diff --git a/src/signals/copycat.ts b/src/signals/copycat.ts
new file mode 100644
index 000000000..40aa88717
--- /dev/null
+++ b/src/signals/copycat.ts
@@ -0,0 +1,178 @@
+// Copycat / plagiarism detection engine (#1969) — the deterministic containment/similarity primitive the
+// (currently inert) `gate.copycat.mode` / `gate.copycat.minScore` config would act on. A natural sibling of the
+// deterministic anti-slop signal (src/signals/slop.ts) and duplicate-cluster adjudication
+// (src/signals/duplicate-winner.ts): given a PR's ADDED code and one piece of prior art, it measures how much of
+// the PR's added code is CONTAINED in that prior art, resolves copy DIRECTION by submission timestamp (the
+// earlier submission is the original, never the copier), and maps the result through the configured tier
+// (warn → label → block) into a public-safe finding.
+//
+// PURE / PRECISION-FIRST: no IO, no Date.now(), no randomness — identical inputs always yield the identical
+// verdict. It is deliberately false-accusation-averse: it only ever `wouldAct` when the score clears the
+// threshold AND the candidate is unambiguously the LATER (copying) submission AND a non-`off` mode is set. Any
+// missing/ambiguous timestamp — or the candidate being the earlier work — resolves to "do not act".
+//
+// DEFERRED (a later slice of #1969): the gate call-site that feeds a PR's real added-code + prior-art into this
+// engine and turns a `wouldAct` verdict into an actual label/block plus the cross-repo "strikes" escalation.
+// This slice is the engine + direction + tier mapping only, mirroring how the miner-side self-plagiarism
+// throttle (packages/gittensory-engine/src/governor/self-plagiarism.ts) "gates nothing on its own".
+
+import type { AdvisorySeverity, CopycatGateMode } from "../types";
+import type { SignalFinding } from "./engine";
+
+/** Precision-first default: only a HIGH containment (>= 85% of the PR's added code found in the prior art) trips
+ *  the check when `gate.copycat.minScore` is unset. Mirrors the conservative 0.85 spirit of the miner-side
+ *  self-plagiarism throttle (governor/self-plagiarism.ts's DEFAULT_SELF_PLAGIARISM_SIMILARITY_THRESHOLD). */
+export const DEFAULT_COPYCAT_MIN_SCORE = 85;
+
+/** Shingle width: consecutive normalized lines folded into one token, so containment reflects COPIED PASSAGES
+ *  (multi-line runs) rather than incidental single-line coincidences (a lone `}` / `return null;`) that would
+ *  inflate a naive line-set overlap. */
+const SHINGLE_SIZE = 3;
+
+/** Copy direction between the candidate PR and one prior-art submission, decided purely by submission time. */
+export type CopycatDirection = "candidate_copied" | "candidate_is_prior" | "ambiguous";
+
+/** Normalize one source line for structural comparison: collapse internal whitespace runs, trim, lowercase — so
+ *  pure reformatting/indentation churn never reads as copied content. */
+function normalizeLine(line: string): string {
+  return line.replace(/\s+/g, " ").trim().toLowerCase();
+}
+
+/** Drop blank/whitespace-only lines and normalize the rest, preserving order. */
+function normalizedLines(lines: readonly string[]): string[] {
+  return lines.map(normalizeLine).filter((line) => line.length > 0);
+}
+
+/** Fold normalized lines into the ORDERED MULTISET of SHINGLE_SIZE-line shingles — duplicates preserved, so a
+ *  passage copied twice counts twice toward containment (see {@link containmentScore}, which must divide by the
+ *  candidate's TOTAL shingle count, not its distinct count). Fewer than SHINGLE_SIZE non-trivial lines collapse
+ *  to a single whole-block shingle so tiny snippets still compare (never silently score 0). */
+export function codeShingleList(lines: readonly string[]): string[] {
+  const normalized = normalizedLines(lines);
+  if (normalized.length === 0) return [];
