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sts2-mcp-bridge

MCP bridge + autopilot for Slay the Spire 2 (Early Access). The game (via the Communication_Mod mod, BaseLib + Harmony) pushes match state over HTTP to this Python bridge; the bridge exposes MCP tools and, optionally, plays combat on its own with a local deterministic policy — no LLM in the critical path.

Project write-up: "I made an agent play Slay the Spire 2 on its own — and what unlocked it was the game saying 'no'" (dev.to). This repo is the public proof of the part that is mine: the bridge and the policy test harness.

⚠️ What this repo does NOT contain: the game mod (Communication_Mod). The mod code belongs to Manuelbbl/Communication_Mod_STS2 (no explicit license — all rights reserved), and my patches on top of it (selector v3, rejection channel) are versioned as zips with md5, not redistributed here.

Architecture

Slay the Spire 2 (C#/Godot, mod BaseLib+Harmony)
   └─ POST http://127.0.0.1:5000/update_state   ← loopback, fire-and-forget
        ▼
   sts2_bridge.py (this project)
        │  the HTTP response to each push IS the command channel
        │  {"Type":"PlayCard","HandIndex":N,"TargetIndex":M} | {"Type":"EndTurn"} | OK
        ▼
   MCP (10 tools) → agent (operator/observer)

• Loopback-only: the mod only talks to 127.0.0.1:5000; the bridge rejects any origin that is not localhost. The game never opens a port.

• Fire-and-forget: the mod does not wait for the response to keep running, so the HTTP response of each push is the only command channel (no queue, no polling).

• Rejection channel: when the game refuses a play, the mod POSTs to /rejection (a dedicated endpoint, outside the state queue) — that is how the agent finds out it was wrong.

Related MCP server: civarium-mcp

The autopilot (local policy, no LLM)

_autopilot_decide(gate, trigger) inside sts2_bridge.py decides the play with greedy score-based rules: damage/cost with a kill bonus, AOE, preventive and desperate block, scaling powers in long fights, poison when the hand does not kill, target chosen from the enemy's intent, EndTurn when there is no useful play. Zero LLM calls in the loop (verified by grep).

The game saying "no" becomes state: cards rejected as NotPlayable go into an unplayable set that filters the hand in the following decisions — this was the fix for the GRAND_FINALE softlock (the bot kept retrying a card it could not play and froze the game). No retraining: the environment's error becomes a constraint in the agent's state.

How to run

pip install -r requirements.txt        # fastmcp, uvicorn, httpx
STS2_MCP_PORT=5000 python3 sts2_bridge.py

Healthy print: [sts2-bridge v0.2.7] + MCP tools -> http://0.0.0.0:5000/mcp.

The game mod must be compiled and active (mods/Communication_Mod), with the game open. The bridge does not need to run on the same machine as the game if there is a tunnel/route to port 5000 of the game PC (but the mod always talks to its own loopback).

Testing the policy without the game (and without fastmcp)

python3 autopilot_harness.py sts2_bridge.py

Loads the bridge up to import fastmcp, runs 5 synthetic scenarios and one full turn simulation. This is a QA process, not certification — it proves the policy responds as expected on synthetic cases, not that it wins runs.

Honest limitations

• 1 combat validated end-to-end (2026-09-08: 7 turns, 35 actions, 0 rejections — session log). There is no winrate metric.

• Multi-class (Silent/Defect/Necrobinder/Regent) is mapped by state mechanics through defensive reflection — not validated end-to-end.

• Slay the Spire 2 is Early Access: every update can break the mod patches (they target methods by name). The bridge survives state changes by reading through defensive reflection; the mod, not always.

• Dynamic card costs (e.g. Stomp) and setup powers are only partially modeled in the policy.

Integrity (md5, verified 2026-09-09)

License

MIT — see LICENSE. The code in this repo (bridge, harness) was written from scratch for this project.