diff --git a/docs/flasher/index.html b/docs/flasher/index.html
index 77dccb78..07945600 100644
--- a/docs/flasher/index.html
+++ b/docs/flasher/index.html
@@ -340,6 +340,18 @@
                 <li>Click <strong>Install and Enable</strong>, then choose the mounted SD-card root.</li>
                 <li>Safely eject the SD card, return it to the T-Deck, power it on, and open Maps.</li>
             </ol>
+            <div class="warning map-safety-contract">
+                <strong>Before installing:</strong> close all other Pyxis flasher tabs or
+                windows, and do not run the command-line installer against this same
+                mounted card at the same time. The cross-tab lock coordinates only
+                tabs of this flasher in the same browser profile; it cannot see the
+                CLI or other browsers.
+                <br><br>
+                <strong>After installing:</strong> wait for the verified completion
+                message, then safely eject the card. If activation fails after a pack
+                was published, retry with the exact same ZIP and the same name and
+                pack ID.
+            </div>
             <p class="map-note">Existing unrelated SD-card files are preserved. Map tiles are validated locally and are never uploaded.</p>
             <div class="map-installer-grid">
                 <div class="map-field">
diff --git a/docs/offline-map-packs.md b/docs/offline-map-packs.md
index 5c8d16a5..dfb7fcef 100644
--- a/docs/offline-map-packs.md
+++ b/docs/offline-map-packs.md
@@ -10,7 +10,8 @@ Open the Pyxis web flasher in current Chrome or Edge and use **Install Offline M
 2. Turn the T-Deck off, remove its SD card, and mount the card on the computer.
 3. Choose the ZIP, enter a map name and pack ID, and wait for local validation.
 4. Click **Install and Enable**, then choose the SD-card root.
-5. After the verified completion message, safely eject the card and return it to the T-Deck.
+5. Wait for the verified completion message, then safely eject the card and
+   return it to the T-Deck.
 
 The browser accepts stored ZIP entries rooted at either `<z>/<x>/<y>.png` or
 `maps/<style>/<z>/<x>/<y>.png`. It rejects mixed styles, traversal, duplicate
@@ -19,6 +20,14 @@ ZIP records, unsafe PNGs, quota violations, and noncanonical XYZ paths. Tiles
 are validated and read back one at a time; the complete archive is not loaded
 into JavaScript memory.
 
+**Single writer.** The browser's cross-tab lock serializes only tabs of the
+same flasher origin in the same browser profile. Close all other flasher tabs
+and windows before installing, and do not run the CLI and the browser
+installer against the same mounted card at the same time. Wait for the
+verified completion message, then safely eject the card. A
+published-but-not-activated pack is never deleted: retry activation with the
+exact same ZIP, map set, name, and pack ID.
+
 Installation refuses to overwrite an existing pack. A verified pack from an
 earlier activation failure can be reselected with the exact same ZIP and
 metadata. Tiles are published first, `manifest.pmp` is published last, and only
diff --git a/tests/build_scripts/test_map_web_installer_contract.py b/tests/build_scripts/test_map_web_installer_contract.py
index 1d79e5bb..407fbac1 100644
--- a/tests/build_scripts/test_map_web_installer_contract.py
+++ b/tests/build_scripts/test_map_web_installer_contract.py
@@ -61,6 +61,44 @@ def test_map_installer_explains_the_complete_sd_card_workflow() -> None:
     assert positions == sorted(positions)
 
 
+def test_map_installer_documents_the_single_writer_contract() -> None:
+    """B9: both the flasher UI and the docs must state the supported
+    concurrency model — no CLI+browser at once, close other flasher tabs,
+    wait for verified completion, safe eject, exact retry — and must never
+    tell users to clean up marker files."""
+    flasher = FLASHER.read_text(encoding="utf-8")
+    docs = (ROOT / "docs/offline-map-packs.md").read_text(encoding="utf-8")
+    assert 'class="warning map-safety-contract"' in flasher
+    # Normalize whitespace so phrase checks survive HTML line-wrapping.
+    flasher_norm = " ".join(flasher.split())
+    contract_phrases = (
+        "close all other Pyxis flasher tabs",
+        "command-line installer",
+        "mounted card at the same time",
+        "completion message",
+        "safely eject",
+        "retry with the exact same ZIP",
+    )
+    for phrase in contract_phrases:
+        assert phrase in flasher_norm, f"flasher missing: {phrase!r}"
+    # The docs carry the same contract, plus the lock-scope limitation.
+    docs_norm = " ".join(docs.split())
+    assert "do not run the CLI and the browser installer" in docs_norm
+    assert "Close all other flasher tabs" in docs_norm
+    assert "Wait for the verified completion message" in docs_norm
+    assert "same browser profile" in docs_norm
+    # No marker-file cleanup instructions anywhere in the map installer docs.
+    lower_docs = docs_norm.lower()
+    for forbidden in (
+        "delete the marker",
+        "remove the marker",
+        "clean up the marker",
+        "delete the active-pack",
+        "remove the active-pack",
+    ):
+        assert forbidden not in lower_docs
+
+
 def test_map_installer_has_no_tile_network_fetch_path() -> None:
     source = INSTALLER.read_text(encoding="utf-8").lower()
     for forbidden in ("fetch(", "xmlhttprequest", "websocket", "http://", "https://"):