@@ -1,5 +1,12 @@
-import { PostgresRunStore, RoutingRunStore, type RunStore } from "@internal/run-store";
-import { ownerEngine, type Residency } from "@trigger.dev/core/v3/isomorphic";
+import {
+  PostgresRunStore,
+  RoutingRunStore,
+  type RoutingStoreMetrics,
+  type RunStore,
+} from "@internal/run-store";
+import { resolveShard, type ShardKey } from "@trigger.dev/core/v3/isomorphic";
+import { Counter } from "prom-client";
+import { metricsRegister } from "~/metrics.server";
 import type { PrismaClient, PrismaReplicaClient } from "@trigger.dev/database";
 import type { RunOpsPrismaClient } from "@internal/run-ops-database";
 import {
@@ -29,8 +36,8 @@ type BuildRunStoreDeps = {
   /** Single-DB store handles (control-plane pair). Used verbatim when split is OFF. */
   singleWriter: PrismaClient;
   singleReplica: PrismaReplicaClient;
-  /** Residency classifier; defaults to ownerEngine inside RoutingRunStore. */
-  classify?: (id: string) => Residency;
+  /** Id-to-shard-key resolver; defaults to the core resolveShard inside RoutingRunStore. */
+  resolveShard?: (id: string) => ShardKey;
   /** Per-pool transaction-resilience configs threaded into the store(s) this builds (IoC). */
   singleResilience?: TransactionResilienceConfig;
   newResilience?: TransactionResilienceConfig;
@@ -82,10 +89,31 @@ export function buildRunStore(deps: BuildRunStoreDeps): RunStore {
   return new RoutingRunStore({
     new: newStore,
     legacy: legacyStore,
-    classify: deps.classify ?? ownerEngine,
+    resolveShard: deps.resolveShard ?? resolveShard,
+    metrics: routingStoreMetrics,
   });
 }
 
+// singleton: module-scope Counter registration double-registers under dev HMR.
+const routingStoreMetrics: RoutingStoreMetrics = singleton("routingStoreMetrics", () => {
+  const duplicateId = new Counter({
+    name: "runops_shard_duplicate_id_total",
+    help: "One id was returned by two run-ops shards that must be disjoint (a routing-invariant violation).",
+    labelNames: ["shard_keys"],
+    registers: [metricsRegister],
+  });
+  const probeFallback = new Counter({
+    name: "runops_waitpoint_probe_fallback_total",
+    help: "A waitpoint was not on the run-ops store its id named and was found by a fallback probe.",
+    labelNames: ["from", "to"],
+    registers: [metricsRegister],
+  });
+  return {
+    recordDuplicateId: (shardKeys) => duplicateId.inc({ shard_keys: shardKeys.join(",") }),
+    recordWaitpointProbeFallback: (from, to) => probeFallback.inc({ from, to }),
+  };
+});
+
 // Build the routing store whenever BOTH run-ops DBs are configured, independent of
 // RUN_OPS_SPLIT_ENABLED. Reads must fan out across both DBs so a run that lives on the new
 // DB stays visible even with the flag off (matches the db.server topology factory). The flag
 
@@ -1826,7 +1826,7 @@ export class PostgresRunStore implements RunStore {
     const branches = args.idempotencyKeys.map((key) => {
       const base = params.length;
       params.push(args.runtimeEnvironmentId, args.taskIdentifier, key);
-      return `SELECT "friendlyId", "idempotencyKey", "idempotencyKeyExpiresAt" FROM "TaskRun" WHERE "runtimeEnvironmentId" = $${base + 1} AND "taskIdentifier" = $${base + 2} AND "idempotencyKey" = $${base + 3}`;
+      return `SELECT "id", "createdAt", "friendlyId", "idempotencyKey", "idempotencyKeyExpiresAt" FROM "TaskRun" WHERE "runtimeEnvironmentId" = $${base + 1} AND "taskIdentifier" = $${base + 2} AND "idempotencyKey" = $${base + 3}`;
     });
     return prisma.$queryRawUnsafe<IdempotencyKeyRunMatch[]>(
       branches.join(" UNION ALL "),
 
@@ -3,4 +3,5 @@ export * from "./PostgresRunStore.js";
 export * from "./runOpsStore.js";
 export * from "./readReplicaClient.js";
 export * from "./redisSnapshotStore.js";
+export * from "./routingStoreMetrics.js";
 export * from "./snapshotComparator.js";
@@ -0,0 +1,16 @@
+/**
+ * Counters the routing store emits. Injected the same way RedisSnapshotStore takes its metrics,
+ * so the package stays free of a metrics dependency and a test can assert on a fake.
