diff --git a/orchestrator/src/services/rulesEngine.ts b/orchestrator/src/services/rulesEngine.ts
new file mode 100644
index 0000000..37e4ff5
--- /dev/null
+++ b/orchestrator/src/services/rulesEngine.ts
@@ -0,0 +1,304 @@
+/**
+ * Pluggable Rules Engine (arch §5.2 Rules Engine; gap v2 §5.2 partial).
+ *
+ * Before this PR, business rules were hardcoded at the call sites
+ * (e.g. "plan must have a pay step" baked into iso20022.ts, SoD
+ * matrix hard-coded in transactionState.ts). This module introduces
+ * a minimal, declarative JSON DSL so that ruleSets can be loaded
+ * from env (RULES_FILE) or swapped per-environment.
+ *
+ * Design principles
+ * -----------------
+ * - No eval. The evaluator is a small recursive switch over a
+ *   closed operator set — no runtime code injection.
+ * - Pure, deterministic, side-effect free. Evaluation order is
+ *   explicit so the engine can be reasoned about and replayed.
+ * - Context is a flat name → value map. Callers project whatever
+ *   shape they need ({plan, state, compliance, participants}).
+ * - Failures are collected, not thrown. The caller decides whether
+ *   a single failure aborts, or whether to accumulate and report.
+ */
+
+import { readFileSync } from "fs";
+
+/** Supported primitive operators. */
+export type Operator =
+  | "eq"
+  | "neq"
+  | "gt"
+  | "gte"
+  | "lt"
+  | "lte"
+  | "in"
+  | "not_in"
+  | "exists"
+  | "matches" // regex
+  | "length_gte"
+  | "length_lte";
+
+/** Leaf condition — references a context path against a literal. */
+export interface LeafCondition {
+  path: string; // dotted path into the context object
+  op: Operator;
+  value?: unknown; // not required for `exists`
+  /** Optional human label for failure messages. */
+  message?: string;
+}
+
+/** Combinator — AND / OR / NOT over child conditions. */
+export interface AndCondition {
+  all: Condition[];
+  message?: string;
+}
+export interface OrCondition {
+  any: Condition[];
+  message?: string;
+}
+export interface NotCondition {
+  not: Condition;
+  message?: string;
+}
+
+export type Condition = LeafCondition | AndCondition | OrCondition | NotCondition;
+
+export interface Rule {
+  id: string;
+  description?: string;
+  when?: Condition; // precondition — rule only fires when `when` is true
+  assert: Condition; // the rule passes when `assert` evaluates true
+  /** Optional severity for reporting: "error" (default) blocks, "warn" does not. */
+  severity?: "error" | "warn";
+}
+
+export interface RuleSet {
+  id: string;
+  version?: string;
+  rules: Rule[];
+}
+
+export interface RuleFailure {
+  ruleId: string;
+  severity: "error" | "warn";
+  message: string;
+  path?: string;
+}
+
+export interface EvaluationResult {
+  ok: boolean;
+  failures: RuleFailure[];
+}
+
+/* -----------------------------------------------------------------
+ * Dotted-path resolver. Supports a.b.c and a.b[0].c.
+ * --------------------------------------------------------------- */
+function getPath(ctx: unknown, path: string): unknown {
+  if (!path) return ctx;
+  const parts = path
+    .replace(/\[(\d+)\]/g, ".$1")
+    .split(".")
+    .filter(Boolean);
+  let cur: unknown = ctx;
+  for (const p of parts) {
+    if (cur === null || cur === undefined) return undefined;
+    if (typeof cur === "object") {
+      cur = (cur as Record<string, unknown>)[p];
+    } else {
+      return undefined;
+    }
+  }
+  return cur;
+}
+
+/* -----------------------------------------------------------------
+ * Operator evaluation. Pure — no throws.
