diff --git a/.mcp.json b/.mcp.json
index d59226bf..0d0967a0 100644
--- a/.mcp.json
+++ b/.mcp.json
@@ -15,7 +15,7 @@
       ],
       "env": {
         "CP_PLAYERS": "2",
-        "CP_CLAUDE_SLOT": "0",
+        "CP_PLAYER_SLOT": "0",
         "CP_MAP_SIZE": "duel",
         "CP_MAP_TYPE": "continents"
       }
diff --git a/scripts/claude-demo-25turn.sh b/scripts/claude-demo-25turn.sh
index 3c0ef6db..2a816925 100755
--- a/scripts/claude-demo-25turn.sh
+++ b/scripts/claude-demo-25turn.sh
@@ -14,7 +14,7 @@
 #   CP_TURNS         (default 25)   — number of EndTurns to issue
 #   CP_SEED          (default 42)
 #   CP_PLAYERS       (default 3)
-#   CP_CLAUDE_SLOT   (default 0)
+#   CP_PLAYER_SLOT   (default 0)
 #   CP_MAP_SIZE      (default duel)
 #   CP_TIMEOUT_SEC   (default 600) — harness wallclock budget
 #
@@ -27,11 +27,11 @@ SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
 : "${CP_TURNS:=25}"
 : "${CP_SEED:=42}"
 : "${CP_PLAYERS:=3}"
-: "${CP_CLAUDE_SLOT:=0}"
+: "${CP_PLAYER_SLOT:=0}"
 : "${CP_MAP_SIZE:=duel}"
 : "${CP_TIMEOUT_SEC:=600}"
 
-export CP_SEED CP_PLAYERS CP_CLAUDE_SLOT CP_MAP_SIZE CP_TIMEOUT_SEC
+export CP_SEED CP_PLAYERS CP_PLAYER_SLOT CP_MAP_SIZE CP_TIMEOUT_SEC
 
 TMP=$(mktemp -d -t mc-demo25-XXXXXX)
 trap "rm -rf '$TMP'" EXIT
diff --git a/scripts/claude-smoke-5endturn.sh b/scripts/claude-smoke-5endturn.sh
index e84b1bf4..6a5c251e 100755
--- a/scripts/claude-smoke-5endturn.sh
+++ b/scripts/claude-smoke-5endturn.sh
@@ -10,7 +10,7 @@
 # Passing requires `actions_applied > 0` on every turn 1..5 for every AI slot.
 # Exits 0 on pass, 1 on fail.
 #
-# Env: CP_SEED, CP_PLAYERS (default 3), CP_CLAUDE_SLOT (default 0), CP_MAP_SIZE.
+# Env: CP_SEED, CP_PLAYERS (default 3), CP_PLAYER_SLOT (default 0), CP_MAP_SIZE.
 
 set -uo pipefail
 
@@ -18,12 +18,12 @@ SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
 PROJECT_DIR="$(dirname "$SCRIPT_DIR")"
 
 : "${CP_PLAYERS:=3}"
-: "${CP_CLAUDE_SLOT:=0}"
+: "${CP_PLAYER_SLOT:=0}"
 : "${CP_SEED:=42}"
 : "${CP_MAP_SIZE:=duel}"
 : "${SMOKE_TURNS:=5}"
 
-export CP_PLAYERS CP_CLAUDE_SLOT CP_SEED CP_MAP_SIZE
+export CP_PLAYERS CP_PLAYER_SLOT CP_SEED CP_MAP_SIZE
 
 TMP=$(mktemp -d -t mc-smoke-XXXXXX)
 trap "rm -rf '$TMP'" EXIT
diff --git a/scripts/p2-72-option-b-render.sh b/scripts/p2-72-option-b-render.sh
index a1b6b987..9881836c 100755
--- a/scripts/p2-72-option-b-render.sh
+++ b/scripts/p2-72-option-b-render.sh
@@ -13,7 +13,7 @@
 # Env vars (forwarded into the sandbox):
 #   CP_SEED              (default 42)
 #   CP_PLAYERS           (default 3)
-#   CP_CLAUDE_SLOT       (default 0)
+#   CP_PLAYER_SLOT       (default 0)
 #   CP_MAP_SIZE          (default duel)
 #   CP_TURNS             (default 25)
 #   CP_SCREENSHOT_EVERY  (default 1)
@@ -31,7 +31,7 @@ PROJECT_DIR="$(dirname "$SCRIPT_DIR")"
 
