Simon Willison’s Weblog

Anthropic released Fable 5.2 and Mythos 5.2 today. Fable 5.2 is currently ranked first on Artificial Analysis. Anthropic say it "solved two open problems in the physics of black holes", so clearly their internal benchmarks have become more ambitious!

I've added the new models to llm-anthropic, so you can now run prompts through Fable 5.2 like this:

uv tool install llm
llm install llm-anthropic
llm keys set anthropic
# paste key here
llm -m claude-fable-5.2 "Generate an SVG of a pelican riding a bicycle"

I ran pelicans at all five reasoning levels - low, medium, high, xhigh, and max. You can see them all here - here's the best result I got, from xhigh:

max failed to return an image at all, running out of tokens after spending 128,000 thinking about them. That's the same bug that I saw with Opus 5.5 - but it's new for the Fable series, both Fable 5 and Fable 5.1 returned max effort pelicans just fine.

It's priced at $8/million input and $40/million output, which is a 20% discount on the previous Fable 5.1 ($10/$50).

I also used Fable 5.2 to help identify the source of a mysterious beep.

I was out walking the dog when I got curious about how easy/difficult it would be to write a SQLite C extension that adds a parse_number() function for turning strings like -$1,234.56 or '12.5%' into numeric values that could be summed and sorted.

I fired up regular Claude chat (Opus 5.5) and prompted:

I'm thinking about a SQLite custom function 'parse_number(value)' that strips ',', '$', '£' and '%', handles a leading '-', and returns 'NULL' for anything it can’t parse

Explore the difference between building this as a SQLite extension in C v.s. in Rust - I want to compare performance and code complexity and size of the resulting.so(or whatever) file

This gave me a solid starting point - the Rust extension was 321 KB (using Rusqlite) and 298 KB (using sqlite-loadable-rs), while the C extension was 14 KB. But the Rust extension was faster!

Turns out that's because the Rust equivalent of C's strtod is "twice as fast". So I prompted:

Try writing your own C function for parsing text into a decimal number that catches up to or beats the Rust implementation

And it did, using "Clinger's fast path":

Net cost per row over 2 million rows, lower is better:

	first C	new C	rusqlite	sqlite-loadable
Mixed data	67 ns	19 ns	46 ns	41 ns
Integers only	25 ns	16 ns	36 ns	32 ns
Decimals only	106 ns	18 ns	55 ns	47 ns

Finally, I had it rewrite the prototype as a research report to match the others in simonw/research by prompting this:

Clone simonw/research from GitHub and turn your above investigation into a research report as seen in that repository - you can give me a zip file to download that contains your new folder

Here's the resulting report.

I added four player online mode to Battledrones - previous maps were limited to two.

Here's the initial prompt I used:

Add a new map for a four player battle - four way rotational symmetry, up to four AI players, works with the WebSocket non-audio based multiplayer mode so up to four human players can join (any spare slots use AI)

Claude Opus 5.5 added a new map called "Compass Rose" and modified the WebSockets multiplayer network code to support up to four players for that map, joining via share links or QR codes.

The new teams use robots themed in purple and green, joining the existing blue and red designs. You can preview those colors in the updated model browser.

Here's a replay link to watch a four way AI battle with the "Goliath Armada" mutator turned on, where each side starts with four giant Goliath mech units.

Another game experiment with Claude. This one used the default Claude iPhone app (chat mode, not Claude Code mode), where I prompted Claude Opus 5.5 Max with:

Build a three js game called "Tree appreciator" - it shows you two algorithmically generated trees and the game mechanics is about appreciating the differences between them. Think hard about how to make sure it is both fun and beautiful.

Somewhat embarrassingly given my recent talk, I've found that telling Opus 5.5 "make sure it is fun" works uncomfortably well. In this case Claude crunched for 3 hours and 7 minutes before retuning a cosy little spot-the-difference game with music and animation and achievements that's quite compelling to play.

I wanted to see if Claude Opus 5.5 could compose music, so I tried this:

I want you to write some computer game music for me. First design simple text based format for the music and build an artifact that can play it out loud - include some example tracks in that artifact

I am looking for music of the quality of the original secret of Monkey Island

It leaned a lot harder into the Monkey Island theme than I had intended, but the results are surprisingly good.

I wonder if the ability to compose competent music is similar to the 3D graphics thing - a new capability for text models that emerged in the past few months?

