diff --git a/package.json b/package.json
index dd52be1..508ef28 100644
--- a/package.json
+++ b/package.json
@@ -15,10 +15,12 @@
     "release": "bun run build && bun test && node scripts/publish-all.mjs"
   },
   "devDependencies": {
+    "@agentclientprotocol/sdk": "^0.22.1",
     "@biomejs/biome": "^2.4.14",
     "@changesets/cli": "^2.31.0",
     "@types/node": "^25.7.0",
     "@types/xxhashjs": "^0.2.4",
+    "@uncaged/workflow-agent-hermes": "workspace:*",
     "bun-types": "^1.3.13"
   }
 }
diff --git a/packages/workflow-agent-hermes/__tests__/acp-client.test.ts b/packages/workflow-agent-hermes/__tests__/acp-client.test.ts
new file mode 100644
index 0000000..54db937
--- /dev/null
+++ b/packages/workflow-agent-hermes/__tests__/acp-client.test.ts
@@ -0,0 +1,77 @@
+import { afterEach, beforeEach, describe, expect, it } from "bun:test";
+
+import { HermesAcpClient } from "../src/acp-client.js";
+
+const UUID_RE = /^[0-9a-f]{8}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{12}$/i;
+
+describe("HermesAcpClient", () => {
+  let client: HermesAcpClient;
+
+  beforeEach(() => {
+    client = new HermesAcpClient();
+  });
+
+  afterEach(async () => {
+    await client.close();
+  });
+
+  it(
+    "connect() returns a UUID sessionId",
+    async () => {
+      const sessionId = await client.connect(process.cwd());
+      expect(typeof sessionId).toBe("string");
+      expect(sessionId).toMatch(UUID_RE);
+    },
+    { timeout: 2 * 60 * 1000 },
+  );
+
+  it(
+    "prompt() returns a non-empty text response",
+    async () => {
+      await client.connect(process.cwd());
+      const result = await client.prompt("Reply with exactly the word: PONG");
+      expect(typeof result.text).toBe("string");
+      expect(result.text.length).toBeGreaterThan(0);
+      expect(typeof result.sessionId).toBe("string");
+      expect(result.sessionId).toMatch(UUID_RE);
+    },
+    { timeout: 2 * 60 * 1000 },
+  );
+
+  it(
+    "prompt() can be called twice on the same session (resume)",
+    async () => {
+      await client.connect(process.cwd());
+
+      const first = await client.prompt("Say the word ALPHA and nothing else.");
+      expect(first.text.length).toBeGreaterThan(0);
+
+      const second = await client.prompt("Now say the word BETA and nothing else.");
+      expect(second.text.length).toBeGreaterThan(0);
+
+      expect(first.sessionId).toBe(second.sessionId);
+    },
+    { timeout: 2 * 60 * 1000 },
+  );
+
+  it(
+    "prompt() collects structured messages including tool calls",
+    async () => {
+      await client.connect(process.cwd());
+      const result = await client.prompt("Run this command: echo TOOL_DETAIL_TEST");
+      expect(result.messages.length).toBeGreaterThan(0);
+      // Should have at least one tool message (the echo command)
+      const toolMessages = result.messages.filter((m) => m.role === "tool");
+      expect(toolMessages.length).toBeGreaterThan(0);
+      // Tool message should contain the output
+      const toolContent = toolMessages[0]?.content ?? "";
+      expect(toolContent).toContain("TOOL_DETAIL_TEST");
+      // Should have assistant messages with tool_calls
+      const assistantWithTools = result.messages.filter(
+        (m) => m.role === "assistant" && m.tool_calls !== null,
+      );
+      expect(assistantWithTools.length).toBeGreaterThan(0);
+    },
+    { timeout: 2 * 60 * 1000 },
+  );
+});
diff --git a/packages/workflow-agent-hermes/package.json b/packages/workflow-agent-hermes/package.json
index 62f3bd6..c98d03e 100644
--- a/packages/workflow-agent-hermes/package.json
+++ b/packages/workflow-agent-hermes/package.json
@@ -21,6 +21,7 @@
     "test": "bun test"
   },
   "dependencies": {
+    "@agentclientprotocol/sdk": "^0.22.1",
     "@uncaged/json-cas": "^0.4.0",
     "@uncaged/workflow-agent-kit": "workspace:^"
   },
diff --git a/packages/workflow-agent-hermes/src/acp-client.ts b/packages/workflow-agent-hermes/src/acp-client.ts
new file mode 100644
index 0000000..74d55f1
--- /dev/null
+++ b/packages/workflow-agent-hermes/src/acp-client.ts
@@ -0,0 +1,233 @@
+import type { ChildProcess } from "node:child_process";
+import { spawn } from "node:child_process";
+import { Readable, Writable } from "node:stream";
+import type {
+  Client,
+  RequestPermissionRequest,
+  RequestPermissionResponse,
+  SessionNotification,
+} from "@agentclientprotocol/sdk";
+import { ClientSideConnection, ndJsonStream, PROTOCOL_VERSION } from "@agentclientprotocol/sdk";
+
+import type { HermesSessionMessage } from "./types.js";
+
+const HERMES_COMMAND = "hermes";
+
+/** Tracks in-flight tool calls so we can build complete messages when they finish. */
+type PendingToolCall = {
+  name: string;
+  args: string;
+};
+
+/**
+ * Collects ACP session/update events into a list of {@link HermesSessionMessage}
+ * that mirrors what Hermes writes to its session JSONL files.