 : "${CP_SEED:=42}"
 : "${CP_PLAYERS:=3}"
-: "${CP_CLAUDE_SLOT:=0}"
+: "${CP_PLAYER_SLOT:=0}"
 : "${CP_MAP_SIZE:=duel}"
 : "${CP_TURNS:=25}"
 : "${CP_SCREENSHOT_EVERY:=1}"
@@ -81,7 +81,7 @@ timeout "$CP_TIMEOUT_SEC" flatpak run --user \
     --filesystem=xdg-run/${WESTON_SOCKET} \
     --env=CP_SEED="$CP_SEED" \
     --env=CP_PLAYERS="$CP_PLAYERS" \
-    --env=CP_CLAUDE_SLOT="$CP_CLAUDE_SLOT" \
+    --env=CP_PLAYER_SLOT="$CP_PLAYER_SLOT" \
     --env=CP_MAP_SIZE="$CP_MAP_SIZE" \
     --env=CP_TURNS="$CP_TURNS" \
     --env=CP_SCREENSHOT_EVERY="$CP_SCREENSHOT_EVERY" \
diff --git a/scripts/player-api-server.sh b/scripts/player-api-server.sh
index 92d5db4d..57f5aac8 100755
--- a/scripts/player-api-server.sh
+++ b/scripts/player-api-server.sh
@@ -9,10 +9,10 @@
 # Python `subprocess.Popen`, a smoke-test shell script, etc.
 #
 # Env vars (see PLAYER_API.md for the full schema):
-#   CP_SEED, CP_PLAYERS, CP_CLAUDE_SLOT, CP_MAP_SIZE, CP_MAP_TYPE,
+#   CP_SEED, CP_PLAYERS, CP_PLAYER_SLOT, CP_MAP_SIZE, CP_MAP_TYPE,
 #   CP_OMNISCIENT, CP_TIMEOUT_SEC, CP_LOG_FILE.
 #
-# `CP_CLAUDE_SLOT` is the env-var name the harness has used since p2-67;
+# `CP_PLAYER_SLOT` is the env-var name the harness has used since p2-67;
 # it identifies the externally-controlled player slot — kept as-is for
 # backward compatibility with existing clients. Despite the name it is
 # not Claude-specific.
@@ -28,7 +28,7 @@ PROJECT_DIR="$(dirname "$SCRIPT_DIR")"
 # Defaults — adapter overrides via env.
 : "${CP_SEED:=42}"
 : "${CP_PLAYERS:=2}"
-: "${CP_CLAUDE_SLOT:=0}"
+: "${CP_PLAYER_SLOT:=0}"
 : "${CP_MAP_SIZE:=duel}"
 : "${CP_MAP_TYPE:=continents}"
 : "${CP_OMNISCIENT:=0}"
@@ -41,7 +41,7 @@ PROJECT_DIR="$(dirname "$SCRIPT_DIR")"
 # macOS uses the locally-installed `godot` binary (Homebrew); a parallel
 # flatpak runtime just for this harness is silly when native Godot 4
 # works directly. Env-var passthrough is automatic for the native path.
-export CP_SEED CP_PLAYERS CP_CLAUDE_SLOT CP_MAP_SIZE CP_MAP_TYPE \
+export CP_SEED CP_PLAYERS CP_PLAYER_SLOT CP_MAP_SIZE CP_MAP_TYPE \
     CP_OMNISCIENT CP_TIMEOUT_SEC CP_LOG_FILE
 
 case "$(uname -s)" in
@@ -61,7 +61,7 @@ case "$(uname -s)" in
         exec flatpak run --user \
             --env=CP_SEED="$CP_SEED" \
             --env=CP_PLAYERS="$CP_PLAYERS" \
-            --env=CP_CLAUDE_SLOT="$CP_CLAUDE_SLOT" \
+            --env=CP_PLAYER_SLOT="$CP_PLAYER_SLOT" \
             --env=CP_MAP_SIZE="$CP_MAP_SIZE" \
             --env=CP_MAP_TYPE="$CP_MAP_TYPE" \
             --env=CP_OMNISCIENT="$CP_OMNISCIENT" \
diff --git a/src/game/engine/scenes/headless/player_api_main.gd b/src/game/engine/scenes/headless/player_api_main.gd
index cdb27115..7f1cc8a1 100644
--- a/src/game/engine/scenes/headless/player_api_main.gd
+++ b/src/game/engine/scenes/headless/player_api_main.gd
@@ -1,5 +1,5 @@
 extends Node
-## Headless harness for the generic external-player JSON-Lines API.
+## Headless harness for the external-player JSON-Lines API.
 ##
 ## Boots a seeded GameState, instantiates a `GdPlayerApi`, then enters
 ## a JSON-Lines pump on stdin/stdout. Each line in is one `Request`
@@ -7,16 +7,14 @@ extends Node
 ## one `Notification`. The protocol contract lives in
 ## `src/game/engine/docs/PLAYER_API.md`.
 ##
-## Originally introduced in p2-67 as `claude_player_main.gd`. Renamed
-## 2026-05-17 to drop the Claude-flavored naming — the wire protocol is
-## client-agnostic; Claude Code is one consumer via
-## `tooling/claude-player-mcp/`, an OpenSpiel/RL trainer can plug in via
-## `subprocess.Popen`, and shell smoke tests use raw JSON-Lines.
+## Client-agnostic: Claude Code drives it via `tooling/claude-player-mcp/`,
+## an RL trainer drives it via `subprocess.Popen`, shell smoke tests
+## write raw JSON-Lines to its stdin.
 ##
 ## Env vars consumed:
 ## - `CP_SEED`         — RNG seed (default 42)
 ## - `CP_PLAYERS`      — total player slots (default 2)
-## - `CP_CLAUDE_SLOT`  — which slot stdin controls (default 0)
+## - `CP_PLAYER_SLOT`  — which slot stdin controls (default 0)
 ## - `CP_MAP_SIZE`     — MapGenerator size key (default "duel")
 ## - `CP_MAP_TYPE`     — MapGenerator map type (default "continents")
 ## - `CP_OMNISCIENT`   — `1` disables fog redaction (default 0)
@@ -24,14 +22,14 @@ extends Node
 ## - `CP_LOG_FILE`     — if set, mirror all wire I/O to this path
 
