diff --git a/ts/EFFECT_NATIVE_REWRITE_AUDIT.md b/ts/EFFECT_NATIVE_REWRITE_AUDIT.md
index cbb6ebb8..9bd8feb6 100644
--- a/ts/EFFECT_NATIVE_REWRITE_AUDIT.md
+++ b/ts/EFFECT_NATIVE_REWRITE_AUDIT.md
@@ -1963,6 +1963,26 @@ Notes:
   - `cd ts && bun run lint`
   - `git diff --check`
 
+### 2026-06-04: NATS MutableHashSet Stream Cache Slice
+
+- Status: migrated and package-verified.
+- Completed:
+  - `ts/packages/base/src/backend/nats.ts` now tracks initialized stream names
+    in `MutableHashSet<string>` instead of a native `Set`.
+  - `ensureStream` uses `MutableHashSet.has` and `MutableHashSet.add` while
+    preserving the NATS broker adapter boundary and the selective 404
+    create-on-missing behavior.
+  - `ts/packages/base/src/__tests__/nats-backend.test.ts` now proves repeated
+    producer/consumer creation for subjects in the same stream only performs
+    one stream lookup.
+- Verification:
+  - `cd ts/packages/base && bunx --bun vitest run src/__tests__/nats-backend.test.ts`
+  - `cd ts && bun run check:tsgo`
+  - `cd ts && bun run build`
+  - `cd ts && bun run test`
+  - `cd ts && bun run lint`
+  - `git diff --check`
+
 ## Subagent Findings To Preserve
 
 - MCP/workbench:
@@ -2007,6 +2027,9 @@ Notes:
   - NATS header construction, ack/nak operations, and lookup create-on-missing
     behavior now stay typed. `PubSubBackend` remains an intentional broker
     adapter contract rather than a direct `effect/PubSub` replacement target.
+  - NATS initialized stream cache now uses Effect `MutableHashSet`; keep the
+    rest of the NATS SDK connection/JetStream manager state as an external
+    broker adapter boundary unless moving the whole backend lifecycle.
   - Consumer rate-limit retry timeout behavior is now wired in both legacy and
     Effect-native consumer paths. Effect-native consumer concurrency now owns
     one backend consumer per worker, and request-response pending shutdown now
diff --git a/ts/packages/base/src/__tests__/nats-backend.test.ts b/ts/packages/base/src/__tests__/nats-backend.test.ts
index 66e3fb29..75c0a007 100644
--- a/ts/packages/base/src/__tests__/nats-backend.test.ts
+++ b/ts/packages/base/src/__tests__/nats-backend.test.ts
@@ -128,6 +128,19 @@ describe("NATS backend", () => {
     });
   });
 
+  it("caches initialized streams through the Effect mutable set", async () => {
+    const backend = makeNatsBackend("nats://test");
+
+    await backend.createProducer<string>({ topic: "tg.test.topic" });
+    await backend.createConsumer<string>({
+      topic: "tg.test.other",
+      subscription: "worker",
+    });
+
+    expect(natsMock.streamsInfo).toHaveBeenCalledTimes(1);
+    expect(natsMock.streamsInfo).toHaveBeenCalledWith("tg_test");
+  });
+
   it("does not create streams for non-missing lookup failures", async () => {
     natsMock.streamsInfo.mockRejectedValueOnce(makeNatsError("PERMISSIONS_VIOLATION"));
     const backend = makeNatsBackend("nats://test");
diff --git a/ts/packages/base/src/backend/nats.ts b/ts/packages/base/src/backend/nats.ts
index ffc8dee6..efebd177 100644
--- a/ts/packages/base/src/backend/nats.ts
+++ b/ts/packages/base/src/backend/nats.ts
@@ -24,6 +24,7 @@ import {
   DeliverPolicy,
 } from "nats";
 import { Effect } from "effect";
+import * as MutableHashSet from "effect/MutableHashSet";
 import * as P from "effect/Predicate";
 import * as S from "effect/Schema";
 
@@ -288,7 +289,7 @@ export function makeNatsBackend(url = "nats://localhost:4222"): PubSubBackend {
   let connection: NatsConnection | null = null;
   let js: JetStreamClient | null = null;
   let jsm: JetStreamManager | null = null;
-  const initializedStreams = new Set<string>();
+  const initializedStreams = MutableHashSet.empty<string>();
 
   const ensureConnected = Effect.fn("NatsBackend.ensureConnected")(function* () {
     if (connection === null) {
@@ -313,7 +314,7 @@ export function makeNatsBackend(url = "nats://localhost:4222"): PubSubBackend {
     const parts = subject.split(".");
     const streamName = parts.slice(0, 2).join("_");
 
-    if (initializedStreams.has(streamName)) return streamName;
+    if (MutableHashSet.has(initializedStreams, streamName)) return streamName;
 
     const wildcardSubject = `${parts.slice(0, 2).join(".")}.>`;
 
@@ -338,7 +339,7 @@ export function makeNatsBackend(url = "nats://localhost:4222"): PubSubBackend {
         (error) => Effect.fail(pubSubError(error.operation, error.cause)),
       ),
     );
-    initializedStreams.add(streamName);
+    MutableHashSet.add(initializedStreams, streamName);
     return streamName;
   });