+ *
+ * runops_shard_duplicate_id_total       — one id returned by two shards that should be disjoint
+ * runops_waitpoint_probe_fallback_total — a waitpoint was not on the store its id named
+ */
+export type RoutingStoreMetrics = {
+  recordDuplicateId(shardKeys: string[]): void;
+  recordWaitpointProbeFallback(from: string, to: string): void;
+};
+
+export const noopRoutingStoreMetrics: RoutingStoreMetrics = {
+  recordDuplicateId() {},
+  recordWaitpointProbeFallback() {},
+};
@@ -0,0 +1,305 @@
+// FOUR-STORE MATRIX — proves RoutingRunStore is correct across legacy + new + two gen-2 shards
+// (a, b) against REAL databases (makeNShardRunOpsPostgresTest). NEVER mocked. This is where the
+// §3.4 disjoint-sum fix is proven end-to-end: a double count here strands a blocked run forever.
+//
+// runOpsStore.mixedResidency.test.ts is the TWO-store invariant lock and stays byte-identical; this
+// file is the N-store extension and lives separately.
+
+import { makeNShardRunOpsPostgresTest } from "@internal/testcontainers";
+import type { PrismaClient } from "@trigger.dev/database";
+import type { RunOpsPrismaClient } from "@internal/run-ops-database";
+import { resolveShard } from "@trigger.dev/core/v3/isomorphic";
+import { describe, expect } from "vitest";
+import { PostgresRunStore } from "./PostgresRunStore.js";
+import { RoutingRunStore } from "./runOpsStore.js";
+import type { CreateRunInput } from "./types.js";
+
+type AnyClient = PrismaClient | RunOpsPrismaClient;
+
+const matrixTest = makeNShardRunOpsPostgresTest(2);
+
+// A gen-2 id: 24-char base32hex core, the shard char at index 24, version "2" at index 25.
+// resolveShard(gen2("a", ...)) === "a". A bare cuid-length id classifies "legacy".
+function gen2(shardChar: string, seed: string): string {
+  const core = (seed.replace(/[^0-9a-v]/g, "0") + "k".repeat(24)).slice(0, 24);
+  return `${core}${shardChar}2`;
+}
+function cuid(seed: string): string {
+  return (seed + "c".repeat(25)).slice(0, 25); // 25 chars → LEGACY
+}
+
+function makeStore(prisma: AnyClient, variant: "legacy" | "dedicated") {
+  return new PostgresRunStore({
+    prisma: prisma as never,
+    readOnlyPrisma: prisma as never,
+    schemaVariant: variant,
+  });
+}
+
+// The real four-store split: legacy (full schema) + new + gen-2 a + gen-2 b (dedicated subset),
+// routed by the REAL core resolveShard.
+function makeMatrixRouter(
+  legacyPrisma: PrismaClient,
+  newPrisma: RunOpsPrismaClient,
+  shardPrismas: RunOpsPrismaClient[]
+) {
+  return new RoutingRunStore({
+    new: makeStore(newPrisma, "dedicated"),
+    legacy: makeStore(legacyPrisma, "legacy"),
+    shards: [
+      { key: "a", store: makeStore(shardPrismas[0]!, "dedicated") },
+      { key: "b", store: makeStore(shardPrismas[1]!, "dedicated") },
+    ],
+    resolveShard,
+  });
+}
+
+async function seedLegacyEnv(prisma: PrismaClient, suffix: string) {
+  const organization = await prisma.organization.create({
+    data: { title: `Org ${suffix}`, slug: `org-${suffix}` },
+  });
+  const project = await prisma.project.create({
+    data: {
+      name: `Project ${suffix}`,
+      slug: `project-${suffix}`,
+      externalRef: `proj_${suffix}`,
+      organizationId: organization.id,
+    },
+  });
+  const environment = await prisma.runtimeEnvironment.create({
+    data: {
+      type: "DEVELOPMENT",
+      slug: "dev",
+      projectId: project.id,
+      organizationId: organization.id,
+      apiKey: `tr_dev_${suffix}`,
+      pkApiKey: `pk_dev_${suffix}`,
+      shortcode: `short_${suffix}`,
+    },
+  });
+  return {
+    organizationId: organization.id,
+    projectId: project.id,
+    runtimeEnvironmentId: environment.id,
+    environmentId: environment.id,
+  };
+}
+
+function buildRun(params: {
+  runId: string;
+  runtimeEnvironmentId: string;
+  organizationId: string;
+  projectId: string;
+  createdAt?: Date;
+}): CreateRunInput {
+  return {
+    data: {
+      id: params.runId,
+      engine: "V2",
+      status: "PENDING",
+      friendlyId: `run_${params.runId}`,
+      runtimeEnvironmentId: params.runtimeEnvironmentId,
+      environmentType: "DEVELOPMENT",
+      organizationId: params.organizationId,
+      projectId: params.projectId,
+      taskIdentifier: "my-task",
+      payload: "{}",
+      payloadType: "application/json",
+      context: {},
+      traceContext: {},
+      traceId: `trace_${params.runId}`,
+      spanId: `span_${params.runId}`,
+      runTags: [],
+      queue: "task/my-task",
+      isTest: false,
