diff --git a/tests/gateway/test_matrix.py b/tests/gateway/test_matrix.py
new file mode 100644
index 000000000..31e59caeb
--- /dev/null
+++ b/tests/gateway/test_matrix.py
@@ -0,0 +1,448 @@
+"""Tests for Matrix platform adapter."""
+import json
+import re
+import pytest
+from unittest.mock import MagicMock, patch, AsyncMock
+
+from gateway.config import Platform, PlatformConfig
+
+
+# ---------------------------------------------------------------------------
+# Platform & Config
+# ---------------------------------------------------------------------------
+
+class TestMatrixPlatformEnum:
+    def test_matrix_enum_exists(self):
+        assert Platform.MATRIX.value == "matrix"
+
+    def test_matrix_in_platform_list(self):
+        platforms = [p.value for p in Platform]
+        assert "matrix" in platforms
+
+
+class TestMatrixConfigLoading:
+    def test_apply_env_overrides_with_access_token(self, monkeypatch):
+        monkeypatch.setenv("MATRIX_ACCESS_TOKEN", "syt_abc123")
+        monkeypatch.setenv("MATRIX_HOMESERVER", "https://matrix.example.org")
+
+        from gateway.config import GatewayConfig, _apply_env_overrides
+        config = GatewayConfig()
+        _apply_env_overrides(config)
+
+        assert Platform.MATRIX in config.platforms
+        mc = config.platforms[Platform.MATRIX]
+        assert mc.enabled is True
+        assert mc.token == "syt_abc123"
+        assert mc.extra.get("homeserver") == "https://matrix.example.org"
+
+    def test_apply_env_overrides_with_password(self, monkeypatch):
+        monkeypatch.delenv("MATRIX_ACCESS_TOKEN", raising=False)
+        monkeypatch.setenv("MATRIX_PASSWORD", "secret123")
+        monkeypatch.setenv("MATRIX_HOMESERVER", "https://matrix.example.org")
+        monkeypatch.setenv("MATRIX_USER_ID", "@bot:example.org")
+
+        from gateway.config import GatewayConfig, _apply_env_overrides
+        config = GatewayConfig()
+        _apply_env_overrides(config)
+
+        assert Platform.MATRIX in config.platforms
+        mc = config.platforms[Platform.MATRIX]
+        assert mc.enabled is True
+        assert mc.extra.get("password") == "secret123"
+        assert mc.extra.get("user_id") == "@bot:example.org"
+
+    def test_matrix_not_loaded_without_creds(self, monkeypatch):
+        monkeypatch.delenv("MATRIX_ACCESS_TOKEN", raising=False)
+        monkeypatch.delenv("MATRIX_PASSWORD", raising=False)
+        monkeypatch.delenv("MATRIX_HOMESERVER", raising=False)
+
+        from gateway.config import GatewayConfig, _apply_env_overrides
+        config = GatewayConfig()
+        _apply_env_overrides(config)
+
+        assert Platform.MATRIX not in config.platforms
+
+    def test_matrix_encryption_flag(self, monkeypatch):
+        monkeypatch.setenv("MATRIX_ACCESS_TOKEN", "syt_abc123")
+        monkeypatch.setenv("MATRIX_HOMESERVER", "https://matrix.example.org")
+        monkeypatch.setenv("MATRIX_ENCRYPTION", "true")
+
+        from gateway.config import GatewayConfig, _apply_env_overrides
+        config = GatewayConfig()
+        _apply_env_overrides(config)
+
+        mc = config.platforms[Platform.MATRIX]
+        assert mc.extra.get("encryption") is True
+
+    def test_matrix_encryption_default_off(self, monkeypatch):
+        monkeypatch.setenv("MATRIX_ACCESS_TOKEN", "syt_abc123")
+        monkeypatch.setenv("MATRIX_HOMESERVER", "https://matrix.example.org")
+        monkeypatch.delenv("MATRIX_ENCRYPTION", raising=False)
+
+        from gateway.config import GatewayConfig, _apply_env_overrides
+        config = GatewayConfig()
+        _apply_env_overrides(config)
+
+        mc = config.platforms[Platform.MATRIX]
+        assert mc.extra.get("encryption") is False
+
+    def test_matrix_home_room(self, monkeypatch):
+        monkeypatch.setenv("MATRIX_ACCESS_TOKEN", "syt_abc123")
+        monkeypatch.setenv("MATRIX_HOMESERVER", "https://matrix.example.org")
+        monkeypatch.setenv("MATRIX_HOME_ROOM", "!room123:example.org")
+        monkeypatch.setenv("MATRIX_HOME_ROOM_NAME", "Bot Room")
+
+        from gateway.config import GatewayConfig, _apply_env_overrides
+        config = GatewayConfig()
+        _apply_env_overrides(config)
+
+        home = config.get_home_channel(Platform.MATRIX)
+        assert home is not None
+        assert home.chat_id == "!room123:example.org"
+        assert home.name == "Bot Room"
+
+    def test_matrix_user_id_stored_in_extra(self, monkeypatch):
+        monkeypatch.setenv("MATRIX_ACCESS_TOKEN", "syt_abc123")
+        monkeypatch.setenv("MATRIX_HOMESERVER", "https://matrix.example.org")
+        monkeypatch.setenv("MATRIX_USER_ID", "@hermes:example.org")
+
+        from gateway.config import GatewayConfig, _apply_env_overrides
+        config = GatewayConfig()
+        _apply_env_overrides(config)
+
+        mc = config.platforms[Platform.MATRIX]
+        assert mc.extra.get("user_id") == "@hermes:example.org"
+
+
+# ---------------------------------------------------------------------------
+# Adapter helpers
+# ---------------------------------------------------------------------------
+
+def _make_adapter():
+    """Create a MatrixAdapter with mocked config."""
