create-harness

name

create-harness

description

Create or update an LLM harness that lets a language model play a kaggle-environments game. Use this skill whenever the user wants to write a harness, LLM agent, or game-playing prompt for any kaggle-environments game — including OpenSpiel games, word games, or custom environments. Also use it when the user mentions core_harness.py, GameHarness, ParseResult, or asks how to connect an LLM to a game.

Create an LLM Harness

A harness bridges a game environment and an LLM. It translates game observations into prompts, sends them to the model, and parses the model's response back into a legal game action. The framework in kaggle_environments/core_harness.py handles the LLM call, retry loop, telemetry, and agent lifecycle — you only implement the game-specific logic.

What you're building

A harness module that implements three functions (the GameHarness protocol):

Method	Purpose
get_legal_moves(observation)	Extract legal actions from the observation
make_prompt(observation, move_history, ...)	Build the LLM prompt
parse_response(response, legal_action_strings)	Extract the chosen action from the LLM's text

Production wires these three module-level functions into an agent via an external wrapper template — do not write an adapter class or a create_agent_fn(...) line in harness.py. Tests construct their own local adapter when they want end-to-end coverage; see Step 6 for the pattern.

Step 1: Understand the game

Before writing any code, understand the game you're building a harness for:

1. Read the game's interpreter to understand actions, observations, and turn structure.
2. For OpenSpiel games, check for a proxy (*_proxy.py) — if one exists, the observation will be structured JSON rather than raw OpenSpiel text. The proxy's state_dict() method shows exactly what fields the LLM will see. Non-OpenSpiel environments already provide structured JSON observations directly, so no proxy is needed.
3. Identify the action space type:
   • Enumerable (most games): a fixed set of legal moves per turn (e.g., board coordinates, directions, bids). get_legal_moves returns dict[int, str].
   • Free-form (rare): the agent can submit any structured object (e.g., a clue in a word game). get_legal_moves returns None.
   • Mixed (rarest): some turns are enumerable, others are free-form, determined at runtime.
4. Understand the observation dict. The harness receives an observation dict with these standard fields:
   • observationString — the game state (often JSON from a proxy)
   • legalActions — list of action IDs (ints)
   • legalActionStrings — list of human-readable action strings
   • currentPlayer — whose turn it is (-2 means simultaneous)
   • playerId — this agent's player ID
   • isTerminal — whether the game is over
   • serializedGameAndState — full pyspiel state (OpenSpiel games only)

Step 2: Implement get_legal_moves

This function extracts legal actions from the observation and returns them as {action_id: action_string}.

from typing import Any, Mapping, Sequence
from kaggle_environments.core_harness import ParseResult

def get_legal_moves(observation: Mapping[str, Any]) -> dict[int, str] | None:
    """Return {action_id: action_string} for enumerable games, or None for free-form."""
    legal_actions = observation.get("legalActions")
    legal_action_strings = observation.get("legalActionStrings")
    if legal_actions and legal_action_strings:
        return dict(zip(legal_actions, legal_action_strings))
    return {}

For OpenSpiel games with a fallback (when the environment doesn't always provide legalActions):

def get_legal_moves(observation: Mapping[str, Any]) -> dict[int, str]:
    legal_actions = observation.get("legalActions")
    legal_action_strings = observation.get("legalActionStrings")
    if legal_actions and legal_action_strings:
        return dict(zip(legal_actions, legal_action_strings))
    # Fallback: deserialize pyspiel state
    serialized = observation.get("serializedGameAndState", "")
    if not serialized:
        return {}
    _, state = pyspiel.deserialize_game_and_state(serialized)
    return {a: state.action_to_string(a) for a in state.legal_actions()}

For mixed action spaces (like word_association), return None on free-form turns:

def get_legal_moves(observation: Mapping[str, Any]) -> dict[int, str] | None:
    if observation.get("isCluemaster"):
        return None  # Free-form: LLM submits any valid clue
    # Enumerable: return word choices
    words = observation.get("words", [])
    return {i: f"{i}: {words[i]}" for i in range(len(words))}

Step 3: Write the prompt

The prompt is the most important part of a harness — it determines how well the LLM plays. The make_prompt function (sometimes named generate_prompt in older code) builds the full text sent to the model.

