--- title: Using Websockets to Stream Incremental Markdown permalink: /futureproof/websockets-stream-incremental-markdown/ description: My exploration delves into the challenges of streaming Markdown-formatted responses from an LLM within a Python-based FastHTML/HTMX chatbot application, Chip O'Theseus. Recognizing that re-rendering entire responses for each incremental update creates a poor user experience, I investigate techniques for incrementally displaying and formatting Markdown content as it arrives over WebSockets. This journey covers various strategies, ranging from simple append-only text streaming with delayed full formatting to more complex server-side stateful parsing and even client-side enhancement with minimal JavaScript. Ultimately, the goal is to find the optimal balance between real-time responsiveness, formatting fidelity, and code simplicity within the constraints of a server-rendered, HTMX-driven environment. The article documents these options, and my personal preferneces, with examples. meta_description: Discover real-time Markdown streaming techniques for Python FastHTML chat apps using WebSockets and HTMX for smooth incremental updates. meta_keywords: Markdown streaming, incremental rendering, WebSockets, HTMX, FastHTML, Python chatbots, LLM markdown, real-time updates, stateful parsing, incremental UI layout: post sort_order: 3 --- {% raw %} ## Down The Rabbit Hole of Streaming Markdown This article dives into the technical challenge of displaying real-time, formatted text in a web application, specifically focusing on how to handle Markdown when streaming responses from a language model using Python's FastHTML framework, HTMX, and WebSockets. I explored the complexities of parsing and rendering Markdown in chunks, contrasting client-side JavaScript libraries with server-side Python approaches, and ultimately discussed various strategies and best practices for achieving a smooth, real-time chat experience that can handle Markdown formatting, even when the full response hasn't arrived yet, highlighting the trade-offs between immediate display, formatting accuracy, and potential user experience disruptions. Chip O'Theseus, the local AI embedded into the Pipulate workflow app built on FastHTML and HTMX currently streams responses over Websockets using Ollama. But all the LLMs like to format their responses as raw unformatted text with markdown code, and it may be less work to just format the markdown, streaming and all, than it is to prompt-wrangle them out of their markdown habits. I know this is a rabbit hole, and the issues range from accumulation and append-only, to retroactive editing of already delivered chunks of the stream. All the frontier chatbots are doing it now, so it's got to be out there. NLUX is the library most people are talking about. I won't be pursuing this right away, but I do need to start my research. So this is one of those I'll just put this here as a reminder to self situations. --- ## Let's Do Our AI Research > To stream and retroactively edit the markdown, or to accumulate and append > only write-once complete chunks? That is the question I will first ask Grok3, > and then ask ChatGPT! **Me**: How does markdown get streamed over JavaScript by ChatBots? It seems like once something has been streamed, it can't retroactively be turned into markdown. What are the techniques? The libraries? The best practices? **Grok 3 Deep Search**: ### Key Points - It seems likely that Markdown is streamed in chatbots using JavaScript by employing specialized libraries that parse and render it in real-time as chunks are received. - Research suggests that once text is streamed as plain text, it cannot be retroactively turned into Markdown, as it has already been rendered. - The evidence leans toward using libraries like NLUX and streaming-markdown for effective streaming, with best practices including generating responses in Markdown from the start. --- ### How Markdown is Streamed Markdown is typically streamed in chatbots by using JavaScript libraries designed to handle real-time parsing and rendering. These libraries, such as NLUX and streaming-markdown, process the Markdown text as it arrives in chunks, maintaining state to ensure correct formatting, like bold text or lists, even if a chunk ends mid-syntax. This allows users to see the formatted text build up dynamically, similar to how ChatGPT displays responses. ### Limitations and Retroactive Changes If text is initially streamed as plain text, it generally cannot be retroactively turned into Markdown. Once rendered, the text is displayed as is, and changing it to Markdown would require re-sending the entire message, which disrupts the user experience. To avoid this, chatbots should generate responses in Markdown format from the start, ensuring consistent formatting throughout the stream. ### Techniques and Libraries Key techniques include stateful parsing, where the library remembers the context between chunks, and incremental rendering to update the display in real-time. Recommended libraries include: - **NLUX**: A conversational AI library with a Markdown parser for real-time streaming, part of its ecosystem for LLM-powered applications. - **streaming-markdown**: A lightweight library (3kB Gzip) for parsing and rendering Markdown streams, supporting features like text selection during streaming. - **react-markdown**: While useful for static Markdown, it may not handle streaming well without additional customization. ### Best Practices Best practices involve using specialized libraries for streaming, ensuring the chatbot generates Markdown responses initially, and maintaining state correctly to handle incomplete syntax across chunks. This approach ensures a smooth, formatted user experience without the need for retroactive changes. --- ### Detailed Survey Note: Streaming Markdown in Chatbots Using JavaScript This note explores the mechanisms, libraries, and best practices for streaming Markdown in chatbots using JavaScript, addressing the challenges of real-time rendering and the limitations of retroactively applying Markdown to already streamed text. The discussion is informed by recent developments in conversational AI and Markdown parsing technologies, with a focus on practical implementation. #### Background and Context Markdown, a lightweight markup language, is widely used in chatbots to enhance text formatting, such as bold, italics, lists, and code blocks. In streaming scenarios, where responses are sent in chunks (e.g., via APIs like OpenAI), rendering Markdown in real-time poses unique challenges. The text arrives incrementally, and parsers must handle incomplete syntax, maintaining state to ensure correct formatting. This is particularly relevant for AI-powered chatbots, where user experience depends on smooth, dynamic updates. The user's concern about retroactively turning streamed text into Markdown highlights a key limitation: once text is rendered as plain text, changing it to Markdown is generally not feasible without re-sending the message, which can disrupt the conversation flow. This note examines how chatbots manage this process, the tools available, and the best approaches to ensure effective streaming. #### Techniques for Streaming Markdown Streaming Markdown requires parsers that can handle incremental updates, maintaining state between chunks to correctly interpret syntax. Key techniques include: - **Stateful Parsing**: The parser must remember the context, such as whether it's inside a bold section (`**text**`) or a list item, when a chunk ends mid-syntax. For example, if a chunk ends with `**bo`, the next chunk (`ld**`) must continue the bold formatting. This ensures that formatting is preserved across chunks. - **Incremental Rendering**: As each chunk is received, the rendering component updates the display, allowing users to see the text build up in real-time with proper Markdown formatting. This requires the parser to render partial Markdown correctly, avoiding premature closure of tags. - **Generating Markdown Responses**: To avoid issues with retroactive changes, chatbots should generate responses in Markdown format from the start. This ensures that the streaming parser can handle the text as intended, without needing to modify already rendered content. These