Fix Mistral 'thinking' content parts

lib/chat_models/chat_mistral_ai.ex

@@ -579,12 +579,25 @@ defmodule LangChain.ChatModels.ChatMistralAI do
         _ -> "assistant"
       end
 
+    # Convert Mistral's content format to ContentPart structs
+    content = process_mistral_content(delta_body["content"])
+
+    # Adjust index to prevent thinking and text content from merging.
+    # Thinking is always at index 0. Text content is offset by 1 to avoid collision.
+    # Similar to DeepSeek: thinking at index 0, text starts at index 1.
+    adjusted_index =
+      case content do
+        %ContentPart{type: :thinking} -> 0
+        _ -> (index || 0) + 1
+      end
+
     data =
       delta_body
       |> Map.put("role", role)
-      |> Map.put("index", index)
+      |> Map.put("index", adjusted_index)
       |> Map.put("status", status)
       |> Map.put("tool_calls", tool_calls)
+      |> Map.put("content", content)
 
     case MessageDelta.new(data) do
       {:ok, message} ->
@@ -694,6 +707,33 @@ defmodule LangChain.ChatModels.ChatMistralAI do
      )}
   end
 
+  # Process Mistral's content format for thinking blocks and text in list format.
+  # Mistral can return:
+  # - Thinking: [%{"type" => "thinking", "thinking" => [%{"text" => "...", "type" => "text"}]}]
+  # - Text: [%{"type" => "text", "text" => "..."}]
+  defp process_mistral_content(nil), do: nil
+
+  defp process_mistral_content(content) when is_binary(content), do: content
+
+  defp process_mistral_content([%{"type" => "thinking", "thinking" => thinking_list} | _])
+       when is_list(thinking_list) do
+    # Extract text from thinking array and convert to ContentPart
+    thinking_text =
+      thinking_list
+      |> Enum.filter(&match?(%{"type" => "text"}, &1))
+      |> Enum.map(&Map.get(&1, "text", ""))
+      |> Enum.join("")
+
+    ContentPart.thinking!(thinking_text)
+  end
+
+  defp process_mistral_content([%{"type" => "text", "text" => text} | _]) do
+    ContentPart.text!(text)
+  end
+
+  # For any other content format, pass through unchanged
+  defp process_mistral_content(content), do: content
+
   defp get_token_usage(%{"usage" => usage} = _response_body) do
     # extract out the reported response token usage
     #

test/chat_models/chat_mistral_ai_test.exs

@@ -158,10 +158,100 @@ defmodule LangChain.ChatModels.ChatMistralAITest do
 
       assert delta.role == :assistant
       assert delta.content == "This is the first part of a mes"
+      assert delta.index == 1
+      assert delta.status == :incomplete
+    end
+
+    test "handles receiving MessageDeltas with thinking content", %{model: model} do
+      response = %{
+        "choices" => [
+          %{
+            "delta" => %{
+              "role" => "assistant",
+              "content" => [
+                %{
+                  "type" => "thinking",
+                  "thinking" => [
+                    %{"type" => "text", "text" => "Let me think about this"}
+                  ]
+                }
+              ]
+            },
+            "finish_reason" => nil,
+            "index" => 0
+          }
+        ]
+      }
+
+      assert [%MessageDelta{} = delta] =
+               ChatMistralAI.do_process_response(model, response)
+
+      assert delta.role == :assistant
+      assert %ContentPart{type: :thinking, content: "Let me think about this"} = delta.content
       assert delta.index == 0
       assert delta.status == :incomplete
     end
 
+    test "handles receiving MessageDeltas with multiple thinking text parts", %{model: model} do
+      response = %{
+        "choices" => [
+          %{
+            "delta" => %{
+              "role" => "assistant",
+              "content" => [
+                %{
+                  "type" => "thinking",
+                  "thinking" => [
+                    %{"type" => "text", "text" => "First part "},
+                    %{"type" => "text", "text" => "second part"}
+                  ]
+                }
+              ]
+            },
+            "finish_reason" => nil,
+            "index" => 0
+          }
+        ]
+      }
+
+      assert [%MessageDelta{} = delta] =
+               ChatMistralAI.do_process_response(model, response)
+
+      assert delta.role == :assistant
+      assert %ContentPart{type: :thinking, content: "First part second part"} = delta.content
+      assert delta.index == 0
+      assert delta.status == :incomplete
+    end
+
+    test "handles receiving MessageDeltas with text content in list format", %{model: model} do
+      response = %{
+        "choices" => [
+          %{
+            "delta" => %{
+              "role" => "assistant",
+              "content" => [
+                %{
+                  "type" => "text",
+                  "text" => "This is regular text"
+                }
+              ]
+            },
+            "finish_reason" => nil,
+            "index" => 0
+          }
+        ]
+      }
+
+      assert [%MessageDelta{} = delta] =
+               ChatMistralAI.do_process_response(model, response)
+
+      assert delta.role == :assistant
+      assert %ContentPart{type: :text, content: "This is regular text"} = delta.content
+      # Text content at index 0 is shifted to index 1 to avoid merging with thinking at index 0
+      assert delta.index == 1
+      assert delta.status == :incomplete
+    end
+
     test "handles API error messages", %{model: model} do
       response = %{
         "error" => %{