+    from gateway.platforms.matrix import MatrixAdapter
+    config = PlatformConfig(
+        enabled=True,
+        token="syt_test_token",
+        extra={
+            "homeserver": "https://matrix.example.org",
+            "user_id": "@bot:example.org",
+        },
+    )
+    adapter = MatrixAdapter(config)
+    return adapter
+
+
+# ---------------------------------------------------------------------------
+# mxc:// URL conversion
+# ---------------------------------------------------------------------------
+
+class TestMatrixMxcToHttp:
+    def setup_method(self):
+        self.adapter = _make_adapter()
+
+    def test_basic_mxc_conversion(self):
+        """mxc://server/media_id should become an authenticated HTTP URL."""
+        mxc = "mxc://matrix.org/abc123"
+        result = self.adapter._mxc_to_http(mxc)
+        assert result == "https://matrix.example.org/_matrix/client/v1/media/download/matrix.org/abc123"
+
+    def test_mxc_with_different_server(self):
+        """mxc:// from a different server should still use our homeserver."""
+        mxc = "mxc://other.server/media456"
+        result = self.adapter._mxc_to_http(mxc)
+        assert result.startswith("https://matrix.example.org/")
+        assert "other.server/media456" in result
+
+    def test_non_mxc_url_passthrough(self):
+        """Non-mxc URLs should be returned unchanged."""
+        url = "https://example.com/image.png"
+        assert self.adapter._mxc_to_http(url) == url
+
+    def test_mxc_uses_client_v1_endpoint(self):
+        """Should use /_matrix/client/v1/media/download/ not the deprecated path."""
+        mxc = "mxc://example.com/test123"
+        result = self.adapter._mxc_to_http(mxc)
+        assert "/_matrix/client/v1/media/download/" in result
+        assert "/_matrix/media/v3/download/" not in result
+
+
+# ---------------------------------------------------------------------------
+# DM detection
+# ---------------------------------------------------------------------------
+
+class TestMatrixDmDetection:
+    def setup_method(self):
+        self.adapter = _make_adapter()
+
+    def test_room_in_m_direct_is_dm(self):
+        """A room listed in m.direct should be detected as DM."""
+        self.adapter._joined_rooms = {"!dm_room:ex.org", "!group_room:ex.org"}
+        self.adapter._dm_rooms = {
+            "!dm_room:ex.org": True,
+            "!group_room:ex.org": False,
+        }
+
+        assert self.adapter._dm_rooms.get("!dm_room:ex.org") is True
+        assert self.adapter._dm_rooms.get("!group_room:ex.org") is False
+
+    def test_unknown_room_not_in_cache(self):
+        """Unknown rooms should not be in the DM cache."""
+        self.adapter._dm_rooms = {}
+        assert self.adapter._dm_rooms.get("!unknown:ex.org") is None
+
+    @pytest.mark.asyncio
+    async def test_refresh_dm_cache_with_m_direct(self):
+        """_refresh_dm_cache should populate _dm_rooms from m.direct data."""
+        self.adapter._joined_rooms = {"!room_a:ex.org", "!room_b:ex.org", "!room_c:ex.org"}
+
+        mock_client = MagicMock()
+        mock_resp = MagicMock()
+        mock_resp.content = {
+            "@alice:ex.org": ["!room_a:ex.org"],
+            "@bob:ex.org": ["!room_b:ex.org"],
+        }
+        mock_client.get_account_data = AsyncMock(return_value=mock_resp)
+        self.adapter._client = mock_client
+
+        await self.adapter._refresh_dm_cache()
+
+        assert self.adapter._dm_rooms["!room_a:ex.org"] is True
+        assert self.adapter._dm_rooms["!room_b:ex.org"] is True
+        assert self.adapter._dm_rooms["!room_c:ex.org"] is False
+
+
+# ---------------------------------------------------------------------------
+# Reply fallback stripping
+# ---------------------------------------------------------------------------
+
+class TestMatrixReplyFallbackStripping:
+    """Test that Matrix reply fallback lines ('> ' prefix) are stripped."""
+
+    def setup_method(self):
+        self.adapter = _make_adapter()
+        self.adapter._user_id = "@bot:example.org"
+        self.adapter._startup_ts = 0.0
+        self.adapter._dm_rooms = {}
+        self.adapter._message_handler = AsyncMock()
+
+    def _strip_fallback(self, body: str, has_reply: bool = True) -> str:
+        """Simulate the reply fallback stripping logic from _on_room_message."""
