diff --git a/extensions/acpx/src/runtime-internals/events.test.ts b/extensions/acpx/src/runtime-internals/events.test.ts
new file mode 100644
index 00000000000..e5688361d43
--- /dev/null
+++ b/extensions/acpx/src/runtime-internals/events.test.ts
@@ -0,0 +1,208 @@
+import { describe, expect, it } from "vitest";
+import { parsePromptEventLine, toAcpxErrorEvent } from "./events.js";
+
+function jsonLine(payload: unknown): string {
+  return JSON.stringify(payload);
+}
+
+describe("acpx runtime event parsing", () => {
+  it("maps agent message chunks to output deltas", () => {
+    const event = parsePromptEventLine(
+      jsonLine({
+        jsonrpc: "2.0",
+        method: "session/update",
+        params: {
+          sessionId: "session-1",
+          update: {
+            sessionUpdate: "agent_message_chunk",
+            content: {
+              type: "text",
+              text: "hello world",
+            },
+          },
+        },
+      }),
+    );
+
+    expect(event).toEqual({
+      type: "text_delta",
+      text: "hello world",
+      stream: "output",
+    });
+  });
+
+  it("preserves leading spaces in streamed output chunks", () => {
+    const event = parsePromptEventLine(
+      jsonLine({
+        jsonrpc: "2.0",
+        method: "session/update",
+        params: {
+          sessionId: "session-1",
+          update: {
+            sessionUpdate: "agent_message_chunk",
+            content: {
+              type: "text",
+              text: "  indented",
+            },
+          },
+        },
+      }),
+    );
+
+    expect(event).toEqual({
+      type: "text_delta",
+      text: "  indented",
+      stream: "output",
+    });
+  });
+
+  it("maps agent thought chunks to thought deltas", () => {
+    const event = parsePromptEventLine(
+      jsonLine({
+        jsonrpc: "2.0",
+        method: "session/update",
+        params: {
+          sessionId: "session-1",
+          update: {
+            sessionUpdate: "agent_thought_chunk",
+            content: {
+              type: "text",
+              text: "thinking",
+            },
+          },
+        },
+      }),
+    );
+
+    expect(event).toEqual({
+      type: "text_delta",
+      text: "thinking",
+      stream: "thought",
+    });
+  });
+
+  it("maps tool call updates to tool_call events", () => {
+    const event = parsePromptEventLine(
+      jsonLine({
+        jsonrpc: "2.0",
+        method: "session/update",
+        params: {
+          sessionId: "session-1",
+          update: {
+            sessionUpdate: "tool_call",
+            toolCallId: "call-1",
+            title: "exec",
+            status: "in_progress",
+          },
+        },
+      }),
+    );
+
+    expect(event).toEqual({
+      type: "tool_call",
+      text: "exec (in_progress)",
+    });
+  });
+
+  it("maps prompt response stop reasons to done events", () => {
+    const event = parsePromptEventLine(
+      jsonLine({
+        jsonrpc: "2.0",
+        id: "req-1",
+        result: {
+          stopReason: "end_turn",
+        },
+      }),
+    );
+
+    expect(event).toEqual({
+      type: "done",
+      stopReason: "end_turn",
+    });
+  });
+
+  it("maps json-rpc errors to runtime errors", () => {
+    const event = parsePromptEventLine(
+      jsonLine({
+        jsonrpc: "2.0",
+        id: "req-1",
+        error: {
+          code: -32000,
+          message: "adapter failed",
+        },
+      }),
+    );
+
+    expect(event).toEqual({
+      type: "error",
+      message: "adapter failed",
+      code: "-32000",
+      retryable: undefined,
+    });
+  });
+
+  it("ignores non-prompt response errors when parse context is provided", () => {
+    const context = {
+      promptRequestIds: new Set<string>(),
+    };
+
+    const promptRequest = parsePromptEventLine(
+      jsonLine({
+        jsonrpc: "2.0",
+        id: 3,
+        method: "session/prompt",
+        params: {
+          sessionId: "session-1",
+          prompt: [{ type: "text", text: "hello" }],
+        },
+      }),
+      context,
+    );
+    const loadError = parsePromptEventLine(
+      jsonLine({
+        jsonrpc: "2.0",
+        id: 1,
+        error: {
+          code: -32002,
+          message: "Resource not found",
+        },
+      }),
+      context,
+    );
+    const promptDone = parsePromptEventLine(
+      jsonLine({
+        jsonrpc: "2.0",
+        id: 3,
+        result: {
+          stopReason: "end_turn",
+        },
+      }),
+      context,
+    );
+
+    expect(promptRequest).toBeNull();
+    expect(loadError).toBeNull();
+    expect(promptDone).toEqual({
+      type: "done",
+      stopReason: "end_turn",
+    });
+  });
+});
+
+describe("toAcpxErrorEvent", () => {
+  it("reads control command errors from json output", () => {
+    expect(
+      toAcpxErrorEvent({
+        error: {
+          code: "NO_SESSION",
+          message: "No matching session",
+          retryable: false,
+        },
+      }),
+    ).toEqual({
+      code: "NO_SESSION",
+      message: "No matching session",
+      retryable: false,
+    });
+  });
+});
diff --git a/extensions/acpx/src/runtime-internals/events.ts b/extensions/acpx/src/runtime-internals/events.ts
index 074787b3fdf..8d840ad5457 100644
--- a/extensions/acpx/src/runtime-internals/events.ts
+++ b/extensions/acpx/src/runtime-internals/events.ts
@@ -9,17 +9,72 @@ import {
   isRecord,
 } from "./shared.js";
 
