@ -484,3 +487,29 @@ func openAIInputAudioMimeType(audioFormat string) string {
 		return "audio/" + audioFormat
 	}
 }
+
+// applyOpenAIResponseFormatToGemini maps OpenAI Chat Completions response_format to Gemini
+// generationConfig structured-output fields. The schema is passed through raw (not cleaned)
+// to match the Responses-side translator (bc652c7): Gemini response structured output requires
+// additionalProperties:false on strict schemas and supports $defs/$ref, which the tool-calling
+// schema cleaner (util.CleanJSONSchemaForGemini) would strip.
+func applyOpenAIResponseFormatToGemini(out []byte, rawJSON []byte) []byte {
+	rf := gjson.GetBytes(rawJSON, "response_format")
+	if !rf.Exists() {
+		return out
+	}
+
+	formatType := strings.ToLower(strings.TrimSpace(rf.Get("type").String()))
+	switch formatType {
+	case "json_object":
+		out, _ = sjson.SetBytes(out, "generationConfig.responseMimeType", "application/json")
+	case "json_schema":
+		out, _ = sjson.SetBytes(out, "generationConfig.responseMimeType", "application/json")
+		out, _ = sjson.DeleteBytes(out, "generationConfig.responseSchema")
+		if schema := rf.Get("json_schema.schema"); schema.Exists() {
+			out, _ = sjson.SetRawBytes(out, "generationConfig.responseJsonSchema", []byte(schema.Raw))
+		}
+	}
+
+	return out
+}

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

@@ -182,3 +182,208 @@ func TestConvertOpenAIRequestToGeminiCleansToolSchemaRequiredFields(t *testing.T
 		t.Fatalf("required[1] = %q, want industry. Schema: %s", got, schema.Raw)
 	}
 }
+
+func TestConvertOpenAIRequestToGemini_ResponseFormatJSONSchema(t *testing.T) {
+	inputJSON := `{
+		"model": "gemini-3.1-flash-lite",
+		"temperature": 0.2,
+		"messages": [
+			{
+				"role": "user",
+				"content": [
+					{
+						"type": "text",
+						"text": "Return structured JSON."
+					}
+				]
+			}
+		],
+		"response_format": {
+			"type": "json_schema",
+			"json_schema": {
+				"name": "response",
+				"strict": true,
+				"schema": {
+					"type": "object",
+					"properties": {
+						"cleanedContent": {
+							"type": "string"
+						}
+					},
+					"required": [
+						"cleanedContent"
+					],
+					"additionalProperties": false
+				}
+			}
+		}
+	}`
+
+	output := ConvertOpenAIRequestToGemini("gemini-3.1-flash-lite", []byte(inputJSON), false)
+	result := gjson.ParseBytes(output)
+	genConfig := result.Get("generationConfig")
+
+	if got := genConfig.Get("responseMimeType").String(); got != "application/json" {
+		t.Fatalf("responseMimeType = %q, want application/json. Output: %s", got, output)
+	}
+	schema := genConfig.Get("responseJsonSchema")
+	if !schema.Exists() {
+		t.Fatalf("responseJsonSchema missing. Output: %s", output)
+	}
+	if genConfig.Get("responseSchema").Exists() {
+		t.Fatalf("responseSchema should not be set with responseJsonSchema. Output: %s", output)
+	}
+	if got := schema.Get("type").String(); got != "object" {
+		t.Fatalf("schema type = %q, want object. Output: %s", got, output)
+	}
+	if got := schema.Get("properties.cleanedContent.type").String(); got != "string" {
+		t.Fatalf("cleanedContent type = %q, want string. Output: %s", got, output)
+	}
+	if additionalProperties := schema.Get("additionalProperties"); !additionalProperties.Exists() || additionalProperties.Bool() {
+		t.Fatalf("additionalProperties = %s, want false. Output: %s", additionalProperties.Raw, output)
+	}
+	if got := genConfig.Get("temperature").Float(); got != 0.2 {
+		t.Fatalf("temperature = %v, want 0.2. Output: %s", got, output)
+	}
+}
+
+func TestConvertOpenAIRequestToGemini_ResponseFormatJSONObject(t *testing.T) {
+	inputJSON := `{
+		"model": "gemini-flash-lite",
+		"messages": [{"role": "user", "content": "Return a JSON object."}],
+		"response_format": {
+			"type": "json_object"
+		}
+	}`
+
+	output := ConvertOpenAIRequestToGemini("gemini-3.1-flash-lite", []byte(inputJSON), false)
+	result := gjson.ParseBytes(output)
+	genConfig := result.Get("generationConfig")
+
+	if got := genConfig.Get("responseMimeType").String(); got != "application/json" {
+		t.Fatalf("responseMimeType = %q, want application/json. Output: %s", got, output)
+	}
+	if genConfig.Get("responseJsonSchema").Exists() {
+		t.Fatalf("responseJsonSchema should not be set for json_object. Output: %s", output)
+	}
+}
+
+func TestConvertOpenAIRequestToGemini_ResponseFormatAbsent(t *testing.T) {
+	inputJSON := `{
+		"model": "gemini-flash-lite",
+		"messages": [{"role": "user", "content": "plain text, no response_format"}],
+		"temperature": 0.5
+	}`
+
+	output := ConvertOpenAIRequestToGemini("gemini-3.1-flash-lite", []byte(inputJSON), false)
+	result := gjson.ParseBytes(output)
+
+	if result.Get("generationConfig.responseMimeType").Exists() {
+		t.Fatalf("responseMimeType should not be set when response_format absent. Output: %s", output)
+	}
+	if result.Get("generationConfig.responseJsonSchema").Exists() {
+		t.Fatalf("responseJsonSchema should not be set when response_format absent. Output: %s", output)
+	}
+	if got := result.Get("generationConfig.temperature").Float(); got != 0.5 {
+		t.Fatalf("temperature = %v, want 0.5. Output: %s", got, output)
+	}
+}
+
+func TestConvertOpenAIRequestToGemini_ResponseFormatJSONSchemaNoSchema(t *testing.T) {
+	// json_schema without an inner schema field: responseMimeType is still set,
+	// but responseJsonSchema is not (there is nothing to embed).
