Fix streaming parallel tool calls concatenating JSON arguments

When Anthropic returns multiple tool calls in a single streaming
response, the JSON argument fragments from different tool calls
were concatenated into one invalid string because input_json_delta
events were all routed to @latest_tool_call_id.

Anthropic's streaming API includes an `index` field on every
content_block_delta that identifies which content block a fragment
belongs to. This fix:

1. Passes the block index through extract_tool_calls as a symbol key
2. Registers block index → tool call ID mappings from content_block_start
3. Routes input_json_delta fragments to the correct tool call by index
4. Falls back to @latest_tool_call_id for providers that don't send index

Fixes #710

Author: alexandrigo

.tool-versions

@@ -0,0 +1 @@
+ruby 3.4.7

lib/ruby_llm/providers/anthropic/tools.rb

@@ -69,12 +69,33 @@ def function_for(tool)
 
         def extract_tool_calls(data)
           if json_delta?(data)
-            { nil => ToolCall.new(id: nil, name: nil, arguments: data.dig('delta', 'partial_json')) }
+            # Use the content block index as the hash key so the accumulator
+            # can route fragments to the correct tool call during parallel
+            # streaming. Without this, all fragments go to @latest_tool_call_id
+            # and parallel tool call arguments get concatenated together.
+            block_index = data['index']
+            key = block_index ? "block_idx_#{block_index}" : nil
+            { key => ToolCall.new(id: nil, name: nil, arguments: data.dig('delta', 'partial_json')) }
+          elsif data['type'] == 'content_block_start' && data.dig('content_block', 'type') == 'tool_use'
+            block = data['content_block']
+            build_tool_use_start(block, data['index'])
           else
             parse_tool_calls(data['content_block'])
           end
         end
 
+        def build_tool_use_start(block, block_index)
+          input = block['input']
+          args = input.is_a?(Hash) && input.empty? ? +'' : (input || +'')
+          tool_calls = { block['id'] => ToolCall.new(id: block['id'], name: block['name'], arguments: args) }
+          if block_index
+            tool_calls["register_idx_#{block_index}"] = ToolCall.new(
+              id: block['id'], name: '_register_block_index', arguments: nil
+            )
+          end
+          tool_calls
+        end
+
         def parse_tool_calls(content_blocks)
           return nil if content_blocks.nil?
 

lib/ruby_llm/stream_accumulator.rb

@@ -72,36 +72,51 @@ def tool_calls_from_stream
       end
     end
 
-    def accumulate_tool_calls(new_tool_calls) # rubocop:disable Metrics/PerceivedComplexity
+    def accumulate_tool_calls(new_tool_calls)
+      @block_index_to_tool_call_id ||= {}
       RubyLLM.logger.debug { "Accumulating tool calls: #{new_tool_calls}" } if RubyLLM.config.log_stream_debug
-      new_tool_calls.each_value do |tool_call|
-        if tool_call.id
-          tool_call_id = tool_call.id.empty? ? SecureRandom.uuid : tool_call.id
-          tool_call_arguments = tool_call.arguments
-          if tool_call_arguments.nil? || (tool_call_arguments.respond_to?(:empty?) && tool_call_arguments.empty?)
-            tool_call_arguments = +''
-          end
-          @tool_calls[tool_call.id] = ToolCall.new(
-            id: tool_call_id,
-            name: tool_call.name,
-            arguments: tool_call_arguments,
-            thought_signature: tool_call.thought_signature
-          )
-          @latest_tool_call_id = tool_call.id
+      new_tool_calls.each do |key, tool_call|
+        if register_block_index?(key, tool_call)
+          block_key = key.to_s.sub('register_idx_', 'block_idx_')
+          @block_index_to_tool_call_id[block_key] = tool_call.id
+        elsif tool_call.id
+          register_tool_call(tool_call)
         else
-          existing = @tool_calls[@latest_tool_call_id]
-          if existing
-            fragment = tool_call.arguments
-            fragment = '' if fragment.nil?
-            existing.arguments << fragment
-            if tool_call.thought_signature && existing.thought_signature.nil?
-              existing.thought_signature = tool_call.thought_signature
-            end
-          end
+          append_tool_call_fragment(key, tool_call)
         end
       end
     end
 
