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+// Frozen-repo snapshot builder, pure core (#9259, harness #9216, epic #8534).
+//
+// Scoring an arbitrary agent against realized history is only meaningful if the agent sees EXACTLY the
+// repository state a maintainer saw at commit T and NOTHING that happened after it. A future-information
+// leak silently inflates every score built on top — and it inflates them invisibly, because a leaked
+// snapshot still produces perfectly well-formed numbers. That makes leak-proofing the deliverable of this
+// module rather than a property of it, which is why the filtering is here, pure and exhaustively tested,
+// instead of inline in a CLI where it would be untestable.
+//
+// ── WHAT IS INCLUDED, AND WHAT IS DELIBERATELY NOT ───────────────────────────────────────────────────
+// INCLUDED, each filtered to its state as of `frozenAt`:
+//   • openPullRequests — PRs created at or before T that were still open at T. Labels are filtered to
+//     those applied at or before T; the body/title are the values as of T.
+//   • openIssues — same rule.
+//   • recentDecisions — gate decisions RECORDED at or before T. These are history the maintainer could
+//     genuinely see, and they are what makes the task realistic rather than context-free.
+//
+// NOT INCLUDED, ever:
+//   • Anything created after T. Not "filtered from the output" — never admitted, so a downstream bug
+//     cannot reintroduce it.
+//   • The OUTCOME of any included work unit. A snapshot carries the question, never the answer: an
+//     open PR's eventual merge/close is exactly what the agent is being asked to predict, so a snapshot
+//     that carried `state: "merged"` would not be a hard benchmark, it would be an answer key.
+//   • Comments, labels, reviews, or status changes timestamped after T, even on an included record.
+//
+// ── THE BOUNDARY IS INCLUSIVE AT T ───────────────────────────────────────────────────────────────────
+// An event AT exactly `frozenAt` is included: T is the instant the maintainer is standing at, so what
+// happened at T is what they can see. Everything strictly after is the future. This is stated once, here,
+// and implemented once, in `atOrBefore`, so no field can quietly disagree with another.
+//
+// Pure: no IO, no clock, no randomness. `scripts/frozen-repo-snapshot.ts` is the thin CLI that does the
+// GitHub/DB reads and hands the raw records in. Checksum discipline mirrors `checksumCases` in
+// backtest-corpus-export-core.ts exactly (canonicalize with sorted keys, JSON-stringify, sha256).
+
+import { createHash } from "node:crypto";
+
+export const FROZEN_REPO_SNAPSHOT_SCHEMA_VERSION = 1 as const;
+
+/** A label with the instant it was APPLIED — the timestamp is what makes label filtering possible at all.
+ *  A raw GitHub label carries no application time; the CLI derives it from the issue-events timeline. */
+export type TimestampedLabel = { name: string; appliedAt: string };
+
+/** One PR or issue as the CLI read it, with every time-varying field carrying its own timestamp. */
+export type RawWorkUnitRecord = {
+  /** `owner/repo#123`. */
+  workUnitId: string;
+  number: number;
+  kind: "pull_request" | "issue";
+  title: string;
+  body: string;
+  authorLogin: string;
+  createdAt: string;
+  /** When it was closed/merged, if it ever was. Present in the RAW record and deliberately dropped from
+   *  the snapshot — see this module's header. Used only to decide open-at-T. */
+  closedAt?: string | null | undefined;
+  labels?: readonly TimestampedLabel[] | undefined;
+  /** Changed file paths. A PR's file list is fixed at push time; the CLI supplies the list as of T. */
+  changedPaths?: readonly string[] | undefined;
+};
+
+/** A past gate decision the maintainer could see at T. */
+export type RawDecisionRecord = {
+  workUnitId: string;
+  action: string;
+  reasonCode: string;
+  decidedAt: string;
+};
+
+/** One work unit as it appears IN the snapshot. Note what is absent: no `closedAt`, no state, no outcome
+ *  of any kind — the type itself refuses to carry the answer. */
+export type FrozenWorkUnit = {
+  workUnitId: string;
+  number: number;
+  kind: "pull_request" | "issue";
+  title: string;
+  body: string;
+  authorLogin: string;
+  createdAt: string;
+  /** Label NAMES only, sorted — the application timestamps did their job during filtering and would
+   *  otherwise be one more channel through which post-T information could travel. */
+  labels: string[];
+  changedPaths: string[];
+};
+
+export type FrozenDecision = { workUnitId: string; action: string; reasonCode: string; decidedAt: string };
+
+export type FrozenRepoSnapshot = {
+  schemaVersion: typeof FROZEN_REPO_SNAPSHOT_SCHEMA_VERSION;
+  repoFullName: string;
+  commitSha: string;
+  frozenAt: string;
+  openPullRequests: FrozenWorkUnit[];
+  openIssues: FrozenWorkUnit[];
+  recentDecisions: FrozenDecision[];
+  /** sha256 over the canonicalized snapshot WITHOUT this field — so it commits to its own content and a
+   *  third party can confirm two runs scored the same task. */
+  snapshotChecksum: string;
+};
+
+/**
+ * The single definition of "visible at T": at or before `frozenAt`, inclusive.