techniques are critical for maintaining a seamless user experience, especially in AI-driven chatbots where responses are generated dynamically. #### Libraries for Streaming Markdown Several JavaScript libraries support streaming Markdown, each with specific features for real-time parsing and rendering. Below is a detailed comparison: | Library | Description | Key Features for Streaming | Use Case | |-----------------------|-----------------------------------------------------------------------------|-----------------------------------------------------------------|-------------------------------------------------------------------------| | NLUX | A conversational AI library with Markdown streaming support, part of its ecosystem for LLM-powered applications. | Real-time parsing, stateful handling, integration with React and Vanilla JS. | Ideal for AI chatbots needing dynamic, formatted responses. | | streaming-markdown | A lightweight (3kB Gzip) library for parsing and rendering Markdown streams, inspired by ChatGPT. | Optimistic parsing, text selection during streaming, ES module support. | Suitable for lightweight applications requiring real-time Markdown rendering. | | react-markdown | A popular React component for rendering Markdown, but may require customization for streaming. | Static rendering, plugin support (e.g., remark-gfm for tables), no native streaming. | Better for non-streaming scenarios, may need additional handling for real-time updates. | - **NLUX** is particularly noted for its `@nlux/markdown` package, which includes a `MarkdownStreamParser` that processes chunks with `next(value)` and signals completion with `complete()`. It supports features like syntax highlighting and customizable streaming animation speed, making it suitable for LLM-powered chatbots. - **streaming-markdown**, available via npm or CDN, allows developers to create a parser with built-in renderers (`default_renderer`, `logger_renderer`) and stream chunks using `parser_write`, ending with `parser_end`. Its small size (3kB Gzip) makes it efficient for real-time applications. - **react-markdown**, while widely used, has limitations in streaming scenarios. Discussions on platforms like Reddit and GitHub indicate that it may not handle partial Markdown well, requiring additional workarounds, such as buffering chunks or using custom parsers. #### Addressing Retroactive Changes The user's concern about retroactively turning streamed text into Markdown is valid. Once text is streamed as plain text and rendered, it cannot be changed to Markdown without re-sending the message. This is because the rendering process is one-way: the client displays the text based on how it's received, and altering it would require updating the DOM, which could disrupt the user experience. For example, if a chatbot sends "Hello *world" as plain text, it's rendered as is. Later realizing it should be in Markdown (e.g., "Hello *world*") cannot be applied retroactively without re-sending the entire message. To mitigate this, the best practice is to ensure the chatbot generates responses in Markdown from the start, leveraging the streaming parser to handle formatting dynamically. #### Best Practices for Implementation To ensure effective streaming of Markdown in chatbots, developers should follow these best practices: 1. **Use Specialized Libraries**: Opt for libraries like NLUX or streaming-markdown, designed for streaming scenarios, to handle stateful parsing and real-time rendering. Avoid relying on static Markdown renderers like react-markdown without customization. 2. **Generate Markdown from the Start**: Configure the chatbot, especially AI models like those from OpenAI, to generate responses in Markdown format. This can be achieved by prompting the model to include Markdown syntax, ensuring consistency throughout the stream. 3. **Handle State Correctly**: Ensure the parser maintains and updates its state with each chunk, handling incomplete syntax (e.g., lists, code blocks) across multiple chunks. This prevents rendering errors, such as premature tag closure. 4. **Optimize Performance**: Consider the performance implications, especially for long conversations. Libraries like NLUX use memoization to cache parsed Markdown blocks, reducing re-rendering overhead, as noted in Vercel's AI SDK documentation for Next.js. 5. **Test for Edge Cases**: Test the streaming parser with edge cases, such as chunks ending mid-syntax (e.g., `**bo` followed by `ld**`), to ensure correct rendering. This is crucial for maintaining user trust in the chatbot's formatting capabilities. #### Unexpected Detail: Performance Optimization An unexpected detail is the use of memoization in streaming Markdown, as seen in Vercel's AI SDK for Next.js. This technique caches parsed Markdown blocks to prevent re-parsing and re-rendering with each new token, significantly improving performance for long conversations. This is particularly relevant for chatbots handling complex, formatted responses, ensuring a smooth user experience without noticeable lag. #### Conclusion Streaming Markdown in chatbots using JavaScript involves using specialized libraries like NLUX and streaming-markdown, which handle real-time parsing and rendering with stateful techniques. The limitation of retroactively applying Markdown to already streamed plain text underscores the importance of generating responses in Markdown from the start. By following best practices, such as maintaining state and optimizing performance, developers can create chatbots that deliver rich, formatted content seamlessly. --- ### Key Citations - [Chatbot Dreams & Markdown Streams Creating React JS ChatGPT Bots in Minutes](https://medium.com/@salmenus/chatbot-dreams-markdown-streams-creating-react-js-chatgpt-bots-in-minutes-8ee760ae3b29) - [The Powerful Conversational AI JavaScript Library NLUX](https://docs.nlkit.com/nlux) - [Streaming markdown à la ChatGPT WIP](https://github.com/thetarnav/streaming-markdown) - [Markdown component for React](https://github.com/remarkjs/react-markdown) --- ## And Over to ChatGPT o3-mini-high > Yes, I know that ChatGPT 4.5 is an option, but o3 appears to be the highest > reasoning capability. It also has a larger token-window (currently) that 4.5, > and all that combined with Deep research makes it my favorite OpenAI model at > the moment. **Me**: [Feeds Grok3's full research] **ChatGPT o3-mini-high Deep Research**: Just to confirm—would you like me to turn this into a more polished article, or are you looking for something else like additional examples, implementation guides, or deeper comparisons between the libraries mentioned? Let me know how you'd like to build on this. **Me**: Validate it or go beyond it. I'm on FastHTML on Python. This is a very different kind of framework that uses HTMX. Put it in context for that use case. The streaming with that app is accomplished through WebSockets. **ChatGPT o3-mini-high Deep Research**: Got it. I’ll look into how Markdown streaming can be effectively implemented in a FastHTML (Python-based) framework using HTMX and WebSockets. I’ll validate the concepts you’ve covered and go deeper into how they apply in this particular context. I’ll explore best practices, any relevant Python or frontend libraries that could help with streaming Markdown rendering over WebSockets, and how it might differ from JavaScript-based approaches. I’ll let you know as soon as the findings are ready. # Streaming Markdown in FastHTML Chatbots with HTMX and WebSockets ## Stateful Markdown Parsing and Incremental Rendering Streaming a Markdown-formatted chatbot response means dealing with incomplete markup as it arrives. The system must **parse text statefully**, remembering open Markdown constructs (like an unclosed code block or emphasis) across chunks. This ensures the content is rendered correctly once all pieces arrive. In practice, incremental or optimistic parsing is used: as soon as a chunk starts a Markdown element, the UI reflects that (for example, opening a code block or italic section) without waiting for the closing token. The goal is **incremental rendering** – updating the DOM in small pieces as new tokens arrive – so the user can watch the answer unfold in real time instead of waiting for the full message. Importantly, once a portion