diff --git a/csp_bot/bot.py b/csp_bot/bot.py
index c17097d..4c7a39c 100644
--- a/csp_bot/bot.py
+++ b/csp_bot/bot.py
@@ -212,10 +212,22 @@ def connect(self, channels: GatewayChannels) -> None:
         secondary_commands = csp.unroll(response_outputs.commands)
         channels.set_channel(GatewayChannels.commands, secondary_commands)
 
+        if self.config.allow_send_messages:
+            # Lets a caller POST to /api/v1/send/messages_out. The gateway wires
+            # the send adapter into the channel after every module has
+            # connected, so the merged edge below picks those messages up too.
+            channels.add_send_channel(GatewayChannels.messages_out)
+
         # Publish responses to adapters
         # chatom handles conversion to backend-specific formats
+        #
+        # Read the channel rather than the edge built above, so anything else
+        # feeding messages_out, including the send adapter, reaches a backend.
+        # This is not a cycle: subscribe() is a source and publish() a sink, so
+        # nothing here flows back into what composes the channel.
+        outbound = channels.get_channel(GatewayChannels.messages_out)
         for backend, adapter in self._adapters.items():
-            backend_messages = self._filter_messages_for_backend(backend, messages_out)
+            backend_messages = self._filter_messages_for_backend(backend, outbound)
             adapter.publish(backend_messages)
 
         # Set up presence updates for Symphony
@@ -253,10 +265,17 @@ def _tag_message_backend(self, backend: str, msg: ts[Message]) -> ts[Message]:
 
     @csp.node
     def _filter_messages_for_backend(self, backend: str, msg: ts[Message]) -> ts[Message]:
-        """Filter messages for a specific backend."""
+        """Filter messages for a specific backend.
+
+        Messages the bot routes internally are tagged in metadata, but
+        chatom's Message carries a `backend` field of its own, and that is what
+        a caller sending one over REST naturally sets. Either names the
+        destination, with metadata taking precedence since that is what the
+        inbound tagging writes.
+        """
         if csp.ticked(msg):
             metadata = msg.metadata or {}
-            msg_backend = metadata.get("backend", "")
+            msg_backend = metadata.get("backend") or msg.backend or ""
             if msg_backend == backend:
                 return msg
 
diff --git a/csp_bot/bot_config.py b/csp_bot/bot_config.py
index 943fa66..2a3badb 100644
--- a/csp_bot/bot_config.py
+++ b/csp_bot/bot_config.py
@@ -114,3 +114,13 @@ class BotConfig(BaseModel):
         default=1.0,
         description="Minimum seconds between message outputs.",
     )
+
+    allow_send_messages: bool = Field(
+        default=False,
+        description=(
+            "Expose messages_out as a send channel, so an authenticated caller can POST a message "
+            "to /api/v1/send/messages_out and have it delivered to the backend named on the message. "
+            "Off by default: it turns the bot into a relay for anything that can reach the API, so it "
+            "should be enabled deliberately and only with authentication configured."
+        ),
+    )
diff --git a/csp_bot/tests/test_bot_integration.py b/csp_bot/tests/test_bot_integration.py
index 8ddf263..06f65bf 100644
--- a/csp_bot/tests/test_bot_integration.py
+++ b/csp_bot/tests/test_bot_integration.py
@@ -8,6 +8,7 @@
 """
 
 import asyncio
+from datetime import UTC, datetime, timedelta
 from unittest.mock import MagicMock, patch
 
 import pytest
@@ -100,7 +101,8 @@ class TestMetadataPropagation:
 
