diff --git a/src/shared/detailsRequestParam.test.ts b/src/shared/detailsRequestParam.test.ts
new file mode 100644
index 00000000..d28bd9f1
--- /dev/null
+++ b/src/shared/detailsRequestParam.test.ts
@@ -0,0 +1,162 @@
+import { describe, expect, it } from "vitest";
+import {
+  buildDetailsRequestParam,
+  parseDetailsRequestParam,
+  type DetailsRequest,
+} from "./detailsRequestParam.js";
+
+describe("detailsRequestParam — build", () => {
+  it("4.1.2-R-Request-flight: encodes flight-number parent", () => {
+    const r: DetailsRequest = {
+      area: "onlineboard",
+      kind: "flight",
+      flightNumber: "SU1234",
+      date: "20260515",
+    };
+    expect(buildDetailsRequestParam(r)).toBe("onlineboard-flight-SU1234-20260515");
+  });
+
+  it("4.1.2-R-Request-departure: encodes departure parent without time", () => {
+    expect(
+      buildDetailsRequestParam({
+        area: "onlineboard",
+        kind: "departure",
+        station: "SVO",
+        date: "20260515",
+      }),
+    ).toBe("onlineboard-departure-SVO-20260515");
+  });
+
+  it("4.1.2-R-Request-departure-time: encodes departure parent with time range", () => {
+    expect(
+      buildDetailsRequestParam({
+        area: "onlineboard",
+        kind: "departure",
+        station: "SVO",
+        date: "20260515",
+        timeFrom: "0600",
+        timeTo: "2200",
+      }),
+    ).toBe("onlineboard-departure-SVO-20260515-06002200");
+  });
+
+  it("4.1.2-R-Request-arrival: encodes arrival parent", () => {
+    expect(
+      buildDetailsRequestParam({
+        area: "onlineboard",
+        kind: "arrival",
+        station: "SVO",
+        date: "20260515",
+      }),
+    ).toBe("onlineboard-arrival-SVO-20260515");
+  });
+
+  it("4.1.2-R-Request-route: encodes route parent", () => {
+    expect(
+      buildDetailsRequestParam({
+        area: "onlineboard",
+        kind: "route",
+        departure: "MOW",
+        arrival: "LED",
+        date: "20260515",
+      }),
+    ).toBe("onlineboard-route-MOW-LED-20260515");
+  });
+
+  it("4.1.2-R-Request-route-time: encodes route parent with time range", () => {
+    expect(
+      buildDetailsRequestParam({
+        area: "onlineboard",
+        kind: "route",
+        departure: "MOW",
+        arrival: "LED",
+        date: "20260515",
+        timeFrom: "0600",
+        timeTo: "2200",
+      }),
+    ).toBe("onlineboard-route-MOW-LED-20260515-06002200");
+  });
+});
+
+describe("detailsRequestParam — parse", () => {
+  it("returns null for empty or non-matching input", () => {
+    expect(parseDetailsRequestParam("")).toBeNull();
+    expect(parseDetailsRequestParam("garbage")).toBeNull();
+    expect(parseDetailsRequestParam("schedule-flight-SU1234-20260515")).toBeNull();
+  });
+
+  it("parses flight-number form", () => {
+    expect(parseDetailsRequestParam("onlineboard-flight-SU1234-20260515")).toEqual({
+      area: "onlineboard",
+      kind: "flight",
+      flightNumber: "SU1234",
+      date: "20260515",
+    });
+  });
+
+  it("parses departure without time", () => {
+    expect(parseDetailsRequestParam("onlineboard-departure-SVO-20260515")).toEqual({
+      area: "onlineboard",
+      kind: "departure",
+      station: "SVO",
+      date: "20260515",
+    });
+  });
+
+  it("parses departure with time range", () => {
+    expect(parseDetailsRequestParam("onlineboard-departure-SVO-20260515-06002200")).toEqual({
+      area: "onlineboard",
+      kind: "departure",
+      station: "SVO",
+      date: "20260515",
+      timeFrom: "0600",
+      timeTo: "2200",
+    });
+  });
+
+  it("parses arrival", () => {
+    expect(parseDetailsRequestParam("onlineboard-arrival-LED-20260515")).toEqual({
+      area: "onlineboard",
+      kind: "arrival",
+      station: "LED",
+      date: "20260515",
+    });
+  });
+
+  it("parses route without time", () => {
+    expect(parseDetailsRequestParam("onlineboard-route-MOW-LED-20260515")).toEqual({
+      area: "onlineboard",
+      kind: "route",
+      departure: "MOW",
+      arrival: "LED",
+      date: "20260515",
+    });
+  });
+
+  it("parses route with time range", () => {
+    expect(parseDetailsRequestParam("onlineboard-route-MOW-LED-20260515-06002200")).toEqual({
+      area: "onlineboard",
+      kind: "route",
+      departure: "MOW",
+      arrival: "LED",
+      date: "20260515",
+      timeFrom: "0600",
+      timeTo: "2200",
+    });
+  });
+});
+
+describe("detailsRequestParam — roundtrip", () => {
+  it.each<DetailsRequest>([
+    { area: "onlineboard", kind: "flight", flightNumber: "SU1234", date: "20260515" },
+    { area: "onlineboard", kind: "departure", station: "SVO", date: "20260515" },
+    { area: "onlineboard", kind: "departure", station: "SVO", date: "20260515", timeFrom: "0600", timeTo: "2200" },
+    { area: "onlineboard", kind: "arrival", station: "LED", date: "20260515" },
+    { area: "onlineboard", kind: "route", departure: "MOW", arrival: "LED", date: "20260515" },
+    { area: "onlineboard", kind: "route", departure: "MOW", arrival: "LED", date: "20260515", timeFrom: "0600", timeTo: "2200" },
+  ])("round-trips %o", (input) => {
+    const encoded = buildDetailsRequestParam(input);
+    const decoded = parseDetailsRequestParam(encoded);
+    expect(decoded).toEqual(input);
+  });
+});
diff --git a/src/shared/detailsRequestParam.ts b/src/shared/detailsRequestParam.ts
new file mode 100644
index 00000000..fc2254d2
--- /dev/null
+++ b/src/shared/detailsRequestParam.ts
@@ -0,0 +1,113 @@
+/**
+ * Codec for the `?request=...` query parameter on Online-Board flight-details URLs.
