diff --git a/lib/absinthe/plug.ex b/lib/absinthe/plug.ex
index f5a6a45..81f0907 100644
--- a/lib/absinthe/plug.ex
+++ b/lib/absinthe/plug.ex
@@ -155,7 +155,7 @@ defmodule Absinthe.Plug do
   - `:adapter` -- (Optional) Absinthe adapter to use (default: `Absinthe.Adapter.LanguageConventions`).
   - `:context` -- (Optional) Initial value for the Absinthe context, available to resolvers. (default: `%{}`).
   - `:no_query_message` -- (Optional) Message to return to the client if no query is provided (default: "No query document supplied").
-  - `:json_codec` -- (Optional) A `module` or `{module, Keyword.t}` dictating which JSON codec should be used (default: `Jason`). The codec module should implement `encode!/2` (e.g., `module.encode!(body, opts)`).
+  - `:json_codec` -- (Optional) A `module` or `{module, Keyword.t}` dictating which JSON codec should be used (default: `Jason`). The codec module should implement `encode!/1` at minimum. Codecs that accept keyword options (like Jason) can also be configured as `{module, opts}`. Elixir 1.18+'s built-in `JSON` module is supported when passed as a bare module (i.e. `json_codec: JSON`).
   - `:pipeline` -- (Optional) `{module, atom}` reference to a 2-arity function that will be called to generate the processing pipeline. (default: `{Absinthe.Plug, :default_pipeline}`).
   - `:document_providers` -- (Optional) A `{module, atom}` reference to a 1-arity function that will be called to determine the document providers that will be used to process the request. (default: `{Absinthe.Plug, :default_document_providers}`, which configures `Absinthe.Plug.DocumentProvider.Default` as the lone document provider). A simple list of document providers can also be given. See `Absinthe.Plug.DocumentProvider` for more information about document providers, their role in processing requests, and how you can define and configure your own.
   - `:schema` -- (Required, if not handled by Mix.Config) The Absinthe schema to use. If a module name is not provided, `Application.get_env(:absinthe, :schema)` will be attempt to find one.
@@ -602,12 +602,29 @@ defmodule Absinthe.Plug do
       }) do
     conn
     |> put_resp_content_type(content_type)
-    |> send_resp(status, mod.encode!(body, opts))
+    |> send_resp(status, safe_encode!(mod, body, opts))
   end
 
   @doc false
   def encode_json!(value, %{json_codec: json_codec}) do
-    json_codec.module.encode!(value, json_codec.opts)
+    safe_encode!(json_codec.module, value, json_codec.opts)
+  end
+
+  # Safely encode a value using the given codec module and options.
+  #
+  # Elixir 1.18's built-in JSON module has a different `encode!/2` signature
+  # than Jason: `JSON.encode!/2` expects an encoder function as the second
+  # argument, not a keyword list of options. To stay compatible with both
+  # conventions, we call `encode!/1` when opts is empty, and only pass opts
+  # through when they are non-empty (which means the user chose a codec like
+  # Jason that actually supports keyword options).
+  @doc false
+  def safe_encode!(module, value, opts) when opts == [] do
+    module.encode!(value)
+  end
+
+  def safe_encode!(module, value, opts) do
+    module.encode!(value, opts)
   end
 
   @doc false
diff --git a/lib/absinthe/plug/graphiql.ex b/lib/absinthe/plug/graphiql.ex
index 561e74d..28dbc7c 100644
--- a/lib/absinthe/plug/graphiql.ex
+++ b/lib/absinthe/plug/graphiql.ex
@@ -242,12 +242,12 @@ defmodule Absinthe.Plug.GraphiQL do
 
         var_string =
           variables
-          |> config.json_codec.module.encode!(pretty: true)
+          |> pretty_encode!(config.json_codec.module)
           |> js_escape
 
         result =
           result
-          |> config.json_codec.module.encode!(pretty: true)
+          |> pretty_encode!(config.json_codec.module)
           |> js_escape
 
         config =
@@ -274,7 +274,7 @@ defmodule Absinthe.Plug.GraphiQL do
 
         var_string =
           variables
-          |> config.json_codec.module.encode!(pretty: true)
+          |> pretty_encode!(config.json_codec.module)
           |> js_escape
 
         config =
@@ -408,7 +408,7 @@ defmodule Absinthe.Plug.GraphiQL do
         header_string =
           val
           |> Enum.map(fn {k, v} -> %{"name" => k, "value" => v} end)
-          |> config.json_codec.module.encode!(pretty: true)
+          |> pretty_encode!(config.json_codec.module)
 
