diff --git a/EvoScientist/channels/base.py b/EvoScientist/channels/base.py
index 9923cd2..edd475b 100644
--- a/EvoScientist/channels/base.py
+++ b/EvoScientist/channels/base.py
@@ -176,24 +176,31 @@ async def download_attachment(
         local_path = media_path(f"{prefix}{safe_name}")
 
         async with httpx.AsyncClient(proxy=proxy) as client:
-            resp = await client.get(url, headers=headers or {}, timeout=30)
-        if resp.status_code != 200:
-            return None, f"[attachment: {filename} - download failed]"
+            async with client.stream("GET", url, headers=headers or {}, timeout=30) as resp:
+                if resp.status_code != 200:
+                    return None, f"[attachment: {filename} - download failed]"
 
-        # Check Content-Length when file_size was not known beforehand
-        if file_size is None:
-            cl = resp.headers.get("content-length")
-            if cl:
-                try:
-                    too_large = check_attachment_size(int(cl), filename)
-                    if too_large:
-                        return None, too_large
-                except (ValueError, TypeError):
-                    pass
-            if len(resp.content) > MAX_ATTACHMENT_BYTES:
-                return None, check_attachment_size(len(resp.content), filename)
+                # Check Content-Length header before downloading body
+                if file_size is None:
+                    cl = resp.headers.get("content-length")
+                    if cl:
+                        try:
+                            too_large = check_attachment_size(int(cl), filename)
+                            if too_large:
+                                return None, too_large
+                        except (ValueError, TypeError):
+                            pass
 
-        local_path.write_bytes(resp.content)
+                # Stream body with incremental size check
+                chunks: list[bytes] = []
+                total = 0
+                async for chunk in resp.aiter_bytes():
+                    total += len(chunk)
+                    if total > MAX_ATTACHMENT_BYTES:
+                        return None, check_attachment_size(total, filename)
+                    chunks.append(chunk)
+
+        local_path.write_bytes(b"".join(chunks))
         return str(local_path), f"[attachment: {local_path}]"
     except Exception as e:
         _logger.warning(f"Failed to download attachment: {e}")
@@ -670,7 +677,12 @@ class Channel(ChannelPlugin, ABC):
 
     def _should_process(self, raw: RawIncoming) -> bool:
         """Decide whether to process a message based on mention gating."""
-        if not raw.is_group or self.require_mention == "off":
+        if self.require_mention == "off":
+            return True
+        if self.require_mention == "always":
+            return raw.was_mentioned
+        # "group" — require mention only in groups
+        if not raw.is_group:
             return True
         return raw.was_mentioned
 
diff --git a/EvoScientist/channels/capabilities.py b/EvoScientist/channels/capabilities.py
index df624e2..b125a22 100644
--- a/EvoScientist/channels/capabilities.py
+++ b/EvoScientist/channels/capabilities.py
@@ -199,7 +199,7 @@ SIGNAL = ChannelCapabilities(
 
 EMAIL = ChannelCapabilities(
     format_type="html",
-    max_text_length=0,  # no practical limit
+    max_text_length=999_999,  # no practical limit
     media_send=True,
     media_receive=True,
     html=True,
@@ -208,7 +208,7 @@ EMAIL = ChannelCapabilities(
 
 IMESSAGE = ChannelCapabilities(
     format_type="plain",
-    max_text_length=0,
+    max_text_length=999_999,
     typing=False,      # Apple does not expose typing indicator API
     media_send=True,
     media_receive=True,
diff --git a/EvoScientist/channels/channel_manager.py b/EvoScientist/channels/channel_manager.py
index 577614c..bc2ffb0 100644
--- a/EvoScientist/channels/channel_manager.py
+++ b/EvoScientist/channels/channel_manager.py
@@ -346,7 +346,10 @@ def build_inbound_pipeline(
     ))
 
     if plugin.pairing is not None:
-        middlewares.append(PairingMiddleware(channel_name=plugin.id))
+        middlewares.append(PairingMiddleware(
+            channel_name=plugin.id,
+            dm_policy=dm_policy,
+        ))
 
