diff --git a/pkg-r/tests/testthat/apps/basic/app.R b/pkg-r/tests/testthat/apps/basic/app.R
index efa2a9c7f..6a31f7463 100644
--- a/pkg-r/tests/testthat/apps/basic/app.R
+++ b/pkg-r/tests/testthat/apps/basic/app.R
@@ -24,12 +24,27 @@ dbDisconnect(conn)
 # Setup database source and QueryChat instance
 db_conn <- dbConnect(RSQLite::SQLite(), temp_db)
 
+# ellmer 0.5.0 moved model details out of `Provider` and into a new `Model`
+# class: `Provider()` no longer takes `model` as its second positional
+# argument, and `Chat$new()` now requires a separate `model` argument. `Model`
+# doesn't exist before 0.5.0, so branch on its presence to support both.
+mock_chat_client <- if (
+  exists("Model", where = asNamespace("ellmer"), inherits = FALSE)
+) {
+  MockChat$new(
+    ellmer::Provider("test", "test"),
+    model = ellmer::Model(name = "test")
+  )
+} else {
+  MockChat$new(ellmer::Provider("test", "test", "test"))
+}
+
 # Create QueryChat instance
 qc <- QueryChat$new(
   data_source = db_conn,
   table_name = "iris",
   greeting = "Welcome to the test app!",
-  client = MockChat$new(ellmer::Provider("test", "test", "test"))
+  client = mock_chat_client
 )
 
 ui <- page_sidebar(
diff --git a/pkg-r/tests/testthat/helper-fixtures.R b/pkg-r/tests/testthat/helper-fixtures.R
index 5180cf59d..7ce594ce5 100644
--- a/pkg-r/tests/testthat/helper-fixtures.R
+++ b/pkg-r/tests/testthat/helper-fixtures.R
@@ -173,7 +173,23 @@ mock_ellmer_chat_client <- function(
     private = private
   )
 
-  MockChat$new(ellmer::Provider("test", "test", "test"))
+  new_mock_chat(MockChat)
+}
+
+# ellmer 0.5.0 moved model details out of `Provider` and into a new `Model`
+# class: `Provider()` no longer takes `model` as its second positional
+# argument, and `Chat$new()` now requires a separate `model` argument. `Model`
+# doesn't exist before 0.5.0, so branch on its presence to support both.
+new_mock_chat <- function(MockChat, ...) {
+  if (exists("Model", where = asNamespace("ellmer"), inherits = FALSE)) {
+    MockChat$new(
+      ellmer::Provider("test", "test"),
+      model = ellmer::Model(name = "test"),
+      ...
+    )
+  } else {
+    MockChat$new(ellmer::Provider("test", "test", "test"), ...)
+  }
 }
 
 # shinychat::chat_restore() validates that `client` is an ellmer::Chat() R6