+  if (normalized.length < SHINGLE_SIZE) return [normalized.join("\n")];
+  const shingles: string[] = [];
+  for (let i = 0; i + SHINGLE_SIZE <= normalized.length; i += 1) {
+    shingles.push(normalized.slice(i, i + SHINGLE_SIZE).join("\n"));
+  }
+  return shingles;
+}
+
+/** The DISTINCT SHINGLE_SIZE-line shingles of `lines` — a de-duplicated view of {@link codeShingleList}, used as
+ *  the prior-art lookup set (membership only, so duplicates there are irrelevant). */
+export function codeShingles(lines: readonly string[]): Set<string> {
+  return new Set(codeShingleList(lines));
+}
+
+/** Asymmetric containment (0-100): the percentage of the CANDIDATE's added-code shingles that also appear in the
+ *  PRIOR ART. Unlike symmetric Jaccard, this answers "how much of THIS PR is copied FROM prior art" without being
+ *  diluted by a large prior-art corpus. The candidate is a MULTISET (its total shingle count is the denominator,
+ *  so a repeated copied passage is not undercounted), while the prior art is a lookup Set (membership only). 0
+ *  when either side has no comparable content. */
+export function containmentScore(candidateLines: readonly string[], priorArtLines: readonly string[]): number {
+  const candidate = codeShingleList(candidateLines);
+  if (candidate.length === 0) return 0;
+  const prior = codeShingles(priorArtLines);
+  if (prior.size === 0) return 0;
+  let contained = 0;
+  for (const shingle of candidate) {
+    if (prior.has(shingle)) contained += 1;
+  }
+  return Math.round((contained / candidate.length) * 100);
+}
+
+/** Parse an ISO-8601 submission time to epoch ms; null for a missing/empty/unparseable value. */
+function submissionTimeMs(value: string | null | undefined): number | null {
+  if (!value) return null;
+  const parsed = Date.parse(value);
+  return Number.isFinite(parsed) ? parsed : null;
+}
+
+/** Copy direction by submission time: the EARLIER submission is the original, so the LATER one is the potential
+ *  copier. Any missing/unparseable timestamp — or an exact tie — is "ambiguous" (fail-safe: never accuse). */
+export function copycatDirection(
+  candidateAt: string | null | undefined,
+  priorAt: string | null | undefined,
+): CopycatDirection {
+  const candidateMs = submissionTimeMs(candidateAt);
+  const priorMs = submissionTimeMs(priorAt);
+  if (candidateMs === null || priorMs === null) return "ambiguous";
+  if (candidateMs > priorMs) return "candidate_copied";
+  if (candidateMs < priorMs) return "candidate_is_prior";
+  return "ambiguous";
+}
+
+/** Clamp `gate.copycat.minScore` into 0-100; a non-numeric/non-finite value falls back to the engine default. */
+function normalizeMinScore(value: number | null | undefined): number {
+  if (typeof value !== "number" || !Number.isFinite(value)) return DEFAULT_COPYCAT_MIN_SCORE;
+  return Math.min(100, Math.max(0, Math.round(value)));
+}
+
+/** Per-tier finding severity. `off` never produces a finding (see {@link assessCopycat}); it maps to `info` only
+ *  so the lookup is total over {@link CopycatGateMode} without an unreachable branch. */
+const MODE_SEVERITY: Record<CopycatGateMode, AdvisorySeverity> = {
+  off: "info",
+  warn: "info",
+  label: "warning",
+  block: "critical",
+};
+
+/** Public-safe finding — reports the containment score + threshold only, never raw code, filenames, or any
+ *  contributor identity (the caller redacts further; this text is already accusation-neutral). */
+function buildFinding(mode: CopycatGateMode, score: number, minScore: number): SignalFinding {
+  return {
+    code: "copycat_overlap",
+    title: "Potential copied code detected",
+    severity: MODE_SEVERITY[mode],
+    detail: `This pull request's added code reaches ${score}% containment against earlier prior art (threshold ${minScore}%).`,
+    action: "Confirm the overlapping code is original or properly attributed before merging.",