+type JsonRpcId = string | number | null;
+
+export type PromptParseContext = {
+  promptRequestIds: Set<string>;
+};
+
+function isJsonRpcId(value: unknown): value is JsonRpcId {
+  return (
+    value === null ||
+    typeof value === "string" ||
+    (typeof value === "number" && Number.isFinite(value))
+  );
+}
+
+function normalizeJsonRpcId(value: unknown): string | null {
+  if (!isJsonRpcId(value) || value == null) {
+    return null;
+  }
+  return String(value);
+}
+
+function isAcpJsonRpcMessage(value: unknown): value is Record<string, unknown> {
+  if (!isRecord(value) || value.jsonrpc !== "2.0") {
+    return false;
+  }
+
+  const hasMethod = typeof value.method === "string" && value.method.length > 0;
+  const hasId = Object.hasOwn(value, "id");
+
+  if (hasMethod && !hasId) {
+    return true;
+  }
+
+  if (hasMethod && hasId) {
+    return isJsonRpcId(value.id);
+  }
+
+  if (!hasMethod && hasId) {
+    if (!isJsonRpcId(value.id)) {
+      return false;
+    }
+    const hasResult = Object.hasOwn(value, "result");
+    const hasError = Object.hasOwn(value, "error");
+    return hasResult !== hasError;
+  }
+
+  return false;
+}
+
 export function toAcpxErrorEvent(value: unknown): AcpxErrorEvent | null {
   if (!isRecord(value)) {
     return null;
   }
-  if (asTrimmedString(value.type) !== "error") {
+  const error = isRecord(value.error) ? value.error : null;
+  if (!error) {
     return null;
   }
+  const message = asTrimmedString(error.message) || "acpx reported an error";
+  const codeValue = error.code;
   return {
-    message: asTrimmedString(value.message) || "acpx reported an error",
-    code: asOptionalString(value.code),
-    retryable: asOptionalBoolean(value.retryable),
+    message,
+    code:
+      typeof codeValue === "number" && Number.isFinite(codeValue)
+        ? String(codeValue)
+        : asOptionalString(codeValue),
+    retryable: asOptionalBoolean(error.retryable),
   };
 }
 
@@ -42,7 +97,155 @@ export function parseJsonLines(value: string): AcpxJsonObject[] {
   return events;
 }
 