+ * --------------------------------------------------------------- */
+function evalOp(op: Operator, actual: unknown, expected: unknown): boolean {
+  switch (op) {
+    case "eq":
+      return actual === expected;
+    case "neq":
+      return actual !== expected;
+    case "gt":
+      return typeof actual === "number" && typeof expected === "number" && actual > expected;
+    case "gte":
+      return typeof actual === "number" && typeof expected === "number" && actual >= expected;
+    case "lt":
+      return typeof actual === "number" && typeof expected === "number" && actual < expected;
+    case "lte":
+      return typeof actual === "number" && typeof expected === "number" && actual <= expected;
+    case "in":
+      return Array.isArray(expected) && expected.includes(actual as never);
+    case "not_in":
+      return Array.isArray(expected) && !expected.includes(actual as never);
+    case "exists":
+      return actual !== undefined && actual !== null;
+    case "matches":
+      if (typeof actual !== "string" || typeof expected !== "string") return false;
+      try {
+        return new RegExp(expected).test(actual);
+      } catch {
+        return false;
+      }
+    case "length_gte":
+      if (!Array.isArray(actual) && typeof actual !== "string") return false;
+      return (actual as { length: number }).length >= (expected as number);
+    case "length_lte":
+      if (!Array.isArray(actual) && typeof actual !== "string") return false;
+      return (actual as { length: number }).length <= (expected as number);
+    default:
+      return false;
+  }
+}
+
+function isLeaf(c: Condition): c is LeafCondition {
+  return (c as LeafCondition).op !== undefined && (c as LeafCondition).path !== undefined;
+}
+
+export function evaluateCondition(
+  condition: Condition,
+  context: Record<string, unknown>,
+): boolean {
+  if (isLeaf(condition)) {
+    const actual = getPath(context, condition.path);
+    return evalOp(condition.op, actual, condition.value);
+  }
+  if ("all" in condition) {
+    return condition.all.every((c) => evaluateCondition(c, context));
+  }
+  if ("any" in condition) {
+    return condition.any.some((c) => evaluateCondition(c, context));
+  }
+  if ("not" in condition) {
+    return !evaluateCondition(condition.not, context);
+  }
+  return false;
+}
+
+/* -----------------------------------------------------------------
+ * Public evaluate(): runs the full rule set and collects failures.
+ * --------------------------------------------------------------- */
+export function evaluate(
+  ruleSet: RuleSet,
+  context: Record<string, unknown>,
+): EvaluationResult {
+  const failures: RuleFailure[] = [];
+  for (const rule of ruleSet.rules) {
+    if (rule.when && !evaluateCondition(rule.when, context)) continue;
+    const passed = evaluateCondition(rule.assert, context);
+    if (!passed) {
+      failures.push({
+        ruleId: rule.id,
+        severity: rule.severity ?? "error",
+        message: rule.description ?? `rule ${rule.id} failed`,
+        path: isLeaf(rule.assert) ? rule.assert.path : undefined,
+      });
+    }
+  }
+  const blocking = failures.filter((f) => f.severity === "error");
+  return { ok: blocking.length === 0, failures };
+}
+
+/* -----------------------------------------------------------------
+ * Built-in rule sets. These mirror the pre-DSL hardcoded checks so
+ * callers can migrate incrementally.