+ */
+class UwfAcpClient implements Client {
+  private messageChunks: string[] = [];
+  private reasoningChunks: string[] = [];
+  private pendingTools = new Map<string, PendingToolCall>();
+  messages: HermesSessionMessage[] = [];
+
+  resetPerPrompt(): void {
+    this.messageChunks = [];
+    this.reasoningChunks = [];
+  }
+
+  async sessionUpdate(params: SessionNotification): Promise<void> {
+    const { update } = params;
+    switch (update.sessionUpdate) {
+      case "agent_message_chunk":
+        if (update.content.type === "text") {
+          this.messageChunks.push(update.content.text);
+        }
+        break;
+
+      case "agent_thought_chunk":
+        if (update.content.type === "text") {
+          this.reasoningChunks.push(update.content.text);
+        }
+        break;
+
+      case "tool_call": {
+        // Agent is invoking a tool — record the call.
+        const title = update.title ?? "";
+        const rawInput =
+          update.rawInput !== undefined && update.rawInput !== null
+            ? JSON.stringify(update.rawInput)
+            : "";
+        this.pendingTools.set(update.toolCallId, { name: title, args: rawInput });
+
+        // Flush accumulated assistant text + reasoning as an assistant message
+        // (the agent "spoke" before calling the tool).
+        this.flushAssistantMessage();
+        break;
+      }
+
+      case "tool_call_update": {
+        if (update.status === "completed" || update.status === "failed") {
+          const pending = this.pendingTools.get(update.toolCallId);
+          const toolName = pending?.name ?? update.toolCallId;
+          const rawOutput =
+            update.rawOutput !== undefined && update.rawOutput !== null
+              ? typeof update.rawOutput === "string"
+                ? update.rawOutput
+                : JSON.stringify(update.rawOutput)
+              : "";
+          this.messages.push({
+            role: "assistant",
+            content: null,
+            reasoning: null,
+            tool_calls: [{ function: { name: toolName, arguments: pending?.args ?? "" } }],
+          });
+          this.messages.push({
+            role: "tool",
+            content: rawOutput,
+            reasoning: null,
+            tool_calls: null,
+          });
+          this.pendingTools.delete(update.toolCallId);
+        }
+        break;
+      }
+
+      default:
+        break;
+    }
+  }
+
+  /** Flush any accumulated text/reasoning into an assistant message. */
+  flushAssistantMessage(): void {
+    const text = this.messageChunks.join("");
+    const reasoning = this.reasoningChunks.join("");
+    if (text !== "" || reasoning !== "") {
+      this.messages.push({
+        role: "assistant",
+        content: text || null,
+        reasoning: reasoning || null,
+        tool_calls: null,
+      });
+    }
+    this.messageChunks = [];
+    this.reasoningChunks = [];
+  }
+
+  async requestPermission(params: RequestPermissionRequest): Promise<RequestPermissionResponse> {
+    const firstOption = params.options[0];
+    return {
+      outcome: {
+        outcome: "selected",
+        optionId: firstOption?.optionId ?? "",
+      },
+    };
+  }
+}
+
+export type AcpPromptResult = {
+  text: string;
+  sessionId: string;
+  messages: HermesSessionMessage[];
+};
+
+export class HermesAcpClient {
+  private process: ChildProcess | null = null;
+  private connection: ClientSideConnection | null = null;
+  private sessionId: string | null = null;
+  private stderrBuffer = "";
+  private client = new UwfAcpClient();
+
+  /** Spawn hermes acp, initialize, create session */
+  async connect(cwd: string): Promise<string> {
+    const child = spawn(HERMES_COMMAND, ["acp"], {
+      env: process.env,
+      shell: false,
+      stdio: ["pipe", "pipe", "pipe"],
+    });
+
+    this.process = child;
+
+    child.stderr?.on("data", (chunk: Buffer) => {
+      this.stderrBuffer += chunk.toString();
+    });
+
+    if (child.stdin === null || child.stdout === null) {
+      throw new Error("hermes acp process stdio is not available");
+    }
+
+    const input = Writable.toWeb(child.stdin);
+    const output = Readable.toWeb(child.stdout);
+    const stream = ndJsonStream(input, output);
