diff --git a/serve.py b/serve.py
index 30b5b23..212400f 100644
--- a/serve.py
+++ b/serve.py
@@ -1,9 +1,19 @@
 #!/usr/bin/env python3
-"""GODSIGH dev server: static files + same-origin proxy for feeds that block CORS."""
+"""GODSIGH dev server: static files + same-origin proxy for feeds that block CORS.
+
+Also implements the **OpenSky shared cache v1** contract (see SPEC2.md §4), which
+lets GODSIGH and its sibling project godstrument poll OpenSky from the same IP
+without racing each other into the anonymous daily quota. The contract is
+mirrored verbatim in godstrument's brief — keep the two in sync.
+"""
+import os
 import sys
+import tempfile
+import time
 import urllib.error
 import urllib.request
 from http.server import SimpleHTTPRequestHandler, ThreadingHTTPServer
+from pathlib import Path
 
 UPSTREAMS = {
     # OpenSky pins Access-Control-Allow-Origin to its own domain, so the browser
@@ -13,6 +23,67 @@ UPSTREAMS = {
     "celestrak": "https://celestrak.org/NORAD/elements/gp.php",
 }
 
+# ---- OpenSky shared cache v1 ------------------------------------------------
+# Path: ~/.cache/godverse/opensky-states.json (overridable via OPENSKY_CACHE_FILE).
+# Content: the raw, unmodified /states/all JSON body (global, no bbox). Freshness
+# = file mtime; readers treat <120 s as fresh. On upstream 429/failure a reader
+# may serve a stale cache rather than nothing.
+OPENSKY_CACHE_FRESH_SEC = 120
+
+
+def opensky_cache_file():
+    override = os.environ.get("OPENSKY_CACHE_FILE")
+    return Path(override if override else "~/.cache/godverse/opensky-states.json").expanduser()
+
+
+def has_bbox(query):
+    # A global states/all request has no bounding-box params; only those bypass the cache.
+    q = query.lower()
+    return any(k in q for k in ("lamin", "lomin", "lamax", "lomax"))
+
+
+def read_cache_if_fresh(path, max_age):
+    try:
+        st = path.stat()
+    except OSError:
+        return None
+    if time.time() - st.st_mtime >= max_age:
+        return None
+    try:
+        return path.read_bytes()
+    except OSError:
+        return None
+
+
+def read_cache_any(path):
+    try:
+        return path.read_bytes()
+    except OSError:
+        return None
+
+
+def write_cache(path, body):
+    # Atomic write: a unique temp file *beside* the target (same filesystem, so
+    # os.replace is atomic) then os.replace over the target. A unique temp name
+    # means concurrent writers (GODSIGH + godstrument, or two threads here) never
+    # corrupt each other's cache or expose a half-written file. Best-effort —
+    # caching must never break the actual response.
+    try:
+        path.parent.mkdir(parents=True, exist_ok=True)
+        fd, tmp = tempfile.mkstemp(dir=str(path.parent), prefix=path.name + ".", suffix=".tmp")
+        try:
+            with os.fdopen(fd, "wb") as f:
+                f.write(body)
+            os.replace(tmp, path)
+        except BaseException:
+            try:
+                os.unlink(tmp)
+            except OSError:
+                pass
+            raise
+    except OSError:
+        pass
+
 
 class Handler(SimpleHTTPRequestHandler):
     def end_headers(self):
@@ -28,8 +99,18 @@ class Handler(SimpleHTTPRequestHandler):
         base = UPSTREAMS.get(name)
         if not base:
             return self.send_error(404, f"unknown upstream {name!r}")
+
+        # Shared cache applies only to the OpenSky global states feed.
+        opensky_global = name == "opensky" and not has_bbox(query)
+        cache = opensky_cache_file()
+
+        # 1) Serve-from-cache: fresh (<120 s) global request costs zero quota.
+        if opensky_global:
+            fresh = read_cache_if_fresh(cache, OPENSKY_CACHE_FRESH_SEC)
+            if fresh is not None:
+                return self._send(200, "application/json", fresh, {"X-Godsigh-Cache": "hit"})
+
         url = base + ("?" + query if query else "")
-        # Forward these upstream headers so the client can show quota state.
         passthrough = {}
 
         def grab(hdrs):
@@ -43,17 +124,38 @@ class Handler(SimpleHTTPRequestHandler):
                 body = r.read()
                 status, ctype = r.status, r.headers.get("Content-Type", "text/plain")
                 grab(r.headers)
+            # 2) Write-through: a healthy global fetch refreshes the shared cache.
+            if opensky_global and status == 200:
+                write_cache(cache, body)
+                passthrough["X-Godsigh-Cache"] = "miss"
         except urllib.error.HTTPError as e:
+            # 3) Stale-on-error: on 429, keep the client working from last-known cache.
+            if opensky_global and e.code == 429:
+                stale = read_cache_any(cache)
+                if stale is not None:
+                    return self._send(200, "application/json", stale, {"X-Godsigh-Cache": "stale"})
             body = e.read() or str(e).encode()
             status, ctype = e.code, "text/plain"
             grab(e.headers)
         except Exception as e:
+            # Same stale fallback for connection errors/timeouts.
+            if opensky_global:
+                stale = read_cache_any(cache)
+                if stale is not None:
+                    return self._send(200, "application/json", stale, {"X-Godsigh-Cache": "stale"})
             body, status, ctype = str(e).encode(), 502, "text/plain"
+
+        self._send(status, ctype, body, passthrough)
+
+    def _send(self, status, ctype, body, extra=None):
         self.send_response(status)
         self.send_header("Content-Type", ctype)
         self.send_header("Access-Control-Allow-Origin", "*")
-        self.send_header("Access-Control-Expose-Headers", "X-Rate-Limit-Remaining, X-Rate-Limit-Limit, X-Expires-After")
-        for k, v in passthrough.items():
+        self.send_header(
+            "Access-Control-Expose-Headers",
+            "X-Rate-Limit-Remaining, X-Rate-Limit-Limit, X-Expires-After, X-Godsigh-Cache",
+        )
+        for k, v in (extra or {}).items():
             self.send_header(k, v)
         self.send_header("Content-Length", str(len(body)))
         self.end_headers()