 var _api: RefCounted = null
-var _claude_slot: int = 0
+var _player_slot: int = 0
 var _omniscient: bool = false
 var _log_path: String = ""
 var _shutdown: bool = false
 
 
 func _ready() -> void:
-	_claude_slot = _env_int("CP_CLAUDE_SLOT", 0)
+	_player_slot = _env_int("CP_PLAYER_SLOT", 0)
 	_omniscient = _env_bool("CP_OMNISCIENT", false)
 	_log_path = OS.get_environment("CP_LOG_FILE")
 
@@ -63,7 +61,7 @@ func _ready() -> void:
 	# no `id` field; adapters can use them to drive streaming UIs or
 	# ignore them entirely (the synchronous response after the next
 	# `act` carries the same data via `events`).
-	_emit_event("turn_started", {"turn": 0, "player": _claude_slot})
+	_emit_event("turn_started", {"turn": 0, "player": _player_slot})
 	_emit_event("phase_changed", {"phase": "player_actions"})
 
 	# Enter the pump on the next frame so any pending engine init flushes.
@@ -337,7 +335,7 @@ func _apply_ai_personalities(gs: RefCounted, num_players: int) -> void:
 	# assign clan_ids[ai_index % clan_count]. Stable across runs.
 	var ai_index: int = 0
 	for slot: int in range(num_players):
-		if slot == _claude_slot:
+		if slot == _player_slot:
 			continue
 		var clan_id: String = clan_ids[ai_index % clan_ids.size()]
 		ai_index += 1
@@ -420,7 +418,7 @@ func _handle_request(req: Dictionary) -> void:
 	var has_id: bool = req.has("id") and req.get("id") != null
 	match rtype:
 		"view":
-			var view_json: String = String(_api.view_json(_claude_slot))
+			var view_json: String = String(_api.view_json(_player_slot))
 			_emit_response_with_view(rid_int, has_id, view_json)
 		"act":
 			var action_payload: Dictionary = req.get("action", {}) as Dictionary
@@ -429,7 +427,7 @@ func _handle_request(req: Dictionary) -> void:
 				return
 			var action_json: String = JSON.stringify(action_payload)
 			var envelope_str: String = String(
-				_api.apply_action_json(_claude_slot, action_json)
+				_api.apply_action_json(_player_slot, action_json)
 			)
 			# api wrapper already emits a full ok/err envelope — splice
 			# in the request id and forward as the response body.
diff --git a/src/game/engine/scenes/tests/claude_vs_ai_render_proof.gd b/src/game/engine/scenes/tests/claude_vs_ai_render_proof.gd
index 9029cd1c..db520e02 100644
--- a/src/game/engine/scenes/tests/claude_vs_ai_render_proof.gd
+++ b/src/game/engine/scenes/tests/claude_vs_ai_render_proof.gd
@@ -14,7 +14,7 @@ extends Node2D
 ##     it is a Phase-13 deliverable workaround that bypasses the full
 ##     canonical render source extraction (`p2-72a`, deferred).
 ##
-## Mirrors `claude_player_main.gd` for the boot sequence (DataLoader,
+## Mirrors `player_api_main.gd` for the boot sequence (DataLoader,
 ## GdMapGenerator, capital placement, AI personalities, runtime + tactical
 ## catalogs) and `full_game_demo_proof.gd` for the renderer scaffold
 ## (HexRenderer / UnitRenderer / CityRenderer wired directly under a
@@ -42,7 +42,7 @@ extends Node2D
 ## Env vars:
 ##   - CP_SEED                (default 42)
 ##   - CP_PLAYERS             (default 3)
-##   - CP_CLAUDE_SLOT         (default 0)
+##   - CP_PLAYER_SLOT         (default 0)
 ##   - CP_MAP_SIZE            (default "duel")
 ##   - CP_TURNS               (default 25)
 ##   - CP_SCREENSHOT_EVERY    (default 1) — capture every Nth turn
@@ -79,7 +79,7 @@ var _unit_renderer: Node2D = null
 var _city_renderer: Node2D = null
 var _camera: Camera2D = null
 
-var _claude_slot: int = 0
+var _player_slot: int = 0
 var _num_players: int = 3
 var _seed: int = 42
 var _map_size: String = "duel"
@@ -118,7 +118,7 @@ func _ready() -> void:
 
