diff --git a/src/acp/translator.error-kind.test.ts b/src/acp/translator.error-kind.test.ts
index 2f5bde7a98d..87e5cdf3a2d 100644
--- a/src/acp/translator.error-kind.test.ts
+++ b/src/acp/translator.error-kind.test.ts
@@ -1,88 +1,9 @@
-import type { PromptRequest } from "@agentclientprotocol/sdk";
-import { describe, expect, it, vi } from "vitest";
-import type { GatewayClient } from "../gateway/client.js";
-import type { EventFrame } from "../gateway/protocol/index.js";
-import { createInMemorySessionStore } from "./session.js";
-import { AcpGatewayAgent } from "./translator.js";
-import { createAcpConnection, createAcpGateway } from "./translator.test-helpers.js";
-
-type PendingPromptHarness = {
-  agent: AcpGatewayAgent;
-  promptPromise: ReturnType<AcpGatewayAgent["prompt"]>;
-  runId: string;
-};
-
-const DEFAULT_SESSION_ID = "session-1";
-const DEFAULT_SESSION_KEY = "agent:main:main";
-const DEFAULT_PROMPT_TEXT = "hello";
-
-function createSessionAgentHarness(
-  request: GatewayClient["request"],
-  options: { sessionId?: string; sessionKey?: string; cwd?: string } = {},
-) {
-  const sessionId = options.sessionId ?? DEFAULT_SESSION_ID;
-  const sessionKey = options.sessionKey ?? DEFAULT_SESSION_KEY;
-  const sessionStore = createInMemorySessionStore();
-  sessionStore.createSession({
-    sessionId,
-    sessionKey,
-    cwd: options.cwd ?? "/tmp",
-  });
-  const agent = new AcpGatewayAgent(createAcpConnection(), createAcpGateway(request), {
-    sessionStore,
-  });
-
-  return {
-    agent,
-    sessionId,
-    sessionKey,
-    sessionStore,
-  };
-}
-
-function promptAgent(
-  agent: AcpGatewayAgent,
-  sessionId = DEFAULT_SESSION_ID,
-  text = DEFAULT_PROMPT_TEXT,
-) {
-  return agent.prompt({
-    sessionId,
-    prompt: [{ type: "text", text }],
-    _meta: {},
-  } as unknown as PromptRequest);
-}
-
-async function createPendingPromptHarness(): Promise<PendingPromptHarness> {
-  let runId: string | undefined;
-  const request = vi.fn(async (method: string, params?: Record<string, unknown>) => {
-    if (method === "chat.send") {
-      runId = params?.idempotencyKey as string | undefined;
-      return new Promise<never>(() => {});
-    }
-    return {};
-  }) as GatewayClient["request"];
-
-  const { agent, sessionId } = createSessionAgentHarness(request);
-  const promptPromise = promptAgent(agent, sessionId);
-
-  await vi.waitFor(() => {
-    expect(runId).toBeDefined();
-  });
-
-  return {
-    agent,
-    promptPromise,
-    runId: runId!,
-  };
-}
-
-function createChatEvent(payload: Record<string, unknown>): EventFrame {
-  return {
-    type: "event",
-    event: "chat",
-    payload,
-  } as EventFrame;
-}
+import { describe, expect, it } from "vitest";
+import {
+  createChatEvent,
+  createPendingPromptHarness,
+  DEFAULT_SESSION_KEY,
+} from "./translator.prompt-harness.test-support.js";
 
 describe("acp translator errorKind mapping", () => {
   it("maps errorKind: refusal to stopReason: refusal", async () => {
diff --git a/src/acp/translator.prompt-harness.test-support.ts b/src/acp/translator.prompt-harness.test-support.ts
new file mode 100644
index 00000000000..ef384c60e11
--- /dev/null
+++ b/src/acp/translator.prompt-harness.test-support.ts
@@ -0,0 +1,94 @@
+import type { PromptRequest } from "@agentclientprotocol/sdk";
+import { expect, vi } from "vitest";
+import type { GatewayClient } from "../gateway/client.js";
+import type { EventFrame } from "../gateway/protocol/index.js";
+import { createInMemorySessionStore } from "./session.js";
+import { AcpGatewayAgent } from "./translator.js";
+import { createAcpConnection, createAcpGateway } from "./translator.test-helpers.js";
+
+export type PendingPromptHarness = {
+  agent: AcpGatewayAgent;
+  promptPromise: ReturnType<AcpGatewayAgent["prompt"]>;
+  runId: string;
+};
+
+export const DEFAULT_SESSION_ID = "session-1";
+export const DEFAULT_SESSION_KEY = "agent:main:main";
+export const DEFAULT_PROMPT_TEXT = "hello";
+
+export function createSessionAgentHarness(
+  request: GatewayClient["request"],
+  options: { sessionId?: string; sessionKey?: string; cwd?: string } = {},
+) {
+  const sessionId = options.sessionId ?? DEFAULT_SESSION_ID;
+  const sessionKey = options.sessionKey ?? DEFAULT_SESSION_KEY;
+  const sessionStore = createInMemorySessionStore();
+  sessionStore.createSession({
+    sessionId,
+    sessionKey,
+    cwd: options.cwd ?? "/tmp",
+  });
+  const agent = new AcpGatewayAgent(createAcpConnection(), createAcpGateway(request), {
+    sessionStore,
+  });
+
+  return {
+    agent,
+    sessionId,
+    sessionKey,
+    sessionStore,
+  };
+}
+
+export function promptAgent(
+  agent: AcpGatewayAgent,