+ *
+ * An unparseable timestamp is NOT visible. That direction is deliberate and is the fail-safe one: a record
+ * whose date cannot be read might be from after T, and admitting it would risk a leak, while excluding it
+ * only costs a task some context. Every filter in this module routes through here, so the boundary cannot
+ * drift between fields.
+ */
+export function atOrBefore(timestamp: string | null | undefined, frozenAt: string): boolean {
+  if (!timestamp) return false;
+  const at = Date.parse(timestamp);
+  const cutoff = Date.parse(frozenAt);
+  if (!Number.isFinite(at) || !Number.isFinite(cutoff)) return false;
+  return at <= cutoff;
+}
+
+/** Was this unit still OPEN at T? Closed strictly after T means it was open at T; closed at or before T
+ *  means it was already closed and is not part of the task. Never closed at all ⇒ open. */
+export function wasOpenAt(record: RawWorkUnitRecord, frozenAt: string): boolean {
+  if (!atOrBefore(record.createdAt, frozenAt)) return false; // not created yet at T
+  if (!record.closedAt) return true;
+  // Closed at or before T ⇒ not open at T. An unparseable closedAt is treated as "still open", which is
+  // the safe direction here: it keeps a question in the task rather than leaking a resolution.
+  const closedAt = Date.parse(record.closedAt);
+  if (!Number.isFinite(closedAt)) return true;
+  return closedAt > Date.parse(frozenAt);
+}
+
+/** Project one raw record onto its state at T. Labels applied after T are dropped; the outcome fields are
+ *  not carried at all. Sorting makes the output canonical, which is what lets the checksum be stable. */
+export function freezeWorkUnit(record: RawWorkUnitRecord, frozenAt: string): FrozenWorkUnit {
+  const labels = (record.labels ?? [])
+    .filter((label) => atOrBefore(label.appliedAt, frozenAt))
+    .map((label) => label.name)
+    .filter((name) => name.length > 0);
+  return {
+    workUnitId: record.workUnitId,
+    number: record.number,
+    kind: record.kind,
+    title: record.title,
+    body: record.body,
+    authorLogin: record.authorLogin,
+    createdAt: record.createdAt,
+    labels: [...new Set(labels)].sort(),
+    changedPaths: [...new Set(record.changedPaths ?? [])].sort(),
+  };
+}
+
+/** Total-order fallback: compare the canonicalized JSON. Reached only when every named key ties, and it
+ *  guarantees the sort is total no matter what fields the record type grows. */
+function compareCanonical(a: object, b: object): number {
+  const left = JSON.stringify(canonicalize(a as Record<string, unknown>));
+  const right = JSON.stringify(canonicalize(b as Record<string, unknown>));
+  return left < right ? -1 : left > right ? 1 : 0;
+}
+
+/** Sort key for every collection in a snapshot: the work-unit id, which is unique and stable. Sorting
+ *  rather than preserving input order is what makes two builds from differently-ordered reads identical. */
+function byWorkUnitId<T extends { workUnitId: string }>(a: T, b: T): number {
+  return a.workUnitId < b.workUnitId ? -1 : a.workUnitId > b.workUnitId ? 1 : 0;
+}
+
+/** Canonicalize with sorted keys — mirrors backtest-corpus-export-core.ts's `canonicalizeCase`. Two-armed
+ *  rather than the usual three: object keys are unique by definition, so an "equal keys" arm would be dead
+ *  code that no test could ever reach. */
+function canonicalize(value: Record<string, unknown>): Record<string, unknown> {
+  return Object.fromEntries(Object.entries(value).sort(([a], [b]) => (a < b ? -1 : 1)));
+}
+
+/**
+ * Deterministic SHA-256 over the canonicalized snapshot body (every field except the checksum itself).
+ *
+ * The committed fields are listed EXPLICITLY rather than spread from the argument. Spreading made this a
+ * foot-gun: handing it a whole `FrozenRepoSnapshot` (the natural thing to do when re-verifying) silently
+ * folded the existing `snapshotChecksum` into the preimage and returned a different, wrong digest with no
+ * error. Naming the fields makes the function total over both shapes and makes an added field a compile
+ * error here — which is the right place to notice that the commitment needs updating.
+ */
+export function checksumSnapshot(snapshot: Omit<FrozenRepoSnapshot, "snapshotChecksum">): string {
+  const canonical = canonicalize({
+    schemaVersion: snapshot.schemaVersion,
+    repoFullName: snapshot.repoFullName,
+    commitSha: snapshot.commitSha,
+    frozenAt: snapshot.frozenAt,
+    openPullRequests: snapshot.openPullRequests.map((unit) => canonicalize(unit as unknown as Record<string, unknown>)),
+    openIssues: snapshot.openIssues.map((unit) => canonicalize(unit as unknown as Record<string, unknown>)),
+    recentDecisions: snapshot.recentDecisions.map((decision) => canonicalize(decision as unknown as Record<string, unknown>)),
+  });
+  return createHash("sha256").update(JSON.stringify(canonical)).digest("hex");
+}
+
+/**
+ * Build a leak-proof snapshot of a repo at commit T.