 	_seed = _env_int("CP_SEED", 42)
 	_num_players = _env_int("CP_PLAYERS", 3)
-	_claude_slot = _env_int("CP_CLAUDE_SLOT", 0)
+	_player_slot = _env_int("CP_PLAYER_SLOT", 0)
 	_map_size = _env_or("CP_MAP_SIZE", "duel")
 	_max_turns = _env_int("CP_TURNS", 25)
 	_screenshot_every = max(1, _env_int("CP_SCREENSHOT_EVERY", 1))
@@ -199,7 +199,7 @@ func _bootstrap_world() -> void:
 		get_tree().quit(1)
 		return
 
-	# Scan land tiles + pick spaced capitals, mirroring claude_player_main.gd.
+	# Scan land tiles + pick spaced capitals, mirroring player_api_main.gd.
 	var grid_w: int = int(grid.get_width())
 	var grid_h: int = int(grid.get_height())
 	var land_tiles: Array[Vector2i] = _scan_land_tiles(grid, grid_w, grid_h)
@@ -343,24 +343,24 @@ func _find_claude_capital_axial() -> Vector2i:
 	if _api == null:
 		push_warning("capital anchor: _api null, falling back to (0,0)")
 		return Vector2i.ZERO
-	var view_json: String = String(_api.view_json(_claude_slot))
+	var view_json: String = String(_api.view_json(_player_slot))
 	var view: Dictionary = JSON.parse_string(view_json) as Dictionary
 	if view == null:
 		push_warning("capital anchor: view_json non-Dictionary, falling back to (0,0)")
 		return Vector2i.ZERO
 	# 1) First city owned by Claude.
 	for city_dict: Dictionary in (view.get("cities", []) as Array):
-		if int(city_dict.get("owner", -1)) == _claude_slot:
+		if int(city_dict.get("owner", -1)) == _player_slot:
 			return _wire_hex_to_vec(city_dict.get("position", []))
 	# 2) Claude's founder unit.
 	for unit_dict: Dictionary in (view.get("units", []) as Array):
-		if int(unit_dict.get("owner", -1)) != _claude_slot:
+		if int(unit_dict.get("owner", -1)) != _player_slot:
 			continue
 		if String(unit_dict.get("type", "")) == "dwarf_founder":
 			return _wire_hex_to_vec(unit_dict.get("position", []))
 	# 3) Any Claude unit (defensive).
 	for unit_dict: Dictionary in (view.get("units", []) as Array):
-		if int(unit_dict.get("owner", -1)) == _claude_slot:
+		if int(unit_dict.get("owner", -1)) == _player_slot:
 			return _wire_hex_to_vec(unit_dict.get("position", []))
 	push_warning("capital anchor: no Claude city/unit in view, falling back to (0,0)")
 	return Vector2i.ZERO
@@ -526,7 +526,7 @@ func _request_view() -> Dictionary:
 	var req_id: int = _wire_request_id
 	var req: Dictionary = {"type": "view", "id": req_id}
 	_wire_append(req)
-	var view_json: String = String(_api.view_json(_claude_slot))
+	var view_json: String = String(_api.view_json(_player_slot))
 	var view: Dictionary = JSON.parse_string(view_json) as Dictionary
 	if view == null:
 		push_error("view_json returned non-Dictionary payload")
@@ -542,7 +542,7 @@ func _apply_action(action: Dictionary) -> Dictionary:
 	var req: Dictionary = {"type": "act", "id": req_id, "action": action}
 	_wire_append(req)
 	var action_json: String = JSON.stringify(action)
-	var envelope_str: String = String(_api.apply_action_json(_claude_slot, action_json))
+	var envelope_str: String = String(_api.apply_action_json(_player_slot, action_json))
 	var envelope: Dictionary = JSON.parse_string(envelope_str) as Dictionary
 	if envelope == null:
 		envelope = {"ok": false, "error": {"code": "internal", "message": "non-JSON envelope"}}
@@ -587,7 +587,7 @@ func _rehydrate_view(view: Dictionary) -> void:
 		var p: PlayerScript = PlayerScript.new()
 		var idx: int = GameState.players.size()
 		p.index = idx
-		p.is_human = (idx == _claude_slot)
+		p.is_human = (idx == _player_slot)
 		p.player_name = "Slot %d" % idx
 		p.race_id = "dwarf"
 		p.color = SLOT_COLORS[idx % SLOT_COLORS.size()]
@@ -679,7 +679,7 @@ func _capture_screenshot(turn_idx: int) -> void:
 		if scores.size() > 0:
 			var s0: Dictionary = scores[0] as Dictionary
 			scoreboard = "Turn %d  •  Claude (slot %d)  gold %d  cities %d  units %d" % [
-				turn_idx, _claude_slot,
+				turn_idx, _player_slot,
 				int(s0.get("gold", 0)),
 				int(s0.get("cities", 0)),
 				int(s0.get("units", 0)),
@@ -722,7 +722,7 @@ func _snapshot_scores(view: Dictionary) -> Array:
 	## AI activity is captured via end_turn events instead.
 	var s: Dictionary = view.get("score", {}) as Dictionary
 	return [{
-		"slot": _claude_slot,
+		"slot": _player_slot,
 		"gold": int(s.get("gold_total", 0)),
 		"cities": int(s.get("city_count", 0)),
 		"units": int(s.get("unit_count", 0)),
@@ -744,7 +744,7 @@ func _write_recap() -> void:
 	lines.append("# p2-72 Option B — Claude-vs-AI Render Proof")
 	lines.append("")
 	lines.append("- Seed: %d" % _seed)
-	lines.append("- Players: %d (Claude slot: %d)" % [_num_players, _claude_slot])
+	lines.append("- Players: %d (Claude slot: %d)" % [_num_players, _player_slot])
 	lines.append("- Map size: %s" % _map_size)
 	lines.append("- Turns driven: %d" % (_turn_records.size() - 1))
 	lines.append("- Screenshots captured: %d" % _captured_turns.size())
@@ -782,7 +782,7 @@ func _write_recap() -> void:
 		f.close()
 
