Streaming Code Blocks in Real-Time Chat

§1 Code blocks arrive before the code is complete

A streaming chat interface displays an AI reply while the model is still generating it. That feels natural for ordinary sentences, but code introduces extra challenges. A function may arrive without its closing bracket, and the Markdown fence that marks a code block may be incomplete for several updates.

The basic rule is simple: treat the reply as unfinished text until the stream ends. Your interface should make partial code readable without pretending that it is complete, valid, or ready to run.

§2 Token streams are not lines of code

Models generate tokens, which are pieces of text rather than guaranteed words or lines. A network update can contain part of a word, several tokens, or a larger chunk collected along the way. Do not assume that each update contains a complete programming statement.

A common delivery format is Server-Sent Events, or SSE. Your application receives structured events and extracts incremental text from them. Network reads do not necessarily align with complete SSE events, so use an SDK or a proper event parser instead of treating every incoming chunk as a complete message.

Keep the accumulated assistant text as your source of truth. Add each text delta to that buffer, then update the visible reply from the accumulated content.

§3 Markdown fences need an unfinished state

Markdown commonly marks code blocks with three backticks, optionally followed by a language name. During streaming, those backticks and the language label can arrive across separate updates. The closing fence may not appear until much later.

A streaming-aware renderer should distinguish ordinary text, a possible opening fence, and an active code block. For a newcomer, a Markdown library with documented support for incomplete input is usually easier than building a parser from scratch.

• – Keep a small trailing buffer when a fence marker might still be arriving.
• – Display an open code block as code even before its closing fence arrives.
• – Preserve indentation and line breaks exactly as the model sends them.
• – Reconcile the final message when generation finishes, including any unclosed fences.

§4 Syntax highlighting should follow the stream

Highlighting every character across an entire long answer can make the interface feel slower than the model. Start with plain, readable code, then add highlighting at a controlled pace. Updating on an animation frame or after a short batch of text often produces smoother results than rendering every individual delta.

Do not require valid code to render code. Incomplete strings, missing braces, and unfinished comments are normal during generation. Use a highlighter that tolerates partial input, and keep the code container stable to avoid distracting layout jumps.

§5 Model choice does not replace rendering logic

Whether you try Claude Sonnet 5.5 or Gemini 2.5 Flash, the interface still needs to handle partial Markdown. Ask the model to use fenced code blocks and include language labels, but treat that request as guidance rather than a guarantee.

Fluxio is a practical option when you want to test this experience through a streaming-first, pay-per-token AI API relay. One key provides access to multiple model families, and its OpenAI-compatible interface can fit an existing SDK-based application. OpenAI-style clients use https://api.fluxioapi.com/v1 as the base URL. Keep the key on your server, not in browser code.

§6 Stream completion controls copy and error states

Show a visible generating state while code is arriving. If you offer a copy button during generation, make it clear that it copies only the code received so far. A dropped connection should leave the partial answer visible with an interrupted status, not silently label it complete.

Render code as text rather than executable HTML, and sanitize any HTML your Markdown pipeline permits. Never automatically run model-generated code. Test split fences, missing language labels, multiple blocks, cancellation, and interrupted streams before shipping.

Sign up for Fluxio to try streaming replies.