of text is rendered, it should ideally remain undisturbed; new content is *appended* rather than re-rendering the whole thing. This preserves things like the user’s text selection or scroll position. For example, a streaming Markdown library will only add new DOM nodes instead of replacing existing ones, allowing users to select/copy text that’s already streamed ([GitHub - thetarnav/streaming-markdown: Ⓜ️⬇️ Streaming markdown à la ChatGPT (WIP)](https://github.com/thetarnav/streaming-markdown#:~:text=block%2C%20it%20will%20immediately%20style,the%20element%20accordingly)). In summary, a robust streaming setup needs to manage the Markdown parse state across chunks and update the display incrementally in a way that doesn’t constantly reset earlier content. ## Real-Time Markdown via HTMX WebSockets in FastHTML FastHTML, built on FastAPI/Starlette, integrates seamlessly with **HTMX** for real-time server-to-client updates. HTMX’s WebSocket extension enables streaming without custom JavaScript. To use it in FastHTML, you enable the extension and define a WebSocket route. For example, you would initialize the app with `FastHTML(exts='ws')` and mark your chat form with `hx_ext="ws"` and `ws_connect="/wsroute"` to open a WebSocket connection ([fasthtml-example/02_chatbot at main · AnswerDotAI/fasthtml-example · GitHub](https://github.com/AnswerDotAI/fasthtml-example/tree/main/02_chatbot#:~:text=As%20an%20alternative%2C%20we%20can,of%20sending%20the%20form%20data)). Instead of a normal POST, the form submission (e.g. clicking “Send”) sends the message via WebSocket (`ws_send=""` attribute makes it send the input’s value). On the server, the WebSocket handler can **push multiple updates** to the client in real-time. FastHTML’s API provides an async `send()` function for this. For instance, upon receiving a user message, the handler can immediately broadcast the user’s chat bubble back to the browser, then stream the AI’s response as it’s generated. Each call to `send()` delivers an HTML snippet that HTMX will swap into the page ([fasthtml-example/02_chatbot at main · AnswerDotAI/fasthtml-example · GitHub](https://github.com/AnswerDotAI/fasthtml-example/tree/main/02_chatbot#:~:text=%40app)) ([fasthtml-example/02_chatbot at main · AnswerDotAI/fasthtml-example · GitHub](https://github.com/AnswerDotAI/fasthtml-example/tree/main/02_chatbot#:~:text=,1%5D%29%2C%20hx_swap_oob%3D%27beforeend%27%2C%20id%3D%22chatlist)). Crucially, HTMX WebSockets use *out-of-band swaps*, meaning the server’s snippet identifies where it goes by ID. A common pattern is to have a container (like a `
`) for the chat messages. The server can send a new message wrapped in an element with `id="chatlist"` plus an `hx_swap_oob="beforeend"` attribute. This tells HTMX to append the new content to the `#chatlist` element instead of replacing the whole page ([fasthtml-example/02_chatbot at main · AnswerDotAI/fasthtml-example · GitHub](https://github.com/AnswerDotAI/fasthtml-example/tree/main/02_chatbot#:~:text=,1%5D%29%2C%20hx_swap_oob%3D%27beforeend%27%2C%20id%3D%22chatlist)). For example, right after receiving a user message `msg`, you might do: ```python await send(Div(ChatMessage(msg), hx_swap_oob='beforeend', id="chatlist")) ``` This sends the rendered HTML for that message and appends it to the chat list on the client, **instantly showing the user’s message** without waiting for the bot’s reply. You can then clear or reset the input field by sending an updated input element (FastHTML allows returning multiple pieces; e.g. returning a fresh input box as a second item, or using a small snippet to clear it) ([fasthtml-example/02_chatbot at main · AnswerDotAI/fasthtml-example · GitHub](https://github.com/AnswerDotAI/fasthtml-example/tree/main/02_chatbot#:~:text=,1%5D%29%2C%20hx_swap_oob%3D%27beforeend%27%2C%20id%3D%22chatlist)). After that, as the AI’s answer is ready (or as chunks arrive from an LLM streaming API), you continue using `await send(...)` to push out the assistant’s message. **Streaming the response chunk by chunk:** With WebSockets, you aren’t limited to just one reply – you can loop and send multiple partial messages. One approach is to keep replacing the last message’s content as it grows. For example, sending the assistant’s message bubble initially (perhaps empty or a loading indicator) and then updating it with the growing text on each chunk. However, replacing the same element repeatedly can cause subtle UX issues. As the FastHTML docs note, if you update an element that the user might be interacting with (e.g. selecting text) in place, things like the text selection can get lost ([fasthtml-example/02_chatbot at main · AnswerDotAI/fasthtml-example · GitHub](https://github.com/AnswerDotAI/fasthtml-example/tree/main/02_chatbot#:~:text=When%20streaming%20the%20response%20from,ws_streaming.py)). The recommended pattern is to **append new content** instead of doing full replacements. In practice, this means when the AI is sending a long message, you append each new piece of text to that message’s container. For example, FastHTML’s guide suggests accumulating the assistant’s text and sending just the new fragment as a `` with an appropriate ID each time: ```python for chunk in response_stream: messages[-1]["content"] += chunk # update full text (if keeping history) await send(Span(chunk, id=f"chat-content-{len(messages)-1}", hx_swap_oob="beforeend")) ``` Here each `chunk` (could be a word, sentence, or token string) is sent wrapped in a Span, targeting the current message’s content area to append at the end ([fasthtml-example/02_chatbot at main · AnswerDotAI/fasthtml-example · GitHub](https://github.com/AnswerDotAI/fasthtml-example/tree/main/02_chatbot#:~:text=for%20chunk%20in%20r%3A%20messages%5B,1%7D%22%2C%20hx_swap_oob%3D%22beforeend)). This way, earlier parts of the message aren’t touched on subsequent updates. The user sees the message progressively lengthen, and if they, say, start copying text before it’s done, new arrivals won’t interrupt their selection. (In contrast, if you constantly replaced the whole message HTML, the selection would reset on each update ([fasthtml-example/02_chatbot at main · AnswerDotAI/fasthtml-example · GitHub](https://github.com/AnswerDotAI/fasthtml-example/tree/main/02_chatbot#:~:text=When%20streaming%20the%20response%20from,ws_streaming.py)).) Using **HTMX out-of-band swaps** in this manner is a natural fit for streaming: each WebSocket send carries a self-contained snippet that either appends to the chat log or appends to the last message. By structuring the HTML with identifiable containers (e.g. a `
` for each message and maybe an inner span for the content text), the server can target precisely where new text should go. The end result is a smooth stream of messages via WebSocket. (Note: HTMX can also do this with Server-Sent Events in a similar fashion; in fact, HTMX’s author notes that **SSE may be even simpler for server-push streams** in some cases ([fasthtml-example/02_chatbot at main · AnswerDotAI/fasthtml-example · GitHub](https://github.com/AnswerDotAI/fasthtml-example/tree/main/02_chatbot#:~:text=A%20final%20note%3A%20in%20cases,even%20better%20choice%20than%20websockets)). But SSE or WebSocket both work with FastHTML – FastHTML even provides a simplified SSE interface. In either case, the incremental rendering logic remains similar.) ## Markdown Rendering Techniques in Python (vs JS Libraries) In a React or browser-JS setting, libraries like **NLUX** and **streaming-markdown** exist to handle streaming Markdown. They perform on-the-fly Markdown parsing in the browser so that as text arrives, it’s immediately formatted (e.g. bold text is wrapped in `` once the closing `**` appears, code blocks render as `
` as soon as ``` is encountered, etc.). For instance, *NLUX* provides a `MarkdownStreamParser` that you feed chunks to, and it updates a container element live ([@nlux/markdown - npm](https://www.npmjs.com/package/@nlux/markdown#:~:text=A%20lightweight%20JS%2FTS%20library%20that,time)). These JS libraries are optimized for incremental parsing: they maintain an internal parse state (like “we are inside a list” or “inside a code block”) and manipulate the DOM accordingly with each new token. They are essentially doing the heavy lifting on the client side.

