[BUG] Content Streaming from Mistral results in `TypeError (no implicit conversion of Array into String)`

### Basic checks

- [x] I searched existing issues - this hasn't been reported
- [x] I can reproduce this consistently
- [x] This is a RubyLLM bug, not my application code

### What's broken?

Content extraction fails for Mistral because the chunk of data returned has the following structure.
choices/delta/content is an array, which isn't expected

```json
{
"id": "836a02cf63f7429793b2a62e63dfce29", 
"object": "chat.completion.chunk", 
"created": 1758093959, 
"model": "magistral-small-latest", 
"choices": [{
   "index": 0, 
   "delta": {
     "content": [{
       "type": "thinking", 
       "thinking": [{"type": "text", "text": "Okay"}] 
     }]
   }, 
   "finish_reason": null
}], 
"p": "abcdefghijklmnopqr"
}
```

### How to reproduce

1. Configure RubyLLM with mistral and use mistral-small-latest
2. Ask `calculate 1+1`
3. exception `TypeError (no implicit conversion of Array into String)`  is raised

Note curiously enough, once in while the problematic message isn't included and the question succeeds. 

### Expected behavior

Process the streaming response without an exception.

### What actually happened

## The exception:

```
TypeError (no implicit conversion of Array into String):
/gems/ruby_llm-1.8.0/lib/ruby_llm/stream_accumulator.rb:23:in 'RubyLLM::StreamAccumulator#add'
/gems/ruby_llm-1.8.0/lib/ruby_llm/streaming.rb:20:in 'block (2 levels) in RubyLLM::Streaming#stream_response'
/gems/ruby_llm-1.8.0/lib/ruby_llm/streaming.rb:41:in 'block in RubyLLM::Streaming#handle_stream'
/gems/ruby_llm-1.8.0/lib/ruby_llm/streaming.rb:129:in 'block in RubyLLM::Streaming#handle_sse'
/gems/event_stream_parser-1.0.0/lib/event_stream_parser.rb:217:in 'EventStreamParser::Parser#dispatch_event'
/gems/event_stream_parser-1.0.0/lib/event_stream_parser.rb:73:in 'EventStreamParser::Parser#process_line'
/gems/event_stream_parser-1.0.0/lib/event_stream_parser.rb:46:in 'EventStreamParser::Parser#feed'
/gems/ruby_llm-1.8.0/lib/ruby_llm/streaming.rb:124:in 'RubyLLM::Streaming#handle_sse'
/gems/ruby_llm-1.8.0/lib/ruby_llm/streaming.rb:74:in 'RubyLLM::Streaming#process_stream_chunk'
/gems/ruby_llm-1.8.0/lib/ruby_llm/streaming.rb:87:in 'block in RubyLLM::Streaming#stream_processor'
/gems/faraday-2.13.4/lib/faraday/options/env.rb:176:in 'block in Faraday::Env#stream_response'
/gems/net-protocol-0.2.2/lib/net/protocol.rb:535:in 'Net::ReadAdapter#call_block'
/gems/net-protocol-0.2.2/lib/net/protocol.rb:526:in 'Net::ReadAdapter#<<'
/gems/net-protocol-0.2.2/lib/net/protocol.rb:168:in 'Net::BufferedIO#read'
/gems/net-http-0.6.0/lib/net/http/response.rb:631:in 'Net::HTTPResponse#read_chunked'
/gems/net-http-0.6.0/lib/net/http/response.rb:595:in 'block in Net::HTTPResponse#read_body_0'
/gems/net-http-0.6.0/lib/net/http/response.rb:588:in 'Net::HTTPResponse#inflater'
/gems/net-http-0.6.0/lib/net/http/response.rb:593:in 'Net::HTTPResponse#read_body_0'
/gems/net-http-0.6.0/lib/net/http/response.rb:363:in 'Net::HTTPResponse#read_body'
/gems/faraday-net_http-3.4.1/lib/faraday/adapter/net_http.rb:116:in 'block (2 levels) in Faraday::Adapter::NetHttp#request_with_wrapped_block'
/gems/net-http-0.6.0/lib/net/http.rb:2433:in 'block in Net::HTTP#transport_request'