+      taskEventStore: "taskEvent",
+      depth: 0,
+      createdAt: params.createdAt ?? new Date("2024-01-01T00:00:00.000Z"),
+    },
+    snapshot: {
+      engine: "V2",
+      executionStatus: "RUN_CREATED",
+      description: "Run was created",
+      runStatus: "PENDING",
+      environmentId: params.runtimeEnvironmentId,
+      environmentType: "DEVELOPMENT",
+      projectId: params.projectId,
+      organizationId: params.organizationId,
+    },
+  };
+}
+
+async function seedPendingWaitpoint(
+  prisma: AnyClient,
+  params: { id: string; projectId: string; environmentId: string }
+) {
+  await (prisma as PrismaClient).waitpoint.create({
+    data: {
+      id: params.id,
+      friendlyId: `wp_${params.id}`,
+      type: "MANUAL",
+      status: "PENDING",
+      idempotencyKey: `idem_${params.id}`,
+      userProvidedIdempotencyKey: false,
+      projectId: params.projectId,
+      environmentId: params.environmentId,
+    },
+  });
+}
+
+describe("RoutingRunStore four-store matrix — disjoint sum on real databases", () => {
+  matrixTest(
+    "countPendingWaitpoints unions across a gen-2 shard and the gen-1 pair with no double count",
+    async ({ legacyPrisma, newPrisma, shardPrismas }) => {
+      const env = await seedLegacyEnv(legacyPrisma, "disjoint");
+      const router = makeMatrixRouter(legacyPrisma, newPrisma, shardPrismas);
+
+      // The blocked run lives on shard a.
+      const runId = gen2("a", "run");
+      // Its blocking waitpoints: a gen-2 waitpoint on shard b, a cuid on legacy, and a cuid MIRRORED
+      // onto both gen-1 stores (the drain-mirror case that must count once).
+      const wpB = gen2("b", "wpb");
+      const wpCuid = cuid("wpcuid");
+      const wpMirror = cuid("wpmirror");
+
+      const dedicatedEnv = { projectId: env.projectId, environmentId: env.environmentId };
+      await seedPendingWaitpoint(shardPrismas[1]!, { id: wpB, ...dedicatedEnv });
+      await seedPendingWaitpoint(legacyPrisma, { id: wpCuid, ...dedicatedEnv });
+      await seedPendingWaitpoint(legacyPrisma, { id: wpMirror, ...dedicatedEnv });
+      await seedPendingWaitpoint(newPrisma, { id: wpMirror, ...dedicatedEnv });
+
+      // b:wpB (1) + legacy:wpCuid (1) + wpMirror (once, though on both gen-1 stores) = 3.
+      const count = await router.countPendingWaitpoints([wpB, wpCuid, wpMirror], undefined, runId);
+      expect(count).toBe(3);
+    }
+  );
+
+  matrixTest(
+    "a gen-2 waitpoint on the run's own shard contributes exactly once, not twice",
+    async ({ legacyPrisma, newPrisma, shardPrismas }) => {
+      const env = await seedLegacyEnv(legacyPrisma, "ownshard");
+      const router = makeMatrixRouter(legacyPrisma, newPrisma, shardPrismas);
+      const runId = gen2("a", "run2");
+      const wpA = gen2("a", "wpa");
+      await seedPendingWaitpoint(shardPrismas[0]!, {
+        id: wpA,
+        projectId: env.projectId,
+        environmentId: env.environmentId,
+      });
+      // wpA lives on the run's own shard a → found by the presence query, never re-queried elsewhere.
+      expect(await router.countPendingWaitpoints([wpA], undefined, runId)).toBe(1);
+    }
+  );
+});
+
+describe("RoutingRunStore four-store matrix — alias topology", () => {
+  matrixTest(
+    "an aliased gen-2 shard counts its database ONCE in a sum (declaration, not identity)",
+    async ({ legacyPrisma, newPrisma, shardPrismas }) => {
+      const env = await seedLegacyEnv(legacyPrisma, "alias");
+      // Shard "a" aliases "new" over the SAME database, but via a SEPARATE store object built over
+      // the same client — exactly how the wiring layer will construct it. Identity dedupe would see
+      // two objects and double-count; declaration dedupe counts the database once.
+      const newStore = makeStore(newPrisma, "dedicated");
+      const aStoreSameDb = makeStore(newPrisma, "dedicated"); // distinct object, same DB
+      const router = new RoutingRunStore({
+        new: newStore,
+        legacy: makeStore(legacyPrisma, "legacy"),
+        shards: [{ key: "a", store: aStoreSameDb, aliasOf: "new" }],
+        resolveShard,
+      });
+
+      const wp = cuid("aliaswp");
+      await seedPendingWaitpoint(newPrisma, {
+        id: wp,
+        projectId: env.projectId,
+        environmentId: env.environmentId,
+      });
+      // No runId → the id-less sum fans over DISTINCT databases. The aliased "a" must not add a
+      // second leg over the "new" database, or the one pending waitpoint counts twice.