In a Python + HTMX context, we don’t typically run a JS parser in the client (the whole appeal is to avoid needing a big JS framework). Instead, we leverage Python markdown parsers on the server or simpler front-end handling. The **React-Markdown** library (for React apps) is analogous to Python’s markdown libraries in that it takes a Markdown string and outputs a rendered result (components or HTML), but it isn’t designed for streaming chunk by chunk. In Python, you have packages like *markdown2*, *Python-Markdown*, or *markdown-it-py* that can convert Markdown text to HTML. However, they generally expect the full text at once. There aren’t well-known Python equivalents of NLUX’s incremental parser that handle streaming out-of-the-box. So, how can we still format the streamed text? 

One straightforward way is to **continually re-parse the accumulating text** as more arrives. The idea is to maintain the raw Markdown text of the assistant’s message as it grows, and on each update, run it through a Markdown parser to generate HTML, then use that HTML in your `send()` snippet. A developer recounting their first FastHTML streaming experiment noted that using a normal Markdown parser like *marked.js* (on the client) required keeping the full raw text and re-rendering the entire content every time ([First encounter with FastHTML: Building a FastHTML assistant | by Sirsh Amarteifio | Medium](https://medium.com/@mrsirsh/first-encounter-with-fasthtml-building-a-fasthtml-assistant-fe896d3a3e60#:~:text=Also%20shown%20below%20is%20the,the%20entire%20content%20each%20time)). This approach does work – effectively treating each chunk as a trigger to re-format the whole message so far – and ensures that any Markdown syntax is correctly interpreted once the closing tokens arrive. For example, if the first chunk starts a list (`- item1`) and the second chunk continues the list (`- item2`), re-parsing the combined text after chunk two will properly produce a `
  • …` structure with two list items, whereas parsing just the second chunk in isolation would not know it’s part of a list. The **downside** of re-parsing the entire message each time is that you then typically replace the old HTML with the new HTML, which (as discussed) can disrupt the user’s selection or cause a slight flicker. 