-export function parsePromptEventLine(line: string): AcpRuntimeEvent | null {
+function parsePromptStopReason(message: Record<string, unknown>): string | undefined {
+  if (!Object.hasOwn(message, "result")) {
+    return undefined;
+  }
+  const result = isRecord(message.result) ? message.result : null;
+  if (!result) {
+    return undefined;
+  }
+  const stopReason = asString(result.stopReason);
+  return stopReason && stopReason.trim().length > 0 ? stopReason : undefined;
+}
+
+function parseSessionUpdateEvent(message: Record<string, unknown>): AcpRuntimeEvent | null {
+  if (asTrimmedString(message.method) !== "session/update") {
+    return null;
+  }
+  const params = isRecord(message.params) ? message.params : null;
+  if (!params) {
+    return null;
+  }
+  const update = isRecord(params.update) ? params.update : null;
+  if (!update) {
+    return null;
+  }
+
+  const sessionUpdate = asTrimmedString(update.sessionUpdate);
+  switch (sessionUpdate) {
+    case "agent_message_chunk": {
+      const content = isRecord(update.content) ? update.content : null;
+      if (!content || asTrimmedString(content.type) !== "text") {
+        return null;
+      }
+      const text = asString(content.text);
+      if (!text) {
+        return null;
+      }
+      return {
+        type: "text_delta",
+        text,
+        stream: "output",
+      };
+    }
+    case "agent_thought_chunk": {
+      const content = isRecord(update.content) ? update.content : null;
+      if (!content || asTrimmedString(content.type) !== "text") {
+        return null;
+      }
+      const text = asString(content.text);
+      if (!text) {
+        return null;
+      }
+      return {
+        type: "text_delta",
+        text,
+        stream: "thought",
+      };
+    }
+    case "tool_call":
+    case "tool_call_update": {
+      const title =
+        asTrimmedString(update.title) ||
+        asTrimmedString(update.toolCallId) ||
+        asTrimmedString(update.kind) ||
+        "tool";
+      const status = asTrimmedString(update.status);
+      return {
+        type: "tool_call",
+        text: status ? `${title} (${status})` : title,
+      };
+    }
+    case "plan": {
+      const entries = Array.isArray(update.entries) ? update.entries : [];
+      const first = entries.find((entry) => isRecord(entry)) as Record<string, unknown> | undefined;
+      const content = asTrimmedString(first?.content);
+      if (!content) {
+        return { type: "status", text: "plan updated" };
+      }
+      const status = asTrimmedString(first?.status);
+      return {
+        type: "status",
+        text: status ? `plan: [${status}] ${content}` : `plan: ${content}`,
+      };
+    }
+    case "available_commands_update": {
+      const commands = Array.isArray(update.availableCommands)
+        ? update.availableCommands.length
+        : 0;
+      return {
+        type: "status",
+        text: `available commands updated (${commands})`,
+      };
+    }
+    case "current_mode_update": {
+      const modeId = asTrimmedString(update.currentModeId);
+      return {
+        type: "status",
+        text: modeId ? `mode updated: ${modeId}` : "mode updated",
+      };
+    }
+    case "config_option_update": {
+      const options = Array.isArray(update.configOptions) ? update.configOptions.length : 0;
+      return {
+        type: "status",
+        text: `config options updated (${options})`,
+      };
+    }
+    case "session_info_update": {
+      const title = asTrimmedString(update.title);
+      return {
+        type: "status",
+        text: title ? `session info updated: ${title}` : "session info updated",
+      };
+    }
+    case "usage_update": {
+      const used =
+        typeof update.used === "number" && Number.isFinite(update.used) ? update.used : null;
+      const size =
+        typeof update.size === "number" && Number.isFinite(update.size) ? update.size : null;
+      if (used == null || size == null) {
+        return { type: "status", text: "usage updated" };
+      }
+      return {
+        type: "status",
+        text: `usage updated: ${used}/${size}`,
+      };
+    }
+    default:
+      return null;
+  }
+}
+
+function shouldHandlePromptResponse(params: {
+  message: Record<string, unknown>;
+  context?: PromptParseContext;
+}): boolean {
+  const id = normalizeJsonRpcId(params.message.id);
+  if (!id) {
+    return false;
+  }
+  if (!params.context) {
+    return true;
+  }
+  return params.context.promptRequestIds.has(id);
+}
+
+export function parsePromptEventLine(
+  line: string,
+  context?: PromptParseContext,
+): AcpRuntimeEvent | null {
   const trimmed = line.trim();
   if (!trimmed) {
     return null;
@@ -61,80 +264,60 @@ export function parsePromptEventLine(line: string): AcpRuntimeEvent | null {
     return null;
   }
 
