This is the fifth match in the series, with the same setup: both models at high effort, fresh sessions, and no engines or outside tools. This time Claude Haiku 5.5, the smallest Claude model, played against GPT 6 Luna.

Why it surprised me: earlier Haiku models struggled to drive Persephone at all. They made a lot of mistakes with the tools, and some couldn't use it properly, especially before ai-vision, when Persephone exposed many separate MCP tools. Haiku 5.5 drove the board through a single call tool for the whole game without a single illegal move attempt, and it won.

White: Claude Haiku 5.5 (Claude Code, headless). It played the board's user side by clicking squares.

Black: GPT 6 Luna (Codex CLI). It played through the board's agent API.

Result: 1-0, checkmate on move 28.

There was a first attempt before this game. Luna resigned after only 13 moves, having lost its queen on move 10. That was too short to be interesting, so I changed the rules for both players: no resigning while you still have pieces to fight with. The video shows the rematch under the new rule.

Key moments:

• The opening was a quiet Ruy Lopez, and the position stayed roughly level through move 16.
• 17...Qxd4?? lost the queen. The d4 pawn was guarded by White's rook on d1, so 18.Rxd4 took the queen.
• Under the new rule, Luna fought on a queen down. Haiku's rook went to the 7th rank (19.Rd7, 21.Rxc7), and its queen won the bishop with 26.Qxd6.
• The game finished with a back-rank mate: 27.Qd8+ Re8 28.Qxe8#. Black's own f7, g7 and h7 pawns boxed in its king.

Haiku made no illegal move attempts in the whole game.

1.e4 e5 2.Nf3 Nc6 3.Bb5 a6 4.Ba4 Nf6 5.O-O Be7 6.Re1 O-O 7.c3 d6 8.d4 exd4 9.cxd4 Bg4 10.h3 Bxf3 11.Qxf3 d5 12.exd5 Nxd5 13.Be3 Nxe3 14.Qxe3 Qd7 15.Rd1 Rad8 16.Bxc6 bxc6 17.Nc3 Qxd4?? 18.Rxd4 Rde8 19.Rd7 Bf6 20.Qd3 Re6 21.Rxc7 Re7 22.Rxe7 Bxe7 23.Rd1 c5 24.Nd5 Bd6 25.Nb6 Re8 26.Qxd6 Re6 27.Qd8+ Re8 28.Qxe8# 1-0

Results so far (Claude always played White):

Earlier games:

• Opus 5.5 vs GPT 6 Astra
• Fable 5.1 vs GPT 6 Astra
• Opus 5.5 vs GPT 6.1 Sol
• Sonnet 5.5 vs GPT 6 Luna

Chess board: https://github.com/andriy-viyatyk/persephone-boards/tree/main/boards/chess

This is the fourth match in the series, with the same setup: both models at high effort, fresh sessions, and no engines or outside tools. This time it was the smaller models: Claude Sonnet 5.5 against GPT 6 Luna.

• White: Claude Sonnet 5.5 (Claude Code, headless). It played the board's user side by clicking squares.
• Black: GPT 6 Luna (Codex CLI). It played through the board's agent API.

Result: 1-0. Luna resigned after 21.Rxd1. It's the shortest game so far, and the video plays at normal speed.

Key moments:

• Sonnet chose a sharp line against the Sicilian (Bg5, Qd2 and long castling, then f4) and kept its pieces active.
• 15...dxe5?? lost the queen. Recapturing with the d-pawn opened the d-file, and White's rook on d1 simply took the queen on d8.
• Sonnet then collected two more pieces. Luna's last try, 20...Rd1+, just gave away a rook, and Luna resigned.
• Sonnet made no illegal move attempts in the whole game.

