diff --git a/ts/EFFECT_NATIVE_REWRITE_AUDIT.md b/ts/EFFECT_NATIVE_REWRITE_AUDIT.md
index 19429f61..164beaab 100644
--- a/ts/EFFECT_NATIVE_REWRITE_AUDIT.md
+++ b/ts/EFFECT_NATIVE_REWRITE_AUDIT.md
@@ -2254,6 +2254,24 @@ Notes:
   - `cd ts && bun run lint`
   - `git diff --check`
 
+### 2026-06-04: Prompt Template MutableHashMap Cache Slice
+
+- Status: migrated and package-verified.
+- Completed:
+  - `ts/packages/flow/src/prompt/template.ts` now stores loaded prompt
+    templates in `MutableHashMap` instead of a native `Map`.
+  - Config reload clears and repopulates the Effect collection, request lookup
+    narrows through `Option`, and logging uses `MutableHashMap.size` / `keys`.
+  - New focused coverage verifies a config-loaded prompt template renders
+    through the service request/response flow.
+- Verification:
+  - `cd ts/packages/flow && bunx --bun vitest run src/__tests__/prompt-template.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:
@@ -2440,9 +2458,9 @@ Notes:
     The workbench random id helper is complete; the remaining workbench
     `Effect.gen` match is a local one-shot command effect value.
   - Remaining real long-lived native collection targets include base processor
-    registries, Librarian service state, prompt template cache, and a workbench
-    module cache. The standalone Librarian collection manager is complete.
-    Local traversal sets and test fakes remain no-op boundaries.
+    registries, Librarian service state, and a workbench module cache. The
+    standalone Librarian collection manager and prompt template cache are
+    complete. Local traversal sets and test fakes remain no-op boundaries.
 
