diff --git a/packages/harness/package.json b/packages/harness/package.json
new file mode 100644
index 0000000..4a4407c
--- /dev/null
+++ b/packages/harness/package.json
@@ -0,0 +1,17 @@
+{
+  "name": "harness",
+  "private": true,
+  "type": "module",
+  "scripts": {
+    "typecheck": "tsgo --noEmit",
+    "test": "vitest run"
+  },
+  "dependencies": {
+    "sync": "workspace:*"
+  },
+  "devDependencies": {
+    "@types/node": "^25.5.0",
+    "tsx": "^4.19.3",
+    "vitest": "^4.1.2"
+  }
+}
diff --git a/packages/harness/src/github.test.ts b/packages/harness/src/github.test.ts
new file mode 100644
index 0000000..4300e53
--- /dev/null
+++ b/packages/harness/src/github.test.ts
@@ -0,0 +1,74 @@
+import { mkdtempSync, rmSync } from "node:fs";
+import { tmpdir } from "node:os";
+import { join } from "node:path";
+import { afterEach, expect, it, vi } from "vitest";
+import { createSync } from "sync";
+import { startGitHub } from "./github.js";
+
+afterEach(() => vi.unstubAllEnvs());
+
+it("syncs queue transitions into SQLite, preserves the last snapshot on failure, and survives restart", async () => {
+  const root = mkdtempSync(join(tmpdir(), "ghd-simulator-"));
+  vi.stubEnv("XDG_CACHE_HOME", root);
+  const github = await startGitHub();
+  const options = { loadInstances: () => [github.instance] };
+  let engine = createSync(options);
+  try {
+    for (const [phase, inMergeQueue, ciStatus] of [
+      ["in-flight", false, "success"],
+      ["queued", true, "pending"],
+      ["queue-failed", false, "failure"],
+    ] as const) {
+      github.setPhase(phase);
+      const result = await engine.sync();
+      expect(result.results[0].fetches.map((f) => f.status)).toEqual([
+        "updated",
+        "updated",
+        "updated",
+      ]);
+      const prs = engine.listPullRequests(github.instance.id, "prs");
+      expect(prs).toHaveLength(1);
+      expect(prs[0]).toMatchObject({ number: 101, inMergeQueue, ciStatus });
+    }
+    github.setPhase("offline");
+    const failed = await engine.sync();
+    expect(failed.results[0].fetches.every((f) => f.status === "failed")).toBe(
+      true,
+    );
+    expect(engine.listPullRequests(github.instance.id, "prs")[0].ciStatus).toBe(
+      "failure",
+    );
+    await engine.close();
+    engine = createSync(options);
+    expect(engine.listPullRequests(github.instance.id, "prs")).toHaveLength(1);
+    expect(engine.listPullRequests(github.instance.id, "prs")[0]).toMatchObject(
+      { inMergeQueue: false, ciStatus: "failure" },
+    );
+    github.setPhase("in-flight");
+    await engine.sync();
+    expect(engine.listPullRequests(github.instance.id, "prs")[0].ciStatus).toBe(
+      "success",
+    );
+    expect(github.requests.some((r) => r.path === "/api/graphql")).toBe(true);
+  } finally {
+    await engine.close();
+    await github.close();
+    rmSync(root, { recursive: true, force: true });
+  }
+});
+
+it("rejects unexpected calls instead of pretending they succeeded", async () => {
+  const github = await startGitHub();
+  try {
+    expect((await fetch(`${github.domain}/api/v3/user`)).status).toBe(401);
+    expect(
+      (
+        await fetch(`${github.domain}/api/v3/unknown`, {
+          headers: { authorization: `token ${github.instance.token}` },
+        })
+      ).status,
+    ).toBe(501);
+  } finally {
+    await github.close();
+  }
+});
diff --git a/packages/harness/src/github.ts b/packages/harness/src/github.ts
new file mode 100644
index 0000000..d23b35f
--- /dev/null
+++ b/packages/harness/src/github.ts
@@ -0,0 +1,159 @@
+import { createServer } from "node:http";
+
+export type Phase = "in-flight" | "queued" | "queue-failed" | "offline";
+export const SCENARIO_TITLE = "Ship the status board";
+export const SCENARIO_TOKEN = "scenario-token";
+export const SCENARIO_USER = "scenario-user";
+
+/** A deliberately small GitHub boundary; unsupported requests fail loudly. */
+export async function startGitHub() {
+  let phase: Phase = "in-flight";
+  const requests: { method: string; path: string; phase: Phase }[] = [];
+  const server = createServer(async (req, res) => {
+    const path = req.url ?? "/";
+    requests.push({ method: req.method ?? "GET", path, phase });
+    res.setHeader("content-type", "application/json");
+    const send = (status: number, body: unknown) => {
+      res.writeHead(status).end(JSON.stringify(body));
+    };
+    if (
+      req.headers.authorization !== `token ${SCENARIO_TOKEN}` &&
+      req.headers.authorization !== `bearer ${SCENARIO_TOKEN}`
+    ) {
+      send(401, { message: "Only scenario credentials are accepted" });
+      return;
+    }
+    if (phase === "offline") {
+      send(503, { message: "Scenario: GitHub unavailable" });
+      return;
+    }
+    if (req.method === "GET" && path === "/api/v3/user") {
+      send(200, { login: SCENARIO_USER });
+      return;
+    }
+    if (req.method === "GET" && path.startsWith("/api/v3/notifications?")) {
+      send(200, []);
