Opening traps in 9x9 gomoku: why AI always plays center first
While building Tic-Five I played roughly 500 matches against my own AI across all three difficulties. 9x9 and standard 15x15 share rules but are different games in practice. Here's what I learned about openings on 9x9.
Why the AI opens center every time
9x9 has 81 points. The center, (4,4), is exactly 4 steps from each edge in every one of 8 directions (2 horizontal, 2 vertical, 4 diagonal). That means:
From center you can launch a "four-in-a-row threat" in 8 different directions. Each direction can run a full 9-point line across the board.
From a corner (e.g., (1,1)), only 3 directions launch threats (down, right, down-right). Each maxes out at an 8-point line. That's a 37.5% reduction in attack vectors — devastating on a board this small.
So if any AI knows anything about first-mover advantage, its opening move is forced to be the center. Not because BverGame's AI is unusually smart — because the math demands it.
How to respond as second player, first four moves
The hardest problem for second player on 9x9: don't let the AI build a "double open-three" near the center. Once that's set, you can only block one side; the other becomes a five.
In practice, my best opening responses look like this:
Move 1 (your first stone): hug the center diagonally
Most reliable: drop on (3,3) or (5,5). A direct adjacent stone (up/down/left/right) doesn't disrupt the AI's diagonal threats. A diagonal hug interferes with two diagonal lines at once.
Move 2: read the AI's second move, then commit
The AI's second move is most likely (3,5) or (5,3) — forming an L with its center stone to set up an open-two. Don't rush to block. Place your stone on (5,5) or its symmetric counterpart to align with your first move and build your own attack.
Move 3: start the counter
By move 3, six stones are on the board. Watch: has the AI formed an open-two yet? If yes, blocking is top priority. If no, keep building — usually a third stone on the extension line of your first two.
Move 4: form your own open-three
This is the critical move. If you can build an open-three within four moves, the AI must spend a move blocking, breaking its attack tempo. If you can't, the AI sets up a "four + open-three" double threat on moves 5–6 and you lose.
9x9 gomoku is decided 80% within the first 6 moves. The remaining 20% is cleanup.
The AI's vulnerabilities by difficulty
Tic-Five has three AI levels. I built each with distinct weaknesses for different players:
Beginner: Evaluates only its own connections; ignores your threats entirely. As long as you start building open-twos and open-threes, it won't actively block — it just keeps doing its own thing. Push fast threes; you'll usually win within 6 moves.
Medium: 70% optimal play, 30% suboptimal. The randomness is deliberate to keep matches interesting. It also means roughly one in three games you can catch its 30% deviation and convert it into a fast-attack win.
Challenge: 2-ply search + threat evaluation. I won't lie — this one forces mistakes out of you. But I left it one deliberate weakness: it's under-tuned for "mid-term layouts where neither side has an open-three." Translation: if you can patiently build a "potential double open-three" over moves 6–10 (looks like an open-two with two upgrade paths), it gets caught off-guard. That's the only repeatable strategy that beats Challenge on our site.
Common ways to die
The three most common deaths on 9x9 for beginners:
- Corner retreat death. Some players think corners are "safe ground." On 9x9, corners are the weakest squares. The AI ignores them and builds dominance at center.
- Follower death. AI plays, you follow. AI plays, you follow again. The board is too small for this to work — letting the AI lead tempo means it punches out an open-three in 4–5 moves and kills you.
- Double-block death. The AI threatens two points simultaneously. You can only block one. The other becomes a four-then-five. Avoid this by building your own counter-line on moves 3–4 rather than always reacting.
Closing: 9x9 isn't "mini 15x15"
The assumption that being good at 15x15 transfers to 9x9 is wrong. They're separate games. 15x15 favors long-game positional players. 9x9 is decided in the opening — full fast-attack.
So if Challenge AI is destroying you, don't doubt your gomoku skill. 9x9 is brutal for second player by design. Beating Beginner and Medium already makes you a competent player.
If you can beat Challenge reliably — please don't tell us. We'll get out of your way.
Leo is BverGame's co-engineer; all 24 games on the site are his code. Strategies here come from his own 500+ matches and his reflection on the AI's evaluation function. They don't apply to competitive gomoku formats like Japanese Renju.