+        reply_to = "some_event_id" if has_reply else None
+        if reply_to and body.startswith("> "):
+            lines = body.split("\n")
+            stripped = []
+            past_fallback = False
+            for line in lines:
+                if not past_fallback:
+                    if line.startswith("> ") or line == ">":
+                        continue
+                    if line == "":
+                        past_fallback = True
+                        continue
+                    past_fallback = True
+                stripped.append(line)
+            body = "\n".join(stripped) if stripped else body
+        return body
+
+    def test_simple_reply_fallback(self):
+        body = "> <@alice:ex.org> Original message\n\nActual reply"
+        result = self._strip_fallback(body)
+        assert result == "Actual reply"
+
+    def test_multiline_reply_fallback(self):
+        body = "> <@alice:ex.org> Line 1\n> Line 2\n\nMy response"
+        result = self._strip_fallback(body)
+        assert result == "My response"
+
+    def test_no_reply_fallback_preserved(self):
+        body = "Just a normal message"
+        result = self._strip_fallback(body, has_reply=False)
+        assert result == "Just a normal message"
+
+    def test_quote_without_reply_preserved(self):
+        """'> ' lines without a reply_to context should be preserved."""
+        body = "> This is a blockquote"
+        result = self._strip_fallback(body, has_reply=False)
+        assert result == "> This is a blockquote"
+
+    def test_empty_fallback_separator(self):
+        """The blank line between fallback and actual content should be stripped."""
+        body = "> <@alice:ex.org> hi\n>\n\nResponse"
+        result = self._strip_fallback(body)
+        assert result == "Response"
+
+    def test_multiline_response_after_fallback(self):
+        body = "> <@alice:ex.org> Original\n\nLine 1\nLine 2\nLine 3"
+        result = self._strip_fallback(body)
+        assert result == "Line 1\nLine 2\nLine 3"
+
+
+# ---------------------------------------------------------------------------
+# Thread detection
+# ---------------------------------------------------------------------------
+
+class TestMatrixThreadDetection:
+    def test_thread_id_from_m_relates_to(self):
+        """m.relates_to with rel_type=m.thread should extract the event_id."""
+        relates_to = {
+            "rel_type": "m.thread",
+            "event_id": "$thread_root_event",
+            "is_falling_back": True,
+            "m.in_reply_to": {"event_id": "$some_event"},
+        }
+        # Simulate the extraction logic from _on_room_message
+        thread_id = None
+        if relates_to.get("rel_type") == "m.thread":
+            thread_id = relates_to.get("event_id")
+        assert thread_id == "$thread_root_event"
+
+    def test_no_thread_for_reply(self):
+        """m.in_reply_to without m.thread should not set thread_id."""
+        relates_to = {
+            "m.in_reply_to": {"event_id": "$reply_event"},
+        }
+        thread_id = None
+        if relates_to.get("rel_type") == "m.thread":
+            thread_id = relates_to.get("event_id")
+        assert thread_id is None
+
+    def test_no_thread_for_edit(self):
+        """m.replace relation should not set thread_id."""
+        relates_to = {
+            "rel_type": "m.replace",
+            "event_id": "$edited_event",
+        }
+        thread_id = None
+        if relates_to.get("rel_type") == "m.thread":
+            thread_id = relates_to.get("event_id")
+        assert thread_id is None
+
+    def test_empty_relates_to(self):
+        """Empty m.relates_to should not set thread_id."""
+        relates_to = {}
+        thread_id = None
+        if relates_to.get("rel_type") == "m.thread":
+            thread_id = relates_to.get("event_id")
+        assert thread_id is None
+
+
+# ---------------------------------------------------------------------------
+# Format message
+# ---------------------------------------------------------------------------
+
+class TestMatrixFormatMessage:
+    def setup_method(self):
+        self.adapter = _make_adapter()
+
+    def test_image_markdown_stripped(self):
+        """![alt](url) should be converted to just the URL."""
+        result = self.adapter.format_message("![cat](https://img.example.com/cat.png)")
+        assert result == "https://img.example.com/cat.png"
+
+    def test_regular_markdown_preserved(self):
+        """Standard markdown should be preserved (Matrix supports it)."""
+        content = "**bold** and *italic* and `code`"
+        assert self.adapter.format_message(content) == content
+
+    def test_plain_text_unchanged(self):
+        content = "Hello, world!"
+        assert self.adapter.format_message(content) == content
+
+    def test_multiple_images_stripped(self):
+        content = "![a](http://a.com/1.png) and ![b](http://b.com/2.png)"
+        result = self.adapter.format_message(content)
+        assert "![" not in result
+        assert "http://a.com/1.png" in result
+        assert "http://b.com/2.png" in result
+
+
+# ---------------------------------------------------------------------------
+# Markdown to HTML conversion
+# ---------------------------------------------------------------------------
+
+class TestMatrixMarkdownToHtml:
+    def setup_method(self):
+        self.adapter = _make_adapter()
+
+    def test_bold_conversion(self):
+        """**bold** should produce <strong> tags."""
