diff --git a/llm/oai/oai_test.go b/llm/oai/oai_test.go
new file mode 100644
index 0000000..7bea552
--- /dev/null
+++ b/llm/oai/oai_test.go
@@ -0,0 +1,96 @@
+package oai
+
+import (
+	"math"
+	"testing"
+
+	"sketch.dev/llm"
+)
+
+// TestCalculateCostFromTokens tests the calculateCostFromTokens method
+func TestCalculateCostFromTokens(t *testing.T) {
+	tests := []struct {
+		name                string
+		model               Model
+		cacheCreationTokens uint64
+		cacheReadTokens     uint64
+		outputTokens        uint64
+		want                float64
+	}{
+		{
+			name:                "Zero tokens",
+			model:               GPT41,
+			cacheCreationTokens: 0,
+			cacheReadTokens:     0,
+			outputTokens:        0,
+			want:                0,
+		},
+		{
+			name:                "1000 input tokens, 500 output tokens",
+			model:               GPT41,
+			cacheCreationTokens: 1000,
+			cacheReadTokens:     0,
+			outputTokens:        500,
+			// GPT41: Input: 200 per million, Output: 800 per million
+			// (1000 * 200 + 500 * 800) / 1_000_000 / 100 = 0.006
+			want: 0.006,
+		},
+		{
+			name:                "10000 input tokens, 5000 output tokens",
+			model:               GPT41,
+			cacheCreationTokens: 10000,
+			cacheReadTokens:     0,
+			outputTokens:        5000,
+			// (10000 * 200 + 5000 * 800) / 1_000_000 / 100 = 0.06
+			want: 0.06,
+		},
+		{
+			name:                "1000 input tokens, 500 output tokens Gemini",
+			model:               Gemini25Flash,
+			cacheCreationTokens: 1000,
+			cacheReadTokens:     0,
+			outputTokens:        500,
+			// Gemini25Flash: Input: 15 per million, Output: 60 per million
+			// (1000 * 15 + 500 * 60) / 1_000_000 / 100 = 0.00045
+			want: 0.00045,
+		},
+		{
+			name:                "With cache read tokens",
+			model:               GPT41,
+			cacheCreationTokens: 500,
+			cacheReadTokens:     500, // 500 tokens from cache
+			outputTokens:        500,
+			// (500 * 200 + 500 * 50 + 500 * 800) / 1_000_000 / 100 = 0.00525
+			want: 0.00525,
+		},
+		{
+			name:                "With all token types",
+			model:               GPT41,
+			cacheCreationTokens: 1000,
+			cacheReadTokens:     1000,
+			outputTokens:        1000,
+			// (1000 * 200 + 1000 * 50 + 1000 * 800) / 1_000_000 / 100 = 0.0105
+			want: 0.0105,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			// Create a service with the test model
+			svc := &Service{Model: tt.model}
+
+			// Create a usage object
+			usage := llm.Usage{
+				CacheCreationInputTokens: tt.cacheCreationTokens,
+				CacheReadInputTokens:     tt.cacheReadTokens,
+				OutputTokens:             tt.outputTokens,
+			}
+
+			totalCost := svc.calculateCostFromTokens(usage)
+			if math.Abs(totalCost-tt.want) > 0.0001 {
+				t.Errorf("calculateCostFromTokens(%s, cache_creation=%d, cache_read=%d, output=%d) = %v, want %v",
+					tt.model.ModelName, tt.cacheCreationTokens, tt.cacheReadTokens, tt.outputTokens, totalCost, tt.want)
+			}
+		})
+	}
+}