+ * --------------------------------------------------------------- */
+
+/** Preconditions check — arch §8 PRECONDITIONS_PENDING -> READY_FOR_PREPARE. */
+export const BUILTIN_PRECONDITIONS: RuleSet = {
+  id: "preconditions.builtin",
+  version: "1",
+  rules: [
+    {
+      id: "plan.exists",
+      description: "plan must be present on the context",
+      assert: { path: "plan", op: "exists" },
+    },
+    {
+      id: "plan.steps.non_empty",
+      description: "plan must contain at least one step",
+      assert: { path: "plan.steps", op: "length_gte", value: 1 },
+    },
+    {
+      id: "plan.pay_step_present",
+      description: "plan must contain at least one pay step (ISO-20022 envelope)",
+      assert: {
+        any: [
+          { path: "plan.steps[0].type", op: "eq", value: "pay" },
+          { path: "plan.steps[1].type", op: "eq", value: "pay" },
+          { path: "plan.steps[2].type", op: "eq", value: "pay" },
+          { path: "plan.steps[3].type", op: "eq", value: "pay" },
+        ],
+      },
+    },
+    {
+      id: "participants.at_least_one",
+      description: "participant registry must not be empty",
+      assert: { path: "participants", op: "length_gte", value: 1 },
+    },
+    {
+      id: "compliance.kyc_ok",
+      description: "compliance KYC status must be ok",
+      when: { path: "compliance", op: "exists" },
+      assert: { path: "compliance.kyc", op: "eq", value: "ok" },
+    },
+  ],
+};
+
+/** Commit rule — arch §9.2. */
+export const BUILTIN_COMMIT: RuleSet = {
+  id: "commit.builtin",
+  version: "1",
+  rules: [
+    {
+      id: "dlt.tx_hash",
+      description: "DLT leg must produce a 0x + 64-hex tx hash",
+      assert: { path: "dlt.txHash", op: "matches", value: "^0x[0-9a-fA-F]{64}$" },
+    },
+    {
+      id: "bank.iso_message_id",
+      description: "bank leg must produce a non-empty ISO message id",
+      assert: { path: "bank.isoMessageId", op: "exists" },
+    },
+    {
+      id: "state.is_validating",
+      description: "commit is only valid from VALIDATING",
+      assert: { path: "state", op: "eq", value: "VALIDATING" },
+    },
+    {
+      id: "no_exception_holds",
+      description: "no exception may be outstanding",
+      assert: { path: "exceptions.active", op: "length_lte", value: 0 },
+    },
+  ],
+};
+
+/* -----------------------------------------------------------------
+ * Loader: RULES_FILE env points at a JSON file containing a map
+ * {ruleSetId: RuleSet}. Falls back to built-ins on any error.
+ * --------------------------------------------------------------- */
+
+let cachedOverrides: Record<string, RuleSet> | undefined;
+
+export function getRuleSet(id: string): RuleSet {
+  if (cachedOverrides === undefined) {
+    cachedOverrides = {};
+    const path = process.env.RULES_FILE;
+    if (path) {
+      try {
+        const raw = readFileSync(path, "utf8");
+        const parsed = JSON.parse(raw) as Record<string, RuleSet>;
+        if (parsed && typeof parsed === "object") cachedOverrides = parsed;
+      } catch {
+        // leave empty — silent fall-through to built-ins
+      }
+    }
+  }
+  if (cachedOverrides[id]) return cachedOverrides[id];
+  if (id === BUILTIN_PRECONDITIONS.id) return BUILTIN_PRECONDITIONS;
+  if (id === BUILTIN_COMMIT.id) return BUILTIN_COMMIT;
+  return { id, rules: [] };
+}
+
+export function __resetRulesCacheForTests(): void {
+  cachedOverrides = undefined;
+}
diff --git a/orchestrator/tests/unit/rulesEngine.test.ts b/orchestrator/tests/unit/rulesEngine.test.ts
new file mode 100644
index 0000000..885a5ef
--- /dev/null
+++ b/orchestrator/tests/unit/rulesEngine.test.ts
@@ -0,0 +1,245 @@
+/**
+ * PR P — Pluggable Rules Engine (gap-analysis v2 §5.2 partial).