+        result = self.adapter._markdown_to_html("**bold**")
+        assert "<strong>" in result or "<b>" in result
+        assert "bold" in result
+
+    def test_italic_conversion(self):
+        """*italic* should produce <em> tags."""
+        result = self.adapter._markdown_to_html("*italic*")
+        assert "<em>" in result or "<i>" in result
+
+    def test_inline_code(self):
+        """`code` should produce <code> tags."""
+        result = self.adapter._markdown_to_html("`code`")
+        assert "<code>" in result
+
+    def test_plain_text_returns_html(self):
+        """Plain text should still be returned (possibly with <br> or <p>)."""
+        result = self.adapter._markdown_to_html("Hello world")
+        assert "Hello world" in result
+
+
+# ---------------------------------------------------------------------------
+# Helper: display name extraction
+# ---------------------------------------------------------------------------
+
+class TestMatrixDisplayName:
+    def setup_method(self):
+        self.adapter = _make_adapter()
+
+    def test_get_display_name_from_room_users(self):
+        """Should get display name from room's users dict."""
+        mock_room = MagicMock()
+        mock_user = MagicMock()
+        mock_user.display_name = "Alice"
+        mock_room.users = {"@alice:ex.org": mock_user}
+
+        name = self.adapter._get_display_name(mock_room, "@alice:ex.org")
+        assert name == "Alice"
+
+    def test_get_display_name_fallback_to_localpart(self):
+        """Should extract localpart from @user:server format."""
+        mock_room = MagicMock()
+        mock_room.users = {}
+
+        name = self.adapter._get_display_name(mock_room, "@bob:example.org")
+        assert name == "bob"
+
+    def test_get_display_name_no_room(self):
+        """Should handle None room gracefully."""
+        name = self.adapter._get_display_name(None, "@charlie:ex.org")
+        assert name == "charlie"
+
+
+# ---------------------------------------------------------------------------
+# Requirements check
+# ---------------------------------------------------------------------------
+
+class TestMatrixRequirements:
+    def test_check_requirements_with_token(self, monkeypatch):
+        monkeypatch.setenv("MATRIX_ACCESS_TOKEN", "syt_test")
+        monkeypatch.setenv("MATRIX_HOMESERVER", "https://matrix.example.org")
+        from gateway.platforms.matrix import check_matrix_requirements
+        try:
+            import nio  # noqa: F401
+            assert check_matrix_requirements() is True
+        except ImportError:
+            assert check_matrix_requirements() is False
+
+    def test_check_requirements_without_creds(self, monkeypatch):
+        monkeypatch.delenv("MATRIX_ACCESS_TOKEN", raising=False)
+        monkeypatch.delenv("MATRIX_PASSWORD", raising=False)
+        monkeypatch.delenv("MATRIX_HOMESERVER", raising=False)
+        from gateway.platforms.matrix import check_matrix_requirements
+        assert check_matrix_requirements() is False
+
+    def test_check_requirements_without_homeserver(self, monkeypatch):
+        monkeypatch.setenv("MATRIX_ACCESS_TOKEN", "syt_test")
+        monkeypatch.delenv("MATRIX_HOMESERVER", raising=False)
+        from gateway.platforms.matrix import check_matrix_requirements
+        assert check_matrix_requirements() is False
diff --git a/tests/gateway/test_mattermost.py b/tests/gateway/test_mattermost.py
new file mode 100644
index 000000000..6b0fbd899
--- /dev/null
+++ b/tests/gateway/test_mattermost.py
@@ -0,0 +1,574 @@
+"""Tests for Mattermost platform adapter."""
