diff --git a/internal/reasoning/eino.go b/internal/reasoning/eino.go
index 0d7a0a30..7dbc1306 100644
--- a/internal/reasoning/eino.go
+++ b/internal/reasoning/eino.go
@@ -56,6 +56,7 @@ func ApplyToEinoChatModelConfig(cfg *einoopenai.ChatModelConfig, oa *config.Open
 	}
 
 	if mode == "off" {
+		applyThinkingDisabled(cfg)
 		return
 	}
 	effort := effectiveEffort(sr, client, allowClient)
@@ -185,11 +186,21 @@ func resolveWireProfile(oa *config.OpenAIConfig, sr *config.OpenAIReasoningConfi
 	}
 }
 
-func applyDeepseek(cfg *einoopenai.ChatModelConfig, mode, effort string) {
-	// auto: enable thinking for DeepSeek line; on: same; auto without effort still opens thinking.
-	if mode == "off" {
+func applyThinkingDisabled(cfg *einoopenai.ChatModelConfig) {
+	if cfg == nil {
 		return
 	}
+	if cfg.ExtraFields == nil {
+		cfg.ExtraFields = make(map[string]any)
+	}
+	if _, exists := cfg.ExtraFields["thinking"]; exists {
+		return
+	}
+	cfg.ExtraFields["thinking"] = map[string]any{"type": "disabled"}
+}
+
+func applyDeepseek(cfg *einoopenai.ChatModelConfig, mode, effort string) {
+	// auto: enable thinking for DeepSeek line; on: same; auto without effort still opens thinking.
 	if mode == "auto" || mode == "on" {
 		if cfg.ExtraFields == nil {
 			cfg.ExtraFields = make(map[string]any)
diff --git a/internal/reasoning/eino_test.go b/internal/reasoning/eino_test.go
index 1aae209e..5f23646f 100644
--- a/internal/reasoning/eino_test.go
+++ b/internal/reasoning/eino_test.go
@@ -49,6 +49,22 @@ func TestApplyOpenAICompat_xhighExtraField(t *testing.T) {
 	}
 }
 
+func TestApplyReasoningOff_disablesThinking(t *testing.T) {
+	cfg := &einoopenai.ChatModelConfig{}
+	oa := &config.OpenAIConfig{
+		BaseURL: "https://api.openai.com/v1",
+		Model:   "gpt-4o",
+		Reasoning: config.OpenAIReasoningConfig{
+			Mode: "off",
+		},
+	}
+	ApplyToEinoChatModelConfig(cfg, oa, nil)
+	th, ok := cfg.ExtraFields["thinking"].(map[string]any)
+	if !ok || th["type"] != "disabled" {
+		t.Fatalf("expected thinking disabled, got %#v", cfg.ExtraFields)
+	}
+}
+
 func TestApplyOpenAICompat_maxPassthrough(t *testing.T) {
 	cfg := &einoopenai.ChatModelConfig{}
 	oa := &config.OpenAIConfig{