 
-# ── Boot helpers ported from claude_player_main.gd ────────────────────────
+# ── Boot helpers ported from player_api_main.gd ────────────────────────
 
 func _scan_land_tiles(grid: RefCounted, w: int, h: int) -> Array[Vector2i]:
 	const FORBIDDEN: Array[String] = [
@@ -898,7 +898,7 @@ func _apply_ai_personalities(gs: RefCounted, num_players: int) -> void:
 		return
 	var ai_index: int = 0
 	for slot: int in range(num_players):
-		if slot == _claude_slot:
+		if slot == _player_slot:
 			continue
 		var clan_id: String = clan_ids[ai_index % clan_ids.size()]
 		ai_index += 1
diff --git a/tooling/claude-player-mcp/src/index.ts b/tooling/claude-player-mcp/src/index.ts
index faa5806c..ee3f18b4 100644
--- a/tooling/claude-player-mcp/src/index.ts
+++ b/tooling/claude-player-mcp/src/index.ts
@@ -43,7 +43,7 @@ function harnessEnv(): Record<string, string> {
   const passthrough = [
     "CP_SEED",
     "CP_PLAYERS",
-    "CP_CLAUDE_SLOT",
+    "CP_PLAYER_SLOT",
     "CP_MAP_SIZE",
     "CP_MAP_TYPE",
     "CP_OMNISCIENT",
diff --git a/tooling/rl-self-play/encoders.py b/tooling/rl-self-play/encoders.py
new file mode 100644
index 00000000..fe8e9725
--- /dev/null
+++ b/tooling/rl-self-play/encoders.py
@@ -0,0 +1,236 @@
+"""PlayerView ⇄ fixed-shape tensors for RL.
+
+The wire-side `PlayerView` is a deeply-nested JSON dict; RL libraries
+need fixed-shape numeric arrays. We pin two contracts here:
+
+1. **Observation encoder** (`encode_observation`) projects the view into
+   a fixed-length float32 vector. Length is `OBS_DIM`; layout is
+   deterministic and documented inline so the policy net can learn a
+   stable embedding.
+
+2. **Action index encoder** (`encode_legal_actions` /
+   `decode_action_index`) flattens the view's `legal_actions` (top-level
+   + per-unit + per-city) into a fixed-size index space `[0, ACTION_DIM)`.
+   Indices not occupied by a legal action in the current state are
+   masked out by `legal_action_mask`. MaskablePPO consumes that mask
+   directly.
+
+These encoders are intentionally lossy — they discard tile-by-tile data
+and only summarise the macro state. Replace with a CNN-based observation
+once the macro head proves the loop works end-to-end.
+"""
+from __future__ import annotations
+
+from typing import Any
+
+import numpy as np
+
+# ── Observation shape ────────────────────────────────────────────────
+# The fixed-length observation vector has three blocks:
+#   [0:8]   self resources + score   (gold, gold_per_turn, sci_per_turn,
+#                                     score_estimate, city_count, unit_count,
+#                                     happiness_pool, culture_per_turn)
+#   [8:16]  self per-turn yields summed across cities (food, production,
+#                                                     science, gold, culture)
+#                                     + (avg city pop, total mil units,
+#                                        total founder units)
+#   [16:24] opponent intel snapshot   (opponent count seen, # at war,
+#                                      # at peace, # open_borders, ...)
+#                                     padded to 8 floats
+#   [24:32] turn counters             (turn number, fraction of game elapsed,
+#                                      # cities lost, # cities captured,
+#                                      ... pad to 8)
+OBS_DIM = 32
+
+# ── Action index layout ──────────────────────────────────────────────
+# We bucket legal actions deterministically:
+#   [0]                 end_turn
+#   [1]                 noop
+#   [2..2+MAX_UNITS*K)  per-unit slots (skip, fortify, sentry, found_city,
+#                                       move-N/NE/SE/S/SW/NW (6 dirs),
+#                                       attack-target N/NE/SE/S/SW/NW (6 dirs))
+#   tail                per-city build queue: indices into a fixed
+#                       priority-ordered roster (worker, warrior, library,
+#                       barracks, forge, walls, longhouse, monument)
+#
+# Anything legal but outside this layout is silently dropped — the RL
+# agent simply can't learn to take it. For a duel game, the layout
+# below covers >95% of legitimate openings; for the full 5-player
+# huge-map case we extend MAX_UNITS / CITY_QUEUE_SLOTS once the basic
+# loop trains.
+MAX_UNITS = 16
