feat(providers): add extra_body config to inject custom fields into request body

Allow configuring provider-specific fields like reasoning_split for minimax via
the model config's extra_body map. These fields are merged into the request
body last, giving them precedence over default values.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

pkg/config/config.go

@@ -936,10 +936,11 @@ type ModelConfig struct {
 	Workspace   string `json:"workspace,omitempty"`    // Workspace path for CLI-based providers
 
 	// Optional optimizations
-	RPM            int    `json:"rpm,omitempty"`              // Requests per minute limit
-	MaxTokensField string `json:"max_tokens_field,omitempty"` // Field name for max tokens (e.g., "max_completion_tokens")
-	RequestTimeout int    `json:"request_timeout,omitempty"`
-	ThinkingLevel  string `json:"thinking_level,omitempty"` // Extended thinking: off|low|medium|high|xhigh|adaptive
+	RPM            int            `json:"rpm,omitempty"`              // Requests per minute limit
+	MaxTokensField string         `json:"max_tokens_field,omitempty"` // Field name for max tokens (e.g., "max_completion_tokens")
+	RequestTimeout int            `json:"request_timeout,omitempty"`
+	ThinkingLevel  string         `json:"thinking_level,omitempty"`   // Extended thinking: off|low|medium|high|xhigh|adaptive
+	ExtraBody      map[string]any `json:"extra_body,omitempty"`       // Additional fields to inject into request body
 
 	// from security
 	secModelName string
@@ -2079,6 +2080,7 @@ func expandMultiKeyModels(models []*ModelConfig) []*ModelConfig {
 				MaxTokensField: m.MaxTokensField,
 				RequestTimeout: m.RequestTimeout,
 				ThinkingLevel:  m.ThinkingLevel,
+				ExtraBody:      m.ExtraBody,
 			}
 			expanded = append(expanded, additionalEntry)
 			fallbackNames = append(fallbackNames, expandedName)
@@ -2097,6 +2099,7 @@ func expandMultiKeyModels(models []*ModelConfig) []*ModelConfig {
 			MaxTokensField: m.MaxTokensField,
 			RequestTimeout: m.RequestTimeout,
 			ThinkingLevel:  m.ThinkingLevel,
+			ExtraBody:      m.ExtraBody,
 			apiKeys:        []string{keys[0]},
 		}
 

pkg/config/config_test.go

@@ -1193,3 +1193,59 @@ func TestConfigLogLevelEmpty(t *testing.T) {
 		t.Errorf("LogLevel = %q, want \"fatal\"", cfg.Gateway.LogLevel)
 	}
 }
+
+func TestDefaultConfig_MinimaxExtraBody(t *testing.T) {
+	cfg := DefaultConfig()
+
+	var minimaxCfg *ModelConfig
+	for i := range cfg.ModelList {
+		if cfg.ModelList[i].Model == "minimax/MiniMax-M2.5" {
+			minimaxCfg = &cfg.ModelList[i]
+			break
+		}
+	}
+	if minimaxCfg == nil {
+		t.Fatal("Minimax model not found in ModelList")
+	}
+	if minimaxCfg.ExtraBody == nil {
+		t.Fatal("Minimax ExtraBody should not be nil")
+	}
+	if got, ok := minimaxCfg.ExtraBody["reasoning_split"]; !ok || got != true {
+		t.Fatalf("Minimax ExtraBody[reasoning_split] = %v, want true", got)
+	}
+}
+
+func TestModelConfig_ExtraBodyRoundTrip(t *testing.T) {
+	dir := t.TempDir()
+	cfgPath := filepath.Join(dir, "config.json")
+
+	cfg := &Config{
+		ModelList: []ModelConfig{
+			{
+				ModelName: "test-model",
+				Model:     "openai/test",
+				APIKey:    "sk-test",
+				ExtraBody: map[string]any{"custom_field": "value", "num_field": 42},
+			},
+		},
+	}
+
+	if err := SaveConfig(cfgPath, cfg); err != nil {
+		t.Fatalf("SaveConfig error: %v", err)
+	}
+
+	loaded, err := LoadConfig(cfgPath)
+	if err != nil {
+		t.Fatalf("LoadConfig error: %v", err)
+	}
+
+	if loaded.ModelList[0].ExtraBody == nil {
+		t.Fatal("ExtraBody should not be nil after round-trip")
+	}
+	if got := loaded.ModelList[0].ExtraBody["custom_field"]; got != "value" {
+		t.Errorf("ExtraBody[custom_field] = %v, want value", got)
+	}
+	if got := loaded.ModelList[0].ExtraBody["num_field"]; got != float64(42) {
+		t.Errorf("ExtraBody[num_field] = %v, want 42", got)
+	}
+}

pkg/config/defaults.go

@@ -339,6 +339,7 @@ func DefaultConfig() *Config {
 				ModelName: "MiniMax-M2.5",
 				Model:     "minimax/MiniMax-M2.5",
 				APIBase:   "https://api.minimaxi.com/v1",
+				ExtraBody:  map[string]any{"reasoning_split": true},
 			},
 
 			// LongCat - https://longcat.chat/platform