diff --git a/web/src/location/chain.ts b/web/src/location/chain.ts
new file mode 100644
index 0000000..9edbb83
--- /dev/null
+++ b/web/src/location/chain.ts
@@ -0,0 +1,77 @@
+/*
+ * chain.ts — the D-04 three-source acquisition state machine.
+ * --------------------------------------------------------
+ * Order is locked: telegram → webview → chat-pin. Freshness is a
+ * SEPARATE axis from availability (RESEARCH Pitfall 2): a telegram fix
+ * that is byte-identical to the last accepted one routes to the nudge
+ * inside nudgeAndRetryTelegram — it is never returned as success. The
+ * first honest fix wins; denial is a state, not an exception.
+ *
+ * Cadence contract (RESEARCH Open Question 2, resolved open-by-design):
+ * this module exposes a Fix-shaped abstraction with a PLUGGABLE cadence
+ * and hard-codes NO continuous-mode interval (no setInterval, no
+ * watchPosition). Phase 6 chooses between bounded getLocation() polling
+ * and watchPosition() without a rewrite. Post-live re-acquisition
+ * re-runs acquireFix; it does not stream.
+ *
+ * The 45000 ms staleness threshold is the UI-SPEC's 45s signal-lost
+ * trigger; fixAgeSeconds feeds the FIX Ns AGO strip telemetry — the
+ * honesty surface for the Android static-fix (issue #56).
+ */
+import type { Fix, LocationState } from "./types.ts";
+import type { TelegramWebAppLike } from "../telegram/webapp.ts";
+import { nudgeAndRetryTelegram, telegramLocationSupported } from "./telegram.ts";
+import { readWebviewFix, webviewLocationSupported } from "./webview.ts";
+import { pollChatPin } from "./chat-pin.ts";
+
+export interface ChainDeps {
+  wa: TelegramWebAppLike | null; // null off-Telegram (plain browser) — telegramLocationSupported handles it
+  getToken: () => string | null;
+  readWebview?: (o?: { timeoutMs?: number }) => Promise<Fix | null>;
+  pollPin?: (token: string, o?: { timeoutMs?: number }) => Promise<Fix | null>;
+  nowMs?: () => number;
+  telegramTimeoutMs?: number;
+}
+
+export async function acquireFix(deps: ChainDeps, lastAccepted: Fix | null): Promise<{ fix: Fix | null; state: LocationState }> {
+  const now = deps.nowMs ?? Date.now;
+  const readWebview = deps.readWebview ?? readWebviewFix;
+  const pollPin = deps.pollPin ?? pollChatPin;
+
+  // LINK 1 — Telegram-native (GEO-01 primary). Version-gated FIRST, and
+  // staleness is routed to the nudge, never to success (RESEARCH Pitfall 2).
+  if (telegramLocationSupported(deps.wa)) {
+    const fix = await nudgeAndRetryTelegram(deps.wa!, readWebview, lastAccepted, {
+      timeoutMs: deps.telegramTimeoutMs ?? 8000,
+      nowMs: now,
+    });
+    if (fix !== null) return { fix, state: { kind: "live", fix } };
+  }
+
+  // LINK 2 — WebView navigator.geolocation.
+  if (webviewLocationSupported()) {
+    const fix = await readWebview({ timeoutMs: 8000 });
+    if (fix !== null) return { fix, state: { kind: "live", fix } };
+  }
+
+  // LINK 3 — chat-keyboard pin relay (GEO-02). The Desktop Telegram path:
+  // Desktop has NO geolocation at all (issue #77), so this is not a
+  // degraded backup there — it is the ONLY origin path.
+  const token = deps.getToken();
+  if (token !== null) {
+    const fix = await pollPin(token, { timeoutMs: 60000 });
+    if (fix !== null) return { fix, state: { kind: "live", fix } };
+    return { fix: null, state: { kind: "denied", reason: "timeout" } };
+  }
+  return { fix: null, state: { kind: "denied", reason: "no-permission" } };
+}
+
+/** The UI-SPEC's 45s signal-lost trigger — `LIVE` past this age is a lie
+ *  (T-03-09): the strip flips to SIGNAL LOST and the last marker dims. */
+export function isFixStale(fix: Fix, nowMs: number): boolean {
+  return nowMs - fix.ts > 45000;
+}
+
+export function fixAgeSeconds(fix: Fix, nowMs: number): number {
+  return Math.max(0, Math.round((nowMs - fix.ts) / 1000));
+}
diff --git a/web/src/session/client.ts b/web/src/session/client.ts
new file mode 100644
index 0000000..f37a492
--- /dev/null
+++ b/web/src/session/client.ts
@@ -0,0 +1,37 @@
+/*
+ * client.ts — the /api/session client (T-03-08, RESEARCH §Security V2).