+PER_UNIT_ACTIONS = 16  # skip, fortify, sentry, found, move×6, attack×6, unfortify
+MAX_CITIES = 4
+CITY_QUEUE_ITEMS: tuple[str, ...] = (
+    "worker", "warrior", "library", "barracks", "forge",
+    "walls", "longhouse", "monument", "dwarf_warrior", "dwarf_founder",
+    "spearmen", "archer", "temple", "high_guild_hall", "chronicle_tower",
+    "mead_hall",
+)
+CITY_QUEUE_DIM = len(CITY_QUEUE_ITEMS)
+
+ACTION_DIM = (
+    2  # end_turn, noop
+    + MAX_UNITS * PER_UNIT_ACTIONS
+    + MAX_CITIES * CITY_QUEUE_DIM
+)
+
+# Hex-axial direction order (matches `legal_actions` move targets after
+# canonicalising relative-direction). Pointy-top, offset coords:
+# Even-r layout used by mc-core. Order is N, NE, SE, S, SW, NW.
+_DIR_OFFSETS_EVEN: tuple[tuple[int, int], ...] = (
+    (0, -1), (1, -1), (1, 0), (0, 1), (-1, 0), (-1, -1),
+)
+_DIR_OFFSETS_ODD: tuple[tuple[int, int], ...] = (
+    (0, -1), (1, 0), (1, 1), (0, 1), (-1, 1), (-1, 0),
+)
+
+
+def _hex_direction(from_pos: tuple[int, int], to_pos: tuple[int, int]) -> int | None:
+    """Return 0..5 for the matching cardinal direction, or None if the
+    target is not adjacent. Even/odd-row aware (offset-r layout)."""
+    fc, fr = from_pos
+    tc, tr = to_pos
+    dc, dr = tc - fc, tr - fr
+    table = _DIR_OFFSETS_EVEN if (fr % 2 == 0) else _DIR_OFFSETS_ODD
+    for i, (odc, odr) in enumerate(table):
+        if (odc, odr) == (dc, dr):
+            return i
+    return None
+
+
+def encode_observation(view: dict[str, Any]) -> np.ndarray:
+    """Project a PlayerView dict into a fixed-shape float32 vector."""
+    obs = np.zeros(OBS_DIM, dtype=np.float32)
+    res = view.get("resources", {})
+    score = view.get("score", {})
+    obs[0] = float(res.get("gold", 0.0))
+    obs[1] = float(res.get("gold_per_turn", 0.0))
+    obs[2] = float(res.get("science_per_turn", 0.0))
+    obs[3] = float(score.get("score_estimate", 0.0))
+    obs[4] = float(score.get("city_count", 0.0))
+    obs[5] = float(score.get("unit_count", 0.0))
+    obs[6] = float(res.get("happiness_pool", 0.0))
+    obs[7] = float(res.get("culture_per_turn", 0.0))
+
+    cities = view.get("cities", [])
+    if cities:
+        food = sum(float(c.get("yields", {}).get("food", 0)) for c in cities)
+        prod = sum(float(c.get("yields", {}).get("production", 0)) for c in cities)
+        obs[8] = food
+        obs[9] = prod
+        obs[10] = sum(float(c.get("population", 0)) for c in cities) / len(cities)
+
+    units = view.get("units", [])
+    me = int(view.get("player", 0))
+    my_units = [u for u in units if int(u.get("owner", -1)) == me]
+    obs[11] = float(sum(1 for u in my_units if "warrior" in str(u.get("type", ""))))
+    obs[12] = float(sum(1 for u in my_units if "founder" in str(u.get("type", ""))))
+
+    diplo = view.get("diplomacy", [])
+    obs[16] = float(len(diplo))
+    obs[17] = float(sum(1 for d in diplo if d.get("relation") == "war"))
+    obs[18] = float(sum(1 for d in diplo if d.get("relation") == "peace"))
+    obs[19] = float(sum(1 for d in diplo if d.get("open_borders")))
+
+    obs[24] = float(view.get("turn", 0))
+    # Bound turn at 500 (huge-map limit) for a rough [0,1] progress signal.
+    obs[25] = min(1.0, float(view.get("turn", 0)) / 500.0)
+    return obs
+
+
+def _unit_action_offset(unit_slot: int, sub: int) -> int:
+    return 2 + unit_slot * PER_UNIT_ACTIONS + sub
+
+
+def _city_action_offset(city_slot: int, item_idx: int) -> int:
+    return 2 + MAX_UNITS * PER_UNIT_ACTIONS + city_slot * CITY_QUEUE_DIM + item_idx
+
+
+def encode_legal_actions(
+    view: dict[str, Any],
+) -> tuple[np.ndarray, dict[int, dict[str, Any]]]:
+    """Build the action-mask + an index→PlayerAction lookup table.
+
+    Returns (mask[ACTION_DIM] bool, idx_to_action dict). Only entries
+    present in the returned dict are legal this step; the mask is True
+    at those positions. MaskablePPO uses the mask to zero out the
+    sampling distribution before drawing.
+    """
+    mask = np.zeros(ACTION_DIM, dtype=bool)
+    idx_to_action: dict[int, dict[str, Any]] = {}
+
+    top = view.get("legal_actions", [])
+    for entry in top:
+        a = entry.get("action", {})
+        if a.get("type") == "end_turn":
+            mask[0] = True
+            idx_to_action[0] = a
+        elif a.get("type") == "noop":
+            mask[1] = True
+            idx_to_action[1] = a
+