         Map.put(config, :default_headers, header_string)
 
@@ -473,4 +473,26 @@ defmodule Absinthe.Plug.GraphiQL do
   defp normalize_socket_url(%{socket_url: url} = config, _) do
     %{config | socket_url: "'#{url}'"}
   end
+
+  # Encode a value with pretty-printing when the codec supports keyword options.
+  # Jason and Poison accept `encode!(value, pretty: true)`, but Elixir 1.18's
+  # built-in JSON module uses a different `encode!/2` signature where the second
+  # argument is an encoder function, not keyword options.
+  defp pretty_encode!(value, module) do
+    if supports_keyword_opts?(module) do
+      module.encode!(value, pretty: true)
+    else
+      module.encode!(value)
+    end
+  end
+
+  @supported_keyword_codecs [Jason, Poison]
+
+  defp supports_keyword_opts?(module) when module in @supported_keyword_codecs, do: true
+
+  defp supports_keyword_opts?(module) do
+    # For unknown codecs, check if encode!/2 exists and is not the Elixir
+    # built-in JSON module (which uses encode!/2 with a function argument).
+    function_exported?(module, :encode!, 2) and module != JSON
+  end
 end
diff --git a/test/lib/absinthe/plug_test.exs b/test/lib/absinthe/plug_test.exs
index 47136bc..e0fb844 100644
--- a/test/lib/absinthe/plug_test.exs
+++ b/test/lib/absinthe/plug_test.exs
@@ -752,4 +752,61 @@ defmodule Absinthe.PlugTest do
   defp basic_opts(context) do
     Map.put(context, :opts, Absinthe.Plug.init(schema: TestSchema))
   end
+
+  describe "safe_encode!/3" do
+    test "calls encode!/1 when opts is empty" do
+      assert Absinthe.Plug.safe_encode!(Jason, %{a: 1}, []) == ~s({"a":1})
+    end
+
+    test "calls encode!/2 when opts is non-empty" do
+      result = Absinthe.Plug.safe_encode!(Jason, %{a: 1}, pretty: true)
+      assert result =~ "\"a\""
+      assert result =~ "\n"
+    end
+
+    if Code.ensure_loaded?(JSON) do
+      test "works with Elixir 1.18 built-in JSON module and empty opts" do
+        assert Absinthe.Plug.safe_encode!(JSON, %{a: 1}, []) == ~s({"a":1})
+      end
+    end
+  end
+
+  if Code.ensure_loaded?(JSON) do
+    describe "Elixir 1.18 JSON codec integration" do
+      @query """
+      {
+        item(id: "foo") {
+          name
+        }
+      }
+      """
+
+      test "works as json_codec for basic queries" do
+        opts = Absinthe.Plug.init(schema: TestSchema, json_codec: JSON)
+
+        assert %{status: 200, resp_body: resp_body} =
+                 conn(:post, "/", @query)
+                 |> put_req_header("content-type", "application/graphql")
+                 |> plug_parser
+                 |> Absinthe.Plug.call(opts)
+
+        assert Jason.decode!(resp_body) == %{"data" => %{"item" => %{"name" => "Foo"}}}
+      end
+
+      test "works as json_codec with JSON content type" do
+        opts = Absinthe.Plug.init(schema: TestSchema, json_codec: JSON)
+
+        query_str = ~S|{ item(id: "foo") { name } }|
+        body = JSON.encode!(%{query: query_str})
+
+        assert %{status: 200, resp_body: resp_body} =
+                 conn(:post, "/", body)
+                 |> put_req_header("content-type", "application/json")
+                 |> plug_parser
+                 |> Absinthe.Plug.call(opts)
+
+        assert Jason.decode!(resp_body) == %{"data" => %{"item" => %{"name" => "Foo"}}}
+      end
+    end
+  end
 end