+ * Per TZ §4.1.2 Table 5 row 6. The param encodes the parent search context so
+ * the mini-list can rebuild the list on refresh / deep-link entry.
+ *
+ * Forms (canonical):
+ *   onlineboard-flight-{flightNumber}-{yyyyMMdd}
+ *   onlineboard-departure-{iata}-{yyyyMMdd}[-{HHmmHHmm}]
+ *   onlineboard-arrival-{iata}-{yyyyMMdd}[-{HHmmHHmm}]
+ *   onlineboard-route-{depIata}-{arrIata}-{yyyyMMdd}[-{HHmmHHmm}]
+ */
+
+export type DetailsRequest =
+  | { area: "onlineboard"; kind: "flight"; flightNumber: string; date: string }
+  | {
+      area: "onlineboard";
+      kind: "departure" | "arrival";
+      station: string;
+      date: string;
+      timeFrom?: string;
+      timeTo?: string;
+    }
+  | {
+      area: "onlineboard";
+      kind: "route";
+      departure: string;
+      arrival: string;
+      date: string;
+      timeFrom?: string;
+      timeTo?: string;
+    };
+
+const AREA = "onlineboard";
+
+function isDate(s: string | undefined): s is string {
+  return typeof s === "string" && /^\d{8}$/.test(s);
+}
+
+function isTimeRange(s: string | undefined): s is string {
+  return typeof s === "string" && /^\d{8}$/.test(s);
+}
+
+export function buildDetailsRequestParam(r: DetailsRequest): string {
+  switch (r.kind) {
+    case "flight":
+      return `${r.area}-flight-${r.flightNumber}-${r.date}`;
+    case "departure":
+    case "arrival": {
+      const base = `${r.area}-${r.kind}-${r.station}-${r.date}`;
+      return r.timeFrom && r.timeTo ? `${base}-${r.timeFrom}${r.timeTo}` : base;
+    }
+    case "route": {
+      const base = `${r.area}-route-${r.departure}-${r.arrival}-${r.date}`;
+      return r.timeFrom && r.timeTo ? `${base}-${r.timeFrom}${r.timeTo}` : base;
+    }
+  }
+}
+
+export function parseDetailsRequestParam(raw: string): DetailsRequest | null {
+  if (!raw) return null;
+  const parts = raw.split("-");
+  if (parts.length < 4) return null;
+  if (parts[0] !== AREA) return null;
+
+  const kind = parts[1];
+
+  if (kind === "flight") {
+    const [, , flightNumber, date] = parts;
+    if (!flightNumber || !isDate(date) || parts.length !== 4) return null;
+    return { area: "onlineboard", kind: "flight", flightNumber, date };
+  }
+
+  if (kind === "departure" || kind === "arrival") {
+    const [, , station, date, maybeTime] = parts;
+    if (!station || !isDate(date)) return null;
+    if (parts.length === 4) {
+      return { area: "onlineboard", kind, station, date };
+    }
+    if (parts.length === 5 && isTimeRange(maybeTime)) {
+      return {
+        area: "onlineboard",
+        kind,
+        station,
+        date,
+        timeFrom: maybeTime.slice(0, 4),
+        timeTo: maybeTime.slice(4, 8),
+      };
+    }
+    return null;
+  }
+
+  if (kind === "route") {
+    const [, , departure, arrival, date, maybeTime] = parts;
+    if (!departure || !arrival || !isDate(date)) return null;
+    if (parts.length === 5) {
+      return { area: "onlineboard", kind: "route", departure, arrival, date };
+    }
+    if (parts.length === 6 && isTimeRange(maybeTime)) {
+      return {
+        area: "onlineboard",
+        kind: "route",
+        departure,
+        arrival,
+        date,
+        timeFrom: maybeTime.slice(0, 4),
+        timeTo: maybeTime.slice(4, 8),
+      };
+    }
+    return null;
+  }
+
+  return null;
+}