/gems/net-http-0.6.0/lib/net/http/response.rb:320:in 'Net::HTTPResponse#reading_body'
/gems/net-http-0.6.0/lib/net/http.rb:2430:in 'Net::HTTP#transport_request'
/gems/net-http-0.6.0/lib/net/http.rb:2384:in 'Net::HTTP#request'
/gems/rack-mini-profiler-4.0.1/lib/patches/net_patches.rb:19:in 'block in Net::HTTP#request_with_mini_profiler'
/gems/rack-mini-profiler-4.0.1/lib/mini_profiler/profiling_methods.rb:51:in 'Rack::MiniProfiler::ProfilingMethods#step'
/gems/rack-mini-profiler-4.0.1/lib/patches/net_patches.rb:18:in 'Net::HTTP#request_with_mini_profiler'
/gems/airbrake-13.0.5/lib/airbrake/rails/net_http.rb:11:in 'block in Airbrake::Rails::NetHttp#request'
/gems/airbrake-13.0.5/lib/airbrake/rack.rb:21:in 'Airbrake::Rack.capture_timing'
/gems/airbrake-13.0.5/lib/airbrake/rails/net_http.rb:10:in 'Airbrake::Rails::NetHttp#request'
/gems/faraday-net_http-3.4.1/lib/faraday/adapter/net_http.rb:113:in 'block in Faraday::Adapter::NetHttp#request_with_wrapped_block'
/gems/net-http-0.6.0/lib/net/http.rb:1632:in 'Net::HTTP#start'
/gems/faraday-net_http-3.4.1/lib/faraday/adapter/net_http.rb:112:in 'Faraday::Adapter::NetHttp#request_with_wrapped_block'
/gems/faraday-net_http-3.4.1/lib/faraday/adapter/net_http.rb:98:in 'block in Faraday::Adapter::NetHttp#perform_request'
/gems/faraday-2.13.4/lib/faraday/options/env.rb:172:in 'Faraday::Env#stream_response'
/gems/faraday-net_http-3.4.1/lib/faraday/adapter/net_http.rb:97:in 'Faraday::Adapter::NetHttp#perform_request'
/gems/faraday-net_http-3.4.1/lib/faraday/adapter/net_http.rb:66:in 'block in Faraday::Adapter::NetHttp#call'
/gems/faraday-2.13.4/lib/faraday/adapter.rb:45:in 'Faraday::Adapter#connection'
/gems/faraday-net_http-3.4.1/lib/faraday/adapter/net_http.rb:65:in 'Faraday::Adapter::NetHttp#call'
/gems/ruby_llm-1.8.0/lib/ruby_llm/error.rb:39:in 'RubyLLM::ErrorMiddleware#call'
/gems/faraday-2.13.4/lib/faraday/middleware.rb:56:in 'Faraday::Middleware#call'
/gems/faraday-2.13.4/lib/faraday/middleware.rb:56:in 'Faraday::Middleware#call'
/gems/faraday-retry-2.3.2/lib/faraday/retry/middleware.rb:171:in 'block in Faraday::Retry::Middleware#call'
/gems/faraday-retry-2.3.2/lib/faraday/retry/retryable.rb:7:in 'Faraday::Retryable#with_retries'
/gems/faraday-retry-2.3.2/lib/faraday/retry/middleware.rb:167:in 'Faraday::Retry::Middleware#call'
/gems/faraday-2.13.4/lib/faraday/middleware.rb:56:in 'Faraday::Middleware#call'
/gems/faraday-2.13.4/lib/faraday/response/logger.rb:25:in 'Faraday::Response::Logger#call'
/gems/faraday-2.13.4/lib/faraday/rack_builder.rb:153:in 'Faraday::RackBuilder#build_response'
/gems/faraday-2.13.4/lib/faraday/connection.rb:452:in 'Faraday::Connection#run_request'
/gems/faraday-2.13.4/lib/faraday/connection.rb:280:in 'Faraday::Connection#post'
/gems/ruby_llm-1.8.0/lib/ruby_llm/connection.rb:38:in 'RubyLLM::Connection#post'
/gems/ruby_llm-1.8.0/lib/ruby_llm/streaming.rb:11:in 'RubyLLM::Streaming#stream_response'
/gems/ruby_llm-1.8.0/lib/ruby_llm/provider.rb:56:in 'RubyLLM::Provider#complete'
/gems/ruby_llm-1.8.0/lib/ruby_llm/chat.rb:125:in 'RubyLLM::Chat#complete'
/gems/ruby_llm-1.8.0/lib/ruby_llm/active_record/chat_methods.rb:190:in 'RubyLLM::ActiveRecord::ChatMethods#complete'
/gems/ruby_llm-1.8.0/lib/ruby_llm/active_record/chat_methods.rb:184:in 'RubyLLM::ActiveRecord::ChatMethods#ask'
/app/jobs/llm_chat_job.rb:72:in 'LlmChatJob#execute'
```


