diff --git a/package.json b/package.json
index 61de316..eab134b 100644
--- a/package.json
+++ b/package.json
@@ -16,7 +16,8 @@
     "test": "pnpm --filter sync build && pnpm --filter cli build && pnpm -r test",
     "prepare": "husky",
     "cli": "node packages/cli/dist/index.js",
-    "dev:cli": "pnpm --filter cli dev"
+    "dev:cli": "pnpm --filter cli dev",
+    "scenario": "pnpm --filter web build && pnpm --filter harness scenario"
   },
   "devDependencies": {
     "@typescript/native-preview": "7.0.0-dev.20260420.1",
diff --git a/packages/harness/README.md b/packages/harness/README.md
new file mode 100644
index 0000000..920fbde
--- /dev/null
+++ b/packages/harness/README.md
@@ -0,0 +1,9 @@
+# Scenario harness
+
+Run `pnpm scenario` from the repo root. It builds the renderer and prints a local board URL. This is the real Hono API and sync engine, backed by a temporary SQLite database and a loopback GitHub simulator. It does not use the demo-data bypass or read your GitHub credentials.
+
+Enter `queued`, then `queue-failed`, to move the fixture PR into and out of the merge queue. `offline` makes GitHub return 503; `in-flight` restores it. Each command requests a fresh sync through the API. The board polls every 10 seconds; reload it for an immediate update. Use `quit` or Ctrl-C to stop and remove temporary files.
+
+The current board changes its queue icon; the dedicated merge-queue section in #84 is a subsequent product change. This harness supplies the repeatable state transitions for developing it.
+
+`pnpm --filter harness test` checks the simulator through the real engine: transitions, outage, recovery, and persisted snapshots across restart. Unsupported GitHub calls fail with 501 so new engine behavior requires an explicit fixture update.
diff --git a/packages/harness/package.json b/packages/harness/package.json
index 4a4407c..86fb4ff 100644
--- a/packages/harness/package.json
+++ b/packages/harness/package.json
@@ -4,7 +4,8 @@
   "type": "module",
   "scripts": {
     "typecheck": "tsgo --noEmit",
-    "test": "vitest run"
+    "test": "vitest run",
+    "scenario": "tsx --conditions=development src/run.ts"
   },
   "dependencies": {
     "sync": "workspace:*"
diff --git a/packages/harness/src/run.ts b/packages/harness/src/run.ts
new file mode 100644
index 0000000..57a4a35
--- /dev/null
+++ b/packages/harness/src/run.ts
@@ -0,0 +1,56 @@
+import { createInterface } from "node:readline/promises";
+import { startScenario } from "./runtime.js";
+import type { Phase } from "./github.js";
+
+const scenario = await startScenario();
+const terminal = createInterface({
+  input: process.stdin,
+  output: process.stdout,
+});
+const stop = async () => {
+  terminal.close();
+  await scenario.close();
+};
+process.once("SIGINT", () => void stop());
+process.once("SIGTERM", () => void stop());
+console.log(`Scenario board: ${scenario.url}`);
+console.log("Commands: in-flight, queued, queue-failed, offline, sync, quit");
+console.log(
+  "Each phase command syncs the real API. The board polls every 10s; reload for an immediate update.",
+);
+try {
+  for await (const line of terminal) {
+    const command = line.trim();
+    if (command === "quit") break;
+    if (["in-flight", "queued", "queue-failed", "offline"].includes(command)) {
+      scenario.setPhase(command as Phase);
+    } else if (command !== "sync") {
+      console.log(
+        "Unknown command. Use in-flight, queued, queue-failed, offline, sync, or quit.",
+      );
+      continue;
+    }
+    try {
+      const prs = await scenario.syncPrs();
+      console.log(
+        JSON.stringify(
+          prs.map(
+            (pr: {
+              number: number;
+              inMergeQueue: boolean;
+              ciStatus: string;
+            }) => ({
+              number: pr.number,
+              queued: pr.inMergeQueue,
+              ci: pr.ciStatus,
+            }),
+          ),
+        ),
+      );
+    } catch (error) {
+      console.log(String(error));
+    }
+  }
+} finally {
+  await stop();
+}
diff --git a/packages/harness/src/runtime.ts b/packages/harness/src/runtime.ts
new file mode 100644
index 0000000..b9f4318
--- /dev/null
+++ b/packages/harness/src/runtime.ts
@@ -0,0 +1,146 @@
+import { spawn, type ChildProcess } from "node:child_process";
+import { mkdtempSync, mkdirSync, rmSync, writeFileSync } from "node:fs";
+import { createServer } from "node:net";
+import { tmpdir } from "node:os";
+import { join, resolve } from "node:path";
+import { setTimeout as delay } from "node:timers/promises";
+import { createSync } from "sync";
+import { startGitHub, SCENARIO_TOKEN, type Phase } from "./github.js";
+
