cow-backtester

Offline backtester, A/B harness, and counterfactual scorecard for CoW Protocol solvers.

Point it at your own solver's /solve endpoint and it replays recent CoW auctions against it, then scores each of your solutions against the set of solutions that actually won on-chain. Point it at two endpoints and it runs a head-to-head A/B on identical auctions, so a routing or config change can be judged before it goes to production.

Nothing touches production: no shadow mode, no staging deployment, no keys.

Why this exists

Today a solver can only be evaluated live: shadow mode consumes the production auction stream, and the local playground runs against a chain fork. Neither lets you take a fixed set of recent auctions, run your solver against them offline, and ask "would I have out-surplused the winners, and by how much?" the way you'd backtest a trading strategy. This tool does.

Quick start

Python 3.10+.

pip install cow-backtester   # installs the `cow-backtester` command (one dependency: eth_abi)

# 1. Baseline — what the field actually captured (no solver needed)
cow-backtester --chain base --blocks 2000 --rpc-url <your-rpc>

# 2. Counterfactual — replay through your solver
cow-backtester --chain base --blocks 2000 --rpc-url <your-rpc> \
        --solver-url http://localhost:8080 --json-out results.jsonl

# 3. A/B — two solvers, same auctions, head-to-head
cow-backtester --chain base --blocks 5000 --rpc-url <your-rpc> \
        --solver-url http://localhost:8080 --solver-name baseline \
        --solver-url http://localhost:8081 --solver-name candidate \
        --html-out ab.html

From a source checkout without installing, python3 -m cow_backtester ... works identically. Your endpoint only needs the standard CoW solver-engine API (POST /solve).

Readiness check

If you are bringing up a new solver and want a fast "is this endpoint healthy enough to face production auctions?" read, --readiness prints a one-screen report instead of the full field scorecard. We hold ourselves to it: our own solvers' reports, run past the 500-auction floor with 0.11.0 on 2026-09-14, were kept under docs/readiness/ here until 2026-09-21 and now live in a separate, checksummed record of their own (KaiserSolver/kaisersolver-readiness). This repository ships the tool; that one is one solver's record of applying it to itself. The versions cited on the forum remain in this repository's history at commit e7b3b73 (git show e7b3b73:docs/readiness/kaisersolver-base-2026-09-14.md).

cow-backtester --chain base --blocks 2000 --rpc-url <your-rpc> \
        --solver-url http://localhost:8080 --solver-name mine --readiness

It replays recent auctions against your endpoint and reports up to eleven checks across four dimensions — does it answer, is it fast enough, are its solutions valid, are they competitive with the on-chain winners — plus how complete the field it was measured against actually was, as pass/warn checks with a READY / REVIEW / NOT READY verdict. Real output, our own Base solver, a 300-block run on 2026-09-14 with 0.11.0 (RPC redacted, nothing else):

====================================================================
  READINESS — kaisersolver   [REVIEW]
  base · prod · blocks 51302436..51302735
====================================================================
  window  : blocks 51302436..51302735 | attempted auctions span 2026-09-14T13:57:09Z → 2026-09-14T14:06:25Z (0.15 h, settlement-block timestamps; 187.77 attempted/h)
            settlements found 32 / auctions formed 29 / attempted 29 / replayed 29 / returned 7
            excluded 0
  budget  : 4.620 s (observed, BUDGETS_S[base].settle) | original-deadline upper bound n/a (needs --compete)
  thresholds: profile 'default' — answer_rate >=90/50% · transport 0/<=1% · deadline_miss 0/<=1% · latency p95 <=0.5x/1x budget · validity >=90/50% · capture >=50/>0% · min_evidence 500
  INSUFFICIENT SAMPLE (attempted 29 < 500)
  [PASS] reached auctions         29 auctions attempted
  [PASS] no transport errors      0 transport errors
  [PASS] answers reliably         100% returned a parseable response inside the budget (incl. legitimate empty solutions)
  [WARN] bid coverage             24% of answered auctions carried >=1 solution
  [PASS] inside the deadline      0 deadline misses
  [PASS] latency headroom         p95 1107 ms of a 4620 ms budget (observed); PASS <= 0.5x, WARN <= 1x
  [PASS] solutions are valid      100% of bid auctions had >=1 valid solution [feasibility+eligibility+udcp+fairness:not-evaluated; fairness_filtered 0, zero_surplus 0, udcp 25 checked / 0 violations]
  [PASS] competitive vs winners   91% of winner surplus captured (coverage-adjusted; 91% conditional on answering; basis mix {'exact_uniform': 23, 'wrapper_lower_bound': 6})
  [PASS] scan coverage            every block in the window was scanned
  [PASS] field coverage           32 settlements, 29 auctions formed, 0 excluded (0% of field; top reasons [])
  ----------------------------------------------------------------
  answered            : 29/29  (100%)   bids: 7   transport: 0   deadline misses: 0
  latency             : p50 348 ms / p95 1107 ms / max 1460 ms   (budget 4620 ms; answered-only incl. late answers)
  surplus vs winners  : 91% captured (coverage-adjusted) / 91% conditional   (7/29 valid)
  winner basis mix    : {'exact_uniform': 23, 'wrapper_lower_bound': 6}
  validity basis      : feasibility+eligibility+udcp+fairness:not-evaluated