+	inputJSON := `{
+		"model": "gemini-flash-lite",
+		"messages": [{"role": "user", "content": "shape only"}],
+		"response_format": {
+			"type": "json_schema",
+			"json_schema": {
+				"name": "response"
+			}
+		}
+	}`
+
+	output := ConvertOpenAIRequestToGemini("gemini-3.1-flash-lite", []byte(inputJSON), false)
+	result := gjson.ParseBytes(output)
+	genConfig := result.Get("generationConfig")
+
+	if got := genConfig.Get("responseMimeType").String(); got != "application/json" {
+		t.Fatalf("responseMimeType = %q, want application/json. Output: %s", got, output)
+	}
+	if genConfig.Get("responseJsonSchema").Exists() {
+		t.Fatalf("responseJsonSchema should not be set when no schema provided. Output: %s", output)
+	}
+}
+
+func TestConvertOpenAIRequestToGemini_ResponseFormatUnknownType(t *testing.T) {
+	// An unrecognized response_format.type is a no-op: no responseMimeType,
+	// no responseJsonSchema, no spurious generationConfig created. Sibling
+	// fields like temperature still pass through.
+	inputJSON := `{
+		"model": "gemini-flash-lite",
+		"messages": [{"role": "user", "content": "plain text"}],
+		"temperature": 0.7,
+		"response_format": {
+			"type": "text"
+		}
+	}`
+
+	output := ConvertOpenAIRequestToGemini("gemini-3.1-flash-lite", []byte(inputJSON), false)
+	result := gjson.ParseBytes(output)
+
+	if result.Get("generationConfig.responseMimeType").Exists() {
+		t.Fatalf("responseMimeType should not be set for unknown type. Output: %s", output)
+	}
+	if result.Get("generationConfig.responseJsonSchema").Exists() {
+		t.Fatalf("responseJsonSchema should not be set for unknown type. Output: %s", output)
+	}
+	if got := result.Get("generationConfig.temperature").Float(); got != 0.7 {
+		t.Fatalf("temperature = %v, want 0.7. Output: %s", got, output)
+	}
+}
+
+func TestConvertOpenAIRequestToGemini_ResponseFormatPreservesUserGenerationConfig(t *testing.T) {
+	// A user-supplied generationConfig must be preserved; response_format only
+	// adds responseMimeType/responseJsonSchema on top rather than replacing it.
+	inputJSON := `{
+		"model": "gemini-flash-lite",
+		"messages": [{"role": "user", "content": "merge check"}],
+		"generationConfig": {
+			"temperature": 0.9,
+			"topP": 0.8
+		},
+		"response_format": {
+			"type": "json_schema",
+			"json_schema": {
+				"schema": {
+					"type": "object",
+					"properties": {"answer": {"type": "string"}},
+					"required": ["answer"],
+					"additionalProperties": false
+				}
+			}
+		}
+	}`
+
+	output := ConvertOpenAIRequestToGemini("gemini-3.1-flash-lite", []byte(inputJSON), false)
+	result := gjson.ParseBytes(output)
+	genConfig := result.Get("generationConfig")
+
+	if got := genConfig.Get("responseMimeType").String(); got != "application/json" {
+		t.Fatalf("responseMimeType = %q, want application/json. Output: %s", got, output)
+	}
+	schema := genConfig.Get("responseJsonSchema")
+	if !schema.Exists() {
+		t.Fatalf("responseJsonSchema missing. Output: %s", output)
+	}
+	if got := schema.Get("type").String(); got != "object" {
+		t.Fatalf("schema type = %q, want object. Output: %s", got, output)
+	}
+	// User-supplied generationConfig fields survive the merge.
+	if got := genConfig.Get("temperature").Float(); got != 0.9 {
+		t.Fatalf("user temperature = %v, want 0.9. Output: %s", got, output)
+	}
+	if got := genConfig.Get("topP").Float(); got != 0.8 {
+		t.Fatalf("user topP = %v, want 0.8. Output: %s", got, output)
+	}
+}