+    def register_block_index?(key, tool_call)
+      tool_call.name == '_register_block_index' && key.to_s.start_with?('register_idx_')
+    end
+
+    def register_tool_call(tool_call)
+      tool_call_id = tool_call.id.empty? ? SecureRandom.uuid : tool_call.id
+      tool_call_arguments = tool_call.arguments
+      if tool_call_arguments.nil? || (tool_call_arguments.respond_to?(:empty?) && tool_call_arguments.empty?)
+        tool_call_arguments = +''
+      end
+      @tool_calls[tool_call.id] = ToolCall.new(
+        id: tool_call_id,
+        name: tool_call.name,
+        arguments: tool_call_arguments,
+        thought_signature: tool_call.thought_signature
+      )
+      @latest_tool_call_id = tool_call.id
+    end
+
+    def append_tool_call_fragment(key, tool_call)
+      target_id = @block_index_to_tool_call_id[key] || @latest_tool_call_id
+      existing = @tool_calls[target_id]
+      return unless existing
+
+      existing.arguments << (tool_call.arguments || '')
+      return unless tool_call.thought_signature && existing.thought_signature.nil?
+
+      existing.thought_signature = tool_call.thought_signature
+    end
+
     def find_tool_call(tool_call_id)
       if tool_call_id.nil?
         @tool_calls[@latest_tool_call]

spec/ruby_llm/stream_accumulator_spec.rb

@@ -14,5 +14,62 @@
       message = accumulator.to_message(nil)
       expect(message.tool_calls['call_1'].arguments).to eq({})
     end
+
+    context 'with parallel streaming tool calls' do
+      it 'keeps arguments separate when block index is provided' do
+        accumulator = described_class.new
+
+        register_tool_call(accumulator, id: 'toolu_A', name: 'get_market_data', block_index: 1)
+        register_tool_call(accumulator, id: 'toolu_B', name: 'web_search', block_index: 2)
+
+        add_delta(accumulator, 'block_idx_1', '{"symbol":')
+        add_delta(accumulator, 'block_idx_2', '{"query":')
+        add_delta(accumulator, 'block_idx_1', '"MNQM26"}')
+        add_delta(accumulator, 'block_idx_2', '"market news"}')
+
+        message = accumulator.to_message(nil)
+
+        expect(message.tool_calls['toolu_A'].arguments).to eq({ 'symbol' => 'MNQM26' })
+        expect(message.tool_calls['toolu_B'].arguments).to eq({ 'query' => 'market news' })
+      end
+    end
+
+    it 'falls back to latest_tool_call_id when no block index is provided' do
+      accumulator = described_class.new
+
+      tc = RubyLLM::ToolCall.new(id: 'call_1', name: 'weather', arguments: +'')
+      accumulator.add(
+        RubyLLM::Chunk.new(role: :assistant, content: nil, tool_calls: { 'call_1' => tc })
+      )
+
+      delta = RubyLLM::ToolCall.new(id: nil, name: nil, arguments: '{"city":"NYC"}')
+      accumulator.add(
+        RubyLLM::Chunk.new(role: :assistant, content: nil, tool_calls: { nil => delta })
+      )
+
+      message = accumulator.to_message(nil)
+      expect(message.tool_calls['call_1'].arguments).to eq({ 'city' => 'NYC' })
+    end
+  end
+
+  private
+
+  def register_tool_call(accumulator, id:, name:, block_index:)
+    tc = RubyLLM::ToolCall.new(id: id, name: name, arguments: +'')
+    register = RubyLLM::ToolCall.new(id: id, name: '_register_block_index', arguments: nil)
+    accumulator.add(
+      RubyLLM::Chunk.new(
+        role: :assistant,
+        content: nil,
+        tool_calls: { id => tc, "register_idx_#{block_index}" => register }
+      )
+    )
+  end
+
+  def add_delta(accumulator, block_key, json_fragment)
+    delta = RubyLLM::ToolCall.new(id: nil, name: nil, arguments: json_fragment)
+    accumulator.add(
+      RubyLLM::Chunk.new(role: :assistant, content: nil, tool_calls: { block_key => delta })
+    )
   end
 end