diff --git a/harness/bridge/README.md b/harness/bridge/README.md
index 05aa7c6..0edb208 100644
--- a/harness/bridge/README.md
+++ b/harness/bridge/README.md
@@ -64,6 +64,20 @@ carries JSON between the two languages is **not here** — it is orchestration (
 `observe()` against the canonical schema — proving the contract holds with **no
 game attached**. That is the T1 acceptance bar.
 
+### Reference implementations
+
+| Impl | What | For |
+|---|---|---|
+| `NullBridge` | connects to no world; empty `Perception`, no-op `act()` | the contract smoke test (T1) |
+| `SyntheticBridge` | a **deterministic in-memory toy world** (places, energy, gathering); not a game | developing the loop + drives (T3/T4/T5) before a real adapter exists, and CI |
+
+`SyntheticBridge` has no wall clock and no RNG (`observedAt` is a logical tick), so
+a given action sequence always yields the same Perceptions. It also exposes
+`operatorAct()` — a second actor mutating the *same* world — to exercise
+**dual-control** deterministically (the Stardew P1 model: two actors, one world;
+see `decisions/adr-0007-substrate-ladder.md`). Real game adapters still live in
+`../adapters/`; `SyntheticBridge` is a fixture, not an adapter.
+
 ## Out of scope — by design
 
 | Not here | Where |
diff --git a/harness/bridge/index.ts b/harness/bridge/index.ts
index 902a719..a723e43 100644
--- a/harness/bridge/index.ts
+++ b/harness/bridge/index.ts
@@ -4,3 +4,5 @@ export type { Action, ActionResult, ActionStatus } from "./action";
 export type { Bridge } from "./bridge";
 export { NullBridge } from "./null-bridge";
 export type { NullState } from "./null-bridge";
+export { SyntheticBridge } from "./synthetic-bridge";
+export type { SyntheticState } from "./synthetic-bridge";
diff --git a/harness/bridge/synthetic-bridge.test.ts b/harness/bridge/synthetic-bridge.test.ts
new file mode 100644
index 0000000..ea3398c
--- /dev/null
+++ b/harness/bridge/synthetic-bridge.test.ts
@@ -0,0 +1,96 @@
+// Tests for SyntheticBridge — the deterministic toy-world fixture.
+// Covers every affordance branch, the rejection paths, schema conformance, and
+// dual-control (operatorAct mutating the shared world the agent then perceives).
+
+import { test } from "node:test";
+import assert from "node:assert/strict";
+import { readFileSync } from "node:fs";
+import { fileURLToPath } from "node:url";
+import { dirname, join } from "node:path";
+import Ajv2020 from "ajv/dist/2020.js";
+import { SyntheticBridge } from "./synthetic-bridge";
+
+const here = dirname(fileURLToPath(import.meta.url));
+const schema = (name: string): object =>
+  JSON.parse(readFileSync(join(here, "schema", name), "utf8")) as object;
+
+const ajv = new Ajv2020({ strict: true, allErrors: true });
+const validatePerception = ajv.compile(schema("perception.schema.json"));
+const validateActionResult = ajv.compile(schema("action-result.schema.json"));
+
+test("observe() conforms to the canonical schema and is deterministic", async () => {
+  const world = new SyntheticBridge();
+  const p = await world.observe();
+  assert.ok(validatePerception(p), ajv.errorsText(validatePerception.errors));
+  assert.equal(p.substrate, "synthetic");
+  assert.equal(p.observedAt, 0); // logical clock starts at 0
+  assert.equal(p.affordances.length, 4);
+  assert.deepEqual(p.state, { tick: 0, place: "meadow", energy: 100, gathered: 0 });
+});
+
+test("each affordance mutates state as expected; results validate", async () => {
+  const world = new SyntheticBridge();
+
+  const move = await world.act({ affordance: "move", reason: "what is over there?" });
+  assert.ok(validateActionResult(move), ajv.errorsText(validateActionResult.errors));
+  assert.equal(move.status, "ok");
+  let s = (await world.observe()).state;
+  assert.equal(s.place, "stream");
+  assert.equal(s.energy, 95);
+  assert.equal(s.tick, 1);
+
+  await world.act({ affordance: "gather" });
+  s = (await world.observe()).state;
+  assert.equal(s.gathered, 1);
+  assert.equal(s.energy, 85);
+  assert.equal(s.tick, 2);
+
+  await world.act({ affordance: "rest" });
+  s = (await world.observe()).state;
+  assert.equal(s.energy, 100); // capped at 100
+  assert.equal(s.tick, 3);
+
+  await world.act({ affordance: "look" });
+  s = (await world.observe()).state;
+  assert.equal(s.tick, 4); // look advances only the clock
+  assert.equal(s.energy, 100);
+});
+
+test("move wraps around the ring of places", async () => {
+  const world = new SyntheticBridge();
+  for (let i = 0; i < 4; i++) await world.act({ affordance: "move" });
+  assert.equal((await world.observe()).state.place, "meadow");
+});
+
+test("gather is rejected when energy is exhausted, leaving state unchanged", async () => {
+  const world = new SyntheticBridge({ energy: 0 });