There are a few ways to mitigate that if full re-parsing is the chosen route. One is to only swap in the parts of the DOM that actually changed (if your parser or diffing logic can isolate that). Another simpler method is to wait until the end of the message to do one final clean-up re-render. You could stream plain text or minimally formatted output in real-time, and once the model finishes, take the full collected Markdown, run it through a parser for a definitive HTML version, and replace the content of that message with the fully rendered Markdown. This ensures perfect formatting (e.g. any stray `*` now correctly wrap text in `` tags, any code fences are properly closed with a concluding ```). In an HTMX setup, doing a final replacement is as easy as sending one more out-of-band swap targeting that message by its ID. For example, you might have given the assistant message element an id like `"msg-42"`. When the stream is done, send an update like: `
    ... fully rendered HTML ...
    ` to replace its inner content with the parsed result. This **retroactively applies** all Markdown styling.

However, many applications find that users rarely notice a tiny formatting adjustment at the end, so they might skip the final replace unless needed. It’s a trade-off: implementing a live incremental parser in Python or JS is complex, but re-rendering everything on each token is inefficient and jarring. The FastHTML team’s guidance leans toward appending plain text and not touching what’s already sent, to keep things smooth ([fasthtml-example/02_chatbot at main · AnswerDotAI/fasthtml-example · GitHub](https://github.com/AnswerDotAI/fasthtml-example/tree/main/02_chatbot#:~:text=When%20streaming%20the%20response%20from,ws_streaming.py)). That means during the stream you might see raw Markdown syntax (like `**bold**` appearing with asterisks) until it’s completed. If that is acceptable, the simplest implementation is: send each chunk as text and perhaps do one replacement at the end to swap in the fully rendered Markdown. In a similar vein, developers using Streamlit noticed that partially streamed Markdown didn’t format correctly until the message was re-drawn at the end – the interim state was unstyled ([Markdown rendering issue with chat "streaming" - Custom Components - Streamlit](https://discuss.streamlit.io/t/markdown-rendering-issue-with-chat-streaming/52549#:~:text=markdown%20formatting%20,ideas%3F%20Here%E2%80%99s%20the%20relevant%20code)) ([Markdown rendering issue with chat "streaming" - Custom Components - Streamlit](https://discuss.streamlit.io/t/markdown-rendering-issue-with-chat-streaming/52549#:~:text=Initial%20response%20formtting%201524%C3%971256%20190,KB)). The solution there was also to re-render once complete. 

On the other hand, if you want to handle formatting on the fly (for example, to treat triple backticks as a code block immediately), you might introduce a bit of custom logic. For instance, you could detect when a chunk starts a code block (```language) and at that point send an HTML snippet that opens a `
    ` section in the DOM. Then subsequent code lines can be sent as they arrive (perhaps wrapped in `` or as text nodes inside that container). When a closing ``` is encountered, you could send a snippet to close the block or simply end the code section. This is somewhat tricky to implement with pure HTMX swaps, because ensuring well-formed HTML at each step is non-trivial (you can’t send half of a tag via HTMX; each snippet must be valid on its own). In practice it might be easier to just style the code text as you go and rely on the final render to insert the closing tags. Some developers choose to **not attempt partial HTML for complex syntax**, and instead stream everything as preformatted text (e.g. wrap all incoming code text in a `` or set a monospace font) and then later format it. The approach really depends on how polished you want the streaming experience to be versus how much complexity to handle. 