  Reproduce this run:
    cow-backtester --chain base --env prod --from-block 51302436 --to-block 51302735 --bodies-dir <bodies-dir: rerun with --archive-bodies DIR to make this reproducible> --solve-timeout 4.62 --min-evidence 500 \
        --rpc-url <rpc> --solver-url http://127.0.0.1:11090/prod/base --solver-name kaisersolver --readiness
    # cow-backtester 0.11.0 · engine build sha: <fill in: the endpoint's boot-line git_sha (not exposed over /solve)>

  Note: replays live liquidity against archived auctions — a readiness
  signal, not a settlement guarantee. Pair with self-hosted shadow before prod.

Twenty-nine auctions is a smoke test, not evidence: the verdict is held at REVIEW by the 500-auction floor no matter how the checks read, and the header says so. The full-window reports, run past the floor on all three chains the same day, are the three files linked above.

Two readings the sum-weighted capture hides (0.11.1). When one auction's decoded winner surplus exceeds 100× the rest of the window combined it is a reference-price valuation artefact, not delivered value (BNB auction 25459284 decoded to 154,228 BNB and alone read the chain's capture as 0 %). Several auctions valued at the same bogus price are tested as a set (0.11.2): a minority of the window whose smallest member exceeds 100× the window outside the set and 10⁹× the largest auction outside it (three Plasma auctions at ≈7.4e32 wei each hid one another from a one-vs-rest test). Each flagged auction is listed with its ratio under a warn-level winner surplus plausible check, an ex-artefact capture line prints under surplus vs winners, and the competitive vs winners verdict reads the ex-artefact figure with both numbers shown; the rule needs 20 attempted auctions. A per-bid median line gives ours ÷ the winner's surplus at the median over the unflagged auctions we bid on — a solver can match winners bid for bid and still capture little when it never enters the largest auctions. In the readiness dict: artefact_auctions, capture_ex_artefact_pct, per_bid_median_ratio.

It prints the exact --from-block/--to-block command to reproduce the run, and the same data lands in --json-out/--html-out under readiness (including unscanned_blocks and every skip reason). --fail-on not-ready (or review) turns it into a CI gate: exit code 4 when the verdict trips. The screen prints even under --quiet, which silences progress only. Works on any of the supported chains, so you can readiness-check an endpoint for a chain you are not yet onboarded on. It is a signal, not a settlement guarantee — pair it with a self-hosted shadow run before going to production.

Consistency economics (--reward-ev)

On most CoW chains the money is not in winning — it is in the CIP-85 consistency pool, which pays success_rate × Σ executed orders ( your best fair bid's surplus / everyone's ). --reward-ev computes the Σ term of that metric from the competition records: your solver's counterfactual metric and pool share, every field solver's real historical metric (a consistency leaderboard for the window — this needs no --solver-url at all), and, with --consistency-budget <COW>, a COW/week estimate. success_rate is not folded in silently: the win floor is reported as a flag — v2 pays zero on a chain where a solver won nothing in the period — and the estimate assumes it is met. Bases are matched: a field solution carrying one order contributes its official score, the same basis the replayed challenger is scored on (multi-order solutions fall back to the net-of-fee per-order computation and are counted as such); auctions where your solver errored stay in the denominator with a zero term; challenger fairness is assumed while the field uses CoW's own filteredOut flags.

