diff --git a/app.py b/app.py
index 95f6605..fac6317 100644
--- a/app.py
+++ b/app.py
@@ -21,6 +21,7 @@ import audit
 import climate
 import content
 import digest
+import discord_interactions as discord_interactions_mod
 import db
 import grid
 import metrics
@@ -679,6 +680,22 @@ app.include_router(v1, prefix=f"{BASE}/api")        # legacy unversioned == v1 (
 app.include_router(v1, prefix=f"{BASE}/api/v1")
 app.include_router(v2, prefix=f"{BASE}/api/v2")
 
+
+# --- Discord slash commands (HTTP interactions) ----------------------------
+async def discord_interactions(request: Request) -> Response:
+    """Discord posts each slash-command interaction here. Verification runs over the
+    RAW body, so this must read bytes, not request.json()."""
+    raw = await request.body()
+    status, payload = discord_interactions_mod.handle(
+        raw,
+        request.headers.get("x-signature-ed25519", ""),
+        request.headers.get("x-signature-timestamp", ""),
+    )
+    return Response(json.dumps(payload), status_code=status, media_type="application/json")
+
+app.add_api_route(f"{BASE}/discord/interactions", discord_interactions,
+                  methods=["POST"], include_in_schema=False)
+
 # --- accounts (fastapi-users) ----------------------------------------------
 # Library-provided routers: /auth/login + /auth/logout (cookie session),
 # /auth/register (signup), and /users/me (session check / profile). All under the
diff --git a/discord_interactions.py b/discord_interactions.py
new file mode 100644
index 0000000..1f0f951
--- /dev/null
+++ b/discord_interactions.py
@@ -0,0 +1,124 @@
+"""Discord slash commands over the HTTP Interactions endpoint.
+
+Discord POSTs each interaction (a slash command, a button, etc.) to a URL we
+register in the developer portal, so commands are just an ordinary FastAPI route
+(app.py wires POST {BASE}/discord/interactions) with no gateway connection and no
+persistent bot process. Every request is signed; we verify the Ed25519 signature
+over the raw body before trusting anything (Discord actively probes this with bad
+signatures and disables the endpoint if we don't reject them).
+
+Commands so far:
+  /grade <city>  -> today's grade for a curated city, from the warm cache.
+
+Register the command definitions with scripts/register_discord_commands.py.
+"""
+from __future__ import annotations
+
+import datetime
+import json
+import os
+
+from nacl.exceptions import BadSignatureError
+from nacl.signing import VerifyKey
+
+import cities
+import discord
+import homepage
+
+# App's Ed25519 public key (Developer Portal -> General Information). Unset =>
+# every request fails verification, which is the safe default for an unconfigured
+# deployment.
+PUBLIC_KEY = os.environ.get("THERMOGRAPH_DISCORD_PUBLIC_KEY", "").strip()
+
+# Interaction types (Discord).
+_PING = 1
+_APPLICATION_COMMAND = 2
+# Response types.
+_PONG = 1
+_CHANNEL_MESSAGE = 4
+# Message flags: 64 = ephemeral (only the caller sees it).
+_EPHEMERAL = 64
+
+
+def verify(signature: str, timestamp: str, body: bytes, public_key: str | None = None) -> bool:
+    """True iff `signature` is a valid Ed25519 signature of timestamp+body under the
+    app public key. Any malformed input is a verification failure, never an error."""
+    key = public_key if public_key is not None else PUBLIC_KEY
+    if not key or not signature or not timestamp:
+        return False
+    try:
+        VerifyKey(bytes.fromhex(key)).verify(timestamp.encode() + body, bytes.fromhex(signature))
+        return True
+    except (BadSignatureError, ValueError):
+        return False
+
+
+def _resolve_city(query: str) -> dict | None:
+    """Best curated-city match for a free-text name: exact name first, then a
+    prefix match, preferring the largest (cities.json is population-sorted)."""
+    q = (query or "").strip().casefold()
+    if not q:
+        return None
+    exact, prefix = None, None
+    for c in cities.all_cities():          # already sorted by population, desc
+        name = c["name"].casefold()
+        if name == q:
+            exact = exact or c
+        elif prefix is None and name.startswith(q):
+            prefix = c
+    return exact or prefix
+
+
+def _grade_message(query: str) -> dict:
+    """The message data for /grade <city>: an embed with today's grade line, or a
+    plain ephemeral note when the city is unknown or not yet warmed."""