+  const r = await world.act({ affordance: "gather" });
+  assert.equal(r.status, "rejected");
+  assert.match(r.detail ?? "", /tired/);
+  const s = (await world.observe()).state;
+  assert.equal(s.gathered, 0);
+  assert.equal(s.tick, 0);
+});
+
+test("an unknown affordance is rejected", async () => {
+  const world = new SyntheticBridge();
+  const r = await world.act({ affordance: "teleport" });
+  assert.equal(r.status, "rejected");
+  assert.match(r.detail ?? "", /unknown affordance: teleport/);
+});
+
+test("operatorAct mutates the shared world — dual-control (agent perceives a change it didn't cause)", async () => {
+  const world = new SyntheticBridge();
+  const before = (await world.observe()).state;
+
+  const r = world.operatorAct("gather");
+  assert.equal(r.status, "ok");
+
+  const after = (await world.observe()).state;
+  assert.equal(after.gathered, before.gathered + 1);
+  assert.equal(after.tick, before.tick + 1);
+});
+
+test("close() resolves", async () => {
+  await assert.doesNotReject(() => new SyntheticBridge().close());
+});
diff --git a/harness/bridge/synthetic-bridge.ts b/harness/bridge/synthetic-bridge.ts
new file mode 100644
index 0000000..f6f29a4
--- /dev/null
+++ b/harness/bridge/synthetic-bridge.ts
@@ -0,0 +1,100 @@
+// SyntheticBridge — a deterministic in-memory toy world. NOT a game: a test
+// fixture / synthetic substrate for developing the loop (T3/T4/T5) and the drive
+// layer before a real adapter exists, and for deterministically exercising
+// dual-control (the operator acting on the shared world — the Stardew P1 model).
+//
+// Determinism: there is no wall clock and no RNG. `observedAt` is a logical tick,
+// so a given sequence of actions always yields the same Perceptions — which is
+// what makes it usable in CI and in assertions.
+
+import type { Bridge } from "./bridge";
+import type { Perception, ActionSpec } from "./perception";
+import type { Action, ActionResult } from "./action";
+
+/** The toy world's state. JSON-serializable, like any substrate's state. */
+export interface SyntheticState {
+  /** logical clock — advances by one per applied action */
+  tick: number;
+  /** current location (one of PLACES) */
+  place: string;
+  /** 0..100; falls on move/gather, rises on rest */
+  energy: number;
+  /** resources collected so far */
+  gathered: number;
+}
+
+const PLACES = ["meadow", "stream", "grove", "ridge"] as const;
+
+const AFFORDANCES: ActionSpec[] = [
+  { id: "move", label: "move to the next place" },
+  { id: "gather", label: "gather here" },
+  { id: "rest", label: "rest" },
+  { id: "look", label: "look around" },
+];
+
+export class SyntheticBridge implements Bridge<SyntheticState> {
+  private state: SyntheticState;
+
+  constructor(init?: Partial<SyntheticState>) {
+    this.state = { tick: 0, place: PLACES[0], energy: 100, gathered: 0, ...init };
+  }
+
+  async observe(): Promise<Perception<SyntheticState>> {
+    return {
+      substrate: "synthetic",
+      observedAt: this.state.tick,
+      state: { ...this.state },
+      affordances: AFFORDANCES.map((a) => ({ ...a })),
+    };
+  }
+
+  /** The agent acts. */
+  async act(action: Action): Promise<ActionResult> {
+    return this.applyAffordance(action.affordance);
+  }
+
+  /**
+   * Dual-control: the operator acts on the *same* world (the co-op model — two
+   * actors, one world). The agent perceives the change on its next observe(),
+   * having not caused it. Synchronous because the fixture has no transport.
+   */
+  operatorAct(affordance: string): ActionResult {
+    return this.applyAffordance(affordance);
+  }
+
+  async close(): Promise<void> {
+    // nothing to release
+  }
+
+  private applyAffordance(id: string): ActionResult {
+    switch (id) {
+      case "move": {
+        const idx = Math.max(0, PLACES.indexOf(this.state.place as (typeof PLACES)[number]));
+        this.state.place = PLACES[(idx + 1) % PLACES.length]!;
+        this.state.energy = Math.max(0, this.state.energy - 5);
+        this.state.tick++;
+        return { status: "ok" };
+      }
+      case "gather": {
+        if (this.state.energy <= 0) {
+          return { status: "rejected", detail: "too tired to gather" };
+        }
+        this.state.gathered++;
+        this.state.energy = Math.max(0, this.state.energy - 10);
+        this.state.tick++;
+        return { status: "ok" };
+      }
+      case "rest": {
+        this.state.energy = Math.min(100, this.state.energy + 20);
+        this.state.tick++;
+        return { status: "ok" };
+      }
+      case "look": {
+        this.state.tick++;
+        return { status: "ok" };
+      }
+      default:
+        return { status: "rejected", detail: `unknown affordance: ${id}` };
+    }
+  }
+}