diff --git a/openai/responses.go b/openai/responses.go
index 4caa564b6..38e070e0e 100644
--- a/openai/responses.go
+++ b/openai/responses.go
@@ -525,6 +525,18 @@ func FromResponsesRequest(r ResponsesRequest) (*api.ChatRequest, error) {
 		options["num_predict"] = *r.MaxOutputTokens
 	}
 
+	var think *api.ThinkValue
+	if effort := r.Reasoning.Effort; effort != "" {
+		switch effort {
+		case "none":
+			think = &api.ThinkValue{Value: false}
+		case "low", "medium", "high", "max":
+			think = &api.ThinkValue{Value: effort}
+		default:
+			return nil, fmt.Errorf("invalid reasoning value: %q (must be \"high\", \"medium\", \"low\", \"max\", or \"none\")", effort)
+		}
+	}
+
 	// Convert tools from Responses API format to api.Tool format
 	var tools []api.Tool
 	for _, t := range r.Tools {
@@ -552,6 +564,7 @@ func FromResponsesRequest(r ResponsesRequest) (*api.ChatRequest, error) {
 		Options:  options,
 		Tools:    tools,
 		Format:   format,
+		Think:    think,
 	}, nil
 }
 
diff --git a/openai/responses_test.go b/openai/responses_test.go
index bcc79ce19..cb4f3bdbf 100644
--- a/openai/responses_test.go
+++ b/openai/responses_test.go
@@ -415,6 +415,86 @@ func TestFromResponsesRequest_Tools(t *testing.T) {
 	}
 }
 
+func TestFromResponsesRequest_ReasoningEffort(t *testing.T) {
+	tests := []struct {
+		name      string
+		effort    string
+		wantThink any
+		wantErr   bool
+	}{
+		{
+			name: "unset",
+		},
+		{
+			name:      "low",
+			effort:    "low",
+			wantThink: "low",
+		},
+		{
+			name:      "medium",
+			effort:    "medium",
+			wantThink: "medium",
+		},
+		{
+			name:      "high",
+			effort:    "high",
+			wantThink: "high",
+		},
+		{
+			name:      "max",
+			effort:    "max",
+			wantThink: "max",
+		},
+		{
+			name:      "none",
+			effort:    "none",
+			wantThink: false,
+		},
+		{
+			name:    "invalid",
+			effort:  "extreme",
+			wantErr: true,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			req := ResponsesRequest{
+				Model: "deepseek-v4-flash",
+				Input: ResponsesInput{Text: "hi"},
+			}
+			if tt.effort != "" {
+				req.Reasoning.Effort = tt.effort
+			}
+
+			chatReq, err := FromResponsesRequest(req)
+			if tt.wantErr {
+				if err == nil {
+					t.Fatal("expected error, got nil")
+				}
+				return
+			}
+			if err != nil {
+				t.Fatalf("unexpected error: %v", err)
+			}
+
+			if tt.wantThink == nil {
+				if chatReq.Think != nil {
+					t.Fatalf("Think = %#v, want nil", chatReq.Think)
+				}
+				return
+			}
+
+			if chatReq.Think == nil {
+				t.Fatalf("Think = nil, want %v", tt.wantThink)
+			}
+			if chatReq.Think.Value != tt.wantThink {
+				t.Errorf("Think.Value = %v, want %v", chatReq.Think.Value, tt.wantThink)
+			}
+		})
+	}
+}
+
 func TestFromResponsesRequest_FunctionCallOutput(t *testing.T) {
 	// Test a complete tool call round-trip:
 	// 1. User message asking about weather