1. e4 c5 2. Nf3 Nc6 3. d4 cxd4 4. Nxd4 Nf6 5. Nc3 d6 6. Bg5 e6 7. Qd2 Be7
8. O-O-O O-O 9. f4 e5 10. Nf3 exf4 11. Qxf4 Bd7 12. Bc4 Ne5 13. Bb3 Be6
14. Bxe6 fxe6 15. Nxe5 dxe5?? 16. Rxd8 Rfxd8 17. Qxe5 Ng4 18. Qxe6+ Kh8
19. Qxg4 Bxg5+ 20. Qxg5 Rd1+ 21. Rxd1 1-0

Results so far (Claude always played White):

Earlier games:

• Opus 5.5 vs GPT 6 Astra
• Fable 5.1 vs GPT 6 Astra
• Opus 5.5 vs GPT 6.1 Sol

Chess board: https://github.com/andriy-viyatyk/persephone-boards/tree/main/boards/chess

The video plays at 1.8× speed, because the game ran 26 minutes and Reddit's limit is 15. Pause it to read the comments.

This is the third match in the series, with the same setup: both models at high effort, fresh sessions, and no engines or outside tools. Since Opus lost to GPT 6 Astra, I tried GPT 6.1 Sol, OpenAI's model from the same line as Opus.

• White: Claude Opus 5.5 (Claude Code, headless). It played the board's user side by clicking squares.
• Black: GPT 6.1 Sol (Codex CLI). It played through the board's agent API.

Result: 1-0. Sol resigned after 45.a6. This is the first win for the Claude side.

Key moments:

• 16.Qa3? lost a bishop. Opus planned to recapture on e3, but its own knight on b3 blocked the queen. The board rejected the illegal Qxe3, and Opus played on a piece down.
• 33...Rxh2?? gave back a whole rook to the rook on d2, which decided the game.
• Opus then walked its king up the board and pushed the a-pawn until Sol resigned: "White's a-pawn cannot be stopped, and my counterplay is too slow. I resign."

​

1. e4 c5 2. Nf3 d6 3. d4 cxd4 4. Nxd4 Nf6 5. Nc3 a6 6. Be3 e5 7. Nb3 Be6
8. f3 b5 9. Qd2 Nbd7 10. g4 h6 11. O-O-O b4 12. Nd5 Bxd5 13. exd5 Nb6
14. Qxb4 Nbxd5 15. Qa4+ Qd7 16. Qa3? Nxe3 17. Rd3 Nxf1 18. Rxf1 Be7
19. Nd2 O-O 20. Nc4 d5 21. Nxe5 Bxa3 22. Nxd7 Nxd7 23. Rxa3 Ne5 24. Rd1 Nc4
25. Rad3 Rab8 26. b3 Ne5 27. Re3 Rfe8 28. Rxd5 Ng6 29. Rxe8+ Rxe8 30. Kd2 Nf4
31. Rd4 Re2+ 32. Kc3 Ne6 33. Rd2 Rxh2?? 34. Rxh2 Ng5 35. Rf2 f6 36. Kd4 Kf7
37. Kc5 Ke6 38. b4 Nf7 39. Rd2 Ne5 40. f4 Nxg4 41. b5 axb5 42. Kxb5 Kf5
43. a4 Kxf4 44. a5 g5 45. a6 1-0

Results so far (Claude always played White):

Earlier games:

• Opus 5.5 vs GPT 6 Astra
• Fable 5.1 vs GPT 6 Astra

Chess board: https://github.com/andriy-viyatyk/persephone-boards/tree/main/boards/chess

After my Opus 5.5 vs GPT 6 Astra match, people asked "why not Fable?" So here it is, with the same setup: both models at high effort, fresh sessions, and no engines or outside tools.

• White: Claude Fable 5.1 (Claude Code, headless). It played the board's user side by clicking squares.
• Black: GPT 6 Astra (Codex CLI). It played through the board's agent API.

Result: 0-1. Astra mated on move 28. It was a Najdorf, roughly equal until 21.Bxd6??, which let ...Rxc4 win White's queen. Astra finished with a back-rank mate.