Rank against the historical field (--compete)

Capture ratio tells you how much surplus you generate; --compete tells you where you would have ranked. For every scored auction it fetches the historical competition record (every submitted solution with its score, CoW's own fairness-filtering outcome, and the winner) and inserts your solver's result into the fairness-surviving score list:

cow-backtester --chain base --blocks 2000 --rpc-url <your-rpc> \
        --solver-url http://localhost:8080 --solver-name mine --compete \
        --archive-dir ./competition-data

The scorecard gains a field-rank line (rank-1 %, top-3 %, median rank, median gap to the winner in bps) and a rivals table — which solvers beat you, how often, and by how much. Per-auction field_rank lands in the JSON rows. Two honesty notes: historical scores include protocol fees while your replayed surplus does not, so the reported rank is a floor (labeled surplus_vs_score_proxy); and records exist only for auctions that had a winner. --archive-dir keeps every fetched record on disk — the API serves roughly two months of history, so an archive you build today is a dataset you keep. --self-address marks your historical solverAddress so rows show shadow-vs-actual side by side.

Try it without a solver

The mock solver in the source checkout (mock_solver.py; not installed by the wheel) lets you see the full counterfactual/A/B output in about a minute, before wiring up your own engine:

MODE=limit  python3 mock_solver.py 8901 &   # fills exactly at the limit
MODE=better python3 mock_solver.py 8902 &   # fills at 1.5x the limit

cow-backtester --chain base --blocks 1500 --rpc-url <your-rpc> \
        --solver-url http://127.0.0.1:8901 --solver-name at-limit \
        --solver-url http://127.0.0.1:8902 --solver-name better --html-out demo.html
kill %1 %2

You get the per-solver panels, the head-to-head table, and the pair breakdown. The implausible_surplus guard usually fires on the "better" mock: claiming 1.5x the limit on real order flow is exactly what the validator exists to flag.

How it works (all public data)

1. Input: the exact /solve bodies CoW sent solvers, from the public S3 instance bucket (solver-instances.s3.amazonaws.com/<env>/<chain>/auction/<id>.json). The archived file is a valid /solve request body.
2. Winner reconstruction: from the on-chain settlements. (The v1 /solver_competition endpoints were removed in the competition-data migration; v2 exists but is retention-bounded. Reconstructing from chain data is trustless and independent of API retention — and --verify-api cross-checks it against v2 where available.) Settlements are grouped by auction: since combinatorial auctions (CIP-67), one auction can settle through several winning transactions, so the baseline is the combined winning set.
3. Replay and score: each auction body is POSTed to each solver; responses are validated (below) and scored with the same function, the same limits, and the same price convention as the winners. Your valid solutions are combined the CIP-67 way (best-first, disjoint directed token pairs).

The comparison basis (read this before quoting numbers)

Both sides are scored on before-fee surplus over the signed order limits, at uniform clearing prices, converted to the chain's native token at the auction's own reference prices.

• Signed limits: CoW's accounting scores against the order's signed amounts (fullSellAmount/fullBuyAmount), not the remaining/fee-adjusted amounts.
• Uniform prices: on-chain settlements carry per-trade post-fee "custom" prices; the first occurrence of each token in the price vector is the uniform (pre-fee) price, which is what autopilot itself resolves. Scoring the winner post-fee but a challenger pre-fee would bias every comparison.
• Surplus token: buy token for sell orders, sell token for buy orders, converted at that token's referencePrice (matches official accounting).
• What the number is: measured against the v2 competition API's official score on seven live records (Arbitrum and Base, September 2026) this basis agrees to within 0.2% — on the pinned reference settlement, 767,957,704,005 wei here vs an official 769,523,899,186. The CIP-38 score is after-fee surplus plus protocol fees, and the uniform-vs-custom price wedge this basis includes is the protocol fee (on Arbitrum and Base the driver bakes it into the custom prices; solvers report fee: 0). Earlier releases described this basis as "overstating the score by the network fee"; that comparison used the after-fee surplus as the reference, not the score. Residual deviations: a solver-determined fee (zero on those chains) is included here and not in the score, and buy-order surplus is valued at the sell token's reference price where the score converts at the limit ratio into the buy token (about 8% of Base orders are buy-kind). It is the only basis we found that can be computed symmetrically offline for both sides.