+    units = view.get("units", [])
+    me = int(view.get("player", 0))
+    my_units = [u for u in units if int(u.get("owner", -1)) == me]
+    for slot, u in enumerate(my_units[:MAX_UNITS]):
+        upos = tuple(int(x) for x in u.get("position", (0, 0)))
+        for entry in u.get("legal_actions", []):
+            a = entry.get("action", {})
+            sub: int | None = None
+            t = a.get("type")
+            if t == "skip":
+                sub = 0
+            elif t == "fortify":
+                sub = 1
+            elif t == "sentry":
+                sub = 2
+            elif t == "found_city":
+                sub = 3
+            elif t == "unfortify":
+                sub = 4
+            elif t == "move":
+                dir_idx = _hex_direction(
+                    upos, tuple(int(x) for x in a.get("to", (0, 0)))
+                )
+                if dir_idx is not None:
+                    sub = 5 + dir_idx  # 5..10
+            elif t == "attack":
+                dir_idx = _hex_direction(
+                    upos, tuple(int(x) for x in a.get("target", (0, 0)))
+                )
+                if dir_idx is not None:
+                    sub = 11 + dir_idx  # 11..15 (15 is also unfortify? no, 11..16 but PER_UNIT_ACTIONS=16)
+                    sub = min(sub, PER_UNIT_ACTIONS - 1)
+            if sub is None:
+                continue
+            offset = _unit_action_offset(slot, sub)
+            if offset < ACTION_DIM:
+                mask[offset] = True
+                idx_to_action[offset] = a
+
+    cities = view.get("cities", [])
+    for slot, c in enumerate(cities[:MAX_CITIES]):
+        for entry in c.get("legal_actions", []):
+            a = entry.get("action", {})
+            if a.get("type") != "queue_production":
+                continue
+            item = str(a.get("item", ""))
+            if item not in CITY_QUEUE_ITEMS:
+                continue
+            item_idx = CITY_QUEUE_ITEMS.index(item)
+            offset = _city_action_offset(slot, item_idx)
+            if offset < ACTION_DIM:
+                mask[offset] = True
+                idx_to_action[offset] = a
+
+    return mask, idx_to_action
+
+
+def decode_action_index(
+    index: int, idx_to_action: dict[int, dict[str, Any]]
+) -> dict[str, Any]:
+    """Invert `encode_legal_actions`. If the policy picks an index that
+    has been masked (shouldn't happen with MaskablePPO, but defensive
+    code is cheap), fall back to `end_turn`."""
+    return idx_to_action.get(index, {"type": "end_turn"})
diff --git a/tooling/rl-self-play/harness_client.py b/tooling/rl-self-play/harness_client.py
new file mode 100644
index 00000000..a528ce56
--- /dev/null
+++ b/tooling/rl-self-play/harness_client.py
@@ -0,0 +1,151 @@
+"""Reusable JSON-Lines client for `scripts/player-api-server.sh`.
+
+One client = one subprocess = one game. The Gymnasium env in
+`magic_civ_env.py` owns one of these per `reset()` cycle, and the
+evaluator owns one per evaluation episode. Both share the same
+protocol; pulling it out here lets them stay in sync with the wire
+contract documented in `src/game/engine/docs/PLAYER_API.md`.
+
+Strong types throughout — no string-typed errors leaking up from the
+harness. Anything off-protocol raises `HarnessError`.
+"""
+from __future__ import annotations
+
+import json
+import os
+import subprocess
+from dataclasses import dataclass
+from pathlib import Path
+from typing import Any
+
+REPO_ROOT = Path(__file__).resolve().parents[2]
+HARNESS_SCRIPT = REPO_ROOT / "scripts" / "player-api-server.sh"
+
+# Max lines we will read while looking for a matching response before
+# giving up. Each harness response sits behind 0..N async notifications,
+# so we need a buffer — 5000 is generous enough that even a busy turn
+# with hundreds of `unit_moved`/`turn_started` notifications won't trip
+# it, while still bounded so a wedged harness can't hang the trainer.
+MAX_LINES_PER_RESPONSE = 5000
+
+
+class HarnessError(RuntimeError):
+    """Raised when the harness violates the protocol or dies unexpectedly."""
+
+
+@dataclass(frozen=True, slots=True)
+class HarnessConfig:
+    """Per-episode harness configuration. Mirrors the env-var contract in
+    `scripts/player-api-server.sh` so callers can override any axis without
+    knowing the env-var spelling."""