+import json
+import time
+import pytest
+from unittest.mock import MagicMock, patch, AsyncMock
+
+from gateway.config import Platform, PlatformConfig
+
+
+# ---------------------------------------------------------------------------
+# Platform & Config
+# ---------------------------------------------------------------------------
+
+class TestMattermostPlatformEnum:
+    def test_mattermost_enum_exists(self):
+        assert Platform.MATTERMOST.value == "mattermost"
+
+    def test_mattermost_in_platform_list(self):
+        platforms = [p.value for p in Platform]
+        assert "mattermost" in platforms
+
+
+class TestMattermostConfigLoading:
+    def test_apply_env_overrides_mattermost(self, monkeypatch):
+        monkeypatch.setenv("MATTERMOST_TOKEN", "mm-tok-abc123")
+        monkeypatch.setenv("MATTERMOST_URL", "https://mm.example.com")
+
+        from gateway.config import GatewayConfig, _apply_env_overrides
+        config = GatewayConfig()
+        _apply_env_overrides(config)
+
+        assert Platform.MATTERMOST in config.platforms
+        mc = config.platforms[Platform.MATTERMOST]
+        assert mc.enabled is True
+        assert mc.token == "mm-tok-abc123"
+        assert mc.extra.get("url") == "https://mm.example.com"
+
+    def test_mattermost_not_loaded_without_token(self, monkeypatch):
+        monkeypatch.delenv("MATTERMOST_TOKEN", raising=False)
+        monkeypatch.delenv("MATTERMOST_URL", raising=False)
+
+        from gateway.config import GatewayConfig, _apply_env_overrides
+        config = GatewayConfig()
+        _apply_env_overrides(config)
+
+        assert Platform.MATTERMOST not in config.platforms
+
+    def test_connected_platforms_includes_mattermost(self, monkeypatch):
+        monkeypatch.setenv("MATTERMOST_TOKEN", "mm-tok-abc123")
+        monkeypatch.setenv("MATTERMOST_URL", "https://mm.example.com")
+
+        from gateway.config import GatewayConfig, _apply_env_overrides
+        config = GatewayConfig()
+        _apply_env_overrides(config)
+
+        connected = config.get_connected_platforms()
+        assert Platform.MATTERMOST in connected
+
+    def test_mattermost_home_channel(self, monkeypatch):
+        monkeypatch.setenv("MATTERMOST_TOKEN", "mm-tok-abc123")
+        monkeypatch.setenv("MATTERMOST_URL", "https://mm.example.com")
+        monkeypatch.setenv("MATTERMOST_HOME_CHANNEL", "ch_abc123")
+        monkeypatch.setenv("MATTERMOST_HOME_CHANNEL_NAME", "General")
+
+        from gateway.config import GatewayConfig, _apply_env_overrides
+        config = GatewayConfig()
+        _apply_env_overrides(config)
+
+        home = config.get_home_channel(Platform.MATTERMOST)
+        assert home is not None
+        assert home.chat_id == "ch_abc123"
+        assert home.name == "General"
+
+    def test_mattermost_url_warning_without_url(self, monkeypatch):
+        """MATTERMOST_TOKEN set but MATTERMOST_URL missing should still load."""
+        monkeypatch.setenv("MATTERMOST_TOKEN", "mm-tok-abc123")
+        monkeypatch.delenv("MATTERMOST_URL", raising=False)
+
+        from gateway.config import GatewayConfig, _apply_env_overrides
+        config = GatewayConfig()
+        _apply_env_overrides(config)
+
+        assert Platform.MATTERMOST in config.platforms
+        assert config.platforms[Platform.MATTERMOST].extra.get("url") == ""
+
+
+# ---------------------------------------------------------------------------
+# Adapter format / truncate
+# ---------------------------------------------------------------------------
+
+def _make_adapter():
+    """Create a MattermostAdapter with mocked config."""
+    from gateway.platforms.mattermost import MattermostAdapter
+    config = PlatformConfig(
+        enabled=True,
+        token="test-token",
+        extra={"url": "https://mm.example.com"},
+    )
+    adapter = MattermostAdapter(config)
+    return adapter
+
+
+class TestMattermostFormatMessage:
+    def setup_method(self):
+        self.adapter = _make_adapter()
+
+    def test_image_markdown_to_url(self):
+        """![alt](url) should be converted to just the URL."""
+        result = self.adapter.format_message("![cat](https://img.example.com/cat.png)")
+        assert result == "https://img.example.com/cat.png"
+
+    def test_image_markdown_strips_alt_text(self):
+        result = self.adapter.format_message("Here: ![my image](https://x.com/a.jpg) done")
+        assert "![" not in result
+        assert "https://x.com/a.jpg" in result
+
+    def test_regular_markdown_preserved(self):
+        """Regular markdown (bold, italic, code) should be kept as-is."""
+        content = "**bold** and *italic* and `code`"
+        assert self.adapter.format_message(content) == content
+
+    def test_regular_links_preserved(self):
+        """Non-image links should be preserved."""
+        content = "[click](https://example.com)"
+        assert self.adapter.format_message(content) == content
+
+    def test_plain_text_unchanged(self):
+        content = "Hello, world!"
+        assert self.adapter.format_message(content) == content
+
+    def test_multiple_images(self):
+        content = "![a](http://a.com/1.png) text ![b](http://b.com/2.png)"
+        result = self.adapter.format_message(content)
+        assert "![" not in result
+        assert "http://a.com/1.png" in result
+        assert "http://b.com/2.png" in result
+
+
+class TestMattermostTruncateMessage:
+    def setup_method(self):
+        self.adapter = _make_adapter()
+
+    def test_short_message_single_chunk(self):
+        msg = "Hello, world!"
+        chunks = self.adapter.truncate_message(msg, 4000)
+        assert len(chunks) == 1
+        assert chunks[0] == msg
+
+    def test_long_message_splits(self):
+        msg = "a " * 2500  # 5000 chars
+        chunks = self.adapter.truncate_message(msg, 4000)
+        assert len(chunks) >= 2
+        for chunk in chunks:
+            assert len(chunk) <= 4000
+
+    def test_custom_max_length(self):
+        msg = "Hello " * 20
+        chunks = self.adapter.truncate_message(msg, max_length=50)
+        assert all(len(c) <= 50 for c in chunks)
+
+    def test_exactly_at_limit(self):
+        msg = "x" * 4000
+        chunks = self.adapter.truncate_message(msg, 4000)
+        assert len(chunks) == 1
+
+
+# ---------------------------------------------------------------------------
+# Send
+# ---------------------------------------------------------------------------
+
+class TestMattermostSend:
+    def setup_method(self):
+        self.adapter = _make_adapter()
+        self.adapter._session = MagicMock()
+
+    @pytest.mark.asyncio
+    async def test_send_calls_api_post(self):
+        """send() should POST to /api/v4/posts with channel_id and message."""