+ * -------------------------------------------------------------------
+ * The request body is the VERBATIM Telegram.WebApp.initData string —
+ * never a re-serialization of its parsed fields (Telegram's own docs
+ * warn that the rebuilt form breaks HMAC validation). No credentials
+ * option, no custom header carrying initData: it IS the body. Non-200
+ * and a non-string token both read as failure — there is no
+ * client-side shortcut that accepts an unvalidated identity; HMAC
+ * validation stays entirely server-side. A 401 carries exactly
+ * bot/src/webhook.ts's rejection vocabulary (malformed | tampered |
+ * stale); "network" covers fetch-level failures (offline, DNS).
+ */
+import { setSession } from "./store.ts";
+
+export type SessionResult =
+  | { ok: true; token: string }
+  | { ok: false; reason: "malformed" | "tampered" | "stale" | "network" };
+
+export async function createSession(initData: string): Promise<SessionResult> {
+  try {
+    const res = await fetch("/api/session", { method: "POST", body: initData });
+    if (res.status === 401) {
+      const body: unknown = await res.json().catch(() => ({ error: "malformed" }));
+      const reason = (body as { error?: string }).error;
+      return { ok: false, reason: reason === "tampered" || reason === "stale" ? reason : "malformed" };
+    }
+    if (!res.ok) return { ok: false, reason: "malformed" };
+    const body: unknown = await res.json();
+    const t = (body as { token?: unknown }).token;
+    if (typeof t !== "string" || t.length === 0) return { ok: false, reason: "malformed" };
+    setSession(t); // module scope only (D-03/D-07) — store.ts writes no storage
+    return { ok: true, token: t };
+  } catch {
+    return { ok: false, reason: "network" };
+  }
+}
diff --git a/web/src/session/store.ts b/web/src/session/store.ts
new file mode 100644
index 0000000..f8ec2bc
--- /dev/null
+++ b/web/src/session/store.ts
@@ -0,0 +1,40 @@
+/*
+ * store.ts — the memory-only session token holder (D-03, D-07).
+ * ----------------------------------------------------------
+ * WHAT is stored: the opaque session token and its issuance second, in
+ * module scope (closure over `let`). WHAT is never stored: anything
+ * else — no coordinates, and NO persistence call anywhere in this file
+ * (no localStorage, no sessionStorage, no cookie, no IndexedDB, no
+ * caches). That absence is the D-07 invariant the grep gate pins
+ * (web/test/no-storage.test.mjs — comments stripped, every web/src file).
+ *
+ * The 900 s TTL mirrors bot/src/kv/sessions.ts by NAME (D-03): a
+ * mid-session stall at or beyond SESSION_TTL_SECONDS is stale, so the
+ * caller re-runs createSession with fresh initData rather than posting
+ * a doomed Bearer header. Revalidation happens on every fresh boot and
+ * on every TTL stall — never from any persisted copy, because none
+ * exists.
+ */
+export const SESSION_TTL_SECONDS = 900;
+
+let token: string | null = null;
+let issuedAtSeconds: number | null = null;
+
+export function setSession(nextToken: string, issuedAtSec: number = Math.floor(Date.now() / 1000)): void {
+  token = nextToken;
+  issuedAtSeconds = issuedAtSec;
+}
+
+export function getSession(): string | null {
+  return token;
+}
+
+export function isSessionStale(nowSeconds: number = Math.floor(Date.now() / 1000)): boolean {
+  if (token === null || issuedAtSeconds === null) return true;
+  return nowSeconds - issuedAtSeconds >= SESSION_TTL_SECONDS;
+}
+
+export function clearSession(): void {
+  token = null;
+  issuedAtSeconds = null;
+}