diff --git a/lib/active_agent/providers/open_ai/chat_provider.rb b/lib/active_agent/providers/open_ai/chat_provider.rb
index 4c1bd656..d4b498bc 100644
--- a/lib/active_agent/providers/open_ai/chat_provider.rb
+++ b/lib/active_agent/providers/open_ai/chat_provider.rb
@@ -102,7 +102,7 @@ def process_stream_chunk(api_response_event)
 
             # If we have a delta, we need to update a message in the stack
             message = find_or_create_message(api_message.index)
-            message = message_merge_delta(message, api_message.delta.as_json.deep_symbolize_keys)
+            message_merge_delta(message, api_message.delta.deep_to_h)
 
             # Stream back content changes as they come in
             if api_message.delta.content
@@ -138,7 +138,7 @@ def process_stream_chunk(api_response_event)
         def process_function_calls(api_function_calls)
           api_function_calls.each do |api_function_call|
             content = instrument("tool_call.active_agent", tool_name: api_function_call.dig(:function, :name)) do
-              case api_function_call[:type]
+              case api_function_call[:type].to_s
               when "function"
                 process_tool_call_function(api_function_call[:function])
               else
@@ -243,7 +243,7 @@ def hash_merge_delta(hash, delta)
                 end
               end
             when String
-              hash[key] += value
+              hash[key] += value.to_s
             else
               hash[key] = value
             end
diff --git a/test/providers/open_ai/chat_provider_test.rb b/test/providers/open_ai/chat_provider_test.rb
new file mode 100644
index 00000000..294f2974
--- /dev/null
+++ b/test/providers/open_ai/chat_provider_test.rb
@@ -0,0 +1,126 @@
+# frozen_string_literal: true
+
+require "test_helper"
+
+begin
+  require "openai"
+rescue LoadError
+  puts "OpenAI gem not available, skipping OpenAI Chat provider tests"
+  return
+end
+
+require_relative "../../../lib/active_agent/providers/open_ai/chat_provider"
+
+module Providers
+  module OpenAI
+    module Chat
+      class ChatProviderTest < ActiveSupport::TestCase
+
+        include WebMock::API
+
+        setup do
+          WebMock.enable!
+          @client = ::OpenAI::Client.new(base_url: "http://localhost", api_key: "test-key")
+        end
+
+        teardown do
+          WebMock.disable!
+        end
+
+        test "accumulates streaming tool call deltas into message_stack" do
+          stub_streaming_response(tool_calls_sse_response)
+
+          stream = @client.chat.completions.stream(
+            messages: [{ content: "What's the weather in Boston?", role: :user }],
+            model: "qwen-plus",
+            tools: weather_tool
+          )
+
+          chat_provider = ActiveAgent::Providers::OpenAI::ChatProvider.new
+
+          stream.each do |event|
+            chat_provider.send(:process_stream_chunk, event)
+          end
+
+          expected_message = {
+            index: 0,
+            role: :assistant,
+            tool_calls: [
+              {
+                index: 0,
+                id: "call_123",
+                function: {
+                  name: "get_weather",
+                  arguments: '{"city":"Paris","units":"celsius"}'
+                },
+                type: :function
+              }
+            ]
+          }
+
+          assert_equal(
+            [expected_message],
+            chat_provider.send(:message_stack),
+            "message_stack should contain one assistant message with merged tool_calls"
+          )
+        end
+
+        private
+
+        def stub_streaming_response(response_body, request_options = {})
+          default_request = {
+            messages: [{ content: "What's the weather in Boston?",role: "user" }],
+            model: "qwen-plus",
+            stream: true
+          }
+
+          stub_request(:post, "http://localhost/chat/completions")
+            .with(body: hash_including(default_request.merge(request_options)))
+            .to_return(
+              status: 200,
+              headers: { "Content-Type" => "text/event-stream" },
+              body: response_body
+            )
+        end
+
+        def tool_calls_sse_response
+          <<~SSE
+            data: {"id":"chatcmpl-1","object":"chat.completion.chunk","created":1234567890,"model":"qwen-plus","choices":[{"index":0,"content":null,"delta":{"role":"assistant","tool_calls":[{"index":0,"id":"call_123","type":"function","function":{"name":"get_weather","arguments":""}}]},"finish_reason":null}]}
+
+            data: {"id":"chatcmpl-1","object":"chat.completion.chunk","created":1234567890,"model":"qwen-plus","choices":[{"index":0,"content":null,"delta":{"tool_calls":[{"index":0,"type":"function","function":{"arguments":"{\\"city\\":"}}]},"finish_reason":null}]}
+
+            data: {"id":"chatcmpl-1","object":"chat.completion.chunk","created":1234567890,"model":"qwen-plus","choices":[{"index":0,"content":null,"delta":{"tool_calls":[{"index":0,"type":"function","function":{"arguments":"\\"Paris\\","}}]},"finish_reason":null}]}
+
+            data: {"id":"chatcmpl-1","object":"chat.completion.chunk","created":1234567890,"model":"qwen-plus","choices":[{"index":0,"content":null,"delta":{"tool_calls":[{"index":0,"type":"function","function":{"arguments":"\\"units\\":\\"celsius\\"}"}}]},"finish_reason":null}]}
+
+            data: {"id":"chatcmpl-1","object":"chat.completion.chunk","created":1234567890,"model":"qwen-plus","choices":[{"index":0,"delta":{},"finish_reason":"tool_calls"}]}
+
+            data: [DONE]
+
+          SSE
+        end
+
+        def weather_tool
+          [
+            {
+              type: :function,
+              function: {
+                name: "get_weather",
+                parameters: {
+                  type: "object",
+                  properties: {
+                    city: { type: "string" },
+                    units: { type: "string" }
+                  },
+                  required: ["city", "units"],
+                  additionalProperties: false
+                },
+                strict: true
+              }
+            }
+          ]
+        end
+      end
+    end
+  end
+end