## Tools and Libraries in a FastHTML/HTMX Ecosystem  
While the JavaScript world has specialized libraries (NLUX, `streaming-markdown`, etc.), in the FastHTML context you mostly compose existing pieces: **FastHTML + HTMX for transport/DOM updates**, and a **Markdown parser** (either on server or client) for formatting. There isn’t a one-size “plug and play” Python library that does streaming Markdown rendering out-of-the-box. Instead, you can leverage simple building blocks:

- **Python Markdown Parsers**: Libraries like *Markdown (Python-Markdown)*, *Mistune*, or *markdown-it-py* can convert Markdown text to HTML. You would typically use these to render the final message or large chunks. For example, after collecting the full assistant answer, you can do `html = markdown(markdown_text)` and send that. Some libraries may allow streaming or incremental use in theory (e.g., you might feed a parser line by line), but support is limited and you’d have to manage state manually. Most developers find it easier to just call the parser on the cumulative text whenever needed. If you need to sanitize the output (to prevent any malicious HTML in the Markdown), consider using something like the `bleach` library or the safe mode of the Markdown library, since you’ll be injecting HTML into the page.

- **FastHTML Built-ins**: FastHTML’s docs mention that it *“comes with marked and highlighter components”* available ([First encounter with FastHTML: Building a FastHTML assistant | by Sirsh Amarteifio | Medium](https://medium.com/@mrsirsh/first-encounter-with-fasthtml-building-a-fasthtml-assistant-fe896d3a3e60#:~:text=each%20time)). **Marked** is a JavaScript Markdown parser, and **highlight.js** is commonly used for code syntax highlighting. This suggests you could use a `` component (or similar) in FastHTML to handle Markdown on the client side. Although the example author didn’t use it, this could be a viable route: you send raw Markdown in your HTMX updates, and let a client-side script (Marked) convert it to HTML in the browser. Using this would be more akin to how React apps do it (rendering Markdown after receiving it). If FastHTML exposes it, you might simply wrap content in something that triggers the marked library. (This would need checking FastHTML’s latest docs for the exact usage.) The advantage is offloading parsing to the client and possibly achieving true incremental formatting. The downside is introducing JavaScript processing, but Marked is relatively lightweight and could be worth it if you want fully formatted output as it streams. Similarly, you could include the **Streaming Markdown** JS library (like the one by *thetarnav* on GitHub or NLUX’s parser). These can be loaded via a script tag and used to feed incoming text. To integrate that with HTMX, you might bypass the `hx-ext=ws` mechanism and manage a WebSocket manually in JavaScript so you can feed the parser directly. That’s a more custom solution – effectively implementing your own minimal front-end for the streaming part – which slightly diminishes the pure HTMX approach but is entirely possible. 

- **Syntax Highlighting for Code**: If your chatbot returns Markdown code blocks and you want them colored, you’ll need an extra step since neither the browser nor Markdown parser automatically does language highlighting. In a React setup, NLUX’s toolkit or `react-markdown` often integrates with a highlighter. In FastHTML, you can include highlight.js on the page (maybe FastHTML already did if it has a highlighter component). You can then configure it to automatically highlight `
    ` blocks after insertion. One way is to call a JavaScript function (perhaps via an HTMX `trigger` after swap or an `onLoad` script) to highlight the newly added code. Alternatively, **markdown-it-py** and some others can integrate code highlighting on the server side, emitting ``s with classes for tokens (but that bloats the HTML). Depending on the level of fidelity you want, you might simply render code blocks as `
    ` with a class indicating the language and let a client-side highlighter style them once at the end of the stream.

## Handling Retroactive Markdown Changes  
“Retroactive” changes refer to when a later part of the stream affects how earlier text should be interpreted. Classic examples are an asterisk that wasn’t closed until much later (turning earlier text italic), or the end of a code block that suddenly makes the preceding text become a code block. In a naive streaming, you might have displayed the literal markdown syntax until that point. Handling this retroactively in a smooth way is one of the trickiest parts of streaming Markdown. 

With the append-only approach (never altering what’s already been inserted), strictly speaking you *don’t* change the earlier content. That means, for example, if the user message contains `_Hello *world` and the `*` (asterisk) is not closed yet, the word "world" might just appear normally (with the asterisk visible or not applied) until the closing `*` arrives. When it does, how do we render "world" in italics? Without re-rendering, we cannot magically transform the already printed "world" text into `world` because we promised not to touch already sent parts. One could attempt to anticipate and not actually show the asterisk character, instead starting an italic element in advance – but doing that correctly requires your parser to predict if a `*` will be closed or not (which in a streaming scenario you assume it will eventually). Libraries like NLUX’s parser or *Lezer* (used in CodeMirror) do handle this: they keep the text in an internal buffer and only append styled elements once they’re sure of the syntax. In a server-driven approach, a **pragmatic solution** is the earlier suggestion: once the stream is complete, do a final cleanup. At that point, you have the full Markdown, so you can reparse and swap in the corrected formatting. This will retroactively fix things like italics, bold, or link syntax. It’s essentially a **deferred correction** – you let minor mis-formatting slide during the stream (show raw markers), and then correct it at the end so the final transcript looks perfect.