+ */
+
+import { describe, it, expect, beforeEach } from "@jest/globals";
+import {
+  evaluate,
+  evaluateCondition,
+  getRuleSet,
+  BUILTIN_PRECONDITIONS,
+  BUILTIN_COMMIT,
+  __resetRulesCacheForTests,
+  type RuleSet,
+} from "../../src/services/rulesEngine";
+
+describe("rulesEngine — primitive operators", () => {
+  it("eq / neq / gt / gte / lt / lte", () => {
+    expect(
+      evaluateCondition({ path: "a", op: "eq", value: 1 }, { a: 1 }),
+    ).toBe(true);
+    expect(
+      evaluateCondition({ path: "a", op: "neq", value: 1 }, { a: 2 }),
+    ).toBe(true);
+    expect(
+      evaluateCondition({ path: "a", op: "gt", value: 1 }, { a: 2 }),
+    ).toBe(true);
+    expect(
+      evaluateCondition({ path: "a", op: "lte", value: 3 }, { a: 3 }),
+    ).toBe(true);
+  });
+
+  it("in / not_in / exists / matches", () => {
+    expect(
+      evaluateCondition(
+        { path: "role", op: "in", value: ["approver", "releaser"] },
+        { role: "approver" },
+      ),
+    ).toBe(true);
+    expect(
+      evaluateCondition(
+        { path: "role", op: "not_in", value: ["approver"] },
+        { role: "operator" },
+      ),
+    ).toBe(true);
+    expect(
+      evaluateCondition({ path: "x", op: "exists" }, { x: 0 }),
+    ).toBe(true);
+    expect(
+      evaluateCondition(
+        { path: "hash", op: "matches", value: "^0x[0-9a-f]+$" },
+        { hash: "0xabc" },
+      ),
+    ).toBe(true);
+  });
+
+  it("length_gte / length_lte work on arrays and strings", () => {
+    expect(
+      evaluateCondition({ path: "a", op: "length_gte", value: 2 }, { a: [1, 2] }),
+    ).toBe(true);
+    expect(
+      evaluateCondition({ path: "a", op: "length_lte", value: 5 }, { a: "abcd" }),
+    ).toBe(true);
+  });
+
+  it("dotted + indexed path resolution", () => {
+    expect(
+      evaluateCondition(
+        { path: "plan.steps[1].type", op: "eq", value: "pay" },
+        { plan: { steps: [{ type: "issue" }, { type: "pay" }] } },
+      ),
+    ).toBe(true);
+  });
+});
+
+describe("rulesEngine — combinators", () => {
+  const ctx = { role: "approver", amount: 1000 };
+
+  it("all (AND) — every child must pass", () => {
+    expect(
+      evaluateCondition(
+        {
+          all: [
+            { path: "role", op: "eq", value: "approver" },
+            { path: "amount", op: "gt", value: 500 },
+          ],
+        },
+        ctx,
+      ),
+    ).toBe(true);
+    expect(
+      evaluateCondition(
+        {
+          all: [
+            { path: "role", op: "eq", value: "approver" },
+            { path: "amount", op: "gt", value: 5000 },
+          ],
+        },
+        ctx,
+      ),
+    ).toBe(false);
+  });
+
+  it("any (OR) — at least one child must pass", () => {
+    expect(
+      evaluateCondition(
+        {
+          any: [
+            { path: "role", op: "eq", value: "releaser" },
+            { path: "amount", op: "gt", value: 500 },
+          ],
+        },
+        ctx,
+      ),
+    ).toBe(true);
+  });
+
+  it("not — inverts the child", () => {
+    expect(
+      evaluateCondition(
+        { not: { path: "role", op: "eq", value: "releaser" } },
+        ctx,
+      ),
+    ).toBe(true);
+  });
+});
+
+describe("rulesEngine — evaluate() and failure reporting", () => {
+  const ruleSet: RuleSet = {
+    id: "test.rs",
+    rules: [
+      {
+        id: "amount_positive",
+        description: "amount must be > 0",
+        assert: { path: "amount", op: "gt", value: 0 },
+      },
+      {
+        id: "role_listed",
+        description: "role must be in the allowed list",
