fix: TypeError: undefined is not an object

packages/opencode/src/session/index.ts

@@ -432,8 +432,8 @@ export namespace Session {
           new Decimal(0)
             .add(new Decimal(tokens.input).mul(costInfo?.input ?? 0).div(1_000_000))
             .add(new Decimal(tokens.output).mul(costInfo?.output ?? 0).div(1_000_000))
-            .add(new Decimal(tokens.cache.read).mul(costInfo?.cache.read ?? 0).div(1_000_000))
-            .add(new Decimal(tokens.cache.write).mul(costInfo?.cache.write ?? 0).div(1_000_000))
+            .add(new Decimal(tokens.cache.read).mul(costInfo?.cache?.read ?? 0).div(1_000_000))
+            .add(new Decimal(tokens.cache.write).mul(costInfo?.cache?.write ?? 0).div(1_000_000))
             // TODO: update models.dev to have better pricing model, for now:
             // charge reasoning tokens at the same rate as output tokens
             .add(new Decimal(tokens.reasoning).mul(costInfo?.output ?? 0).div(1_000_000))