## The cause 

The accumulator adds content with the following code (stream_accumulator.rb:23).
This fails because chunk.content is an array

```ruby
@content << (chunk.content || '')
```

## Workaround / Possible Solution

The problem that needs to be resolved is that content from the choices chunk is an array. This seems to be unexpected. This can be resolved by not accepting arrays as content. (Or at least the thinking message from Mistral)

I'm not sure what the best solution is for this problem.
In the solution below, I've added a custom streaming handler to Mistral.
Perhaps the global implementation should ignore array content items.
Because if another provider responds with array, it also breaks.

The workaround I've implemented is described below.

(Perhaps this can be implemented better by only fixing the content extraction for Mistral, but I don't know the internals of RubyLLM good enough)

Include a custom streaming provider to the mistral provider `providers/mistral.rb`


```ruby
include Mistral::Streaming
```

Create a provider chunk builder `providers/mistral/streaming.rb`
(Copied the used OpenAI implementation of `build_chunk` and changed the content to invoke the `extract_content` method)

```ruby
# frozen_string_literal: true

module RubyLLM
  module Providers
    class Mistral
      # Streaming methods of the OpenAI API integration
      module Streaming
        module_function

        def build_chunk(data)
          Chunk.new(
            role: :assistant,
            model_id: data['model'],
            content: extract_content(data),
            tool_calls: parse_tool_calls(data.dig('choices', 0, 'delta', 'tool_calls'), parse_arguments: false),
            input_tokens: data.dig('usage', 'prompt_tokens'),
            output_tokens: data.dig('usage', 'completion_tokens')
          )
        end

        def extract_content(data)
          data = data.dig('choices', 0, 'delta', 'content')
          return '' if data.is_a?(Array) || data.is_a?(Hash)  # the workaround

          data.to_s
        end
      end
    end
  end
end
```



### Environment

- Ruby 3.4.1
- RubyLLM 1.8.0
- Provider Mistral
- OS Mac OS X

Provide feedback

Saved searches

Use saved searches to filter your results more quickly

Uh oh!

Uh oh!

[BUG] Content Streaming from Mistral results in `TypeError (no implicit conversion of Array into String)` #420

Basic checks

What's broken?

How to reproduce

Expected behavior

What actually happened

The exception:

The cause

Workaround / Possible Solution

Environment

Metadata

Assignees

Labels

Projects

Milestone

Relationships

Development

Uh oh!

[BUG] Content Streaming from Mistral results in TypeError (no implicit conversion of Array into String) #420

Description

Basic checks

What's broken?

How to reproduce

Expected behavior

What actually happened

The exception:

The cause

Workaround / Possible Solution

Environment

Metadata

Metadata

Assignees

Labels

Projects

Milestone

Relationships

Development

Issue actions

[BUG] Content Streaming from Mistral results in `TypeError (no implicit conversion of Array into String)` #420