fix(gateway): remove DM thread session seeding to prevent cross-thread contamination

The session store was copying the ENTIRE parent DM transcript into new
thread sessions. This caused unrelated conversations to bleed across
threads in Slack DMs.

The Slack adapter already handles thread context correctly via
_fetch_thread_context() (conversations.replies API), which fetches
only the actual thread messages. The session-level seeding was both
redundant and harmful.

No other platform (Telegram, Discord) uses DM threads, so the seeding
code path was only triggered by Slack — where it conflicted with the
adapter-level context.

Tests updated to assert thread isolation: all thread sessions start
empty, platform adapters are responsible for injecting thread context.

Salvage of PR #5868 (jarvisxyz). Reported by norbert on Discord.

gateway/session.py

@@ -770,41 +770,6 @@ class SessionStore:
             except Exception as e:
                 print(f"[gateway] Warning: Failed to create SQLite session: {e}")
 
-        # Seed new DM thread sessions with parent DM session history.
-        # When a bot reply creates a Slack thread and the user responds in it,
-        # the thread gets a new session (keyed by thread_ts).  Without seeding,
-        # the thread session starts with zero context — the user's original
-        # question and the bot's answer are invisible.  Fix: copy the parent
-        # DM session's transcript into the new thread session so context carries
-        # over while still keeping threads isolated from each other.
-        if (
-            source.chat_type == "dm"
-            and source.thread_id
-            and entry.created_at == entry.updated_at  # brand-new session
-            and not was_auto_reset
-        ):
-            parent_source = SessionSource(
-                platform=source.platform,
-                chat_id=source.chat_id,
-                chat_type="dm",
-                user_id=source.user_id,
-                # no thread_id — this is the parent DM session
-            )
-            parent_key = self._generate_session_key(parent_source)
-            with self._lock:
-                parent_entry = self._entries.get(parent_key)
-            if parent_entry and parent_entry.session_id != entry.session_id:
-                try:
-                    parent_history = self.load_transcript(parent_entry.session_id)
-                    if parent_history:
-                        self.rewrite_transcript(entry.session_id, parent_history)
-                        logger.info(
-                            "[Session] Seeded DM thread session %s with %d messages from parent %s",
-                            entry.session_id, len(parent_history), parent_entry.session_id,
-                        )
-                except Exception as e:
-                    logger.warning("[Session] Failed to seed thread session: %s", e)
-
         return entry
 
     def update_session(

tests/gateway/test_session_dm_thread_seeding.py

@@ -1,19 +1,17 @@
-"""Tests for DM thread session seeding.
+"""Tests for DM thread session isolation.
 
-When a bot reply creates a thread in a DM (e.g. Slack), the user's reply
-in that thread gets a new session (keyed by thread_ts). The seeding logic
-copies the parent DM session's transcript into the new thread session so
-the bot retains context of the original conversation.
+DM thread sessions must start empty — no parent transcript seeding.
+Thread context is handled by platform adapters (e.g. Slack's
+_fetch_thread_context fetches actual thread replies via the API).
+Session-level seeding was removed because it copied the ENTIRE parent
+DM transcript, causing unrelated conversations to bleed across threads.
 
 Covers:
-- Basic seeding: parent transcript copied to new thread session
-- No seeding for group/channel chats
-- No seeding when parent session doesn't exist
-- No seeding on auto-reset sessions
-- No seeding on existing (non-new) thread sessions
-- Parent transcript is not mutated by seeding
-- Multiple threads from same parent each get independent copies
-- Cross-platform: works for any platform with DM threads (Slack, Telegram, Discord)
+- Thread sessions start empty (no parent seeding)
+- Group/channel thread sessions also start empty
+- Multiple threads from same parent are independent
+- Existing thread sessions are not mutated on re-access
+- Cross-platform: consistent behavior for Slack, Telegram, Discord
 """
 
 import pytest
@@ -60,48 +58,41 @@ PARENT_HISTORY = [
 ]
 
 
-class TestDMThreadSeeding:
-    """Core seeding behavior."""
+class TestDMThreadIsolation:
+    """Thread sessions must start empty — no parent transcript seeding."""
 
-    def test_thread_session_seeded_from_parent(self, store):
-        """New DM thread session should contain the parent's transcript."""
-        # Create parent DM session with history
+    def test_thread_session_starts_empty(self, store):
+        """New DM thread session should NOT inherit parent's transcript."""
         parent_source = _dm_source()
         parent_entry = store.get_or_create_session(parent_source)
         for msg in PARENT_HISTORY:
             store.append_to_transcript(parent_entry.session_id, msg)
 
-        # Create thread session (user replied in thread)
         thread_source = _dm_source(thread_id="1234567890.000001")
         thread_entry = store.get_or_create_session(thread_source)
 
-        # Thread should have parent's history
         thread_transcript = store.load_transcript(thread_entry.session_id)
-        assert len(thread_transcript) == 2
-        assert thread_transcript[0]["content"] == "What's the weather?"
-        assert thread_transcript[1]["content"] == "It's sunny and 72°F."
+        assert len(thread_transcript) == 0
 
-    def test_parent_transcript_not_mutated(self, store):
-        """Seeding should not alter the parent session's transcript."""
+    def test_parent_transcript_unaffected_by_thread(self, store):
+        """Creating a thread session should not alter parent's transcript."""
         parent_source = _dm_source()
         parent_entry = store.get_or_create_session(parent_source)
         for msg in PARENT_HISTORY:
             store.append_to_transcript(parent_entry.session_id, msg)
 