-  const type = asTrimmedString(parsed.type);
-  switch (type) {
-    case "text": {
-      const content = asString(parsed.content);
-      if (content == null || content.length === 0) {
-        return null;
-      }
-      return {
-        type: "text_delta",
-        text: content,
-        stream: "output",
-      };
-    }
-    case "thought": {
-      const content = asString(parsed.content);
-      if (content == null || content.length === 0) {
-        return null;
-      }
-      return {
-        type: "text_delta",
-        text: content,
-        stream: "thought",
-      };
-    }
-    case "tool_call": {
-      const title = asTrimmedString(parsed.title) || asTrimmedString(parsed.toolCallId) || "tool";
-      const status = asTrimmedString(parsed.status);
-      return {
-        type: "tool_call",
-        text: status ? `${title} (${status})` : title,
-      };
-    }
-    case "client_operation": {
-      const method = asTrimmedString(parsed.method) || "operation";
-      const status = asTrimmedString(parsed.status);
-      const summary = asTrimmedString(parsed.summary);
-      const text = [method, status, summary].filter(Boolean).join(" ");
-      if (!text) {
-        return null;
-      }
-      return { type: "status", text };
-    }
-    case "plan": {
-      const entries = Array.isArray(parsed.entries) ? parsed.entries : [];
-      const first = entries.find((entry) => isRecord(entry)) as Record<string, unknown> | undefined;
-      const content = asTrimmedString(first?.content);
-      if (!content) {
-        return null;
-      }
-      return { type: "status", text: `plan: ${content}` };
-    }
-    case "update": {
-      const update = asTrimmedString(parsed.update);
-      if (!update) {
-        return null;
-      }
-      return { type: "status", text: update };
-    }
-    case "done": {
-      return {
-        type: "done",
-        stopReason: asOptionalString(parsed.stopReason),
-      };
-    }
-    case "error": {
-      const message = asTrimmedString(parsed.message) || "acpx runtime error";
+  if (!isAcpJsonRpcMessage(parsed)) {
+    const fallbackError = toAcpxErrorEvent(parsed);
+    if (fallbackError) {
       return {
         type: "error",
-        message,
-        code: asOptionalString(parsed.code),
-        retryable: asOptionalBoolean(parsed.retryable),
+        message: fallbackError.message,
+        code: fallbackError.code,
+        retryable: fallbackError.retryable,
       };
     }
-    default:
-      return null;
+    return null;
   }
+
+  const updateEvent = parseSessionUpdateEvent(parsed);
+  if (updateEvent) {
+    return updateEvent;
+  }
+
+  if (asTrimmedString(parsed.method) === "session/prompt") {
+    const id = normalizeJsonRpcId(parsed.id);
+    if (id && context) {
+      context.promptRequestIds.add(id);
+    }
+    return null;
+  }
+
+  if (Object.hasOwn(parsed, "error")) {
+    if (!shouldHandlePromptResponse({ message: parsed, context })) {
+      return null;
+    }
+    const error = isRecord(parsed.error) ? parsed.error : null;
+    const message = asTrimmedString(error?.message);
+    const codeValue = error?.code;
+    return {
+      type: "error",
+      message: message || "acpx runtime error",
+      code:
+        typeof codeValue === "number" && Number.isFinite(codeValue)
+          ? String(codeValue)
+          : asOptionalString(codeValue),
+      retryable: asOptionalBoolean(error?.retryable),
+    };
+  }
+
+  const stopReason = parsePromptStopReason(parsed);
+  if (stopReason) {
+    if (!shouldHandlePromptResponse({ message: parsed, context })) {
+      return null;
+    }
+    return {
+      type: "done",
+      stopReason,
+    };
+  }
+
+  return null;
 }
diff --git a/extensions/acpx/src/runtime.test.ts b/extensions/acpx/src/runtime.test.ts
index f9096bbc73e..d05c4f0c8aa 100644
--- a/extensions/acpx/src/runtime.test.ts
+++ b/extensions/acpx/src/runtime.test.ts
@@ -23,6 +23,13 @@ const writeLog = (entry) => {
   if (!logPath) return;
   fs.appendFileSync(logPath, JSON.stringify(entry) + "\n");
 };
+const emitJson = (payload) => process.stdout.write(JSON.stringify(payload) + "\n");
+const emitUpdate = (sessionId, update) =>
+  emitJson({
+    jsonrpc: "2.0",
+    method: "session/update",
+    params: { sessionId, update },
+  });
 