Response validation (your solver can't accidentally cheat)

A solution is scored only if every fulfillment trade is feasible:

• known order uid (case-insensitive) and strict U256 numerics (decimal or 0x-hex; negatives and malformed values rejected);
• prices present for both tokens;
• at most one fulfillment per order (GPv2 accumulates fills and the driver rejects duplicate trades, so N copies of a trade can't score N times the surplus);
• no over-fill (executed + fee vs sellAmount for sell orders, executed vs buyAmount for buy orders; exact for fill-or-kill);
• on-chain feasibility at the fee-adjusted terms: the net delivery must still cover the gross-scaled limit, which is what GPv2 enforces, so a padded fee cannot manufacture surplus.

Violations mark the whole solution INVALID (tallied with a reason), never a silent zero. Solutions scoring >10× the winning set (or >0.001 native when the winning set scored 0) are flagged implausible_surplus. Malformed responses are tallied, never crash a run. Prices are claimed, not simulated — the tool checks feasibility; it does not execute routes.

What a run prints

• COVERAGE: settlements found, auctions formed/scored, every skip with its reason, unscanned block spans, auction ages, cache stats, --verify-api results.
• FIELD SURPLUS: winner surplus by USD size bucket, plus the winners' fee take.
• WINNING SUBMITTERS: who is actually winning (label them with --solver-map).
• COUNTERFACTUAL (per solver): returned / valid / positive / beat the winning set, surplus sums, capture ratio, solve latency p50+p95, invalid reasons, error classes.
• HEAD TO HEAD (exactly two solvers): side-by-side metrics, per-auction win counts, and the surplus delta.
• TOP PAIRS: winner surplus by token pair with each solver's surplus beside it: where you win and lose, not just by how much.

A real single-solver run (Arbitrum, eight auctions, trimmed):

====================================================================
  COUNTERFACTUAL — kaisersolver  (8 replayed)
====================================================================
  returned a solution : 7/8
  valid solutions     : 7/8
  positive surplus    : 7/8
  beat the winning set: 0/8
  our surplus (sum)   : 0.001643 ETH
  winners (sum)       : 0.001748 ETH  (replayed auctions only)
  capture ratio       : 94.0% of the winning set
  solve latency       : p50 1174 ms / p95 2471 ms

====================================================================
  TOP PAIRS BY WINNER SURPLUS (top 2)
====================================================================
                        pair  trades         winner   kaisersolver
                   WETH->MOR       7       0.001722       0.001643
                 USDC->USD₮0       1       0.000026       0.000000

Eight auctions is a small sample, but the shape is the point: this solver is consistently a few percent behind one competitor on one pair, and the table says which pair. That took minutes to learn here; it took weeks from logs.

--json-out streams one row per auction (block, timestamp, age, winner txs + submitters + surplus/fees, per-solver validation detail and latency, expired_orders_pct, flags). Under --watch the file is appended to, so a restart continues the same JSON Lines file (one _meta line per cycle; the first cycle after a restart re-scans --blocks from head, so dedupe by auction_id when summing); a one-shot run overwrites it. --html-out writes a single self-contained HTML report: no external assets, light/dark aware, fine to attach to a PR or post.

Flags

Progress goes to stderr, the scorecard to stdout (2>/dev/null gives clean results; --json-out/--html-out are unaffected). Exit codes: 0 success, 1 runtime error, 2 usage error, 4 --fail-on readiness gate tripped, 130 interrupted mid-run (a partial scorecard is printed when auctions had already been processed; stopping --watch during its idle sleep is a clean stop and exits 0).

A/B runs post byte-identical bodies to every solver concurrently under one shared deadline, so each endpoint gets the same inputs and the same wall-clock budget. Any HTTP 200 that is not {"solutions": [...]} is an error (bad_schema), never a healthy abstention; {"solutions": []} is the only legitimate empty answer. The JSONL _meta line carries failed_ranges, unscanned_blocks, competition_missing, validto_clamped_auctions and winner_surplus_artefacts (rows are streamed before the window total is known, so the artefact flag lives here — join on auction_id) so a pipeline inherits the run's coverage caveats.

Caching

Only immutable facts are cached: archived auction bodies, settlements below the reorg margin, and block timestamps. Nothing derived from live liquidity is ever cached, so a warm re-run is faster (3.8x on a measured six-auction, two-solver run) without changing a single number. Delete .cowbt-cache any time, or pass --no-cache.