Signature

def generate_prompt(
    observation: Mapping[str, Any],
    move_history: list[str],
    previous_response: str | None = None,
    previous_action: str | None = None,
) -> str:

• observation — the current game state
• move_history — list of this agent's past action strings (persists across turns)
• previous_response — the LLM's last response if this is a retry after an illegal move
• previous_action — the illegal move that was attempted

Prompt structure

A good prompt follows this pattern:

1. Game rules — concise explanation of how the game works
2. Current state — the board/situation, parsed from observation["observationString"]
3. Move history — what's happened so far
4. Player identity — which side the LLM is playing
5. Output format — tell the LLM exactly how to format its move
6. Rethink suffix — appended on retries to explain what went wrong

Do not enumerate the legal moves in the prompt. The framework already validates the LLM's response against legalActionStrings and triggers a retry with the rethink suffix on an illegal move, so listing every legal action just bloats the prompt, encourages the model to pick mechanically from the list instead of reasoning about the position, and trivializes the benchmark for games whose challenge is partly finding the legal moves (e.g. checkers' forced captures). Instead, describe the move-legality rules clearly and let the model derive legality from the state. The lone exception is when the legal-action set is not derivable from the visible state at all (e.g. a hidden hand of cards the LLM owns but the rules text cannot enumerate) — in that case, list only what the model cannot infer.

Prompt template example

Read kaggle_environments/envs/open_spiel_env/games/checkers/harness.py — it's the canonical shape: a single *_PROMPT_TEMPLATE constant with explicit {field} placeholders, plus two named rethink templates (RETHINK_ILLEGAL, RETHINK_UNPARSABLE) selected by render_rethink_suffix from core_harness.

The required pieces:

• A main template that interpolates game state, player identity, move history, and a JSON output spec with a concrete For example: line.
• RETHINK_ILLEGAL — fires when the parser extracted a move that wasn't legal. Leads with {previous_action} ("You suggested … but this is not a legal move"). Does NOT include the previous response.
• RETHINK_UNPARSABLE — fires when the parser extracted nothing. Leads with {previous_response} and restates the JSON format with a clean placeholder + concrete example.
• A generate_prompt that builds the main prompt and appends render_rethink_suffix(RETHINK_ILLEGAL, RETHINK_UNPARSABLE, previous_response, previous_action). The helper returns "" on the first attempt, picks the right branch otherwise, and truncates previous_response to the last 500 chars.

For an imperfect-information variant (per-player observation, custom matcher=), use dark_hex/harness.py as the model instead.

Prompt writing tips

• Parse the observation string. If the game has a proxy, observation["observationString"] is JSON — parse it and present the state clearly rather than dumping raw JSON.
• Be specific about the output format. The LLM's response needs to be parseable. JSON with a clear field name (like "move") works well.
• Explain the coordinate system. If the game uses a board, explain notation clearly (e.g., "columns are letters a-h, rows are numbers 1-9").
• Include rules that affect strategy. Don't just list rules mechanically — highlight the ones that matter for making good decisions.
• Don't give strategy advice. The prompt should explain rules and mechanics, not coach the model on how to play. Saying "capture pieces to win" is fine (that's a rule); saying "control the center early" or "prefer defensive moves" is not (that's strategy). The LLM should reason about strategy on its own from the rules and game state.
• Keep the rethink suffix concise. Truncate the previous response to ~500 characters. Include the illegal move attempt and remind the LLM to re-derive a legal move from the state and rules (do not paste in a legal-moves list on retry either — same reasoning as above).
• Branch the rethink on whether previous_action was extracted. Each case wants a different signal back at the model:
  • previous_action is set (parser pulled a value from the JSON) → the action string itself is the most useful signal. Lead with "You suggested move {previous_action} but this is not legal." Do NOT also include the full previous response — that's noise the model has to skim past to find the actual correction signal. Tail with a brief reminder to keep using the same JSON format.
  • previous_action is None (parser found nothing) → there's no action string to show. Lead with the previous response (last 500 chars, not first 500 — the model's conclusion is at the end, not the preamble) so the model can see what it tried, then restate the JSON format with a concrete example. Tail with a brief reminder that the move must also be legal.
  • Pick the suffix at render time based on previous_action. A single one-size-fits-all suffix that always restates the format teaches the wrong fix on illegal-move retries; a single suffix that never restates the format leaves the model in the dark on unparseable retries.
• Make the JSON example unambiguous. In the rethink-unparseable suffix, write the JSON example so the placeholder and the are clearly separated. reads like the value should literally be that whole string. Instead, use a clean placeholder in the fenced block and put a concrete example on its own line right after:

move_history_str = ", ".join(move_history) if move_history else "None"

Second pass: compact the prose

Shorter prompts have matched or beaten longer ones across games; padding buries the signal. After the first working draft, do a dedicated compaction pass at a real rendered observation:

• Collapse restatements (same rule in the rules section and output-format section — keep one).
• Cut filler ("Note that…", "Please…") and hedging ("Try your best to…").
• Replace paragraphs with a single declarative sentence where possible.
• Keep every concrete rule, phase distinction, coordinate-system explanation, and format example.

Start compact; only add clarifying prose or worked examples back in when a later evaluation shows the model actually struggling.

Step 4: Implement parse_response

The parser extracts the LLM's chosen move from its text response and matches it to a legal action. This is where robustness matters — LLMs don't always follow instructions perfectly.

Signature and return type

def parse_response(
    response: str,
    legal_action_strings: Sequence[str] | None,
) -> ParseResult:

ParseResult is a frozen dataclass with three fields:

@dataclasses.dataclass(frozen=True)
class ParseResult:
    legal_action: str | None = None   # Matched legal move string (enumerable)
    raw_action: str | None = None     # What the model actually said (for rethink context)
    submission: Any = None            # Parsed object (free-form only)

For enumerable actions: set legal_action to the matched string from legal_action_strings, and raw_action to what the model originally said.

For free-form actions: set submission to the parsed object (e.g., a dict), and raw_action to a string representation.

On failure: return ParseResult(raw_action=<what_was_attempted>) — the framework will retry with the rethink prompt.

The default: parse_json_action from core_harness

For an enumerable harness, your parse_response should be one line — delegate to parse_json_action:

from kaggle_environments.core_harness import ParseResult, parse_json_action


def parse_response(
    response: str, legal_action_strings: Sequence[str],
) -> ParseResult:
    """Trust the model's JSON answer; let the rethink loop fix anything else."""
    return parse_json_action(response, legal_action_strings)

parse_json_action enforces the design rule that the parser has exactly one intent surface: the model's structured JSON answer. It uses extract_last_json_object under the hood (last-block-wins, both fenced and bare), and:

• If the model wrote no JSON answer at all → legal_action=None, raw_action=None → rethink loop asks for one.
• If the JSON is parseable but the move is illegal → legal_action=None, raw_action=<what_the_model_said> → rethink loop shows the model what it tried and asks it to pick a legal move.
• If the JSON is legal → legal_action=<matched>, raw_action=<raw> → submit.

Do not write a prose-scan fallback. Coord regex, keyword regex, response.rfind(legal), anything that picks a token mentioned in the reasoning text — these silently substitute moves the model never explicitly chose (almost always a rejected option from the prose). The review-harness skill's "Ghost-fallback / prose-scan rescue" entry has the empirical case: across 12 harnesses, 7,477 substitutions in one game's replay archive touched 74% of episodes.

