diff --git a/harness/tests/fake_pi/fake_pi_rpc.mjs b/harness/tests/fake_pi/fake_pi_rpc.mjs
new file mode 100644
index 00000000..43b8d21e
--- /dev/null
+++ b/harness/tests/fake_pi/fake_pi_rpc.mjs
@@ -0,0 +1,33 @@
+// harness/tests/fake_pi/fake_pi_rpc.mjs — scripted RPC test double (LF-only JSONL).
+import fs from "node:fs";
+const script = JSON.parse(fs.readFileSync(process.argv[2], "utf8"));
+const out = (evt) => process.stdout.write(JSON.stringify(evt) + "\n");
+
+let buf = "";
+process.stdin.on("data", (chunk) => {
+  buf += chunk.toString("utf8");
+  for (let nl; (nl = buf.indexOf("\n")) !== -1; ) {
+    const line = buf.slice(0, nl).replace(/\r$/, ""); buf = buf.slice(nl + 1);
+    if (!line) continue;
+    let cmd; try { cmd = JSON.parse(line); } catch { continue; }
+    if (cmd.type === "prompt") {
+      for (const step of script.on_prompt ?? []) {
+        if (step.ui_request_descriptor) {
+          const d = step.ui_request_descriptor;
+          out({ type: "extension_ui_request", id: d.id, method: "input", title: JSON.stringify(d) });
+        } else { out(step); }  // eventi non-UI (text_delta, agent_end, …) passano tali e quali
+      }
+    } else if (cmd.type === "extension_ui_response") {
+      for (const evt of (script.on_response ?? {})[cmd.id] ?? []) out(evt);
+    } else if (cmd.type === "get_available_models") {
+      out({ type: "response", command: "get_available_models", id: cmd.id, success: true,
+            data: { models: script.available_models ?? [] } });
+    } else if (cmd.type === "steer") {
+      out({ type: "response", command: "steer", id: cmd.id, success: true });
+    } else if (cmd.type === "get_state") {
+      out({ type: "response", command: "get_state", id: cmd.id, success: true,
+            data: { sessionId: "fake", thinkingLevel: "medium", isStreaming: false } });
+    }
+  }
+});
+process.stdin.on("end", () => process.exit(0));
diff --git a/harness/tests/fake_pi/scripts/f1_disambiguation.json b/harness/tests/fake_pi/scripts/f1_disambiguation.json
new file mode 100644
index 00000000..fe27e9cc
--- /dev/null
+++ b/harness/tests/fake_pi/scripts/f1_disambiguation.json
@@ -0,0 +1,11 @@
+{
+  "on_prompt": [
+    { "ui_request_descriptor": { "type": "ui_request", "id": "u1", "schema_version": 1,
+        "phase": "F1_chiarimento", "title": "Disambigua",
+        "widget": "select",
+        "options": [ {"id":"a","label":"interpretazione A"}, {"id":"b","label":"interpretazione B"} ],
+        "reserved": ["back","exit","other"] } }
+  ],
+  "on_response": { "u1": [ { "type": "agent_end" } ] },
+  "available_models": [ {"provider":"zai","id":"glm-5.2"} ]
+}
diff --git a/harness/tests/fake_pi/test_fake_pi_contract.mjs b/harness/tests/fake_pi/test_fake_pi_contract.mjs
new file mode 100644
index 00000000..7a9e59de
--- /dev/null
+++ b/harness/tests/fake_pi/test_fake_pi_contract.mjs
@@ -0,0 +1,38 @@
+// harness/tests/fake_pi/test_fake_pi_contract.mjs  — run: node --test
+import test from "node:test";
+import assert from "node:assert";
+import { spawn } from "node:child_process";
+import path from "node:path";
+
+function drive(scriptPath, commands) {
+  return new Promise((resolve) => {
+    const fp = spawn("node", [path.join(import.meta.dirname, "fake_pi_rpc.mjs"), scriptPath]);
+    const events = []; let buf = "";
+    fp.stdout.on("data", (c) => {
+      buf += c.toString("utf8");
+      for (let nl; (nl = buf.indexOf("\n")) !== -1; ) {
+        const line = buf.slice(0, nl).replace(/\r$/, ""); buf = buf.slice(nl + 1);
+        if (line) events.push(JSON.parse(line));
+      }
+    });
+    fp.on("exit", () => resolve(events));
+    for (const cmd of commands) fp.stdin.write(JSON.stringify(cmd) + "\n");
+    setTimeout(() => fp.stdin.end(), 300);
+  });
+}
+
+test("on prompt emette il widget F1; on response avanza", async () => {
+  const sp = path.join(import.meta.dirname, "scripts/f1_disambiguation.json");
+  const events = await drive(sp, [
+    { type: "prompt", message: "/nuova-domanda \"x\"" },
+    { type: "extension_ui_response", id: "u1",
+      value: JSON.stringify({ id: "u1", choices: ["a"], decision: { type: "concept_clarified" } }) },
+  ]);
+  const widget = events.find((e) => e.type === "extension_ui_request");
+  assert.equal(widget.method, "input");          // shape nativa di Pi
+  assert.equal(widget.id, "u1");
+  const descriptor = JSON.parse(widget.title);    // il descriptor viaggia nel title
+  assert.equal(descriptor.widget, "select");
+  assert.equal(descriptor.id, "u1");
+  assert.ok(events.some((e) => e.type === "agent_end"));
+});