Limitations

• Replay uses live liquidity. Your solver quotes against current chain state, not the historical block. The counterfactual is indicative — it answers "how does my solver handle this real order flow", not "the exact outcome at that block". Ages are printed and a warning fires beyond --max-age-hours (default 6h). Prefer recent, short windows. Each row carries settlement_block — the block the winning settlement landed in, which is later than the auction cut block the bidders actually saw. A fork pinned there is an approximation, not a faithful auction replay (it can even include the settlement itself); true historical-fork replay needs the auction cut block, which this tool does not yet reconstruct.
• The S3 bucket retains roughly one month of auctions (measured Aug 2026). This is a recent-window backtester, not an archive.
• Entrypoint coverage. Direct settle() calls are attributed by the auction id appended to their calldata. Wrapper-routed settlements (solver router contracts — measured Aug 2026 at ~43% of mainnet, ~40% of Base, ~2% of Arbitrum settlements, and larger than direct ones at the median, so they are not a random slice) are attributed via the v2 solver_competition/by_tx_hash endpoint and proven by uid overlap with the S3 auction body before scoring; they score on a DELIVERED basis (Trade events vs signed limits), which understates the direct-path before-fee basis by the settlement's fee wedge (typically a few bps) — rows carry entry: "wrapper" so the bases are distinguishable. Settlements the endpoint cannot resolve are counted under wrapper_unattributed and disclosed in the coverage block, --readiness, and the _meta JSON line.
• "Beat the winning set" is necessary, not sufficient. Real winner selection also applies fairness filters, and bids score net of gas; the tool also takes your solutions at face value while the driver merges and simulates.
• Orders keep their historical validTo (~1%/day expire; more near the retention edge). Engines that filter expired orders look age-degraded — see expired_orders_pct per row, or pass --clamp-validto.
• JIT orders. The winner baseline credits surplus-capturing JIT trades (owners in surplusCapturingJitOrderOwners), per official accounting; the challenger side never credits JIT (a response's JIT order has no verifiable owner). On auctions where winner surplus comes from CoW-AMM-style JIT, the comparison is conservative against the challenger, never in its favor.
• Bring your own RPC. Public defaults rot and rate-limit eth_getLogs; the tool splits and retries failing ranges and always reports any span it could not scan rather than under-counting silently. eth_chainId is checked against --chain on startup.

Provenance of the reconstruction

• Auction id: CoW's driver appends it to settle() calldata; autopilot reads back exactly the last 8 bytes (META_DATA_LEN = 8 in cowprotocol/services). The tool applies the same rule, only when the calldata prefix re-encodes canonically, cross-checked by requiring at least one settled order uid to appear in the fetched body (body_uid_mismatch guard — this also catches staging/prod contamination, which shares the settlement contract).
• Trade events are filtered by emitting address + topic and must align 1:1 with calldata trades, else the settlement is skipped (trade_event_mismatch).
• Fill-or-kill: GPv2 ignores the calldata executedAmount and uses the signed amount — the tool applies the same substitution.
• Response-schema authority: crates/solvers-dto / the solver-engine OpenAPI in cowprotocol/services.
• --verify-api compares the reconstructed winning-tx set per auction against the v2 competition endpoint (match / mismatch / unavailable counters).

Tests

pip install -e ".[dev]"                      # pytest + ruff (one-time)
python3 -m pytest                            # OFFLINE suite (fixtures; no network)
python3 -m cow_backtester.scorer --unittest  # offline core checks
python3 -m cow_backtester.scorer --selftest  # network: winner path on the reference settlement
python3 -m cow_backtester --selftest         # network: counterfactual == winner baseline +
                                             # every exploit class as regression cases

The pinned reference (Arbitrum auction 8339027): decode integrity is exact against the GPv2 Trade event (2385773 atoms), before-fee surplus 1471 atoms, protocol-fee wedge 805 atoms, 767,957,704,005 wei vs the official score of 769,523,899,186. The network self-test picks a recent settlement automatically once the pinned auction ages out of the S3 bucket. mock_solver.py (modes: empty / limit / better / hex / garbage / invalid; from a source checkout, it is not installed by the wheel) exercises the wire path including validation and implausibility guards. CI runs the offline suite on Python 3.10, 3.12 and 3.13.

Files

MIT licensed. Contributions and corrections welcome — especially from CoW core devs on anything where this tool's accounting diverges from the protocol's.