+  sessionId = DEFAULT_SESSION_ID,
+  text = DEFAULT_PROMPT_TEXT,
+) {
+  return agent.prompt({
+    sessionId,
+    prompt: [{ type: "text", text }],
+    _meta: {},
+  } as unknown as PromptRequest);
+}
+
+export function observeSettlement(promise: ReturnType<AcpGatewayAgent["prompt"]>) {
+  const settleSpy = vi.fn();
+  void promise.then(
+    (value) => settleSpy({ kind: "resolve", value }),
+    (error) => settleSpy({ kind: "reject", error }),
+  );
+  return settleSpy;
+}
+
+export async function createPendingPromptHarness(): Promise<PendingPromptHarness> {
+  let runId: string | undefined;
+  const request = vi.fn(async (method: string, params?: Record<string, unknown>) => {
+    if (method === "chat.send") {
+      runId = params?.idempotencyKey as string | undefined;
+      return new Promise<never>(() => {});
+    }
+    return {};
+  }) as GatewayClient["request"];
+
+  const { agent, sessionId } = createSessionAgentHarness(request);
+  const promptPromise = promptAgent(agent, sessionId);
+
+  await vi.waitFor(() => {
+    expect(runId).toBeDefined();
+  });
+
+  return {
+    agent,
+    promptPromise,
+    runId: runId!,
+  };
+}
+
+export function createChatEvent(payload: Record<string, unknown>): EventFrame {
+  return {
+    type: "event",
+    event: "chat",
+    payload,
+  } as EventFrame;
+}
diff --git a/src/acp/translator.stop-reason.test.ts b/src/acp/translator.stop-reason.test.ts
index a30fbcd8a55..f110f23c06c 100644
--- a/src/acp/translator.stop-reason.test.ts
+++ b/src/acp/translator.stop-reason.test.ts
@@ -1,98 +1,17 @@
 import type { PromptRequest } from "@agentclientprotocol/sdk";
 import { describe, expect, it, vi } from "vitest";
 import type { GatewayClient } from "../gateway/client.js";
-import type { EventFrame } from "../gateway/protocol/index.js";
 import { createInMemorySessionStore } from "./session.js";
 import { AcpGatewayAgent } from "./translator.js";
+import {
+  createChatEvent,
+  createPendingPromptHarness,
+  createSessionAgentHarness,
+  observeSettlement,
+  promptAgent,
+} from "./translator.prompt-harness.test-support.js";
 import { createAcpConnection, createAcpGateway } from "./translator.test-helpers.js";
 
-type PendingPromptHarness = {
-  agent: AcpGatewayAgent;
-  promptPromise: ReturnType<AcpGatewayAgent["prompt"]>;
-  runId: string;
-};
-
-const DEFAULT_SESSION_ID = "session-1";
-const DEFAULT_SESSION_KEY = "agent:main:main";
-const DEFAULT_PROMPT_TEXT = "hello";
-
-function createSessionAgentHarness(
-  request: GatewayClient["request"],
-  options: { sessionId?: string; sessionKey?: string; cwd?: string } = {},
-) {
-  const sessionId = options.sessionId ?? DEFAULT_SESSION_ID;
-  const sessionKey = options.sessionKey ?? DEFAULT_SESSION_KEY;
-  const sessionStore = createInMemorySessionStore();
-  sessionStore.createSession({
-    sessionId,
-    sessionKey,
-    cwd: options.cwd ?? "/tmp",
-  });
-  const agent = new AcpGatewayAgent(createAcpConnection(), createAcpGateway(request), {
-    sessionStore,
-  });
-
-  return {
-    agent,
-    sessionId,
-    sessionKey,
-    sessionStore,
-  };
-}
-
-function promptAgent(
-  agent: AcpGatewayAgent,
-  sessionId = DEFAULT_SESSION_ID,
-  text = DEFAULT_PROMPT_TEXT,
-) {
-  return agent.prompt({
-    sessionId,
-    prompt: [{ type: "text", text }],
-    _meta: {},
-  } as unknown as PromptRequest);
-}
-
-function observeSettlement(promise: ReturnType<AcpGatewayAgent["prompt"]>) {
-  const settleSpy = vi.fn();
-  void promise.then(
-    (value) => settleSpy({ kind: "resolve", value }),
-    (error) => settleSpy({ kind: "reject", error }),
-  );
-  return settleSpy;
-}
-
-async function createPendingPromptHarness(): Promise<PendingPromptHarness> {
-  let runId: string | undefined;
-  const request = vi.fn(async (method: string, params?: Record<string, unknown>) => {
-    if (method === "chat.send") {
-      runId = params?.idempotencyKey as string | undefined;
-      return new Promise<never>(() => {});
-    }
-    return {};
-  }) as GatewayClient["request"];
-
-  const { agent, sessionId } = createSessionAgentHarness(request);
-  const promptPromise = promptAgent(agent, sessionId);
-
-  await vi.waitFor(() => {
-    expect(runId).toBeDefined();
-  });
-
-  return {
-    agent,
-    promptPromise,
-    runId: runId!,
-  };
-}
-
-function createChatEvent(payload: Record<string, unknown>): EventFrame {
-  return {
-    type: "event",
-    event: "chat",
-    payload,
-  } as EventFrame;
-}
-
 describe("acp translator stop reason mapping", () => {
   it("error state resolves as end_turn, not refusal", async () => {
     const { agent, promptPromise, runId } = await createPendingPromptHarness();