+      return;
+    }
+    if (req.method === "POST" && path === "/api/graphql") {
+      let raw = "";
+      for await (const chunk of req) raw += chunk;
+      try {
+        const { query, variables } = JSON.parse(raw);
+        if (
+          typeof query !== "string" ||
+          query.includes("mutation") ||
+          !query.includes("fragment PrFields") ||
+          variables.first !== 100 ||
+          ![
+            `author:${SCENARIO_USER} type:pr state:open`,
+            `review-requested:${SCENARIO_USER} type:pr state:open`,
+          ].includes(variables.q)
+        ) {
+          send(501, { message: "Unsupported scenario GraphQL request" });
+          return;
+        }
+        send(200, {
+          data: {
+            search: {
+              nodes: variables.q.startsWith("author:")
+                ? [pullRequest(phase)]
+                : [],
+            },
+            rateLimit: {
+              cost: 1,
+              remaining: 5000,
+              resetAt: "2099-01-01T00:00:00Z",
+            },
+          },
+        });
+      } catch {
+        send(400, { message: "Invalid scenario GraphQL request" });
+      }
+      return;
+    }
+    send(501, {
+      message: `Unsupported scenario request: ${req.method} ${path}`,
+    });
+  });
+  await new Promise<void>((resolve, reject) => {
+    server.once("error", reject);
+    server.listen(0, "127.0.0.1", resolve);
+  });
+  const address = server.address();
+  if (!address || typeof address === "string")
+    throw new Error("Missing simulator address");
+  const domain = `http://127.0.0.1:${address.port}`;
+  return {
+    domain,
+    instance: {
+      id: `127-0-0-1-${address.port}`,
+      label: "Scenario GitHub",
+      baseUrl: `${domain}/api/v3`,
+      token: SCENARIO_TOKEN,
+    },
+    requests,
+    setPhase(next: Phase) {
+      phase = next;
+    },
+    close: () =>
+      new Promise<void>((resolve, reject) => {
+        server.close((error) => (error ? reject(error) : resolve()));
+        server.closeIdleConnections();
+      }),
+  };
+}
+
+function pullRequest(phase: Exclude<Phase, "offline">) {
+  return {
+    id: "PR_scenario",
+    databaseId: 101,
+    number: 101,
+    title: SCENARIO_TITLE,
+    body: "A deterministic PR for testing the status board.",
+    url: "https://github.com/scenario/status-board/pull/101",
+    createdAt: "2026-01-01T12:00:00Z",
+    updatedAt:
+      phase === "in-flight"
+        ? "2026-01-01T12:00:00Z"
+        : phase === "queued"
+          ? "2026-01-01T12:01:00Z"
+          : "2026-01-01T12:02:00Z",
+    isDraft: false,
+    mergeable: "MERGEABLE",
+    mergeStateStatus: "CLEAN",
+    reviewDecision: "APPROVED",
+    additions: 12,
+    deletions: 3,
+    headRefName: "feature/status-board",
+    baseRefName: "main",
+    author: { login: SCENARIO_USER, avatarUrl: "" },
+    repository: {
+      nameWithOwner: "scenario/status-board",
+      autoMergeAllowed: true,
+    },
+    commits: {
+      nodes: [
+        {
+          commit: {
+            statusCheckRollup: {
+              state:
+                phase === "queue-failed"
+                  ? "FAILURE"
+                  : phase === "queued"
+                    ? "PENDING"
+                    : "SUCCESS",
+            },
+          },
+        },
+      ],
+    },
+    reviews: { nodes: [{ state: "APPROVED", author: { login: "reviewer" } }] },
+    reviewThreads: { totalCount: 0, nodes: [] },
+    autoMergeRequest: null,
+    mergeQueueEntry: phase === "queued" ? { id: "queue-entry" } : null,
+    labels: { nodes: [] },
+    comments: { totalCount: 0 },
+    commitsTotal: { totalCount: 2 },
+  };
+}
diff --git a/packages/harness/tsconfig.json b/packages/harness/tsconfig.json
new file mode 100644
index 0000000..d08df5e
--- /dev/null
+++ b/packages/harness/tsconfig.json
@@ -0,0 +1,13 @@
+{
+  "compilerOptions": {
+    "target": "ES2022",
+    "module": "ESNext",
+    "moduleResolution": "bundler",
+    "strict": true,
+    "outDir": "dist",
+    "rootDir": "src",
+    "skipLibCheck": true,
+    "types": ["node"]
+  },
+  "include": ["src"]
+}
diff --git a/pnpm-lock.yaml b/pnpm-lock.yaml
index cb0b6a8..eb7290b 100644
--- a/pnpm-lock.yaml
+++ b/pnpm-lock.yaml
@@ -92,6 +92,22 @@ importers:
         specifier: workspace:*
         version: link:../server
 
+  packages/harness:
+    dependencies:
+      sync:
+        specifier: workspace:*
+        version: link:../sync
+    devDependencies:
+      '@types/node':
+        specifier: ^25.5.0
+        version: 25.5.0
+      tsx:
+        specifier: ^4.19.3
+        version: 4.21.0
+      vitest:
+        specifier: ^4.1.2
+        version: 4.1.2(@types/node@25.5.0)(jsdom@29.0.1(@noble/hashes@1.8.0))(msw@2.12.14(@types/node@25.5.0)(typescript@6.0.2))(vite@8.0.3(@emnapi/core@1.9.1)(@emnapi/runtime@1.9.1)(@types/node@25.5.0)(esbuild@0.27.4)(jiti@2.6.1)(tsx@4.21.0)(yaml@2.8.3))
+
   packages/server:
     dependencies:
       '@hono/node-server':