+    publicText: `High overlap (${score}%) with earlier prior art — please confirm originality or attribution.`,
+  };
+}
+
+export type CopycatAssessmentInput = {
+  /** The PR's ADDED source lines (the candidate). */
+  candidateLines: readonly string[];
+  /** One piece of prior art's source lines to compare against. */
+  priorArtLines: readonly string[];
+  /** ISO-8601 submission time of the candidate PR; absent/unparseable ⇒ ambiguous direction (never acts). */
+  candidateSubmittedAt?: string | null | undefined;
+  /** ISO-8601 submission time of the prior art; absent/unparseable ⇒ ambiguous direction (never acts). */
+  priorSubmittedAt?: string | null | undefined;
+  /** `gate.copycat.mode`; `off`/absent ⇒ never acts (score is still computed for observability). */
+  mode?: CopycatGateMode | null | undefined;
+  /** `gate.copycat.minScore` (0-100); absent/out-of-range ⇒ {@link DEFAULT_COPYCAT_MIN_SCORE}. */
+  minScore?: number | null | undefined;
+};
+
+export type CopycatAssessment = {
+  /** Asymmetric containment of the candidate in the prior art, 0-100. */
+  score: number;
+  direction: CopycatDirection;
+  /** The resolved threshold the score was tested against. */
+  minScore: number;
+  /** True ONLY when a non-`off` mode is set AND score >= threshold AND the candidate is the later (copying) work. */
+  wouldAct: boolean;
+  findings: SignalFinding[];
+};
+
+/**
+ * Assess one PR's added code against one piece of prior art (#1969). Pure and precision-first: the containment
+ * score is always computed for observability, but a finding is emitted ONLY when the configured mode is non-`off`,
+ * the score clears the (resolved) threshold, and the candidate is unambiguously the later submission — so an
+ * earlier-submitted victim, or any ambiguous/missing timing, is never flagged.
+ */
+export function assessCopycat(input: CopycatAssessmentInput): CopycatAssessment {
+  const minScore = normalizeMinScore(input.minScore);
+  const score = containmentScore(input.candidateLines, input.priorArtLines);
+  const direction = copycatDirection(input.candidateSubmittedAt, input.priorSubmittedAt);
+  const mode = input.mode ?? "off";
+  const wouldAct = mode !== "off" && score >= minScore && direction === "candidate_copied";
+  return {
+    score,
+    direction,
+    minScore,
+    wouldAct,
+    findings: wouldAct ? [buildFinding(mode, score, minScore)] : [],
+  };
+}
diff --git a/test/unit/copycat.test.ts b/test/unit/copycat.test.ts
new file mode 100644
index 000000000..66a1bc657
--- /dev/null
+++ b/test/unit/copycat.test.ts
@@ -0,0 +1,202 @@
+import { describe, expect, it } from "vitest";
+import {
+  assessCopycat,
+  codeShingleList,
+  codeShingles,
+  containmentScore,
+  copycatDirection,
+  DEFAULT_COPYCAT_MIN_SCORE,
+} from "../../src/signals/copycat";
+
+// Six distinct normalized lines → four 3-line shingles; used as a reusable "prior art" corpus.
+const BLOCK = [
+  "function add(a, b) {",
+  "const total = a + b;",
+  "logger.debug(total);",
+  "return total;",
+  "}",
+  "export default add;",
+];
+
+// First 4 BLOCK lines (2 contained shingles) + 2 novel lines (2 non-matching shingles) → 50% containment.
+const HALF_COPIED = [
+  "function add(a, b) {",
+  "const total = a + b;",
+  "logger.debug(total);",
+  "return total;",
+  "noveltyOne();",
+  "noveltyTwo();",
+];
+
+describe("codeShingles", () => {
+  it("returns an empty set when every line is blank/whitespace-only", () => {
+    expect(codeShingles(["", "   ", "\t"]).size).toBe(0);
+  });
+
+  it("collapses a sub-shingle-width snippet into a single whole-block shingle", () => {
+    const shingles = codeShingles(["const x = 1;", "const y = 2;"]);
+    expect(shingles.size).toBe(1);
+    expect([...shingles][0]).toBe("const x = 1;\nconst y = 2;");
+  });
+
+  it("produces sliding 3-line shingles for a longer block", () => {
+    // 6 non-trivial lines → 6 - 3 + 1 = 4 shingles.