+    city = _resolve_city(query)
+    if city is None:
+        return {"content": f"I don't track a city called '{query}' yet. "
+                           "Try a major city name.", "flags": _EPHEMERAL}
+    card = homepage._grade_city(city, datetime.date.today())
+    if card is None:
+        return {"content": f"No recent reading is cached for {city['name']} yet. "
+                           "It should appear once the data warms up.", "flags": _EPHEMERAL}
+    color = discord._COLD if card.get("tail") == "cold" else discord._HOT
+    return {"embeds": [{
+        "title": f"{card.get('display') or city['name']} — today",
+        "description": discord._line(card),
+        "url": f"https://thermograph.org/climate/{city['slug']}",
+        "color": color,
+        "footer": {"text": "thermograph.org · graded against ~45 years of local history"},
+    }]}
+
+
+def _command_response(data: dict) -> dict:
+    """Dispatch an application-command interaction to its handler."""
+    name = (data.get("name") or "").lower()
+    opts = {o.get("name"): o.get("value") for o in (data.get("options") or [])}
+    if name == "grade":
+        return {"type": _CHANNEL_MESSAGE, "data": _grade_message(opts.get("city", ""))}
+    # Unknown command: acknowledge rather than error, so a stale registration can't
+    # look broken to the user.
+    return {"type": _CHANNEL_MESSAGE,
+            "data": {"content": "Unknown command.", "flags": _EPHEMERAL}}
+
+
+def handle(body: bytes, signature: str, timestamp: str) -> tuple[int, dict]:
+    """Verify and route one interaction. Returns (http_status, json_body).
+
+    401 on a bad/missing signature (what Discord's security probe expects); a PING
+    is answered with a PONG; an application command is dispatched. The caller reads
+    the RAW body for verification and passes it here unparsed."""
+    if not verify(signature, timestamp, body):
+        return 401, {"error": "bad signature"}
+    try:
+        payload = json.loads(body)
+    except ValueError:
+        return 400, {"error": "bad json"}
+    itype = payload.get("type")
+    if itype == _PING:
+        return 200, {"type": _PONG}
+    if itype == _APPLICATION_COMMAND:
+        return 200, _command_response(payload.get("data") or {})
+    # Components/modals/etc. aren't used yet; a PONG-shaped 200 is harmless.
+    return 200, {"type": _CHANNEL_MESSAGE,
+                 "data": {"content": "Unsupported interaction.", "flags": _EPHEMERAL}}
diff --git a/requirements.txt b/requirements.txt
index 1f14143..7cb9a77 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -9,5 +9,7 @@ aiosqlite==0.22.1
 # Web Push (VAPID) delivery of notifications (see backend/push.py). Pulls in
 # py-vapid, cryptography, and http-ece.
 pywebpush==2.0.0
+# Ed25519 verification of Discord interaction webhooks (see backend/discord_interactions.py).
+PyNaCl==1.5.0
 # Server-rendered SEO content pages (see backend/content.py, templates/).
 jinja2==3.1.6
diff --git a/tests/test_discord_interactions.py b/tests/test_discord_interactions.py
new file mode 100644
index 0000000..3d475ed
--- /dev/null
+++ b/tests/test_discord_interactions.py
@@ -0,0 +1,116 @@
+"""Discord HTTP-interactions: Ed25519 verification, PING/PONG, /grade dispatch,
+and the wired FastAPI route. No live Discord — we hold a test signing key and sign
+requests ourselves."""