+
+    const clientRef = this.client;
+    const connection = new ClientSideConnection((_agent) => clientRef, stream);
+    this.connection = connection;
+
+    connection.signal.addEventListener("abort", () => {
+      if (this.sessionId !== null) {
+        const detail = this.stderrBuffer.trim() !== "" ? ` stderr=${this.stderrBuffer.trim()}` : "";
+        throw new Error(`hermes acp connection closed unexpectedly${detail}`);
+      }
+    });
+
+    await connection.initialize({
+      protocolVersion: PROTOCOL_VERSION,
+      clientCapabilities: {},
+    });
+
+    const sessionResult = await connection.newSession({ cwd, mcpServers: [] });
+    const { sessionId } = sessionResult;
+
+    this.sessionId = sessionId;
+    return sessionId;
+  }
+
+  /** Send prompt and collect full response text + structured messages. */
+  async prompt(text: string): Promise<AcpPromptResult> {
+    if (this.connection === null || this.sessionId === null) {
+      throw new Error("Not connected — call connect() first");
+    }
+
+    this.client.resetPerPrompt();
+
+    await this.connection.prompt({
+      sessionId: this.sessionId,
+      prompt: [{ type: "text", text }],
+    });
+
+    // Flush any trailing assistant text that wasn't followed by a tool call.
+    this.client.flushAssistantMessage();
+
+    // Extract the final assistant text from collected messages.
+    const messages = this.client.messages;
+    let finalText = "";
+    for (let i = messages.length - 1; i >= 0; i--) {
+      const msg = messages[i];
+      if (msg !== undefined && msg.role === "assistant" && msg.content !== null && msg.content.trim() !== "") {
+        finalText = msg.content;
+        break;
+      }
+    }
+
+    return {
+      text: finalText,
+      sessionId: this.sessionId,
+      messages,
+    };
+  }
+
+  /** Close the connection */
+  async close(): Promise<void> {
+    if (this.process === null) {
+      return;
+    }
+    this.sessionId = null;
+    this.process.stdin?.end();
+    const proc = this.process;
+    await new Promise<void>((resolve) => {
+      proc.on("close", () => resolve());
+      setTimeout(resolve, 5000);
+    });
+    this.process = null;
+    this.connection = null;
+  }
+}
diff --git a/packages/workflow-agent-hermes/src/hermes.ts b/packages/workflow-agent-hermes/src/hermes.ts
index 5264934..6d3066f 100644
--- a/packages/workflow-agent-hermes/src/hermes.ts
+++ b/packages/workflow-agent-hermes/src/hermes.ts
@@ -1,4 +1,3 @@
-import { spawn } from "node:child_process";
 import type { Store } from "@uncaged/json-cas";
 
 import {
@@ -8,14 +7,8 @@ import {
   createAgent,
 } from "@uncaged/workflow-agent-kit";
 
-import {
-  loadHermesSession,
-  parseSessionIdFromStdout,
-  storeHermesSessionDetail,
-} from "./session-detail.js";
-
-const HERMES_COMMAND = "hermes";
-const HERMES_MAX_TURNS = 90;
+import { HermesAcpClient } from "./acp-client.js";
+import { storeHermesSessionDetail } from "./session-detail.js";
 
 function buildHistorySummary(steps: AgentContext["steps"]): string {
   if (steps.length === 0) {
@@ -52,110 +45,43 @@ export function buildHermesPrompt(ctx: AgentContext): string {
   return parts.join("\n");
 }
 
-function spawnHermes(args: string[]): Promise<{ stdout: string; stderr: string }> {
-  return new Promise((resolve, reject) => {
-    const child = spawn(HERMES_COMMAND, args, {
-      env: process.env,
-      shell: false,
-      stdio: ["ignore", "pipe", "pipe"],
-    });
-
-    let stdout = "";
-    let stderr = "";
-    child.stdout?.on("data", (chunk: Buffer) => {
-      stdout += chunk.toString();
-    });
-    child.stderr?.on("data", (chunk: Buffer) => {
-      stderr += chunk.toString();
-    });
-
-    child.on("error", (cause) => {
-      const message = cause instanceof Error ? cause.message : String(cause);
-      reject(new Error(`hermes spawn failed: ${message}`));
-    });
-
-    child.on("close", (code) => {
-      if (code === 0) {
-        resolve({ stdout, stderr });
-        return;
-      }
-      const detail = stderr.trim() !== "" ? ` stderr=${stderr.trim()}` : "";
-      reject(new Error(`hermes exited with code ${code ?? "null"}${detail}`));
-    });
-  });
-}
-
-function spawnHermesChat(prompt: string): Promise<{ stdout: string; stderr: string }> {
