diff --git a/apps/s03-indexer/src/queryResultShapes.ts b/apps/s03-indexer/src/queryResultShapes.ts
new file mode 100644
index 0000000..51169f1
--- /dev/null
+++ b/apps/s03-indexer/src/queryResultShapes.ts
@@ -0,0 +1,170 @@
+export type QueryEntityName = "markets" | "orders" | "positions";
+
+export type FieldShape =
+  | "string"
+  | "number"
+  | "boolean"
+  | "date"
+  | "nullableString"
+  | "nullableNumber"
+  | "nullableBoolean"
+  | "nullableDate"
+  | NestedShape;
+
+export type NestedShape = {
+  fields: Record<string, FieldShape>;
+};
+
+export const frontendQueryResultShapes: Record<QueryEntityName, NestedShape> = {
+  markets: {
+    fields: {
+      id: "string",
+      key: "string",
+      name: "nullableString",
+      status: "string",
+      createdBy: "nullableString",
+      createdLedger: "number",
+      createdTimestamp: "date",
+      createdTransactionHash: "string",
+      marketToken: tokenSummaryShape(),
+      indexToken: tokenSummaryShape(),
+      longToken: tokenSummaryShape(),
+      shortToken: tokenSummaryShape(),
+    },
+  },
+  orders: {
+    fields: {
+      id: "string",
+      key: "string",
+      account: "string",
+      orderType: "string",
+      status: "string",
+      isLong: "nullableBoolean",
+      sizeDeltaUsd: "nullableString",
+      collateralDeltaAmount: "nullableString",
+      triggerPrice: "nullableString",
+      acceptablePrice: "nullableString",
+      createdTimestamp: "nullableDate",
+      updatedTimestamp: "nullableDate",
+      frozenTimestamp: "nullableDate",
+      frozenTransactionHash: "nullableString",
+      executedTimestamp: "nullableDate",
+      executedTransactionHash: "nullableString",
+      cancelledTimestamp: "nullableDate",
+      cancelledTransactionHash: "nullableString",
+      cancellationReason: "nullableString",
+      market: marketSummaryShape(),
+      collateralToken: tokenSummaryWithDecimalsShape(),
+    },
+  },
+  positions: {
+    fields: {
+      id: "string",
+      key: "string",
+      account: "string",
+      isLong: "boolean",
+      status: "string",
+      sizeUsd: "nullableString",
+      collateralAmount: "nullableString",
+      averagePrice: "nullableString",
+      entryFundingRate: "nullableString",
+      reserveAmount: "nullableString",
+      realizedPnlUsd: "nullableString",
+      realizedPnlAmount: "nullableString",
+      openedLedger: "nullableNumber",
+      openedTimestamp: "nullableDate",
+      updatedTimestamp: "nullableDate",
+      closedTimestamp: "nullableDate",
+      market: {
+        fields: {
+          ...marketSummaryShape().fields,
+          indexToken: tokenSummaryShape(),
+          longToken: tokenSummaryShape(),
+          shortToken: tokenSummaryShape(),
+        },
+      },
+      collateralToken: tokenSummaryWithDecimalsShape(),
+    },
+  },
+};
+
+export function assertFrontendQueryResultShape(
+  entityName: QueryEntityName,
+  node: Record<string, unknown>,
+): void {
+  assertShape(entityName, frontendQueryResultShapes[entityName], node);
+}
+
+function assertShape(path: string, shape: NestedShape, value: unknown): void {
+  if (!isRecord(value)) {
+    throw new Error(`${path} must be an object`);
+  }
+
+  const expectedKeys = Object.keys(shape.fields).sort();
+  const actualKeys = Object.keys(value).sort();
+  if (JSON.stringify(actualKeys) !== JSON.stringify(expectedKeys)) {
+    throw new Error(
+      `${path} fields changed. Expected ${expectedKeys.join(", ")}; received ${actualKeys.join(", ")}`,
+    );
+  }
+
+  for (const [field, fieldShape] of Object.entries(shape.fields)) {
+    assertField(`${path}.${field}`, fieldShape, value[field]);
+  }
+}
+
+function assertField(path: string, shape: FieldShape, value: unknown): void {
+  if (typeof shape === "object") {
+    assertShape(path, shape, value);
+    return;
+  }
+
+  if (shape.startsWith("nullable") && value === null) {
+    return;
+  }
+
+  const expectedType = shape.replace("nullable", "").toLowerCase();
+  if (expectedType === "date") {
+    if (!(value instanceof Date)) {
+      throw new Error(`${path} must be a Date or null`);
+    }
+    return;
+  }
+
+  if (typeof value !== expectedType) {
+    throw new Error(`${path} must be ${shape}`);
+  }
+}
+
+function tokenSummaryShape(): NestedShape {
+  return {
+    fields: {
+      address: "string",
+      symbol: "nullableString",
+    },
+  };
+}
+
+function tokenSummaryWithDecimalsShape(): NestedShape {
+  return {
+    fields: {
+      address: "string",
+      decimals: "nullableNumber",
+      symbol: "nullableString",
+    },
+  };
+}
+
+function marketSummaryShape(): NestedShape {
+  return {
+    fields: {
+      id: "string",
+      key: "string",
+      name: "nullableString",
+    },
+  };
+}
+
+function isRecord(value: unknown): value is Record<string, unknown> {
+  return typeof value === "object" && value !== null && !Array.isArray(value);
+}
diff --git a/apps/s03-indexer/tests/query-result-shapes.test.ts b/apps/s03-indexer/tests/query-result-shapes.test.ts