Customizing the JSON key

If your game uses a key other than "move", pass json_key=:

return parse_json_action(response, legal_action_strings, json_key="bid")

Customizing the matcher

The default matcher is case-insensitive and strips whitespace. For games that need notation tolerance, alias handling, or canonicalization (e.g. 'A7' → 'a7', 'b1-c2' → 'b1xc2'), pass matcher=:

def _match_move_to_legal(
    raw: str, legal_action_strings: Sequence[str],
) -> str | None:
    """Game-specific normalization, e.g. canonicalize a coord."""
    canonical = _canonicalize(raw)
    return canonical if canonical in set(legal_action_strings) else None


def parse_response(
    response: str, legal_action_strings: Sequence[str],
) -> ParseResult:
    return parse_json_action(
        response, legal_action_strings, matcher=_match_move_to_legal,
    )

The matcher operates on the raw value the model wrote inside the JSON, never the full response. This is the one place where game-specific parsing logic belongs.

Pre-flight check before shipping the default matcher. Print a handful of state.action_to_string(...) outputs at representative states (initial, mid-game, post-capture, multi-component turns). For every non-alphanumeric marker that appears — * (backgammon hit), x (checkers/chess capture), trailing Pass (backgammon per-die filler), - (move separator), O-O (castling) — ask: "would a model naturally omit or add this?" If yes, the default _default_match won't tolerate it and the rethink loop pays for every drift. Backgammon's audit found 97.3% of episodes forfeited; adding just * and Pass tolerance via matcher= recovered 34.2% of forfeit turns. Build the tolerant normalization once, keep it in the matcher.

Free-form harnesses

For free-form turns (where legal_action_strings is None), don't use parse_json_action — write the extraction by hand using extract_last_json_object, since the resulting object goes into submission rather than legal_action:

import json
from kaggle_environments.core_harness import (
    ParseResult, extract_last_json_object, parse_json_action,
)


def parse_response(response, legal_action_strings):
    if legal_action_strings is None:
        data = extract_last_json_object(response, required_keys=("clue",))
        if data and "clue" in data:
            return ParseResult(
                submission=data,
                raw_action=json.dumps(data),
            )
        return ParseResult(raw_action=response[:200])
    return parse_json_action(response, legal_action_strings)

Last-mention-wins — when extracting the structured answer

When the model writes the structured answer surface more than once — a draft, then a revision — the last occurrence is the intent. Models almost always enumerate options ("considered a1, then b2, but going with e5") before stating the final answer. Forward iteration silently picks the rejected first candidate.

parse_json_action and extract_last_json_object already handle this for JSON answers. If your harness uses a different stage-1 surface (e.g. a tagged Final Answer: <x> line), apply the same rule:

Pattern	Wrong	Right
Multiple JSON blocks	_JSON_BLOCK_RE.search(response)	parse_json_action(response, legal_action_strings) (preferred) or extract_last_json_object(response, required_keys=(...))
Single-match regex extract for an answer tag	m = r.search(response)	matches = list(r.finditer(response)); m = matches[-1] if matches else None
Substring of a fixed answer tag	response.find("Final Answer:")	response.rfind("Final Answer:")

Do not scan the response for unstructured candidates. Iterating a regex (finditer / findall) over the response for coords or keywords, or iterating legal_action_strings and checking response.rfind(legal), is the prose-scan rescue antipattern even with the "reverse-iter" fix: the model often discusses several options it doesn't choose, and any loop over those mentions silently substitutes a non-chosen move. If the structured answer is missing or illegal, return legal_action=None and let the rethink loop ask the model to fix its format. See the review-harness "Ghost-fallback / prose-scan rescue" entry for the empirical impact.

Parsing tips

• Prefer parse_json_action over rolling your own. It's the audited single-stage default and removes the temptation to add a "smart" fallback later.
• JSON is the only intent surface. No prose fallback. If the model doesn't give a JSON answer, return legal_action=None and let the rethink loop ask for one. Prose-scan fallbacks reliably substitute moves the model never chose (rejected options it discussed in its reasoning).
• Case-insensitive matching is essential — the default matcher already lowercases and strips whitespace. Pass a custom matcher= only when you need notation tolerance, alias handling, or canonicalization.
• Handle special actions like "PASS" or "STAND" by including them in legal_action_strings; the default matcher will pick them up.
• Adapt the JSON key to your game by passing json_key=: "move" for board games (default), "bid" for auction games, "action" for generic games — whatever matches your prompt.
• For numeric actions (like bids), validate in the matcher: convert raw → int, check membership in the legal-action-encoded integers, and return the canonical legal string.

