diff --git a/R/provider-openai.R b/R/provider-openai.R
index dc9afec13..456249132 100644
--- a/R/provider-openai.R
+++ b/R/provider-openai.R
@@ -323,7 +323,8 @@ method(value_turn, ProviderOpenAI) <- function(
   })
 
   tokens <- value_tokens(provider, result)
-  cost <- get_token_cost(provider, tokens, variant = result$service_tier)
+  variant <- result$service_tier %||% "default"
+  cost <- get_token_cost(provider, tokens, variant = variant)
   AssistantTurn(
     contents = contents,
     json = result,
diff --git a/tests/testthat/test-provider-openai.R b/tests/testthat/test-provider-openai.R
index 2668428c8..44bdd37b2 100644
--- a/tests/testthat/test-provider-openai.R
+++ b/tests/testthat/test-provider-openai.R
@@ -142,3 +142,20 @@ test_that("can extract dummy response from malformed JSON", {
     list(custom_id = "123", response = list(status_code = 500))
   )
 })
+
+test_that("value_turn handles NULL service_tier gracefully", {
+  provider <- chat_openai_test()$get_provider()
+
+  result <- list(
+    output = list(
+      list(type = "message", content = list(list(text = "Hello")))
+    ),
+    usage = NULL,
+    service_tier = NULL
+  )
+
+  turn <- value_turn(provider, result)
+  expect_s7_class(turn, AssistantTurn)
+  expect_equal(turn@tokens[["input"]], 0)
+  expect_equal(turn@tokens[["output"]], 0)
+})