 if (args.includes("--version")) {
   process.stdout.write("mock-acpx ${ACPX_PINNED_VERSION}\\n");
@@ -61,33 +68,33 @@ const setValue = command === "set" ? String(args[commandIndex + 2] || "") : "";
 
 if (command === "sessions" && args[commandIndex + 1] === "ensure") {
   writeLog({ kind: "ensure", agent, args, sessionName: ensureName });
-  process.stdout.write(JSON.stringify({
-    type: "session_ensured",
+  emitJson({
+    action: "session_ensured",
     acpxRecordId: "rec-" + ensureName,
     acpxSessionId: "sid-" + ensureName,
     agentSessionId: "inner-" + ensureName,
     name: ensureName,
     created: true,
-  }) + "\n");
+  });
   process.exit(0);
 }
 
 if (command === "cancel") {
   writeLog({ kind: "cancel", agent, args, sessionName: sessionFromOption });
-  process.stdout.write(JSON.stringify({
+  emitJson({
     acpxSessionId: "sid-" + sessionFromOption,
     cancelled: true,
-  }) + "\n");
+  });
   process.exit(0);
 }
 
 if (command === "set-mode") {
   writeLog({ kind: "set-mode", agent, args, sessionName: sessionFromOption, mode: setModeValue });
-  process.stdout.write(JSON.stringify({
-    type: "mode_set",
+  emitJson({
+    action: "mode_set",
     acpxSessionId: "sid-" + sessionFromOption,
     mode: setModeValue,
-  }) + "\n");
+  });
   process.exit(0);
 }
 
@@ -100,148 +107,167 @@ if (command === "set") {
     key: setKey,
     value: setValue,
   });
-  process.stdout.write(JSON.stringify({
-    type: "config_set",
+  emitJson({
+    action: "config_set",
     acpxSessionId: "sid-" + sessionFromOption,
     key: setKey,
     value: setValue,
-  }) + "\n");
+  });
   process.exit(0);
 }
 
 if (command === "status") {
   writeLog({ kind: "status", agent, args, sessionName: sessionFromOption });
-  process.stdout.write(JSON.stringify({
+  emitJson({
     acpxRecordId: sessionFromOption ? "rec-" + sessionFromOption : null,
     acpxSessionId: sessionFromOption ? "sid-" + sessionFromOption : null,
     agentSessionId: sessionFromOption ? "inner-" + sessionFromOption : null,
     status: sessionFromOption ? "alive" : "no-session",
     pid: 4242,
     uptime: 120,
-  }) + "\n");
+  });
   process.exit(0);
 }
 
 if (command === "sessions" && args[commandIndex + 1] === "close") {
   writeLog({ kind: "close", agent, args, sessionName: closeName });
-  process.stdout.write(JSON.stringify({
-    type: "session_closed",
+  emitJson({
+    action: "session_closed",
     acpxRecordId: "rec-" + closeName,
     acpxSessionId: "sid-" + closeName,
     name: closeName,
-  }) + "\n");
+  });
   process.exit(0);
 }
 