+        mock_resp = AsyncMock()
+        mock_resp.status = 200
+        mock_resp.json = AsyncMock(return_value={"id": "post123"})
+        mock_resp.text = AsyncMock(return_value="")
+        mock_resp.__aenter__ = AsyncMock(return_value=mock_resp)
+        mock_resp.__aexit__ = AsyncMock(return_value=False)
+
+        self.adapter._session.post = MagicMock(return_value=mock_resp)
+
+        result = await self.adapter.send("channel_1", "Hello!")
+
+        assert result.success is True
+        assert result.message_id == "post123"
+
+        # Verify post was called with correct URL
+        call_args = self.adapter._session.post.call_args
+        assert "/api/v4/posts" in call_args[0][0]
+        # Verify payload
+        payload = call_args[1]["json"]
+        assert payload["channel_id"] == "channel_1"
+        assert payload["message"] == "Hello!"
+
+    @pytest.mark.asyncio
+    async def test_send_empty_content_succeeds(self):
+        """Empty content should return success without calling the API."""
+        result = await self.adapter.send("channel_1", "")
+        assert result.success is True
+
+    @pytest.mark.asyncio
+    async def test_send_with_thread_reply(self):
+        """When reply_mode is 'thread', reply_to should become root_id."""
+        self.adapter._reply_mode = "thread"
+
+        mock_resp = AsyncMock()
+        mock_resp.status = 200
+        mock_resp.json = AsyncMock(return_value={"id": "post456"})
+        mock_resp.text = AsyncMock(return_value="")
+        mock_resp.__aenter__ = AsyncMock(return_value=mock_resp)
+        mock_resp.__aexit__ = AsyncMock(return_value=False)
+
+        self.adapter._session.post = MagicMock(return_value=mock_resp)
+
+        result = await self.adapter.send("channel_1", "Reply!", reply_to="root_post")
+
+        assert result.success is True
+        payload = self.adapter._session.post.call_args[1]["json"]
+        assert payload["root_id"] == "root_post"
+
+    @pytest.mark.asyncio
+    async def test_send_without_thread_no_root_id(self):
+        """When reply_mode is 'off', reply_to should NOT set root_id."""
+        self.adapter._reply_mode = "off"
+
+        mock_resp = AsyncMock()
+        mock_resp.status = 200
+        mock_resp.json = AsyncMock(return_value={"id": "post789"})
+        mock_resp.text = AsyncMock(return_value="")
+        mock_resp.__aenter__ = AsyncMock(return_value=mock_resp)
+        mock_resp.__aexit__ = AsyncMock(return_value=False)
+
+        self.adapter._session.post = MagicMock(return_value=mock_resp)
+
+        result = await self.adapter.send("channel_1", "Reply!", reply_to="root_post")
+
+        assert result.success is True
+        payload = self.adapter._session.post.call_args[1]["json"]
+        assert "root_id" not in payload
+
+    @pytest.mark.asyncio
+    async def test_send_api_failure(self):
+        """When API returns error, send should return failure."""
+        mock_resp = AsyncMock()
+        mock_resp.status = 500
+        mock_resp.json = AsyncMock(return_value={})
+        mock_resp.text = AsyncMock(return_value="Internal Server Error")
+        mock_resp.__aenter__ = AsyncMock(return_value=mock_resp)
+        mock_resp.__aexit__ = AsyncMock(return_value=False)
+
+        self.adapter._session.post = MagicMock(return_value=mock_resp)
+
+        result = await self.adapter.send("channel_1", "Hello!")
+
+        assert result.success is False
+
+
+# ---------------------------------------------------------------------------
+# WebSocket event parsing
+# ---------------------------------------------------------------------------
+
+class TestMattermostWebSocketParsing:
+    def setup_method(self):
+        self.adapter = _make_adapter()
+        self.adapter._bot_user_id = "bot_user_id"
+        # Mock handle_message to capture the MessageEvent without processing
+        self.adapter.handle_message = AsyncMock()
+
+    @pytest.mark.asyncio
+    async def test_parse_posted_event(self):
+        """'posted' events should extract message from double-encoded post JSON."""
+        post_data = {
+            "id": "post_abc",
+            "user_id": "user_123",
+            "channel_id": "chan_456",
+            "message": "Hello from Matrix!",
+        }
+        event = {
+            "event": "posted",
+            "data": {
+                "post": json.dumps(post_data),  # double-encoded JSON string
+                "channel_type": "O",
+                "sender_name": "@alice",
+            },
+        }
+
+        await self.adapter._handle_ws_event(event)
+        assert self.adapter.handle_message.called
+        msg_event = self.adapter.handle_message.call_args[0][0]
+        assert msg_event.text == "Hello from Matrix!"