+        assert: {
+          path: "role",
+          op: "in",
+          value: ["approver", "releaser", "operator"],
+        },
+      },
+      {
+        id: "warning_only",
+        description: "low amount warning",
+        severity: "warn",
+        assert: { path: "amount", op: "gte", value: 10_000 },
+      },
+    ],
+  };
+
+  it("returns ok=true when all error-severity rules pass", () => {
+    const res = evaluate(ruleSet, { amount: 1000, role: "approver" });
+    expect(res.ok).toBe(true);
+    // warn still reported even though ok=true
+    expect(res.failures.some((f) => f.ruleId === "warning_only")).toBe(true);
+    expect(res.failures.every((f) => f.severity === "warn")).toBe(true);
+  });
+
+  it("returns ok=false with error failure when a blocking rule fails", () => {
+    const res = evaluate(ruleSet, { amount: -1, role: "approver" });
+    expect(res.ok).toBe(false);
+    const amountFail = res.failures.find((f) => f.ruleId === "amount_positive");
+    expect(amountFail?.severity).toBe("error");
+  });
+
+  it("'when' gates a rule — false when-clause skips the assert", () => {
+    const guarded: RuleSet = {
+      id: "guarded.rs",
+      rules: [
+        {
+          id: "kyc_if_present",
+          when: { path: "compliance", op: "exists" },
+          assert: { path: "compliance.kyc", op: "eq", value: "ok" },
+        },
+      ],
+    };
+    expect(evaluate(guarded, {}).ok).toBe(true);
+    expect(evaluate(guarded, { compliance: { kyc: "ok" } }).ok).toBe(true);
+    expect(evaluate(guarded, { compliance: { kyc: "fail" } }).ok).toBe(false);
+  });
+});
+
+describe("rulesEngine — built-in rule sets", () => {
+  it("preconditions: pay step + non-empty participants passes", () => {
+    const res = evaluate(BUILTIN_PRECONDITIONS, {
+      plan: { steps: [{ type: "pay" }] },
+      participants: [{ id: "p1" }],
+    });
+    expect(res.ok).toBe(true);
+  });
+
+  it("preconditions: missing pay step fails", () => {
+    const res = evaluate(BUILTIN_PRECONDITIONS, {
+      plan: { steps: [{ type: "issueInstrument" }] },
+      participants: [{ id: "p1" }],
+    });
+    expect(res.ok).toBe(false);
+    expect(res.failures.some((f) => f.ruleId === "plan.pay_step_present")).toBe(
+      true,
+    );
+  });
+
+  it("commit: VALIDATING + matching refs + no exceptions passes", () => {
+    const res = evaluate(BUILTIN_COMMIT, {
+      state: "VALIDATING",
+      dlt: { txHash: `0x${"a".repeat(64)}` },
+      bank: { isoMessageId: "MSG-1" },
+      exceptions: { active: [] },
+    });
+    expect(res.ok).toBe(true);
+  });
+
+  it("commit: state != VALIDATING blocks", () => {
+    const res = evaluate(BUILTIN_COMMIT, {
+      state: "EXECUTING",
+      dlt: { txHash: `0x${"a".repeat(64)}` },
+      bank: { isoMessageId: "MSG-1" },
+      exceptions: { active: [] },
+    });
+    expect(res.ok).toBe(false);
+    expect(res.failures.some((f) => f.ruleId === "state.is_validating")).toBe(
+      true,
+    );
+  });
+});
+
+describe("rulesEngine — pluggable loading", () => {
+  beforeEach(() => {
+    __resetRulesCacheForTests();
+    delete process.env.RULES_FILE;
+  });
+
+  it("returns built-ins when RULES_FILE is unset", () => {
+    expect(getRuleSet(BUILTIN_PRECONDITIONS.id).rules.length).toBeGreaterThan(0);
+    expect(getRuleSet(BUILTIN_COMMIT.id).rules.length).toBeGreaterThan(0);
+  });
+
+  it("returns an empty rule set for unknown ids (no throw)", () => {
+    const rs = getRuleSet("nonexistent");
+    expect(rs.rules).toEqual([]);
+  });
+});