 if (command === "prompt") {
   const stdinText = fs.readFileSync(0, "utf8");
   writeLog({ kind: "prompt", agent, args, sessionName: sessionFromOption, stdinText });
-  const acpxSessionId = "sid-" + sessionFromOption;
+  const requestId = "req-1";
+
+  emitJson({
+    jsonrpc: "2.0",
+    id: 0,
+    method: "session/load",
+    params: {
+      sessionId: sessionFromOption,
+      cwd: process.cwd(),
+      mcpServers: [],
+    },
+  });
+  emitJson({
+    jsonrpc: "2.0",
+    id: 0,
+    error: {
+      code: -32002,
+      message: "Resource not found",
+    },
+  });
+
+  emitJson({
+    jsonrpc: "2.0",
+    id: requestId,
+    method: "session/prompt",
+    params: {
+      sessionId: sessionFromOption,
+      prompt: [
+        {
+          type: "text",
+          text: stdinText.trim(),
+        },
+      ],
+    },
+  });
 
   if (stdinText.includes("trigger-error")) {
-    process.stdout.write(JSON.stringify({
-      eventVersion: 1,
-      acpxSessionId,
-      requestId: "req-1",
-      seq: 0,
-      stream: "prompt",
-      type: "error",
-      code: "RUNTIME",
-      message: "mock failure",
-    }) + "\n");
+    emitJson({
+      jsonrpc: "2.0",
+      id: requestId,
+      error: {
+        code: -32000,
+        message: "mock failure",
+      },
+    });
     process.exit(1);
   }
 
   if (stdinText.includes("split-spacing")) {
-    process.stdout.write(JSON.stringify({
-      eventVersion: 1,
-      acpxSessionId,
-      requestId: "req-1",
-      seq: 0,
-      stream: "prompt",
-      type: "text",
-      content: "alpha",
-    }) + "\n");
-    process.stdout.write(JSON.stringify({
-      eventVersion: 1,
-      acpxSessionId,
-      requestId: "req-1",
-      seq: 1,
-      stream: "prompt",
-      type: "text",
-      content: " beta",
-    }) + "\n");
-    process.stdout.write(JSON.stringify({
-      eventVersion: 1,
-      acpxSessionId,
-      requestId: "req-1",
-      seq: 2,
-      stream: "prompt",
-      type: "text",
-      content: " gamma",
-    }) + "\n");
-    process.stdout.write(JSON.stringify({
-      eventVersion: 1,
-      acpxSessionId,
-      requestId: "req-1",
-      seq: 3,
-      stream: "prompt",
-      type: "done",
-      stopReason: "end_turn",
-    }) + "\n");
+    emitUpdate(sessionFromOption, {
+      sessionUpdate: "agent_message_chunk",
+      content: { type: "text", text: "alpha" },
+    });
+    emitUpdate(sessionFromOption, {
+      sessionUpdate: "agent_message_chunk",
+      content: { type: "text", text: " beta" },
+    });
+    emitUpdate(sessionFromOption, {
+      sessionUpdate: "agent_message_chunk",
+      content: { type: "text", text: " gamma" },
+    });
+    emitJson({
+      jsonrpc: "2.0",
+      id: requestId,
+      result: {
+        stopReason: "end_turn",
+      },
+    });
     process.exit(0);
   }
 