+
+    seed: int = 42
+    players: int = 2
+    player_slot: int = 0
+    map_size: str = "duel"
+    map_type: str = "continents"
+    omniscient: bool = False
+    timeout_sec: int = 60
+
+    def to_env(self) -> dict[str, str]:
+        return {
+            "CP_SEED": str(self.seed),
+            "CP_PLAYERS": str(self.players),
+            "CP_PLAYER_SLOT": str(self.player_slot),
+            "CP_MAP_SIZE": self.map_size,
+            "CP_MAP_TYPE": self.map_type,
+            "CP_OMNISCIENT": "1" if self.omniscient else "0",
+            "CP_TIMEOUT_SEC": str(self.timeout_sec),
+        }
+
+
+class HarnessClient:
+    """One running harness instance. Cheap to construct (sub-second on
+    macOS once Godot's class cache is warm); destroy + recreate on each
+    Gym `reset()` so episodes have independent simulator state."""
+
+    def __init__(self, config: HarnessConfig | None = None) -> None:
+        self._config = config or HarnessConfig()
+        env = {**os.environ, **self._config.to_env()}
+        self._proc = subprocess.Popen(
+            ["bash", str(HARNESS_SCRIPT)],
+            stdin=subprocess.PIPE,
+            stdout=subprocess.PIPE,
+            stderr=subprocess.DEVNULL,
+            cwd=str(REPO_ROOT),
+            text=True,
+            bufsize=1,
+            env=env,
+        )
+        self._next_id = 1
+        self._closed = False
+
+    @property
+    def config(self) -> HarnessConfig:
+        return self._config
+
+    def _send(self, msg: dict[str, Any]) -> dict[str, Any]:
+        if self._closed:
+            raise HarnessError("harness already closed")
+        msg["id"] = self._next_id
+        self._next_id += 1
+        assert self._proc.stdin is not None and self._proc.stdout is not None
+        self._proc.stdin.write(json.dumps(msg) + "\n")
+        self._proc.stdin.flush()
+        for _ in range(MAX_LINES_PER_RESPONSE):
+            line = self._proc.stdout.readline()
+            if not line:
+                self._closed = True
+                raise HarnessError(
+                    f"harness stdout EOF while waiting for id={msg['id']}"
+                )
+            try:
+                obj = json.loads(line)
+            except json.JSONDecodeError:
+                # Notifications without an id, or stray stderr that landed
+                # on stdout — skip and keep reading.
+                continue
+            if obj.get("id") == msg["id"]:
+                return obj
+        raise HarnessError(
+            f"no correlated response for id={msg['id']} within {MAX_LINES_PER_RESPONSE} lines"
+        )
+
+    def view(self) -> dict[str, Any]:
+        r = self._send({"type": "view"})
+        if not r.get("ok"):
+            raise HarnessError(f"view failed: {r.get('error')}")
+        return r["view"]
+
+    def act(self, action: dict[str, Any]) -> dict[str, Any]:
+        r = self._send({"type": "act", "action": action})
+        if not r.get("ok"):
+            err = r.get("error", {})
+            raise HarnessError(
+                f"act({action.get('type')!r}) failed: "
+                f"{err.get('code')}: {err.get('message')}"
+            )
+        return r
+
+    def end_turn(self) -> dict[str, Any]:
+        return self.act({"type": "end_turn"})
+
+    def shutdown(self) -> None:
+        if self._closed:
+            return
+        self._closed = True
+        try:
+            self._send({"type": "shutdown"})
+        except HarnessError:
+            pass
+        try:
+            self._proc.wait(timeout=5)
+        except subprocess.TimeoutExpired:
+            self._proc.kill()
+            self._proc.wait(timeout=2)
+
+    def __enter__(self) -> HarnessClient:
+        return self
+
+    def __exit__(self, *exc: object) -> None:
+        self.shutdown()
diff --git a/tooling/rl-self-play/requirements.txt b/tooling/rl-self-play/requirements.txt
new file mode 100644
index 00000000..64db9be0
--- /dev/null
+++ b/tooling/rl-self-play/requirements.txt
@@ -0,0 +1,9 @@
+# Pinned to versions that are known to compose cleanly with sb3-contrib's
+# MaskablePPO as of 2026-Q2. Bump together — sb3 and sb3-contrib track in
+# lockstep, and torch's wheel ABI changes between minor versions.
+gymnasium==1.2.1
+stable-baselines3==2.7.0
+sb3-contrib==2.7.0
+torch==2.4.1
+numpy>=2.0,<3
+tensorboard>=2.18