+    expect(codeShingles(BLOCK).size).toBe(4);
+  });
+
+  it("codeShingleList keeps duplicate shingles that codeShingles (the distinct set) collapses", () => {
+    // Two identical 3-line blocks back-to-back → shingles [ABC, BCA, CAB, ABC]: 4 in the list, 3 distinct.
+    const repeated = ["alpha();", "beta();", "gamma();", "alpha();", "beta();", "gamma();"];
+    expect(codeShingleList(repeated)).toHaveLength(4);
+    expect(codeShingles(repeated).size).toBe(3);
+  });
+
+  it("ignores blank lines and normalizes whitespace/case before shingling", () => {
+    const a = codeShingles(["Const   X = 1;", "", "  const y = 2;  ", "const z = 3;"]);
+    const b = codeShingles(["const x = 1;", "const y = 2;", "const z = 3;"]);
+    expect([...a]).toEqual([...b]);
+  });
+});
+
+describe("containmentScore", () => {
+  it("is 0 when the candidate has no comparable content", () => {
+    expect(containmentScore(["", "  "], BLOCK)).toBe(0);
+  });
+
+  it("is 0 when the prior art has no comparable content", () => {
+    expect(containmentScore(BLOCK, [])).toBe(0);
+  });
+
+  it("is 100 when every candidate shingle appears in the prior art", () => {
+    expect(containmentScore(BLOCK, BLOCK)).toBe(100);
+  });
+
+  it("is 0 when nothing overlaps", () => {
+    expect(containmentScore(["alpha();", "beta();", "gamma();"], BLOCK)).toBe(0);
+  });
+
+  it("reports the partial percentage of candidate shingles found in the prior art", () => {
+    // Candidate = the first 4 BLOCK lines (2 contained shingles) plus 2 novel lines (2 non-matching shingles)
+    // → 4 shingles total, 2 contained → 50%.
+    expect(containmentScore(HALF_COPIED, BLOCK)).toBe(50);
+  });
+
+  it("counts a repeated copied shingle as a MULTISET, not a distinct set (regression)", () => {
+    // Candidate shingles = [ABC, BCA, CAB, ABC]; prior art = {ABC}. Multiset: 2 of 4 contained → 50%.
+    // The earlier distinct-Set denominator undercounted this as 1 of 3 → 33%.
+    const repeatedCopier = ["alpha();", "beta();", "gamma();", "alpha();", "beta();", "gamma();"];
+    const priorArt = ["alpha();", "beta();", "gamma();"];
+    expect(containmentScore(repeatedCopier, priorArt)).toBe(50);
+  });
+});
+
+describe("copycatDirection", () => {
+  it("is candidate_copied when the candidate is submitted AFTER the prior art", () => {
+    expect(copycatDirection("2026-06-02T00:00:00Z", "2026-06-01T00:00:00Z")).toBe("candidate_copied");
+  });
+
+  it("is candidate_is_prior when the candidate is submitted BEFORE the prior art", () => {
+    expect(copycatDirection("2026-06-01T00:00:00Z", "2026-06-02T00:00:00Z")).toBe("candidate_is_prior");
+  });
+
+  it("is ambiguous on an exact timestamp tie", () => {
+    expect(copycatDirection("2026-06-01T00:00:00Z", "2026-06-01T00:00:00Z")).toBe("ambiguous");
+  });
+
+  it("is ambiguous when either timestamp is missing", () => {
+    expect(copycatDirection(null, "2026-06-01T00:00:00Z")).toBe("ambiguous");
+    expect(copycatDirection("2026-06-01T00:00:00Z", undefined)).toBe("ambiguous");
+    expect(copycatDirection("", "2026-06-01T00:00:00Z")).toBe("ambiguous");
+  });
+
+  it("is ambiguous when either timestamp is unparseable", () => {
+    expect(copycatDirection("not-a-date", "2026-06-01T00:00:00Z")).toBe("ambiguous");
+    expect(copycatDirection("2026-06-01T00:00:00Z", "nonsense")).toBe("ambiguous");
+  });
+});
+
+describe("assessCopycat", () => {
+  const laterCopier = {
+    candidateLines: BLOCK,
+    priorArtLines: BLOCK,
+    candidateSubmittedAt: "2026-06-02T00:00:00Z",
