diff --git a/README.md b/README.md
index 36f50be2..85469068 100644
--- a/README.md
+++ b/README.md
@@ -81,6 +81,7 @@ construction, not NIP-44 encryption.
   - `region` (2 chars): Country/large region
 - **Internet**: Required (connects to Nostr relays)
 - **Use Case**: Location-based community chat, local events, regional discussions
+- **External publishers:** relays are chosen by proximity to the geohash — see [Geohash publisher relays](docs/GEOHASH-PUBLISHER-RELAYS.md)
 
 ### Direct Message Routing
 
diff --git a/docs/GEOHASH-PUBLISHER-RELAYS.md b/docs/GEOHASH-PUBLISHER-RELAYS.md
new file mode 100644
index 00000000..e80242f0
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+++ b/docs/GEOHASH-PUBLISHER-RELAYS.md
@@ -0,0 +1,116 @@
+# Geohash Publisher Relay Selection & Delivery
+
+Guide for external tools that publish into BitChat location channels (kind
+`20000` chat, kind `20001` presence). BitChat clients do **not** use a fixed
+global relay set for these channels — they pick relays by proximity to the
+geohash cell. A publisher that targets the wrong relays will never reach those
+clients.
+
+## Event shape
+
+Location chat messages are ephemeral Nostr events:
+
+| Field | Value |
+|-------|--------|
+| `kind` | `20000` (chat) or `20001` (presence heartbeat) |
+| `tags` | Must include `["g", "<geohash>"]`. Chat may also carry `["n", "<nickname>"]`. |
+| `content` | Message text for kind `20000`; empty for kind `20001` |
+| `pubkey` | Ephemeral identity derived for that geohash (not the person's long-term key) |
+
+Clients subscribe with filters equivalent to:
+
+```json
+{ "kinds": [20000, 20001], "#g": ["<geohash>"], "limit": 200 }
+```
+
+See [GeohashPresenceSpec.md](GeohashPresenceSpec.md) for presence broadcast
+rules (precision limits, heartbeat cadence).
+
+## How clients choose relays
+
+On subscribe and publish, BitChat asks `GeoRelayDirectory` for the closest
+relays to the geohash center:
+
+1. Decode the geohash to a lat/lon center.
+2. Rank the geo-relay directory by haversine distance.
+3. Take the nearest **5** relays (`TransportConfig.nostrGeoRelayCount`).
+4. Ties break by hostname so every device with the same directory picks the
+   same set — publishers and subscribers must agree.
+
+### Directory source of truth
+
+Do **not** rely only on the CSV bundled inside an older app build. At runtime
+the client refreshes and caches the reviewed copy on `main`:
+
+https://raw.githubusercontent.com/permissionlesstech/bitchat/refs/heads/main/relays/online_relays_gps.csv
+
+That URL is `GeoRelayDirectory`'s `remoteURL`. External publishers should rank
+against this same file (or a freshly synced cache of it). A snapshot shipped
+with an old binary can diverge after relay GPS rows change, so messages land
+on hosts current clients no longer subscribe to.
+
+### Empty-directory fallback
+
+`closestRelays` returns `[]` only when the in-memory directory has no entries
+(or `count <= 0`). In that case the publish path does **not** refuse — it
+falls back to `NostrRelayManager`'s default relay list (built-in + any custom
+relays). See `ChatPublicConversationCoordinator.sendPublicRaw`: empty
+`targetRelays` → `sendEvent(event)` with no explicit `to:`.
+
+Publishers should still prefer proximity selection against the live CSV.
+Default-relay fallback is a last resort when the geo directory failed to load;
+relying on it intentionally will miss clients that *did* load the directory
+and are subscribed only to up to the nearest five (fewer when the directory
+is thin; none when it is empty — see the fallback above).
+
+## Sparse cells (stale directory rows)
+
+A "sparse" cell is about **directory freshness**, not geographic remoteness.
+Haversine ranking returns *up to* the nearest five hosts from whatever GPS
+rows the CSV currently has — including for oceans or deserts — and fewer
+than five when the directory is thin. When those rows are stale or thin for
+a region (relay moved, GPS never updated, host offline), the selected hosts
+can be the wrong place for live traffic even though they look "close" on
+paper.
+
+The channel UI still opens normally — there is no separate "undeliverable"
+banner — but live messages may not round-trip. Treat an empty timeline in those
+cells as a stale/coverage problem in the relay directory, not a client bug.
+Refreshing from the canonical CSV above is the first fix to try.
+
+## Ephemerality and late joiners
+
+Kind `20000` / `20001` events are ephemeral. Relays typically do not store them
+for historical replay. A client that subscribes minutes later cannot retrieve
+messages that already left the relay's ephemeral buffer. Initial lookback on
+subscribe is on the order of one hour for chat sampling
+(`nostrGeohashInitialLookbackSeconds`), but that only recovers what the chosen
+relays still hold.
+
+For durable geographic notes, BitChat uses persistent kind `1` events tagged
+with `g` (location notes / drops) — a different surface from live channel chat.
+
+## Practical checklist for external publishers
+
+1. Compute the target geohash at the precision the channel uses (`block` ≈ 7,
+   `neighborhood` ≈ 6, `city` ≈ 5, etc.).
+2. Fetch the **current**
+   [`online_relays_gps.csv`](https://raw.githubusercontent.com/permissionlesstech/bitchat/refs/heads/main/relays/online_relays_gps.csv)
+   (same URL the app refreshes from). Rank by haversine to the geohash center
+   and take the nearest ~5 `wss://` hosts (hostname tie-break). Optionally
+   publish to a strict **superset** that covers both that selection and any
+   older bundled snapshot you still support.
+3. Publish the signed kind-`20000` event to those relays with a correct `g` tag.
+4. Expect no backlog for late subscribers; design tools around live delivery.
+5. Do not assume App Store / mesh Bluetooth peers will see Nostr-only publishes
+   — mesh and geohash are separate transports.
+
+## Related code
+
+- `bitchat/Nostr/GeoRelayDirectory.swift` — proximity ranking, `remoteURL`, cache
+- `bitchat/ViewModels/ChatPublicConversationCoordinator.swift` — empty-selection
+  fallback to default relays
+- `bitchat/Services/GeohashPresenceService.swift` — subscribe/publish wiring
+- `bitchat/Services/TransportConfig.swift` — `nostrGeoRelayCount`, lookback limits
+- `bitchat/Nostr/NostrProtocol.swift` — kind `20000` / `20001` builders
+- `relays/online_relays_gps.csv` — reviewed directory checked into this repo
diff --git a/docs/GeohashPresenceSpec.md b/docs/GeohashPresenceSpec.md
index 321a491f..c04c3b4b 100644
--- a/docs/GeohashPresenceSpec.md
+++ b/docs/GeohashPresenceSpec.md
@@ -94,3 +94,9 @@ The presentation of the participant count depends on the geohash precision level
 *   **Privacy:** High-precision location presence is NOT broadcast. Temporal correlation between different levels is obfuscated via random delays.
 *   **Consistency:** "Online" status is maintained globally while the app is open.
 *   **Transparency:** The UI correctly reflects uncertainty (`?`) when privacy rules prevent accurate passive counting.
+
+## External publishers
+
+Tools that publish kind `20000` / `20001` into a geohash channel must target the
+same proximity-selected relays BitChat clients use, or messages will not arrive.
+See [GEOHASH-PUBLISHER-RELAYS.md](GEOHASH-PUBLISHER-RELAYS.md).