new file mode 100644
index 0000000..15363df
--- /dev/null
+++ b/apps/s03-indexer/tests/query-result-shapes.test.ts
@@ -0,0 +1,164 @@
+import { describe, expect, test } from "bun:test";
+import { assertFrontendQueryResultShape } from "../src/queryResultShapes";
+
+const account = "GB6YQTHA5COVDNKLV6B4ISXYE6A2ZY5ENBZJTQVI7RI4KOCC6JFZ6C7E";
+const timestamp = new Date("2026-06-24T12:00:00Z");
+
+const fixtures = seedQueryFixtures();
+
+describe("frontend indexer query result shapes", () => {
+  test("markets query keeps field names and types stable", () => {
+    const [market] = queryMarkets(fixtures);
+
+    assertFrontendQueryResultShape("markets", market);
+    expect(Object.keys(market).sort()).toEqual([
+      "createdBy",
+      "createdLedger",
+      "createdTimestamp",
+      "createdTransactionHash",
+      "id",
+      "indexToken",
+      "key",
+      "longToken",
+      "marketToken",
+      "name",
+      "shortToken",
+      "status",
+    ]);
+  });
+
+  test("positions query keeps frontend-consumed field names and types stable", () => {
+    const [position] = queryPositionsByAccount(fixtures, account);
+
+    assertFrontendQueryResultShape("positions", position);
+    expect(position.market).toEqual({
+      id: "market:BTC-USD",
+      key: "BTC-USD",
+      name: "BTC/USD",
+      indexToken: { address: "CBTCINDEX", symbol: "BTC" },
+      longToken: { address: "CLONGTOKEN", symbol: "BTC" },
+      shortToken: { address: "CSHORTTOKEN", symbol: "USDC" },
+    });
+    expect(typeof position.isLong).toBe("boolean");
+    expect(typeof position.sizeUsd).toBe("string");
+  });
+
+  test("orders query keeps frontend-consumed field names and types stable", () => {
+    const [order] = queryOrdersByAccount(fixtures, account);
+
+    assertFrontendQueryResultShape("orders", order);
+    expect(order.market).toEqual({
+      id: "market:BTC-USD",
+      key: "BTC-USD",
+      name: "BTC/USD",
+    });
+    expect(typeof order.orderType).toBe("string");
+    expect(typeof order.isLong).toBe("boolean");
+    expect(order.cancelledTimestamp).toBeNull();
+  });
+});
+
+type QueryFixtures = ReturnType<typeof seedQueryFixtures>;
+
+function seedQueryFixtures() {
+  const market = {
+    id: "market:BTC-USD",
+    key: "BTC-USD",
+    name: "BTC/USD",
+    status: "ACTIVE",
+    createdBy: account,
+    createdLedger: 12345,
+    createdTimestamp: timestamp,
+    createdTransactionHash: "tx-market",
+    marketToken: { address: "CMARKETTOKEN", symbol: "BTC-USD" },
+    indexToken: { address: "CBTCINDEX", symbol: "BTC" },
+    longToken: { address: "CLONGTOKEN", symbol: "BTC" },
+    shortToken: { address: "CSHORTTOKEN", symbol: "USDC" },
+  };
+
+  const collateralToken = {
+    address: "CSHORTTOKEN",
+    symbol: "USDC",
+    decimals: 7,
+  };
+
+  return {
+    markets: [market],
+    positions: [
+      {
+        id: "position:pos-1",
+        key: "pos-1",
+        account,
+        isLong: true,
+        status: "open",
+        sizeUsd: "500000000000000000000000000000000",
+        collateralAmount: "1000000000",
+        averagePrice: "50000",
+        entryFundingRate: "0",
+        reserveAmount: "250000000",
+        realizedPnlUsd: "0",
+        realizedPnlAmount: "0",
+        openedLedger: 12346,
+        openedTimestamp: timestamp,
+        updatedTimestamp: timestamp,
+        closedTimestamp: null,
+        market: {
+          id: market.id,
+          key: market.key,
+          name: market.name,
+          indexToken: market.indexToken,
+          longToken: market.longToken,
+          shortToken: market.shortToken,
+        },
+        collateralToken,
+      },
+    ],
+    orders: [
+      {
+        id: "order:ord-1",
+        key: "ord-1",
+        account,
+        orderType: "MarketIncrease",
+        status: "created",
+        isLong: true,
+        sizeDeltaUsd: "100000000000000000000000000000000",
+        collateralDeltaAmount: "250000000",
+        triggerPrice: null,
+        acceptablePrice: "51000",
+        createdTimestamp: timestamp,
+        updatedTimestamp: timestamp,
+        frozenTimestamp: null,
+        frozenTransactionHash: null,
+        executedTimestamp: null,
+        executedTransactionHash: null,
+        cancelledTimestamp: null,
+        cancelledTransactionHash: null,
+        cancellationReason: null,
+        market: {
+          id: market.id,
+          key: market.key,
+          name: market.name,
+        },
+        collateralToken,
+      },
+    ],
+  };
+}
+
+function queryMarkets(store: QueryFixtures): Array<Record<string, unknown>> {
+  return store.markets;
+}
+
+function queryPositionsByAccount(
+  store: QueryFixtures,
+  accountId: string,
+): Array<Record<string, unknown>> {
+  return store.positions.filter((position) => position.account === accountId);
+}
+
+function queryOrdersByAccount(
+  store: QueryFixtures,
+  accountId: string,
+): Array<Record<string, unknown>> {
+  return store.orders.filter((order) => order.account === accountId);
+}