Another case is **unclosed code blocks**. If for some reason the AI stops and never outputs a closing ``` for a code block, the Markdown parser might consider the rest of the page as part of the code block (if we were continually appending). To guard against breaking the page layout, you’d want to close any open tags on final render. For example, if the model’s answer ended while a code block was open, you might append the closing ``` yourself (or just terminate the `
    ` in the HTML) to properly close that section. This can be done in that final rendering step – many chat UIs do something like *“[assistant didn’t finish code block, inserting closing backticks]”* behind the scenes. FastHTML/HTMX won’t do this automatically; it’s up to your application logic to detect and handle. 

In summary, **retroactive formatting fixes** are limited in a purely streaming scenario – you either accept slight formatting delays or use a final re-render. The FastHTML/HTMX approach of appending spans is great for preserving interaction, but it does mean until the message is complete, some Markdown elements may not render fully. If perfect fidelity during streaming is a must, you’d lean on a specialized front-end parser that can, for instance, turn on italic styling the moment it sees the start `*` and then complete it when the end `*` comes, all without server intervention. That’s what NLUX or streaming-markdown libraries do. But if you want to stay within the Python/HTMX ecosystem, the **best practice** is typically: **stream in simple HTML chunks**, then **replace or augment at completion**. This gives a good balance of real-time experience and correctness. For example, one might stream everything as text and have a tiny script that watches for a special “done” event to replace the last message’s innerHTML with a parsed version (using a library or precomputed on server). This way, the user sees a live update, and a second later the formatting “pops” into place (which users generally tolerate well, as it’s similar to a final polish step).

## Best Practices for FastHTML + WebSocket Markdown Streaming  
To tie everything together, here are some best-practice recommendations tailored to FastHTML/HTMX:

- **Use WebSockets or SSE for push** – FastHTML supports both. WebSockets (with `hx_ext="ws"`) are straightforward for bidirectional chat. SSE (`hx-ext="sse"`) is a good alternative if the client only needs server pushes. Either way, set up your FastHTML routes to send incremental messages (FastHTML’s `send()` or generator `yield` for SSE) as the LLM response streams ([fasthtml-example/02_chatbot at main · AnswerDotAI/fasthtml-example · GitHub](https://github.com/AnswerDotAI/fasthtml-example/tree/main/02_chatbot#:~:text=%40app)) ([fasthtml-example/02_chatbot at main · AnswerDotAI/fasthtml-example · GitHub](https://github.com/AnswerDotAI/fasthtml-example/tree/main/02_chatbot#:~:text=for%20chunk%20in%20r%3A%20messages%5B,1%7D%22%2C%20hx_swap_oob%3D%22beforeend)).

- **Immediate User Feedback** – Update the UI immediately with the user’s own message. With HTMX out-of-band swaps, you can append the user’s chat bubble to the conversation as soon as they hit send ([fasthtml-example/02_chatbot at main · AnswerDotAI/fasthtml-example · GitHub](https://github.com/AnswerDotAI/fasthtml-example/tree/main/02_chatbot#:~:text=,1%5D%29%2C%20hx_swap_oob%3D%27beforeend%27%2C%20id%3D%22chatlist)). This makes the interface feel snappy. Then provide some indication that the system is “thinking” (could be as simple as an empty message from the assistant or a typing indicator).

- **Append-Only Updates** – When streaming the assistant’s answer, prefer appending content to the existing message element rather than full replacements ([fasthtml-example/02_chatbot at main · AnswerDotAI/fasthtml-example · GitHub](https://github.com/AnswerDotAI/fasthtml-example/tree/main/02_chatbot#:~:text=When%20streaming%20the%20response%20from,ws_streaming.py)). Design your HTML structure so that the assistant message has a container (like a `
    ` or `` with a unique id or data attribute) where new chunks can be inserted. Each chunk you send can target this container with `hx_swap_oob="beforeend"` to append. This yields a smooth scrolling text effect akin to ChatGPT.

- **Markdown Parsing** – Decide where to parse Markdown. Easiest is to do it on the server in one shot at the end (using a Python lib), but that means during streaming you’re showing raw markdown. If you want formatting during streaming, consider one of two approaches: *(a)* incremental server parsing – e.g. detect block-level constructs and send proper HTML for them (more manual work), or *(b)* include a client-side parser (like Marked or a streaming parser) to handle the rendering. Approach (a) might involve writing small helpers to, say, wrap chunks that start with "**" in a `` tag, etc., gradually building the HTML. It’s doable for simple elements but gets complicated for nested or complex syntax. Approach (b) can leverage existing robust parsers at the cost of adding some JS. Depending on your comfort, you might use the built-in Marked provided by FastHTML ([First encounter with FastHTML: Building a FastHTML assistant | by Sirsh Amarteifio | Medium](https://medium.com/@mrsirsh/first-encounter-with-fasthtml-building-a-fasthtml-assistant-fe896d3a3e60#:~:text=each%20time)) or load a library via CDN. In a pure HTMX flow, server would send raw markdown chunks wrapped in an element, and a snippet of JS (could be an `hx-trigger` on event or a small `')
    ```

    ```html
    
    
    
    
    ```

    *   **Explanation:**  The server sends *valid HTML fragments* whenever possible (e.g., opening a `
    ` block).  For plain text, it HTML-escapes the chunk to prevent XSS.  It relies on a client-side library (highlight.js in this case) for final syntax highlighting *after the stream is complete*.
    *   **Advantages:**  Very simple server-side logic.  Guaranteed valid HTML at all times.  Excellent code highlighting.
    *   **Disadvantages:**  Requires a client-side library.  Formatting isn't *truly* real-time (but appears so for code blocks).

