diff --git a/lib/tesla.ex b/lib/tesla.ex
index 556bf0d9..02e13745 100644
--- a/lib/tesla.ex
+++ b/lib/tesla.ex
@@ -3,7 +3,7 @@ defmodule Tesla.Error do
 end
 
 defmodule Tesla.Env do
-  @type client :: Tesla.Client.t() | (t, stack -> t)
+  @type client :: Tesla.Client.t()
   @type method :: :head | :get | :delete | :trace | :options | :post | :put | :patch
   @type url :: binary
   @type param :: binary | [{binary | atom, param}]
@@ -41,7 +41,6 @@ end
 
 defmodule Tesla.Client do
   @type t :: %__MODULE__{
-          fun: (Tesla.Env.t(), Tesla.Env.stack() -> Tesla.Env.t()) | nil,
           pre: Tesla.Env.stack(),
           post: Tesla.Env.stack()
         }
@@ -88,15 +87,12 @@ defmodule Tesla do
   end
 
   @doc false
-  def execute(module, %{fun: fun, pre: pre, post: post} = client, options) do
+  def execute(module, %{pre: pre, post: post} = client, options) do
     env = struct(Env, options ++ [__module__: module, __client__: client])
-    stack = pre ++ wrapfun(fun) ++ module.__middleware__ ++ post ++ [effective_adapter(module)]
+    stack = pre ++ module.__middleware__ ++ post ++ [effective_adapter(module)]
     run(env, stack)
   end
 
-  defp wrapfun(nil), do: []
-  defp wrapfun(fun), do: [{:fn, fun}]
-
   @doc false
   def effective_adapter(module) do
     with nil <- adapter_per_module_from_config(module),
diff --git a/lib/tesla/builder.ex b/lib/tesla/builder.ex
index 22dba265..b8331fc8 100644
--- a/lib/tesla/builder.ex
+++ b/lib/tesla/builder.ex
@@ -175,7 +175,7 @@ defmodule Tesla.Builder do
 
       # fallback to keep backward compatibility
       def unquote(method)(fun, url, body, options) when is_function(fun) and is_list(options) do
-        unquote(method)(%Tesla.Client{fun: fun}, url, body, options)
+        Tesla.Migration.client_function!()
       end
 
       if unquote(docs) do
@@ -199,7 +199,7 @@ defmodule Tesla.Builder do
 
       # fallback to keep backward compatibility
       def unquote(method)(fun, url, body) when is_function(fun) do
-        unquote(method)(%Tesla.Client{fun: fun}, url, body)
+        Tesla.Migration.client_function!()
       end
 
       if unquote(docs) do
@@ -252,7 +252,7 @@ defmodule Tesla.Builder do
 
       # fallback to keep backward compatibility
       def unquote(method)(fun, url, options) when is_function(fun) and is_list(options) do
-        unquote(method)(%Tesla.Client{fun: fun}, url, options)
+        Tesla.Migration.client_function!()
       end
 
       if unquote(docs) do
@@ -275,7 +275,7 @@ defmodule Tesla.Builder do
 
       # fallback to keep backward compatibility
       def unquote(method)(fun, url) when is_function(fun) do
-        unquote(method)(%Tesla.Client{fun: fun}, url)
+        Tesla.Migration.client_function!()
       end
 
       if unquote(docs) do
diff --git a/lib/tesla/migration.ex b/lib/tesla/migration.ex
index 0575e1b4..213efe29 100644
--- a/lib/tesla/migration.ex
+++ b/lib/tesla/migration.ex
@@ -3,6 +3,7 @@ defmodule Tesla.Migration do
 
   @breaking_alias "https://github.com/teamon/tesla/wiki/0.x-to-1.0-Migration-Guide#dropped-aliases-support-159"
   @breaking_headers_map "https://github.com/teamon/tesla/wiki/0.x-to-1.0-Migration-Guide#headers-are-now-a-list-160"
+  @breaking_client_fun "https://github.com/teamon/tesla/wiki/0.x-to-1.0-Migration-Guide#dropped-client-as-function-176"
 
   def breaking_alias!(_kind, _name, nil), do: nil
 
@@ -80,6 +81,19 @@ defmodule Tesla.Migration do
 
   def breaking_headers_map!(_middleware, _opts, _caller), do: nil
 
+  ## CLIENT FUNCTION
+
+  def client_function! do
+    raise RuntimeError,
+      message: """
+
+        Using anonymous function as client has been removed.
+        Use `Tesla.build_client` instead
+
+        See #{@breaking_client_fun}
+      """
+  end
+
   ## UTILS
 
   defp elixir_module?(atom) do
diff --git a/test/tesla/0.x_to_1.0_migration_test.exs b/test/tesla/0.x_to_1.0_migration_test.exs
index 527361d7..ce432402 100644
--- a/test/tesla/0.x_to_1.0_migration_test.exs
+++ b/test/tesla/0.x_to_1.0_migration_test.exs
@@ -109,4 +109,13 @@ defmodule MigrationTest do
       )
     end
   end
+
+  describe "Drop client as function #176" do
+    test "error when passing a function as client" do
+      client = fn env, next -> Tesla.run(env, next) end
+      assert_raise RuntimeError, fn ->
+        Tesla.get(client, "/")
+      end
+    end
+  end
 end
diff --git a/test/tesla_test.exs b/test/tesla_test.exs
index e278d124..3f1285ab 100644
--- a/test/tesla_test.exs
+++ b/test/tesla_test.exs
@@ -181,22 +181,6 @@ defmodule TeslaTest do
       assert response.body == "some-data"
     end
 
-    test "request with client" do
-      client = fn env, next ->
-        env
-        |> Map.put(:url, "/prefix" <> env.url)
-        |> Tesla.run(next)
-      end
-
-      assert {:ok, response} = SimpleClient.get("/")
-      assert response.url == "/"
-      assert response.__client__ == %Tesla.Client{}
-
-      assert {:ok, response} = client |> SimpleClient.get("/")
-      assert response.url == "/prefix/"
-      assert response.__client__ == %Tesla.Client{fun: client}
-    end
-
     test "better errors when given nil opts" do
       assert_raise FunctionClauseError, fn ->
         Tesla.get("/", nil)