Step 6: Write tests

Harness tests follow a consistent 4-class structure. Create your test file at the same relative path as the harness, under tests/.

For example:

• Harness at kaggle_environments/envs/open_spiel_env/games/myg/harness.py
• Tests at tests/envs/open_spiel_env/games/myg/harness_test.py

Copy tests/envs/open_spiel_env/games/checkers/harness_test.py as the starting point. It's the canonical layout: a _make_observation helper that builds a harness-style obs dict from a proxy state, and four test classes that together cover the surface area.

Class	What it covers
ParseResponseTest	Parser in isolation. Cover at minimum: fenced JSON, bare JSON, case-insensitive match, illegal-move-returns-raw, prose-only-returns-None (no ghost fallback), multiple-JSON-last-wins. Add a test_illegal_json_does_not_ghost_substitute_from_prose regression — the model writes a legal token in prose, then commits to an illegal one in JSON; the parser must NOT silently substitute the prose token.
GeneratePromptTest	Prompt contents from a real proxy state. Cover: rules keywords present, board orientation, player-asymmetric text differs between player_id=0 and player_id=1, captures/phase flags render correctly, rethink suffixes appear under the right conditions, the JSON example format is unambiguous. If the harness has multi-branch prompts (roles/phases), assert each branch contains its required rules.
GetLegalMovesTest	Round-trip from legalActions + legalActionStrings, fallback from serializedGameAndState, empty-obs returns {}.
AgentIntegrationTest	Full harness through create_agent_fn with litellm.completion patched. Cover: successful move, retry-on-bad-parse, raise-after-two-failures, terminal-step-returns-inactive, and a short scripted game (first-legal-each-turn) that round-trips through pyspiel without raising. Define a small test-local _MyGameHarness adapter (see the snippet below) at the top of the test file and pass it to create_agent_fn; do NOT import an adapter from harness.py (there isn't one).

Test-local adapter pattern (the only place an adapter class should live):

class _MyGameHarness:
    """Test-local GameHarness adapter; mirrors the prod wrapper shape."""

    def get_legal_moves(self, observation):
        return get_legal_moves(observation)

    def make_prompt(self, observation, move_history,
                    previous_response=None, previous_action=None):
        return generate_prompt(
            observation, move_history, previous_response, previous_action,
        )

    def parse_response(self, response, legal_action_strings, *, observation=None):
        # Most parsers ignore `observation`. If yours forwards it to a
        # module-level parse_response that needs the env state, pass it
        # through here.
        return parse_response(response, legal_action_strings)

The *, observation keyword on parse_response is required by the GameHarness protocol — core_harness always passes the current turn's observation. Most parsers can match the model's output against legal_action_strings alone and can ignore it; for parsers that genuinely need the env state at parse time (e.g. repeated_poker's bet-size soft-matching), declare an opt-in *, observation: Mapping[str, Any] | None = None kwarg on the module-level parse_response too and forward it through. See kaggle_environments/envs/open_spiel_env/games/repeated_poker/harness.py for that pattern.

Mock helpers (_StreamDelta, _StreamChoice, _StreamChunk, _make_mock_response, _ENV) live at the top of checkers' harness_test.py — copy them verbatim; they're game-independent.

Step 7: Write test_llm_game.py

Every harness should include a test_llm_game.py script that runs a full game locally with a real LLM — catches issues mocked unit tests miss (bad prompts, unparseable responses, env interaction bugs).

