Fix OpenAI content encoding for thinking parts and empty arrays

lib/req_llm/provider/defaults.ex

@@ -819,13 +819,18 @@ defmodule ReqLLM.Provider.Defaults do
   defp encode_openai_content(content) when is_list(content) do
     content
     |> Enum.map(&encode_openai_content_part/1)
-    |> maybe_flatten_single_text()
+    |> normalize_encoded_content()
   end
 
-  defp maybe_flatten_single_text(content) do
+  # Normalize encoded content parts: reject nils, flatten single text blocks,
+  # and collapse empty arrays to "" (vLLM/strict OpenAI rejects "content": []).
+  defp normalize_encoded_content(content) do
     filtered = Enum.reject(content, &is_nil/1)
 
     case filtered do
+      [] ->
+        ""
+
       [%{type: "text", text: text} = block] ->
         if map_size(block) == 2, do: text, else: [block]
 
@@ -899,6 +904,10 @@ defmodule ReqLLM.Provider.Defaults do
     }
   end
 
+  # Reasoning model artifacts (e.g. chain-of-thought) — strip from OpenAI encoding
+  # since the format has no standard representation for thinking content.
+  defp encode_openai_content_part(%ReqLLM.Message.ContentPart{type: :thinking}), do: nil
+
   defp encode_openai_content_part(_), do: nil
 
   @passthrough_metadata_keys [:cache_control, "cache_control"]

test/req_llm/provider/defaults_test.exs

@@ -228,7 +228,7 @@ defmodule ReqLLM.Provider.DefaultsTest do
         messages: [
           %{
             role: "assistant",
-            content: [],
+            content: "",
             tool_calls: [
               %{
                 id: "call_123",
@@ -296,6 +296,58 @@ defmodule ReqLLM.Provider.DefaultsTest do
 
       refute Map.has_key?(encoded_message, :reasoning_details)
     end
+
+    test "strips :thinking content parts from encoding" do
+      message = %Message{
+        role: :assistant,
+        content: [
+          %ContentPart{type: :thinking, text: "Let me reason about this..."},
+          %ContentPart{type: :text, text: "Here is the answer."}
+        ]
+      }
+
+      context = %Context{messages: [message]}
+      result = Defaults.encode_context_to_openai_format(context, "gpt-4")
+
+      [encoded_message] = result.messages
+      # :thinking part stripped, single text part flattened to string
+      assert encoded_message.content == "Here is the answer."
+    end
+
+    test "collapses to empty string when all content parts are :thinking" do
+      message = %Message{
+        role: :assistant,
+        content: [
+          %ContentPart{type: :thinking, text: "Internal chain-of-thought"}
+        ],
+        tool_calls: [
+          %{
+            id: "call_abc",
+            type: "function",
+            function: %{name: "get_weather", arguments: ~s({"city":"NYC"})}
+          }
+        ]
+      }
+
+      context = %Context{messages: [message]}
+      result = Defaults.encode_context_to_openai_format(context, "gpt-4")
+
+      [encoded_message] = result.messages
+      # All parts filtered → empty array collapsed to ""
+      assert encoded_message.content == ""
+      # Tool calls still present
+      assert length(encoded_message.tool_calls) == 1
+    end
+
+    test "collapses empty content list to empty string" do
+      message = %Message{role: :assistant, content: []}
+
+      context = %Context{messages: [message]}
+      result = Defaults.encode_context_to_openai_format(context, "gpt-4")
+
+      [encoded_message] = result.messages
+      assert encoded_message.content == ""
+    end
   end
 
   describe "decode_response_body_openai_format/2" do