fix: preserve capabilities when model/1 receives a populated LLMDB.Model struct

lib/req_llm.ex

@@ -288,11 +288,15 @@ defmodule ReqLLM do
   """
   @spec model(model_input()) :: {:ok, LLMDB.Model.t()} | {:error, term()}
   def model(%LLMDB.Model{} = model) do
-    model
-    |> Map.from_struct()
-    |> Enum.reject(fn {_key, value} -> is_nil(value) end)
-    |> Map.new()
-    |> LLMDB.Enrich.enrich_model()
+    non_nil =
+      model
+      |> Map.from_struct()
+      |> Enum.reject(fn {_key, value} -> is_nil(value) end)
+      |> Map.new()
+
+    enriched = LLMDB.Enrich.enrich_model(non_nil)
+
+    Map.merge(enriched, non_nil)
     |> LLMDB.Model.new()
     |> normalize_model_result()
   end

test/req_llm_test.exs

@@ -176,6 +176,28 @@ defmodule ReqLLMTest do
               }} = ReqLLM.model(model)
     end
 
+    test "preserves caller-provided fields from LLMDB.Model structs" do
+      model =
+        LLMDB.Model.new!(%{
+          id: "test-model-not-in-catalog",
+          provider: :openai,
+          base_url: "http://my-custom-endpoint.example.com",
+          capabilities: %{
+            chat: true,
+            json: %{schema: true},
+            tools: %{enabled: true, strict: true},
+            reasoning: %{enabled: true}
+          }
+        })
+
+      {:ok, result} = ReqLLM.model(model)
+
+      assert result.base_url == "http://my-custom-endpoint.example.com"
+      assert result.capabilities.json.schema == true
+      assert result.capabilities.tools.enabled == true
+      assert result.capabilities.reasoning.enabled == true
+    end
+
     test "returns error for map missing required fields" do
       assert {:error, error} = ReqLLM.model(%{id: "no-provider"})
       assert Exception.message(error) =~ "Inline model specs require :provider"