Use run_llm_game from kaggle_environments.local_harness_runner. The helper handles API-key discovery, env-var defaults, --model / --replay-path CLI flags, game execution, per-step printing, and replay save. Per-game files are 3 lines plus a docstring:

"""Run a full Checkers game with LLM agents for local integration testing."""
from kaggle_environments.local_harness_runner import run_llm_game

if __name__ == "__main__":
    run_llm_game("open_spiel_checkers", caller_file=__file__)

For games that need extra config, pass configuration=, replay_filename=, num_agents=, or agent_module=. See havannah/test_llm_game.py (custom board size), word_association/test_llm_game.py (4 agents, custom post-run output), and python_ant_foraging/test_llm_game.py (custom replay filename) for real examples. Capture the returned env if you need to print game-specific results after the run.

Place it next to the harness module:

• OpenSpiel: kaggle_environments/envs/open_spiel_env/games/<name>/test_llm_game.py
• Non-OpenSpiel: kaggle_environments/envs/<name>/test_llm_game.py

File organization

OpenSpiel games

kaggle_environments/envs/open_spiel_env/games/<name>/
├── __init__.py
├── <name>_proxy.py          # proxy (if not already created)
├── harness.py                # <-- your harness
└── test_llm_game.py          # local LLM integration test

tests/envs/open_spiel_env/games/<name>/
└── harness_test.py

Non-OpenSpiel games

kaggle_environments/envs/<name>/
├── harness.py                # <-- your harness
├── test_llm_game.py          # local LLM integration test
└── ...

tests/envs/<name>/
└── harness_test.py

Running tests

uv sync && uv run pytest tests/envs/open_spiel_env/games/<name>/harness_test.py -v

Checklist

• Identified the action space type (enumerable, free-form, or mixed)
• get_legal_moves returns dict[int, str] or None as appropriate
• Prompt includes: rules, state, move history, player identity, output format
• Prompt does not enumerate the legal moves (let the model derive legality from the state and rules)
• Every concrete field/value/rule in the prompt traces to a code path (no phantom claims; rules match the env's actual load_game params)
• Board dimensions and coordinate-system text are derived from the observation / env config — not hardcoded — and the prompt renders correctly at every supported size
• Move history shown to the model covers BOTH players (sourced from the proxy's state_dict() or reconstructed from serializedGameAndState), not just the per-agent move_history argument — and the prompt renders the actual moves (e.g. "a1b1, b3a3, ..."), not just a count like "Moves played so far: 14"
• Prompt rendered for player_id=0 and player_id=1 — directional/orientation language mirrors correctly
• Multi-phase games dispatch on the engine's explicit phase identifier, not legals-shape; unknown phase raises instead of silently falling through
• Prompt has a rethink suffix for retries (uses previous_response and previous_action)
• Compaction pass done on the rendered prompt: restatements, filler, and hedging cut; concrete rules and format examples preserved. Start compact; expand only in response to observed model failures.
• parse_response delegates to parse_json_action (uses last-mention-wins JSON extraction; no prose-scan fallback — that's the ghost-fallback anti-pattern)
• does case-insensitive matching (default matcher) or passes a custom for notation tolerance (check outputs for , , trailing , , etc. that models drop or add)

Reference files

File	What to learn from it
kaggle_environments/core_harness.py	The framework — GameHarness protocol, ParseResult, create_agent_fn, parse_json_action, render_rethink_suffix
Canonical templates — start here:
kaggle_environments/envs/open_spiel_env/games/checkers/harness.py	Modern enumerable shape: delegates to parse_json_action, branches render_rethink_suffix, demonstrates a phase-conditional prompt section (multi-jump continuation)
kaggle_environments/envs/open_spiel_env/games/dark_hex/harness.py	Same modern shape with a custom matcher= callable for notation tolerance; per-player rendering for imperfect-information games
kaggle_environments/envs/word_association/harness.py	Mixed free-form + enumerable harness (non-OpenSpiel)
Test patterns:
tests/envs/open_spiel_env/games/checkers/harness_test.py	Full enumerable test suite (parser stress, prompt assertions, create_agent_fn integration with mocked LLM)
tests/core_harness_test.py	Framework test patterns
kaggle_environments/envs/open_spiel_env/games/checkers/test_llm_game.py	LLM integration test script (OpenSpiel)
kaggle_environments/envs/word_association/test_llm_game.py	LLM integration test script (non-OpenSpiel)