+ *
+ * Every collection is filtered through {@link atOrBefore} and sorted, so the result is a pure function of
+ * (records, frozenAt) — never of when the build ran, nor of the order the CLI happened to read records in.
+ * That is the property the benchmark's reproducibility rests on, and it is asserted directly in the tests.
+ */
+export function buildFrozenRepoSnapshot(input: {
+  repoFullName: string;
+  commitSha: string;
+  frozenAt: string;
+  workUnits: readonly RawWorkUnitRecord[];
+  decisions?: readonly RawDecisionRecord[] | undefined;
+}): FrozenRepoSnapshot {
+  const openAtT = input.workUnits.filter((record) => wasOpenAt(record, input.frozenAt));
+  const openPullRequests = openAtT
+    .filter((record) => record.kind === "pull_request")
+    .map((record) => freezeWorkUnit(record, input.frozenAt))
+    .sort(byWorkUnitId);
+  const openIssues = openAtT
+    .filter((record) => record.kind === "issue")
+    .map((record) => freezeWorkUnit(record, input.frozenAt))
+    .sort(byWorkUnitId);
+  const recentDecisions = (input.decisions ?? [])
+    .filter((decision) => atOrBefore(decision.decidedAt, input.frozenAt))
+    .map((decision) => ({
+      workUnitId: decision.workUnitId,
+      action: decision.action,
+      reasonCode: decision.reasonCode,
+      decidedAt: decision.decidedAt,
+    }))
+    // Decisions can repeat per work unit, so the id alone is not a total order. The tie-break chain must
+    // cover EVERY field, not just the obvious ones: two decisions differing only in `reasonCode` compared
+    // equal under an earlier version of this sort, so their relative order followed input order and the
+    // snapshot checksum moved when the CLI happened to read them the other way round -- which is exactly
+    // the reproducibility property this module exists to guarantee. Comparing the canonical serialization
+    // last makes the order total by construction, so adding a field to FrozenDecision cannot silently
+    // reintroduce the same hole.
+    .sort(
+      (a, b) =>
+        byWorkUnitId(a, b) ||
+        (a.decidedAt < b.decidedAt ? -1 : a.decidedAt > b.decidedAt ? 1 : 0) ||
+        (a.action < b.action ? -1 : a.action > b.action ? 1 : 0) ||
+        compareCanonical(a, b),
+    );
+
+  const body: Omit<FrozenRepoSnapshot, "snapshotChecksum"> = {
+    schemaVersion: FROZEN_REPO_SNAPSHOT_SCHEMA_VERSION,
+    repoFullName: input.repoFullName,
+    commitSha: input.commitSha,
+    frozenAt: input.frozenAt,
+    openPullRequests,
+    openIssues,
+    recentDecisions,
+  };
+  return { ...body, snapshotChecksum: checksumSnapshot(body) };
+}
+
+/** Recompute a snapshot's checksum and compare — the exact check a third party runs to confirm two runs
+ *  scored the same task, exported so our tests exercise the SAME path rather than a parallel one. */
+export function verifySnapshotChecksum(snapshot: FrozenRepoSnapshot): boolean {
+  const { snapshotChecksum, ...body } = snapshot;
+  return checksumSnapshot(body) === snapshotChecksum;
+}
+
+/**
+ * Audit a built snapshot for future information — a belt-and-braces check over the builder's own output.
+ *
+ * The builder already excludes post-T data by construction; this re-derives the property independently, so
+ * a future refactor that breaks the filtering fails loudly here instead of silently inflating scores. It
+ * returns the offending paths rather than a bare boolean, because "which field leaked" is the only useful
+ * form of that answer.
+ */
+export function auditSnapshotForLeaks(snapshot: FrozenRepoSnapshot): string[] {
+  const leaks: string[] = [];
+  const check = (collection: "openPullRequests" | "openIssues") => {
+    for (const unit of snapshot[collection]) {
+      if (!atOrBefore(unit.createdAt, snapshot.frozenAt)) leaks.push(`${collection}/${unit.workUnitId}: createdAt is after frozenAt`);
+      // The snapshot type carries no outcome fields, but a hand-built or deserialized object could.
+      for (const forbidden of ["closedAt", "mergedAt", "state", "merged"]) {
+        if (forbidden in (unit as unknown as Record<string, unknown>)) leaks.push(`${collection}/${unit.workUnitId}: carries outcome field "${forbidden}"`);
+      }
+    }
+  };
+  check("openPullRequests");
+  check("openIssues");
+  for (const decision of snapshot.recentDecisions) {
+    if (!atOrBefore(decision.decidedAt, snapshot.frozenAt)) leaks.push(`recentDecisions/${decision.workUnitId}: decidedAt is after frozenAt`);
+  }
+  return leaks;
+}