+    priorSubmittedAt: "2026-06-01T00:00:00Z",
+  };
+
+  it("acts (emits a finding) on a high-containment later submission with a non-off mode", () => {
+    const result = assessCopycat({ ...laterCopier, mode: "block" });
+    expect(result.score).toBe(100);
+    expect(result.direction).toBe("candidate_copied");
+    expect(result.minScore).toBe(DEFAULT_COPYCAT_MIN_SCORE);
+    expect(result.wouldAct).toBe(true);
+    expect(result.findings).toHaveLength(1);
+    expect(result.findings[0]).toMatchObject({ code: "copycat_overlap", severity: "critical" });
+    expect(result.findings[0]?.publicText).toContain("100%");
+  });
+
+  it("maps each non-off tier to its severity", () => {
+    expect(assessCopycat({ ...laterCopier, mode: "warn" }).findings[0]?.severity).toBe("info");
+    expect(assessCopycat({ ...laterCopier, mode: "label" }).findings[0]?.severity).toBe("warning");
+    expect(assessCopycat({ ...laterCopier, mode: "block" }).findings[0]?.severity).toBe("critical");
+  });
+
+  it("never acts when the mode is off (or absent), but still reports the score", () => {
+    const off = assessCopycat({ ...laterCopier, mode: "off" });
+    expect(off.score).toBe(100);
+    expect(off.wouldAct).toBe(false);
+    expect(off.findings).toEqual([]);
+    const absent = assessCopycat(laterCopier);
+    expect(absent.wouldAct).toBe(false);
+    expect(absent.findings).toEqual([]);
+  });
+
+  it("never acts when the score is below the threshold", () => {
+    const result = assessCopycat({
+      candidateLines: ["alpha();", "beta();", "gamma();"],
+      priorArtLines: BLOCK,
+      candidateSubmittedAt: "2026-06-02T00:00:00Z",
+      priorSubmittedAt: "2026-06-01T00:00:00Z",
+      mode: "block",
+    });
+    expect(result.score).toBe(0);
+    expect(result.wouldAct).toBe(false);
+    expect(result.findings).toEqual([]);
+  });
+
+  it("never acts when the candidate is the EARLIER (victim) submission, even at 100% containment", () => {
+    const result = assessCopycat({
+      candidateLines: BLOCK,
+      priorArtLines: BLOCK,
+      candidateSubmittedAt: "2026-06-01T00:00:00Z",
+      priorSubmittedAt: "2026-06-02T00:00:00Z",
+      mode: "block",
+    });
+    expect(result.score).toBe(100);
+    expect(result.direction).toBe("candidate_is_prior");
+    expect(result.wouldAct).toBe(false);
+    expect(result.findings).toEqual([]);
+  });
+
+  it("honors a custom in-range minScore and reports it back", () => {
+    // 50% containment with a 40 threshold → acts; the same score with the default 85 would not.
+    const acting = assessCopycat({
+      candidateLines: HALF_COPIED,
+      priorArtLines: BLOCK,
+      candidateSubmittedAt: "2026-06-02T00:00:00Z",
+      priorSubmittedAt: "2026-06-01T00:00:00Z",
+      mode: "label",
+      minScore: 40,
+    });
+    expect(acting.score).toBe(50);
+    expect(acting.minScore).toBe(40);
+    expect(acting.wouldAct).toBe(true);
+  });
+
+  it("clamps and rounds an out-of-range or non-numeric minScore", () => {
+    expect(assessCopycat({ ...laterCopier, mode: "off", minScore: -5 }).minScore).toBe(0);
+    expect(assessCopycat({ ...laterCopier, mode: "off", minScore: 150 }).minScore).toBe(100);
+    expect(assessCopycat({ ...laterCopier, mode: "off", minScore: 82.6 }).minScore).toBe(83);
+    expect(assessCopycat({ ...laterCopier, mode: "off", minScore: Number.NaN }).minScore).toBe(DEFAULT_COPYCAT_MIN_SCORE);
+    expect(assessCopycat({ ...laterCopier, mode: "off", minScore: null }).minScore).toBe(DEFAULT_COPYCAT_MIN_SCORE);
+  });
+});