 ## Ranked Findings
 
diff --git a/ts/packages/flow/src/__tests__/prompt-template.test.ts b/ts/packages/flow/src/__tests__/prompt-template.test.ts
new file mode 100644
index 00000000..53f68dc9
--- /dev/null
+++ b/ts/packages/flow/src/__tests__/prompt-template.test.ts
@@ -0,0 +1,219 @@
+import { describe, expect, it } from "@effect/vitest";
+import { ConfigProvider, Effect, Fiber } from "effect";
+import * as S from "effect/Schema";
+import {
+  MessagingRuntimeLive,
+  PubSub,
+  runProcessorScoped,
+  topics,
+  type BackendConsumer,
+  type BackendProducer,
+  type CreateConsumerOptions,
+  type CreateProducerOptions,
+  type Message,
+  type PromptRequest,
+  type PromptResponse,
+  type PubSubBackend,
+} from "@trustgraph/base";
+import { PromptTemplateService } from "../prompt/template.js";
+
+class WaitForTimeout extends S.TaggedErrorClass<WaitForTimeout>()(
+  "WaitForTimeout",
+  { label: S.String },
+) {}
+
+const isWaitForTimeout = S.is(WaitForTimeout);
+
+function createMessage<T>(value: T, properties: Record<string, string> = {}): Message<T> {
+  return {
+    value: () => value,
+    properties: () => properties,
+  };
+}
+
+const waitFor = (condition: () => boolean, label: string) =>
+  Effect.tryPromise({
+    try: () =>
+      new Promise<void>((resolve, reject) => {
+        const deadline = Date.now() + 1000;
+        const check = () => {
+          if (condition()) {
+            resolve();
+            return;
+          }
+          if (Date.now() > deadline) {
+            reject(WaitForTimeout.make({ label }));
+            return;
+          }
+          setTimeout(check, 5);
+        };
+        check();
+      }),
+    catch: (error) => isWaitForTimeout(error) ? error : WaitForTimeout.make({ label }),
+  });
+
+class RecordingProducer<T> implements BackendProducer<T> {
+  readonly sent: Array<{ readonly message: T; readonly properties?: Record<string, string> }> = [];
+
+  async send(message: T, properties?: Record<string, string>): Promise<void> {
+    this.sent.push(properties === undefined ? { message } : { message, properties });
+  }
+
+  async flush(): Promise<void> {}
+
+  async close(): Promise<void> {}
+}
+
+class PushConsumer<T> implements BackendConsumer<T> {
+  readonly acknowledged: Array<Message<T>> = [];
+  private readonly messages: Array<Message<T>> = [];
+  private readonly waiters: Array<(message: Message<T> | null) => void> = [];
+  private closed = false;
+
+  push(message: Message<T>): void {
+    const waiter = this.waiters.shift();
+    if (waiter !== undefined) {
+      waiter(message);
+      return;
+    }
+    this.messages.push(message);
+  }
+
+  async receive(): Promise<Message<T> | null> {
+    const message = this.messages.shift();
+    if (message !== undefined || this.closed) {
+      return message ?? null;
+    }
+    return await new Promise((resolve) => {
+      this.waiters.push(resolve);
+    });
+  }
+
+  async acknowledge(message: Message<T>): Promise<void> {
+    this.acknowledged.push(message);
+  }
+
+  async negativeAcknowledge(): Promise<void> {}
+
+  async unsubscribe(): Promise<void> {}
+
+  async close(): Promise<void> {
+    this.closed = true;
+    for (const waiter of this.waiters.splice(0)) {
+      waiter(null);
+    }
+  }
+}
+
+class PromptBackend implements PubSubBackend {
+  readonly configConsumer = new PushConsumer<{ readonly version: number; readonly config: Record<string, unknown> }>();
+  readonly consumersByTopic = new Map<string, PushConsumer<unknown>>();
+  readonly producersByTopic = new Map<string, RecordingProducer<unknown>>();
+
+  async createProducer<T>(options: CreateProducerOptions): Promise<BackendProducer<T>> {
+    const producer = new RecordingProducer<unknown>();
+    this.producersByTopic.set(options.topic, producer);
+    return producer as BackendProducer<T>;
+  }
+
+  async createConsumer<T>(options: CreateConsumerOptions): Promise<BackendConsumer<T>> {
+    if (options.topic === topics.configPush) {
+      return this.configConsumer as unknown as BackendConsumer<T>;
+    }
+    const consumer = new PushConsumer<unknown>();
+    this.consumersByTopic.set(options.topic, consumer);
+    return consumer as BackendConsumer<T>;
+  }
+
+  async close(): Promise<void> {}
+
+  pushPromptConfig(): void {
+    this.configConsumer.push(createMessage({
+      version: 1,
+      config: {
+        flows: {
+          default: {
+            topics: {
+              "prompt-request": "prompt-request-topic",
+              "prompt-response": "prompt-response-topic",
+            },
+          },
+        },
+        prompt: {
+          greeting: {
+            system: "System for {name}",
+            prompt: "Hello {name} from {place}",
+          },
+        },
+      },
+    }));
+  }
+}
+
+const fastMessagingConfig = ConfigProvider.layer(
+  ConfigProvider.fromEnv({
+    TG_CONSUMER_RECEIVE_TIMEOUT_MS: "1",
+    TG_CONSUMER_ERROR_BACKOFF_MS: "1",
+    TG_RATE_LIMIT_RETRY_MS: "1",
+    TG_REQUEST_TIMEOUT_MS: "250",
+  }),
+);
+
+describe("PromptTemplateService", () => {
+  it.effect(
+    "renders prompt templates loaded from config through MutableHashMap state",
+    Effect.fnUntraced(function* () {
+      const backend = new PromptBackend();
+
+      yield* Effect.scoped(
+        Effect.gen(function* () {
+          const fiber = yield* runProcessorScoped(
+            {
+              id: "prompt",
+              pubsubUrl: "nats://unused:4222",
+              metricsPort: 8000,
+              manageProcessSignals: true,
+            },
+            (config) => new PromptTemplateService(config),
+          ).pipe(
+            Effect.provide(MessagingRuntimeLive),
+            Effect.provide(PubSub.layer(backend)),
+            Effect.provide(fastMessagingConfig),
+            Effect.forkChild,
+          );
+
+          backend.pushPromptConfig();
+          yield* waitFor(() => backend.consumersByTopic.has("prompt-request-topic"), "prompt consumer");
+          yield* waitFor(() => backend.producersByTopic.has("prompt-response-topic"), "prompt producer");
+          yield* waitFor(() => backend.configConsumer.acknowledged.length === 1, "config ack");
+
+          const inputConsumer = backend.consumersByTopic.get("prompt-request-topic") as PushConsumer<PromptRequest>;
+          const outputProducer = backend.producersByTopic.get("prompt-response-topic") as RecordingProducer<PromptResponse>;
+
+          inputConsumer.push(createMessage({
+            name: "greeting",
+            variables: {
+              name: "Ada",
+              place: "TrustGraph",
+            },
+          }, { id: "request-1" }));
+
+          yield* waitFor(() => outputProducer.sent.length === 1, "prompt response");
+
+          expect(inputConsumer.acknowledged.length).toBe(1);
+          expect(outputProducer.sent).toEqual([
+            {
+              message: {
+                system: "System for Ada",
+                prompt: "Hello Ada from TrustGraph",
+              },
+              properties: { id: "request-1" },
+            },
+          ]);
+
+          yield* Fiber.interrupt(fiber);
+        }),
+      );
+    }),
+  );
+});
diff --git a/ts/packages/flow/src/prompt/template.ts b/ts/packages/flow/src/prompt/template.ts
index abbd764d..a82001a5 100644
--- a/ts/packages/flow/src/prompt/template.ts
+++ b/ts/packages/flow/src/prompt/template.ts
@@ -41,6 +41,8 @@ import {
 import { NodeRuntime } from "@effect/platform-node";
 import { makeFlowProcessorProgram } from "@trustgraph/base";
 import { Effect, Layer, ManagedRuntime } from "effect";
+import * as MutableHashMap from "effect/MutableHashMap";
+import * as O from "effect/Option";
 import * as S from "effect/Schema";
 