+      expect(await router.countPendingWaitpoints([wp])).toBe(1);
+    }
+  );
+});
+
+describe("RoutingRunStore four-store matrix — mixed gen-1 and gen-2 reads", () => {
+  matrixTest(
+    "findRunsByIds hydrates a mixed id set across legacy, new and both gen-2 shards",
+    async ({ legacyPrisma, newPrisma, shardPrismas }) => {
+      const env = await seedLegacyEnv(legacyPrisma, "mixed");
+      const router = makeMatrixRouter(legacyPrisma, newPrisma, shardPrismas);
+
+      const legacyId = cuid("mixleg");
+      // A v1 run-ops id (version "1") routes to "new"; gen-2 ids route to their shard char.
+      const newId = ("mixnew".replace(/[^0-9a-v]/g, "0") + "k".repeat(24)).slice(0, 24) + "01";
+      const aId = gen2("a", "mixa");
+      const bId = gen2("b", "mixb");
+      const all = [legacyId, newId, aId, bId];
+
+      for (const runId of all) {
+        await router.createRun(buildRun({ runId, ...env }));
+      }
+
+      const found = await router.findRunsByIds(all, { select: { id: true } });
+      expect(new Set([...found.keys()])).toEqual(new Set(all));
+    }
+  );
+});
+
+describe("RoutingRunStore four-store matrix — cross-tree completion across gen-2 shards", () => {
+  matrixTest(
+    "a gen-2 waitpoint completes on its own shard even under the cross-tree legacy pin",
+    async ({ legacyPrisma, newPrisma, shardPrismas }) => {
+      const env = await seedLegacyEnv(legacyPrisma, "crosstree");
+      const router = makeMatrixRouter(legacyPrisma, newPrisma, shardPrismas);
+      // The waitpoint is owned by a run on shard b; the blocked run is on shard a (cross-tree).
+      const wpB = gen2("b", "ctwp");
+      await seedPendingWaitpoint(shardPrismas[1]!, {
+        id: wpB,
+        projectId: env.projectId,
+        environmentId: env.environmentId,
+      });
+      // isCrossTreeIdempotency pins gen-1 flows to legacy; a gen-2 id must OVERRIDE that pin, or the
+      // completion write lands on legacy, matches zero rows, and strands the run.
+      const store = await router.forWaitpointCompletion(wpB, {
+        isCrossTreeIdempotency: true,
+      } as never);
+      // The returned store finds wpB on its primary — only shard b holds it, so the override worked.
+      const found = await store.findWaitpoint({ where: { id: wpB } }, store.primaryReadClient);
+      expect(found?.id).toBe(wpB);
+    }
+  );
+});
+
+describe("RoutingRunStore four-store matrix — pagination merge", () => {
+  matrixTest(
+    "findRuns merges an open-predicate page across all four stores in orderBy order",
+    async ({ legacyPrisma, newPrisma, shardPrismas }) => {
+      const env = await seedLegacyEnv(legacyPrisma, "paginate");
+      const router = makeMatrixRouter(legacyPrisma, newPrisma, shardPrismas);
+      // One run per store, distinct createdAt so the global sort order is unambiguous.
+      const rows = [
+        { id: cuid("pgleg"), at: new Date("2024-01-01T00:00:00Z") },
+        {
+          id: ("pgnew".replace(/[^0-9a-v]/g, "0") + "k".repeat(24)).slice(0, 24) + "01",
+          at: new Date("2024-01-02T00:00:00Z"),
+        },
+        { id: gen2("a", "pga"), at: new Date("2024-01-03T00:00:00Z") },
+        { id: gen2("b", "pgb"), at: new Date("2024-01-04T00:00:00Z") },
+      ];
+      for (const r of rows) {
+        await router.createRun(buildRun({ runId: r.id, ...env, createdAt: r.at }));
+      }
+      // Open predicate (no id set) → fan out + merge; take 2 skip 1 over createdAt desc.
+      const page = (await router.findRuns({
+        where: { runtimeEnvironmentId: env.runtimeEnvironmentId },
+        select: { id: true, createdAt: true },
+        orderBy: { createdAt: "desc" },
+        take: 2,
+        skip: 1,
+      })) as Array<{ id: string }>;
+      // Global desc order is b, a, new, legacy; skip 1 take 2 → [a, new].
+      expect(page.map((r) => r.id)).toEqual([rows[2]!.id, rows[1]!.id]);
+    }
+  );
+});