     Bug context: Commands returning Message objects directly (like trout/slap)
     were not being routed to backends because metadata["backend"] wasn't set.
-    The _filter_messages_for_backend node filters on metadata, not msg.backend.
+    _execute_command now sets it, and _filter_messages_for_backend also falls
+    back to msg.backend, so either names the destination.
     """
 
     def test_execute_command_sets_metadata_on_message(self, bot_with_symphony, sample_bot_command):
@@ -1076,3 +1078,59 @@ def test_discord_dm_message_uses_metadata_channel_id_without_channel(self, bot_w
         assert channel_id == "dm123"
         assert text == "!help"
         assert mentions == []
+
+
+class TestBackendFiltering:
+    """Which field names a message's destination."""
+
+    @staticmethod
+    def _routed_to(bot, backend, message):
+        """Run the filter node on one message and report whether it passed."""
+        import csp
+
+        @csp.graph
+        def g():
+            edge = csp.unroll(csp.const([message]))
+            csp.add_graph_output("out", bot._filter_messages_for_backend(backend, edge))
+
+        result = csp.run(g, starttime=datetime.now(UTC), endtime=timedelta(seconds=1))
+        return len(result.get("out", [])) == 1
+
+    def test_metadata_routes_a_message(self, bot_with_symphony):
+        message = Message(content="hi", metadata={"backend": "symphony"})
+
+        assert self._routed_to(bot_with_symphony, "symphony", message)
+        assert not self._routed_to(bot_with_symphony, "slack", message)
+
+    def test_backend_field_routes_a_message(self, bot_with_symphony):
+        """A message sent over REST sets `backend`, not metadata.
+
+        Before this fell back to the field, such a message was accepted by the
+        send endpoint and then silently dropped, reaching no backend at all.
+        """
+        message = Message(content="hi", backend="symphony")
+
+        assert self._routed_to(bot_with_symphony, "symphony", message)
+        assert not self._routed_to(bot_with_symphony, "slack", message)
+
+    def test_metadata_wins_over_the_field(self, bot_with_symphony):
+        """Inbound tagging writes metadata, so it stays authoritative."""
+        message = Message(content="hi", backend="slack", metadata={"backend": "symphony"})
+
+        assert self._routed_to(bot_with_symphony, "symphony", message)
+        assert not self._routed_to(bot_with_symphony, "slack", message)
+
+    def test_a_message_naming_no_backend_goes_nowhere(self, bot_with_symphony):
+        message = Message(content="hi")
+
+        assert not self._routed_to(bot_with_symphony, "symphony", message)
+
+
+class TestSendChannelOptIn:
+    """messages_out is only sendable when the config asks for it."""
+
+    def test_send_is_off_by_default(self):
+        assert BotConfig().allow_send_messages is False
+
+    def test_send_can_be_enabled(self):
+        assert BotConfig(allow_send_messages=True).allow_send_messages is True
diff --git a/docs/wiki/Advanced.md b/docs/wiki/Advanced.md
index d4e7f7d..c2895cf 100644
--- a/docs/wiki/Advanced.md
+++ b/docs/wiki/Advanced.md
@@ -9,3 +9,50 @@ More information coming soon!
 ## Running long-running commands in the background
 
 More information coming soon!
+
+## Sending messages over REST
+
+csp-bot runs on csp-gateway, which serves its channels over a REST API. With
+`allow_send_messages` turned on, `messages_out` becomes a send channel, so a job
+can post a message and have the bot deliver it:
+
+```yaml
+# @package _global_
+modules:
+  bot:
+    config:
+      allow_send_messages: true
+```
+
+```bash
+curl -X POST http://localhost:8000/api/v1/send/messages_out \
+  -H 'Content-Type: application/json' \
+  -H "token: $CSP_BOT_API_KEY" \
+  -d '[{"content": "<messageML>deploy finished</messageML>",
+        "backend": "symphony",
+        "channel": {"id": "..."}}]'
+```
+
+The `backend` field names where the message goes; `metadata["backend"]` does the
+same and takes precedence, since that is what inbound tagging writes. A message
+naming no backend reaches none of them.
+
+Content is whatever the destination expects, so build it with chatom and render
+it for the backend rather than writing markup by hand:
+
+```python
+from chatom import Format, MessageBuilder
+
+body = MessageBuilder().heading("Deploy finished", level=3).build()
+content = body.render(Format.SYMPHONY_MESSAGEML)
+```
+
+This is off by default. Turning it on makes the bot relay anything that reaches
+its API, so configure authentication at the same time — `MountAPIKeyMiddleware`
+rejects unauthenticated requests, but only once it has a key:
+
+```yaml
+modules:
+  mount_api_key_middleware:
+    api_key: ${oc.env:CSP_BOT_API_KEY}
+```