+const repo = resolve(import.meta.dirname, "../../..");
+
+/** Runs the real API + built renderer against an isolated upstream and cache. */
+export async function startScenario() {
+  const root = mkdtempSync(join(tmpdir(), "ghd-scenario-"));
+  let github: Awaited<ReturnType<typeof startGitHub>> | undefined;
+  let child: ChildProcess | undefined;
+  let output = "";
+  let closing: Promise<void> | undefined;
+  const close = () =>
+    (closing ??= (async () => {
+      if (child?.pid && child.exitCode === null && child.signalCode === null) {
+        const exited = new Promise<void>((resolve) =>
+          child!.once("exit", () => resolve()),
+        );
+        child.kill("SIGTERM");
+        const force = setTimeout(() => child?.kill("SIGKILL"), 3000);
+        force.unref();
+        await exited;
+        clearTimeout(force);
+      }
+      await github?.close();
+      rmSync(root, { recursive: true, force: true });
+    })());
+  try {
+    github = await startGitHub();
+    const instanceId = github.instance.id;
+    const configHome = join(root, "config");
+    const cacheHome = join(root, "cache");
+    mkdirSync(join(configHome, "github-dashboard"), { recursive: true });
+    writeFileSync(
+      join(configHome, "github-dashboard/config.yml"),
+      `instances:\n  - domain: ${github.domain}\n    label: Scenario GitHub\n    token: ${SCENARIO_TOKEN}\n`,
+      { mode: 0o600 },
+    );
+    const port = await freePort();
+    const url = `http://127.0.0.1:${port}`;
+    child = spawn(
+      process.execPath,
+      [
+        "--import",
+        import.meta.resolve("tsx"),
+        "--conditions=development",
+        join(repo, "packages/server/src/index.ts"),
+      ],
+      {
+        cwd: repo,
+        env: {
+          ...process.env,
+          PORT: String(port),
+          DEMO: "0",
+          XDG_CONFIG_HOME: configHome,
+          XDG_CACHE_HOME: cacheHome,
+          GHD_DATA_DIR: root,
+          GHD_WEB_DIST: join(repo, "packages/web/dist"),
+        },
+        stdio: ["ignore", "pipe", "pipe"],
+      },
+    );
+    child.stdout?.on("data", (chunk) => {
+      output += chunk.toString();
+    });
+    child.stderr?.on("data", (chunk) => {
+      output += chunk.toString();
+    });
+    let spawnError: Error | undefined;
+    child.on("error", (error) => {
+      spawnError = error;
+    });
+    const deadline = Date.now() + 10_000;
+    while (true) {
+      if (spawnError) throw spawnError;
+      if (child.exitCode !== null)
+        throw new Error(`Scenario server exited:\n${output}`);
+      try {
+        const response = await fetch(`${url}/api/config`, {
+          signal: AbortSignal.timeout(1000),
+        });
+        if (response.ok && (await response.json()).status.kind === "ready")
+          break;
+      } catch {
+        /* Server is still starting. */
+      }
+      if (Date.now() > deadline)
+        throw new Error(`Scenario server did not become ready:\n${output}`);
+      await delay(50);
+    }
+    return {
+      url,
+      instanceId,
+      setPhase(phase: Phase) {
+        github!.setPhase(phase);
+      },
+      async syncPrs() {
+        const response = await fetch(`${url}/api/${instanceId}/prs?fresh=1`);
+        if (!response.ok)
+          throw new Error(`Scenario sync failed (${response.status})`);
+        return response.json();
+      },
+      async persistedPrs() {
+        // Open the actual database through the public engine API. Only its
+        // synchronous constructor needs the scoped environment override.
+        const previous = process.env.XDG_CACHE_HOME;
+        let reader: ReturnType<typeof createSync>;
+        try {
+          process.env.XDG_CACHE_HOME = cacheHome;
+          reader = createSync();
+        } finally {
+          if (previous === undefined) delete process.env.XDG_CACHE_HOME;
+          else process.env.XDG_CACHE_HOME = previous;
+        }
+        try {
+          return reader.listPullRequests(instanceId, "prs");
+        } finally {
+          await reader.close();
+        }
+      },
+      diagnostics: () => ({ server: output, requests: [...github!.requests] }),
+      close,
+    };
+  } catch (error) {
+    await close();
+    throw error;
+  }
+}
+
+async function freePort(): Promise<number> {
+  const server = createServer();
+  await new Promise<void>((resolve, reject) => {
+    server.once("error", reject);
+    server.listen(0, "127.0.0.1", resolve);
+  });
+  const address = server.address();
+  await new Promise<void>((resolve) => server.close(() => resolve()));
+  if (!address || typeof address === "string") throw new Error("Missing port");
+  return address.port;
+}