+        assert msg_event.message_id == "post_abc"
+
+    @pytest.mark.asyncio
+    async def test_ignore_own_messages(self):
+        """Messages from the bot's own user_id should be ignored."""
+        post_data = {
+            "id": "post_self",
+            "user_id": "bot_user_id",  # same as bot
+            "channel_id": "chan_456",
+            "message": "Bot echo",
+        }
+        event = {
+            "event": "posted",
+            "data": {
+                "post": json.dumps(post_data),
+                "channel_type": "O",
+            },
+        }
+
+        await self.adapter._handle_ws_event(event)
+        assert not self.adapter.handle_message.called
+
+    @pytest.mark.asyncio
+    async def test_ignore_non_posted_events(self):
+        """Non-'posted' events should be ignored."""
+        event = {
+            "event": "typing",
+            "data": {"user_id": "user_123"},
+        }
+
+        await self.adapter._handle_ws_event(event)
+        assert not self.adapter.handle_message.called
+
+    @pytest.mark.asyncio
+    async def test_ignore_system_posts(self):
+        """Posts with a 'type' field (system messages) should be ignored."""
+        post_data = {
+            "id": "sys_post",
+            "user_id": "user_123",
+            "channel_id": "chan_456",
+            "message": "user joined",
+            "type": "system_join_channel",
+        }
+        event = {
+            "event": "posted",
+            "data": {
+                "post": json.dumps(post_data),
+                "channel_type": "O",
+            },
+        }
+
+        await self.adapter._handle_ws_event(event)
+        assert not self.adapter.handle_message.called
+
+    @pytest.mark.asyncio
+    async def test_channel_type_mapping(self):
+        """channel_type 'D' should map to 'dm'."""
+        post_data = {
+            "id": "post_dm",
+            "user_id": "user_123",
+            "channel_id": "chan_dm",
+            "message": "DM message",
+        }
+        event = {
+            "event": "posted",
+            "data": {
+                "post": json.dumps(post_data),
+                "channel_type": "D",
+                "sender_name": "@bob",
+            },
+        }
+
+        await self.adapter._handle_ws_event(event)
+        assert self.adapter.handle_message.called
+        msg_event = self.adapter.handle_message.call_args[0][0]
+        assert msg_event.source.chat_type == "dm"
+
+    @pytest.mark.asyncio
+    async def test_thread_id_from_root_id(self):
+        """Post with root_id should have thread_id set."""
+        post_data = {
+            "id": "post_reply",
+            "user_id": "user_123",
+            "channel_id": "chan_456",
+            "message": "Thread reply",
+            "root_id": "root_post_123",
+        }
+        event = {
+            "event": "posted",
+            "data": {
+                "post": json.dumps(post_data),
+                "channel_type": "O",
+                "sender_name": "@alice",
+            },
+        }
+
+        await self.adapter._handle_ws_event(event)
+        assert self.adapter.handle_message.called
+        msg_event = self.adapter.handle_message.call_args[0][0]
+        assert msg_event.source.thread_id == "root_post_123"
+
+    @pytest.mark.asyncio
+    async def test_invalid_post_json_ignored(self):
+        """Invalid JSON in data.post should be silently ignored."""
+        event = {
+            "event": "posted",
+            "data": {
+                "post": "not-valid-json{{{",
+                "channel_type": "O",
+            },
+        }
+
+        await self.adapter._handle_ws_event(event)
+        assert not self.adapter.handle_message.called
+
+
+# ---------------------------------------------------------------------------
+# File upload (send_image)
+# ---------------------------------------------------------------------------
+
+class TestMattermostFileUpload:
+    def setup_method(self):
+        self.adapter = _make_adapter()
+        self.adapter._session = MagicMock()
+
+    @pytest.mark.asyncio
+    async def test_send_image_downloads_and_uploads(self):
+        """send_image should download the URL, upload via /api/v4/files, then post."""