     if caps.groups:
         middlewares.append(GroupHistoryMiddleware())
diff --git a/EvoScientist/channels/consumer.py b/EvoScientist/channels/consumer.py
index a3ac15c..5fdfd28 100644
--- a/EvoScientist/channels/consumer.py
+++ b/EvoScientist/channels/consumer.py
@@ -158,7 +158,10 @@ class InboundConsumer:
                 # Evict oldest entry
                 oldest = next(iter(self._sessions))
                 del self._sessions[oldest]
-            self._sessions[sender_id] = self.thread_id or str(uuid.uuid4())
+            if self.thread_id:
+                self._sessions[sender_id] = f"{self.thread_id}:{sender_id}"
+            else:
+                self._sessions[sender_id] = str(uuid.uuid4())
         return self._sessions[sender_id]
 
     def _get_channel(self, channel_name: str) -> Channel | None:
@@ -361,7 +364,7 @@ class InboundConsumer:
                 await self.bus.publish_outbound(OutboundMessage(
                     channel=msg.channel,
                     chat_id=msg.chat_id,
-                    content=f"Error: {e}",
+                    content="Sorry, something went wrong. Please try again later.",
                     metadata=msg.metadata,
                 ))
             finally:
diff --git a/EvoScientist/channels/middleware.py b/EvoScientist/channels/middleware.py
index 7bfbf40..9ab67bc 100644
--- a/EvoScientist/channels/middleware.py
+++ b/EvoScientist/channels/middleware.py
@@ -511,6 +511,12 @@ class FormattingMiddleware(OutboundMiddlewareBase):
         """Convert text to channel format."""
         return self._formatter.format(text)
 
+    async def process_outbound(
+        self, message: OutboundMessage, context: dict[str, Any],
+    ) -> OutboundMessage | None:
+        formatted = self._formatter.format(message.content)
+        return dataclasses.replace(message, content=formatted)
+
 
 # ── Retry ────────────────────────────────────────────────────────────
 
@@ -662,11 +668,13 @@ class MentionGatingMiddleware(InboundMiddleware):
         return raw
 
     def _should_process(self, raw: RawIncoming) -> bool:
-        if not raw.is_group or self.require_mention == "off":
+        if self.require_mention == "off":
             return True
         if self.require_mention == "always":
             return raw.was_mentioned
-        # "group" — require mention in groups
+        # "group" — require mention only in groups
+        if not raw.is_group:
+            return True
         return raw.was_mentioned
 
 
@@ -779,10 +787,12 @@ class PairingMiddleware(InboundMiddleware):
         self,
         channel_name: str,
         send_response_fn: Callable[[str, str], Any] | None = None,
+        dm_policy: str = "allowlist",
     ) -> None:
         self._manager = PairingManager()
         self._channel_name = channel_name
         self._send_response_fn = send_response_fn
+        self._dm_policy = dm_policy
 
     async def process_inbound(
         self, raw: RawIncoming, context: dict[str, Any],
@@ -790,8 +800,7 @@ class PairingMiddleware(InboundMiddleware):
         if raw.is_group:
             return raw  # pairing only applies to DMs
 
-        dm_policy = context.get("dm_policy", "allowlist")
-        if dm_policy != "pairing":
+        if self._dm_policy != "pairing":
             return raw
 
         if self._manager.is_approved(self._channel_name, raw.sender_id):
diff --git a/channel_feature_matrix.xlsx b/channel_feature_matrix.xlsx
new file mode 100644
index 0000000..8fa95ad
Binary files /dev/null and b/channel_feature_matrix.xlsx differ
diff --git a/tests/test_channel_comprehensive.py b/tests/test_channel_comprehensive.py
index 933591b..ee45860 100644
--- a/tests/test_channel_comprehensive.py
+++ b/tests/test_channel_comprehensive.py
@@ -1108,7 +1108,7 @@ class TestInboundConsumer:
         assert tid1 == tid2
 
     def test_shared_thread_id_bug(self):
-        """[B-20] If thread_id is non-empty, all senders share the same session."""
+        """[B-20] If thread_id is non-empty, senders get unique thread IDs with shared prefix."""
         bus = MessageBus()
         mgr = ChannelManager(bus)
         mgr.register(StubChannel())
@@ -1118,8 +1118,10 @@ class TestInboundConsumer:
         )
         tid1 = consumer._get_thread_id("alice")
         tid2 = consumer._get_thread_id("bob")
-        # BUG: Both get the same thread_id
-        assert tid1 == tid2 == "shared_thread"
+        # Fixed: Each sender gets a unique thread_id using thread_id as prefix
+        assert tid1 != tid2
+        assert tid1 == "shared_thread:alice"
+        assert tid2 == "shared_thread:bob"
 
     def test_session_eviction_is_fifo_not_lru(self):
         """[B-19] Sessions evict oldest by insertion, not by access."""