Would need some careful experiments with other recent and not-so-recent models to confirm if this is new or if they've been able to do this for a while.

I presented a closing keynote for the WeAreDevelopers World Congress North America yesterday. As a STAR moment I decided to weave in references to the record breaking kākāpō breeding season we had in 2026.

For my closing slide I wanted to celebrate, and I had seen some buzz around how good Claude Opus 5.5 was at creating pixel art animations. So I rounded up three Kakapo photos from Google image search and dropped them into Claude with this prompt:

Here are some photos of kakapo parrots just to remind you what they look like

I need you to make an animation in animated pixel art on HTML 5 canvas of obviously pixel art kakapo jumping up and down having a party with confetti and suchlike - there should be at least 20 of them

Here's the transcript, and this is the resulting page. It's pretty great!

I wanted to embed it in a Keynote presentation file, so I downloaded the HTML and told a local Claude Code session:

Make me a video of file:///Users/simon/Downloads/kakapo-party.html - you need to load it in a browser and click on it a few times to get the confetti effect, the video should be 15s long

don't start clicking until 3s in

make sure several clicks are spread around the clickable area

Claude Code used Playwright (transcript here) and produced this video, which was exactly what I needed for my final slide:

Here's the full Playwright script it used, which was pleasingly short:

# /// script
# dependencies = ["playwright"]
# ///
import time
from playwright.sync_api import sync_playwright
W, H = 1280, 720
# Canvas fills the viewport; spread clicks across corners, edges and centre
clicks = [
    (3.0, 640, 360),   # centre
    (4.2, 160, 120),   # top-left
    (5.4, 1120, 120),  # top-right
    (6.6, 180, 600),   # bottom-left
    (7.8, 1100, 600),  # bottom-right
    (9.0, 640, 100),   # top-centre
    (10.0, 380, 380),  # mid-left
    (11.0, 900, 380),  # mid-right
    (12.2, 640, 620),  # bottom-centre
    (13.2, 640, 300),  # finale centre
]
with sync_playwright() as p:
    b = p.chromium.launch()
    ctx = b.new_context(viewport={"width":W,"height":H}, record_video_dir="vids", record_video_size={"width":W,"height":H})
    page = ctx.new_page()
    t0 = time.time()
    page.goto("file:///Users/simon/Downloads/kakapo-party.html")
    for t,x,y in clicks:
        time.sleep(max(0, t-(time.time()-t0)))
        page.mouse.click(x,y)
    time.sleep(max(0, 16.0-(time.time()-t0)))
    ctx.close(); b.close()

I recorded a short demo video of my Equal Earth animation on my phone and wanted to publish an optimized version of that video (using FFMPEG) on my blog, so I had Claude Fable 5.1 in Claude Code for web build me this tool using the WebAssembly build of FFMPEG.

Claude Fable 5.1, reasoning traces are now displayed by default for models that support them, plus a new llm_anthropic.ClaudeRefusal exception for when Claude throws a refusal.

This release of the Anthropic plugin for LLM mainly provides compatibility with the recently released anthropic v1.0.0 Python library, which switches from httpx to httpx2. OpenAI made the same change in their v3.0.0 release two weeks ago.

Anthropic provide this migration guide for upgrading to 1.0, so I prompted Fable 5 in Claude Code with:

Upgrade to anthropic>=1 - read https://raw.githubusercontent.com/anthropics/anthropic-sdk-python/refs/heads/main/MIGRATION.md and get the tests passing

Here's the resulting PR.

Includes new features enabled by LLM 0.32:

• New models: claude-fable-5, claude-sonnet-5, and claude-opus-5. #75, #76
• Added server-side tools for WebSearch, WebFetch, CodeExecution, and AnthropicMCP, available through LLM's -T interface or Python tools=. The previous -o web_search* options have been removed in favor of -T WebSearch. #79
• Upgraded to llm>=0.32. Reasoning, tool calls, tool results, and server-side tool results now stream as typed events. Reasoning for llm CLI prompts now displays to standard error unless you pass --hide-reasoning/-R.
• Simplified extended thinking to thinking and thinking_effort (low, medium, high, xhigh, or max). Claude 5 models think by default; -o thinking 0 disables thinking for Sonnet 5 and Opus 5, while Fable 5 always thinks. -R/--hide-reasoning now omits reasoning from responses and logs. The thinking_budget, thinking_display, and thinking_adaptive options have been removed. #80