+        # Mock the download (GET)
+        mock_dl_resp = AsyncMock()
+        mock_dl_resp.status = 200
+        mock_dl_resp.read = AsyncMock(return_value=b"\x89PNG\x00fake-image-data")
+        mock_dl_resp.content_type = "image/png"
+        mock_dl_resp.__aenter__ = AsyncMock(return_value=mock_dl_resp)
+        mock_dl_resp.__aexit__ = AsyncMock(return_value=False)
+
+        # Mock the upload (POST to /files)
+        mock_upload_resp = AsyncMock()
+        mock_upload_resp.status = 200
+        mock_upload_resp.json = AsyncMock(return_value={
+            "file_infos": [{"id": "file_abc123"}]
+        })
+        mock_upload_resp.text = AsyncMock(return_value="")
+        mock_upload_resp.__aenter__ = AsyncMock(return_value=mock_upload_resp)
+        mock_upload_resp.__aexit__ = AsyncMock(return_value=False)
+
+        # Mock the post (POST to /posts)
+        mock_post_resp = AsyncMock()
+        mock_post_resp.status = 200
+        mock_post_resp.json = AsyncMock(return_value={"id": "post_with_file"})
+        mock_post_resp.text = AsyncMock(return_value="")
+        mock_post_resp.__aenter__ = AsyncMock(return_value=mock_post_resp)
+        mock_post_resp.__aexit__ = AsyncMock(return_value=False)
+
+        # Route calls: first GET (download), then POST (upload), then POST (create post)
+        self.adapter._session.get = MagicMock(return_value=mock_dl_resp)
+        post_call_count = 0
+        original_post_returns = [mock_upload_resp, mock_post_resp]
+
+        def post_side_effect(*args, **kwargs):
+            nonlocal post_call_count
+            resp = original_post_returns[min(post_call_count, len(original_post_returns) - 1)]
+            post_call_count += 1
+            return resp
+
+        self.adapter._session.post = MagicMock(side_effect=post_side_effect)
+
+        result = await self.adapter.send_image(
+            "channel_1", "https://img.example.com/cat.png", caption="A cat"
+        )
+
+        assert result.success is True
+        assert result.message_id == "post_with_file"
+
+
+# ---------------------------------------------------------------------------
+# Dedup cache
+# ---------------------------------------------------------------------------
+
+class TestMattermostDedup:
+    def setup_method(self):
+        self.adapter = _make_adapter()
+        self.adapter._bot_user_id = "bot_user_id"
+        # Mock handle_message to capture calls without processing
+        self.adapter.handle_message = AsyncMock()
+
+    @pytest.mark.asyncio
+    async def test_duplicate_post_ignored(self):
+        """The same post_id within the TTL window should be ignored."""
+        post_data = {
+            "id": "post_dup",
+            "user_id": "user_123",
+            "channel_id": "chan_456",
+            "message": "Hello!",
+        }
+        event = {
+            "event": "posted",
+            "data": {
+                "post": json.dumps(post_data),
+                "channel_type": "O",
+                "sender_name": "@alice",
+            },
+        }
+
+        # First time: should process
+        await self.adapter._handle_ws_event(event)
+        assert self.adapter.handle_message.call_count == 1
+
+        # Second time (same post_id): should be deduped
+        await self.adapter._handle_ws_event(event)
+        assert self.adapter.handle_message.call_count == 1  # still 1
+
+    @pytest.mark.asyncio
+    async def test_different_post_ids_both_processed(self):
+        """Different post IDs should both be processed."""
+        for i, pid in enumerate(["post_a", "post_b"]):
+            post_data = {
+                "id": pid,
+                "user_id": "user_123",
+                "channel_id": "chan_456",
+                "message": f"Message {i}",
+            }
+            event = {
+                "event": "posted",
+                "data": {
+                    "post": json.dumps(post_data),
+                    "channel_type": "O",
+                    "sender_name": "@alice",
+                },
+            }
+            await self.adapter._handle_ws_event(event)
+
+        assert self.adapter.handle_message.call_count == 2
+
+    def test_prune_seen_clears_expired(self):
+        """_prune_seen should remove entries older than _SEEN_TTL."""
+        now = time.time()
+        # Fill with enough expired entries to trigger pruning
+        for i in range(self.adapter._SEEN_MAX + 10):
+            self.adapter._seen_posts[f"old_{i}"] = now - 600  # 10 min ago
+
+        # Add a fresh one
+        self.adapter._seen_posts["fresh"] = now
+
+        self.adapter._prune_seen()
+
+        # Old entries should be pruned, fresh one kept
+        assert "fresh" in self.adapter._seen_posts
+        assert len(self.adapter._seen_posts) < self.adapter._SEEN_MAX
+
+    def test_seen_cache_tracks_post_ids(self):
+        """Posts are tracked in _seen_posts dict."""
+        self.adapter._seen_posts["test_post"] = time.time()
+        assert "test_post" in self.adapter._seen_posts
+
+
+# ---------------------------------------------------------------------------
+# Requirements check
+# ---------------------------------------------------------------------------
+
+class TestMattermostRequirements:
+    def test_check_requirements_with_token_and_url(self, monkeypatch):
+        monkeypatch.setenv("MATTERMOST_TOKEN", "test-token")
+        monkeypatch.setenv("MATTERMOST_URL", "https://mm.example.com")
+        from gateway.platforms.mattermost import check_mattermost_requirements
+        assert check_mattermost_requirements() is True
+
+    def test_check_requirements_without_token(self, monkeypatch):
+        monkeypatch.delenv("MATTERMOST_TOKEN", raising=False)
+        monkeypatch.delenv("MATTERMOST_URL", raising=False)
+        from gateway.platforms.mattermost import check_mattermost_requirements
+        assert check_mattermost_requirements() is False
+
+    def test_check_requirements_without_url(self, monkeypatch):
+        monkeypatch.setenv("MATTERMOST_TOKEN", "test-token")
+        monkeypatch.delenv("MATTERMOST_URL", raising=False)
+        from gateway.platforms.mattermost import check_mattermost_requirements
+        assert check_mattermost_requirements() is False