-  return spawnHermes([
-    "chat",
-    "-q",
-    prompt,
-    "--yolo",
-    "--max-turns",
-    String(HERMES_MAX_TURNS),
-    "--quiet",
-  ]);
-}
-
-function spawnHermesResume(
-  sessionId: string,
-  message: string,
-): Promise<{ stdout: string; stderr: string }> {
-  return spawnHermes([
-    "chat",
-    "--resume",
-    sessionId,
-    "-q",
-    message,
-    "--yolo",
-    "--max-turns",
-    String(HERMES_MAX_TURNS),
-    "--quiet",
-  ]);
-}
-
-function parseSessionId(stdout: string, stderr: string): string {
-  const sessionId = parseSessionIdFromStdout(stderr) ?? parseSessionIdFromStdout(stdout);
-  if (sessionId === null) {
-    throw new Error(
-      "Failed to parse session_id from hermes output.\n" +
-        `stderr (first 200 chars): ${stderr.slice(0, 200)}\n` +
-        `stdout (first 200 chars): ${stdout.slice(0, 200)}`,
-    );
-  }
-  return sessionId;
-}
-
-async function buildResultFromSession(sessionId: string, store: Store): Promise<AgentRunResult> {
-  const session = await loadHermesSession(sessionId);
-  if (session === null) {
-    throw new Error(`Failed to load hermes session file for session_id: ${sessionId}`);
-  }
-  const { detailHash, output } = await storeHermesSessionDetail(store, session);
-  return { output, detailHash, sessionId };
-}
-
-async function runHermes(ctx: AgentContext): Promise<AgentRunResult> {
-  const fullPrompt = buildHermesPrompt(ctx);
-  const { stdout, stderr } = await spawnHermesChat(fullPrompt);
-  const sessionId = parseSessionId(stdout, stderr);
-  return buildResultFromSession(sessionId, ctx.store);
-}
-
-async function continueHermes(
-  sessionId: string,
-  message: string,
-  store: Store,
-): Promise<AgentRunResult> {
-  const { stdout, stderr } = await spawnHermesResume(sessionId, message);
-  // Resume may return a new session_id
-  const newSessionId = parseSessionIdFromStdout(stderr) ?? parseSessionIdFromStdout(stdout);
-  const resolvedId = newSessionId ?? sessionId;
-  return buildResultFromSession(resolvedId, store);
-}
-
-/** Agent CLI factory: parses argv, runs Hermes, extracts output, writes StepNode. */
+/**
+ * Agent CLI factory: parses argv, runs Hermes, extracts output, writes StepNode.
+ *
+ * A single ACP client is shared across run() and continue() calls so that
+ * frontmatter retry loops keep the same Hermes session context.  The client
+ * is closed once the agent process exits (via process.on("exit")).
+ */
 export function createHermesAgent(): () => Promise<void> {
+  const client = new HermesAcpClient();
+
+  // Ensure cleanup regardless of how the process exits.
+  process.on("exit", () => {
+    void client.close();
+  });
+
+  async function runHermes(ctx: AgentContext): Promise<AgentRunResult> {
+    const fullPrompt = buildHermesPrompt(ctx);
+    await client.connect(process.cwd());
+    const { text, sessionId, messages } = await client.prompt(fullPrompt);
+    const session = { session_id: sessionId, model: "", session_start: new Date().toISOString(), messages };
+    const { detailHash } = await storeHermesSessionDetail(ctx.store, session);
+    return { output: text, detailHash, sessionId };
+  }
+
+  async function continueHermes(
+    _sessionId: string,
+    message: string,
+    store: Store,
+  ): Promise<AgentRunResult> {
+    // Client is already connected from runHermes — same ACP session,
+    // so the agent sees the full conversation history (crucial for retries).
+    const { text, sessionId, messages } = await client.prompt(message);
+    const session = { session_id: sessionId, model: "", session_start: new Date().toISOString(), messages };
+    const { detailHash } = await storeHermesSessionDetail(store, session);
+    return { output: text, detailHash, sessionId };
+  }
+
   return createAgent({
     name: "hermes",
     run: runHermes,
diff --git a/packages/workflow-agent-hermes/src/index.ts b/packages/workflow-agent-hermes/src/index.ts
index b27f12a..cf147d7 100644
--- a/packages/workflow-agent-hermes/src/index.ts
+++ b/packages/workflow-agent-hermes/src/index.ts
@@ -1 +1,2 @@
+export { HermesAcpClient } from "./acp-client.js";
 export { buildHermesPrompt, createHermesAgent } from "./hermes.js";