-  process.stdout.write(JSON.stringify({
-    eventVersion: 1,
-    acpxSessionId,
-    requestId: "req-1",
-    seq: 0,
-    stream: "prompt",
-    type: "thought",
-    content: "thinking",
-  }) + "\n");
-  process.stdout.write(JSON.stringify({
-    eventVersion: 1,
-    acpxSessionId,
-    requestId: "req-1",
-    seq: 1,
-    stream: "prompt",
-    type: "tool_call",
+  if (stdinText.includes("double-done")) {
+    emitUpdate(sessionFromOption, {
+      sessionUpdate: "agent_message_chunk",
+      content: { type: "text", text: "ok" },
+    });
+    emitJson({
+      jsonrpc: "2.0",
+      id: requestId,
+      result: {
+        stopReason: "end_turn",
+      },
+    });
+    emitJson({
+      jsonrpc: "2.0",
+      id: requestId,
+      result: {
+        stopReason: "end_turn",
+      },
+    });
+    process.exit(0);
+  }
+
+  emitUpdate(sessionFromOption, {
+    sessionUpdate: "agent_thought_chunk",
+    content: { type: "text", text: "thinking" },
+  });
+  emitUpdate(sessionFromOption, {
+    sessionUpdate: "tool_call",
+    toolCallId: "tool-1",
     title: "run-tests",
     status: "in_progress",
-  }) + "\n");
-  process.stdout.write(JSON.stringify({
-    eventVersion: 1,
-    acpxSessionId,
-    requestId: "req-1",
-    seq: 2,
-    stream: "prompt",
-    type: "text",
-    content: "echo:" + stdinText.trim(),
-  }) + "\n");
-  process.stdout.write(JSON.stringify({
-    eventVersion: 1,
-    acpxSessionId,
-    requestId: "req-1",
-    seq: 3,
-    stream: "prompt",
-    type: "done",
-    stopReason: "end_turn",
-  }) + "\n");
+    kind: "command",
+  });
+  emitUpdate(sessionFromOption, {
+    sessionUpdate: "agent_message_chunk",
+    content: { type: "text", text: "echo:" + stdinText.trim() },
+  });
+  emitJson({
+    jsonrpc: "2.0",
+    id: requestId,
+    result: {
+      stopReason: "end_turn",
+    },
+  });
   process.exit(0);
 }
 
 writeLog({ kind: "unknown", args });
-process.stdout.write(JSON.stringify({
-  eventVersion: 1,
-  acpxSessionId: "unknown",
-  seq: 0,
-  stream: "control",
-  type: "error",
-  code: "USAGE",
-  message: "unknown command",
-}) + "\n");
+emitJson({
+  error: {
+    code: "USAGE",
+    message: "unknown command",
+  },
+});
 process.exit(2);
 `;
 
@@ -444,6 +470,28 @@ describe("AcpxRuntime", () => {
     expect(textDeltas.join("")).toBe("alpha beta gamma");
   });
 
+  it("emits done once when ACP stream repeats stop reason responses", async () => {
+    const { runtime } = await createMockRuntime();
+    const handle = await runtime.ensureSession({
+      sessionKey: "agent:codex:acp:double-done",
+      agent: "codex",
+      mode: "persistent",
+    });
+
+    const events = [];
+    for await (const event of runtime.runTurn({
+      handle,
+      text: "double-done",
+      mode: "prompt",
+      requestId: "req-double-done",
+    })) {
+      events.push(event);
+    }
+
+    const doneCount = events.filter((event) => event.type === "done").length;
+    expect(doneCount).toBe(1);
+  });
+
   it("maps acpx error events into ACP runtime error events", async () => {
     const { runtime } = await createMockRuntime();
     const handle = await runtime.ensureSession({
@@ -465,7 +513,7 @@ describe("AcpxRuntime", () => {
     expect(events).toContainEqual({
       type: "error",
       message: "mock failure",
-      code: "RUNTIME",
+      code: "-32000",
       retryable: undefined,
     });
   });
diff --git a/extensions/acpx/src/runtime.ts b/extensions/acpx/src/runtime.ts
index 1256c8903c3..c0d2cca94bc 100644
--- a/extensions/acpx/src/runtime.ts
+++ b/extensions/acpx/src/runtime.ts
@@ -197,6 +197,9 @@ export class AcpxRuntime implements AcpRuntime {
       sessionName: state.name,
       cwd: state.cwd,
     });
+    const parseContext = {
+      promptRequestIds: new Set<string>(),
+    };
 
     const cancelOnAbort = async () => {
       await this.cancel({
@@ -238,11 +241,14 @@ export class AcpxRuntime implements AcpRuntime {
     const lines = createInterface({ input: child.stdout });
     try {
       for await (const line of lines) {
-        const parsed = parsePromptEventLine(line);
+        const parsed = parsePromptEventLine(line, parseContext);
         if (!parsed) {
           continue;
         }
         if (parsed.type === "done") {
+          if (sawDone) {
+            continue;
+          }
           sawDone = true;
         }
         if (parsed.type === "error") {