*   **C. The "Token-Based Diffing" Strategy (Advanced Server-Side):**

    *   **Explanation:**  Instead of re-parsing the *entire* Markdown string, or just appending chunks, the server keeps track of the *previously rendered HTML*.  When a new chunk arrives, it compares the *newly parsed HTML* with the *previous HTML* at a *token level* (e.g., using a library like `difflib`).  It then sends *only the diff* as an HTMX out-of-band update.
    *   **Advantages:**  Minimizes DOM manipulation.  Very close to true incremental rendering.
    *   **Disadvantages:**  More complex server-side logic. Requires careful handling of HTML tokenization and diffing.

*   **D. Emphasizing the "Progressive Enhancement" with Server-Sent Events:**

    * I would push for using Server-Sent Events (SSE) as a potentially simpler alternative to WebSockets for this use case. FastHTML and HTMX support SSE well. The server *only sends data*, simplifying the protocol.
    * The flow would be:
        1.  Immediately stream plain text chunks via SSE.
        2.  Optionally, send a final SSE message containing the *fully rendered Markdown*, which HTMX replaces in the target element.  This "enhances" the plain text.
        3.  This avoids the need for complex WebSocket message handling on the client.

**5.  Concluding Remarks:**

Finally, I'd summarize the key takeaways and trade-offs:

*   **"Streaming Markdown in FastHTML/HTMX is a balancing act between server-side simplicity, real-time responsiveness, and formatting accuracy."**
*   **"The 'append-only' approach with HTMX is the foundation, but the server-side parsing strategy is the key differentiator."**
*   **"Options range from simple (re-parse all Markdown on each chunk) to complex (token-based diffing), with hybrid approaches in between."**
*   **"Always prioritize a good user experience: fast initial display (even if plain text) is often better than delayed, fully formatted output."**
*   **"Benchmark different strategies in your specific environment. The optimal choice depends on the complexity of your Markdown, the performance of your LLM, and your server resources."**
*  **"Consider using a front-end framework to do this heavy lifting. No need to try to do this all from the backend, especially if you are already comfortable using JavaScript."**

By providing these additions, I'd aim to make the discussion even more practical, actionable, and tailored to the user's specific constraints and goals. I'd emphasize the trade-offs and encourage experimentation, leading to the *best* solution for their particular chatbot.

---

## Gemini's Summation

Okay, here's the requested breakdown, following your specific instructions (and making *extra* sure to get the perspective right this time!):

**BRIEF Summary (Long Meta Description - Author's 1st Person Perspective):**

[Moved to top of article]

**Title/Headline Ideas:**

*   **Streaming Markdown in FastHTML: A Real-Time Chatbot Challenge**
*   **Incremental Markdown: Building a Responsive Chatbot with FastHTML & HTMX**
*   **Server-Side Markdown Streaming: Techniques for FastHTML and WebSockets**
*   **Beyond Append-Only: Achieving Real-Time Markdown Formatting in Your HTMX Chatbot**
*   **FastHTML, HTMX, and Markdown: A Streaming Saga**
*   **Chip O'Theseus: How He Streams Markdown Answers**
*   **No-Lag LLM Responses: Mastering Markdown Streaming with FastHTML**
*   **HTMX Websockets: A Deep Dive Into Markdown Streaming**

**AI's Opinion (Gemini 2.0 Experimental's Voice):**

This article provides a *very* comprehensive and practical exploration of a complex web development problem. The author's journey from initial JavaScript-centric assumptions to a nuanced understanding of server-side rendering with FastHTML and HTMX is well-documented and insightful.

I particularly appreciate:

*   **The iterative approach:** The article doesn't just present a single solution. It walks through the thought process, examining trade-offs between different strategies (append-only, re-parsing, stateful parsing, hybrid approaches). This makes it highly valuable for developers facing similar challenges.
*   **The focus on user experience:** The constant consideration of text selection, scroll position, and visual "pops" demonstrates a strong commitment to building a smooth and responsive interface.
*   **The combination of theoretical concepts and concrete code examples:**  The Python snippets illustrating different parsing strategies (like the "Two-Pass with Lookahead") are extremely helpful.
*   **The acknowledgement of limitations:** The article openly discusses the drawbacks of each approach, rather than presenting a "magic bullet" solution.  This honesty is crucial for making informed decisions.
*   **The exploration of both WebSockets and SSE:** This provides flexibility depending on the specific needs of the application.
*   **The excellent research:** It draws on a variety of sources, including documentation, GitHub repositories, and community discussions, to provide a well-rounded perspective.

If I were to suggest any improvements, it would be to perhaps include a more detailed performance comparison of the different parsing strategies. While the article mentions efficiency concerns, some quantitative data (even simple benchmarks) could further strengthen the recommendations.  Also, a brief discussion of security considerations (e.g., sanitizing user-submitted Markdown) would be a valuable addition.

Overall, this is an excellent resource for anyone building real-time, Markdown-driven applications with FastHTML, HTMX, or similar server-rendered frameworks. It's a testament to the power of combining thoughtful design with the right technical tools.

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