 export interface PromptTemplate {
@@ -67,7 +69,7 @@ interface PromptTemplateRuntime {
 const programRuntimes = new WeakMap<PromptTemplateConfig, PromptTemplateRuntime>();
 
 const makePromptTemplateRuntime = (config: PromptTemplateConfig): PromptTemplateRuntime => {
-  const templates = new Map<string, PromptTemplate>();
+  const templates = MutableHashMap.empty<string, PromptTemplate>();
   const configKey = config.configKey ?? "prompt";
   const PromptResponseProducer = makeProducerSpec<PromptResponse>("prompt-response");
 
@@ -93,17 +95,17 @@ const makePromptTemplateRuntime = (config: PromptTemplateConfig): PromptTemplate
     );
     if (decoded === null) return;
 
-    templates.clear();
+    MutableHashMap.clear(templates);
 
     for (const [name, template] of Object.entries(decoded)) {
-      templates.set(name, {
+      MutableHashMap.set(templates, name, {
         system: template.system ?? "",
         prompt: template.prompt ?? "",
       });
     }
 
     yield* Effect.log(
-      `[PromptTemplate] Loaded ${templates.size} template(s): ${[...templates.keys()].join(", ")}`,
+      `[PromptTemplate] Loaded ${MutableHashMap.size(templates)} template(s): ${Array.from(MutableHashMap.keys(templates)).join(", ")}`,
     );
   });
 
@@ -116,7 +118,7 @@ const makePromptTemplateRuntime = (config: PromptTemplateConfig): PromptTemplate
     if (requestId === undefined || requestId.length === 0) return;
 
     const responseProducer = yield* flowCtx.flow.producerEffect(PromptResponseProducer);
-    const template = templates.get(msg.name);
+    const template = O.getOrUndefined(MutableHashMap.get(templates, msg.name));
     if (template === undefined) {
       yield* responseProducer.send(requestId, {
         system: "",