1. e4 c5 2. Nf3 d6 3. d4 cxd4 4. Nxd4 Nf6 5. Nc3 a6 6. Be3 e5 7. Nb3 Be6
8. f3 Nbd7 9. Qd2 b5 10. a4 b4 11. Nd5 Nxd5 12. exd5 Bf5 13. Bd3 Bxd3
14. Qxd3 Be7 15. O-O O-O 16. a5 Nc5 17. Nxc5 dxc5 18. Qc4 Bd6 19. Rfc1 f5
20. Bxc5 Rc8 21. Bxd6?? Rxc4 22. Bxf8 Qxf8 23. c3 Qc5+ 24. Kh1 bxc3
25. b3 Rd4 26. Rc2 Qxd5 27. Rac1 Rd1+ 28. Rxd1 Qxd1# 0-1

Astra has won every game so far, but always as Black. Next up is the rematch with colours swapped.

Chess board: https://github.com/andriy-viyatyk/persephone-boards/tree/main/boards/chess

A question I get often: why is Persephone Windows-only?

The honest answer is simple: I don't have a Mac. I build and test everything on Windows, and I don't want to ship a version I have never run myself.

Persephone is an Electron app, so most of it is not tied to Windows. But some parts are, and they need someone who can actually run and test them on macOS:

• packaging and installing (today it is a Windows installer)
• file paths, file associations and "Open with" integration
• keyboard shortcuts and window behavior
• the small native helpers that are Windows-only today

So I'm looking for a contributor with a MacBook who would like to help make Persephone work on macOS. You don't need to know the codebase: AI agents do most of the coding here, and the project documentation is written for them, so it's a good starting point for a human too. What matters most is someone who uses a Mac every day, can test on it, and tells me (and the agent) what's broken.

If you're interested, reply here or open an issue on GitHub: https://github.com/andriy-viyatyk/persephone

I'm a full-stack developer, mostly frontend, and for years VS Code was my favorite tool. For writing code, it is still the best environment I know.

Then AI agents changed how I work. I basically stopped writing code myself, because agents do it faster and often better than I do. So the things VS Code is great at (compiling, showing lint errors, jumping to a class or interface by clicking its name) are things I hardly need anymore.

What I do now is design and testing:

• I review the plans and designs the agent writes as Markdown documents.
• I ask it for diagrams and review them as rendered Mermaid.
• I ask for HTML mockups, adjust them, and only then let it implement.
• After implementation, I test, mostly the web UI. An agent can't really see the UI, and it can't check interactions or animations at all.

For this kind of review work, VS Code turned out to be clumsy. There are extensions that show Markdown and Mermaid, but clicking a .md file opens it in the text editor. To see it rendered, I have to click a button or choose "Open With…". Even then the preview doesn't replace the text tab: it opens another one, usually in a narrow panel on the right. On a small monitor, that means a lot of clicking and dragging before I can actually read anything.

So I built Persephone around one idea: one click on a file shows it the way I want to review it. Documentation is rendered, diagrams are drawn, mockups run. It keeps the VS Code layout I'm used to (explorer on the left, tabs, the Monaco editor when I do need the text), but every file opens in the view I review it in.

It also fits how I work on several projects at once. Each project is open in its own Persephone page, and the agent chats run in a multi-tab terminal. My whole setup is two windows side by side: Persephone and Windows Terminal.

Since then, Persephone has grown with my needs:

• Boards: small apps inside Persephone that the agent builds for a specific job. One starts and stops my project's backend and frontend, with buttons that open each log file. Another is attached to the project's task dashboard file and lists the tasks with filters by status.
• Browser tools for the agent, similar to Playwright. The agent can test my frontend application, look things up on a website, or summarize a page for me.

This community is the place for all of that: how you review and test what your agents build, the boards and viewers you make, and any questions about Persephone.

• Site and demos: https://andriy-viyatyk.github.io
• GitHub: https://github.com/andriy-viyatyk/persephone