+import datetime
+import json
+
+import pytest
+from fastapi.testclient import TestClient
+from nacl.signing import SigningKey
+
+import app as appmod
+import discord_interactions as di
+import homepage
+
+B = "/thermograph"
+
+_SK = SigningKey.generate()
+_PUBKEY = _SK.verify_key.encode().hex()
+
+
+def _sign(body: bytes, ts: str = "1700000000") -> tuple[str, str]:
+    return _SK.sign(ts.encode() + body).signature.hex(), ts
+
+
+_CARD = {
+    "display": "Tokyo, Japan", "name": "Tokyo", "slug": "tokyo-jp",
+    "metric_label": "High temp", "value": 96.0, "normal": 84.0,
+    "pct_label": "97th percentile", "grade_label": "Very High",
+    "tail": "warm", "date_label": None,
+}
+
+
+# --- signature verification --------------------------------------------------
+
+def test_verify_accepts_a_good_signature():
+    body = b'{"type":1}'
+    sig, ts = _sign(body)
+    assert di.verify(sig, ts, body, _PUBKEY) is True
+
+
+def test_verify_rejects_tampering_and_bad_inputs():
+    body = b'{"type":1}'
+    sig, ts = _sign(body)
+    assert di.verify(sig, ts, body + b" ", _PUBKEY) is False   # body changed
+    assert di.verify(sig, "1700000001", body, _PUBKEY) is False  # timestamp changed
+    assert di.verify("00" * 64, ts, body, _PUBKEY) is False       # wrong signature
+    assert di.verify("nothex", ts, body, _PUBKEY) is False         # malformed
+    assert di.verify(sig, ts, body, "") is False                   # no key configured
+    assert di.verify("", ts, body, _PUBKEY) is False               # no signature
+
+
+# --- handle(): routing -------------------------------------------------------
+
+def test_handle_ping_pongs(monkeypatch):
+    monkeypatch.setattr(di, "PUBLIC_KEY", _PUBKEY)
+    body = b'{"type":1}'
+    sig, ts = _sign(body)
+    status, payload = di.handle(body, sig, ts)
+    assert status == 200 and payload == {"type": 1}
+
+
+def test_handle_rejects_bad_signature(monkeypatch):
+    monkeypatch.setattr(di, "PUBLIC_KEY", _PUBKEY)
+    body = b'{"type":1}'
+    status, payload = di.handle(body, "00" * 64, "1700000000")
+    assert status == 401
+
+
+def test_grade_command_returns_the_citys_grade(monkeypatch):
+    monkeypatch.setattr(di, "PUBLIC_KEY", _PUBKEY)
+    monkeypatch.setattr(homepage, "_grade_city", lambda city, today: dict(_CARD))
+    body = json.dumps({"type": 2, "data": {
+        "name": "grade", "options": [{"name": "city", "value": "Tokyo"}]}}).encode()
+    sig, ts = _sign(body)
+    status, payload = di.handle(body, sig, ts)
+    assert status == 200 and payload["type"] == 4
+    embed = payload["data"]["embeds"][0]
+    assert "Tokyo, Japan" in embed["title"]
+    assert "96°F" in embed["description"] and "97th percentile" in embed["description"]
+    assert embed["url"].endswith("/climate/tokyo-jp")
+
+
+def test_grade_unknown_city_is_ephemeral(monkeypatch):
+    monkeypatch.setattr(di, "PUBLIC_KEY", _PUBKEY)
+    data = di._grade_message("Nowherecityville")
+    assert data["flags"] == di._EPHEMERAL and "don't track" in data["content"]
+
+
+def test_grade_city_not_yet_warm_is_ephemeral(monkeypatch):
+    monkeypatch.setattr(homepage, "_grade_city", lambda city, today: None)
+    data = di._grade_message("Tokyo")           # a real curated city, but no warm cache
+    assert data["flags"] == di._EPHEMERAL and "No recent reading" in data["content"]
+
+
+def test_resolve_city_prefers_exact_then_population():
+    # Real cities.json: an exact name resolves, and it's a real curated entry.
+    tokyo = di._resolve_city("tokyo")
+    assert tokyo is not None and tokyo["name"] == "Tokyo"
+    assert di._resolve_city("   ") is None
+    assert di._resolve_city("Zzznotacity") is None
+
+
+# --- the wired FastAPI route -------------------------------------------------
+
+def test_interactions_route_pings_and_rejects(monkeypatch):
+    monkeypatch.setattr(di, "PUBLIC_KEY", _PUBKEY)
+    client = TestClient(appmod.app)
+    body = b'{"type":1}'
+    sig, ts = _sign(body)
+    # Signed PING -> PONG. Send raw bytes so the signature matches byte-for-byte.
+    r = client.post(f"{B}/discord/interactions", content=body,
+                    headers={"x-signature-ed25519": sig, "x-signature-timestamp": ts})
+    assert r.status_code == 200 and r.json() == {"type": 1}
+    # Unsigned -> 401 (Discord's security probe).
+    r = client.post(f"{B}/discord/interactions", content=body)
+    assert r.status_code == 401