-        # Create thread and add a message to it
         thread_source = _dm_source(thread_id="1234567890.000001")
         thread_entry = store.get_or_create_session(thread_source)
         store.append_to_transcript(thread_entry.session_id, {
             "role": "user", "content": "thread-only message"
         })
 
-        # Parent should still have only its original messages
         parent_transcript = store.load_transcript(parent_entry.session_id)
         assert len(parent_transcript) == 2
         assert all(m["content"] != "thread-only message" for m in parent_transcript)
 
-    def test_multiple_threads_get_independent_copies(self, store):
-        """Each thread from the same parent gets its own copy."""
+    def test_multiple_threads_are_independent(self, store):
+        """Each thread from the same parent starts empty and stays independent."""
         parent_source = _dm_source()
         parent_entry = store.get_or_create_session(parent_source)
         for msg in PARENT_HISTORY:
@@ -118,49 +109,43 @@ class TestDMThreadSeeding:
         thread_b_source = _dm_source(thread_id="2222.000002")
         thread_b_entry = store.get_or_create_session(thread_b_source)
 
-        # Thread B should have parent history, not thread A's additions
+        # Thread B starts empty
         thread_b_transcript = store.load_transcript(thread_b_entry.session_id)
-        assert len(thread_b_transcript) == 2
-        assert all(m["content"] != "thread A message" for m in thread_b_transcript)
+        assert len(thread_b_transcript) == 0
 
-        # Thread A should have parent history + its own message
+        # Thread A has only its own message
         thread_a_transcript = store.load_transcript(thread_a_entry.session_id)
-        assert len(thread_a_transcript) == 3
+        assert len(thread_a_transcript) == 1
+        assert thread_a_transcript[0]["content"] == "thread A message"
 
-    def test_existing_thread_session_not_reseeded(self, store):
-        """Returning to an existing thread session should not re-copy parent history."""
+    def test_existing_thread_session_preserved(self, store):
+        """Returning to an existing thread session should not reset it."""
         parent_source = _dm_source()
         parent_entry = store.get_or_create_session(parent_source)
         for msg in PARENT_HISTORY:
             store.append_to_transcript(parent_entry.session_id, msg)
 
-        # Create thread session
         thread_source = _dm_source(thread_id="1234567890.000001")
         thread_entry = store.get_or_create_session(thread_source)
         store.append_to_transcript(thread_entry.session_id, {
             "role": "user", "content": "follow-up"
         })
 
-        # Add more to parent after thread was created
-        store.append_to_transcript(parent_entry.session_id, {
-            "role": "user", "content": "new parent message"
-        })
-
-        # Get the same thread session again (not new — created_at != updated_at)
+        # Get the same thread session again
         thread_entry_again = store.get_or_create_session(thread_source)
         assert thread_entry_again.session_id == thread_entry.session_id
 
-        # Should still have 3 messages (2 seeded + 1 follow-up), not re-seeded
+        # Should still have only its own message
         thread_transcript = store.load_transcript(thread_entry_again.session_id)
-        assert len(thread_transcript) == 3
-        assert thread_transcript[2]["content"] == "follow-up"
+        assert len(thread_transcript) == 1
+        assert thread_transcript[0]["content"] == "follow-up"
 
 
-class TestDMThreadSeedingEdgeCases:
-    """Edge cases and conditions where seeding should NOT happen."""
+class TestDMThreadIsolationEdgeCases:
+    """Edge cases — threads always start empty regardless of context."""
 
-    def test_no_seeding_for_group_threads(self, store):
-        """Group/channel threads should not trigger seeding."""
+    def test_group_thread_starts_empty(self, store):
+        """Group/channel threads should also start empty."""
         parent_source = _group_source()
         parent_entry = store.get_or_create_session(parent_source)
         for msg in PARENT_HISTORY:
@@ -172,7 +157,7 @@ class TestDMThreadSeedingEdgeCases:
         thread_transcript = store.load_transcript(thread_entry.session_id)
         assert len(thread_transcript) == 0
 
-    def test_no_seeding_without_parent_session(self, store):
+    def test_thread_without_parent_session_starts_empty(self, store):
         """Thread session without a parent DM session should start empty."""
         thread_source = _dm_source(thread_id="1234567890.000001")
         thread_entry = store.get_or_create_session(thread_source)
@@ -180,34 +165,21 @@ class TestDMThreadSeedingEdgeCases:
         thread_transcript = store.load_transcript(thread_entry.session_id)
         assert len(thread_transcript) == 0
 
-    def test_no_seeding_with_empty_parent(self, store):
-        """If parent session exists but has no transcript, thread starts empty."""
-        parent_source = _dm_source()
-        store.get_or_create_session(parent_source)
-        # No messages appended to parent
-
-        thread_source = _dm_source(thread_id="1234567890.000001")
-        thread_entry = store.get_or_create_session(thread_source)
-
-        thread_transcript = store.load_transcript(thread_entry.session_id)
-        assert len(thread_transcript) == 0
-
-    def test_no_seeding_for_dm_without_thread_id(self, store):
-        """Top-level DMs (no thread_id) should not trigger seeding."""
+    def test_dm_without_thread_starts_empty(self, store):
+        """Top-level DMs (no thread_id) should start empty as always."""
         source = _dm_source()
         entry = store.get_or_create_session(source)
 
-        # Should just be a normal empty session
         transcript = store.load_transcript(entry.session_id)
         assert len(transcript) == 0
 
 
-class TestDMThreadSeedingCrossPlatform:
-    """Verify seeding works for platforms beyond Slack."""
+class TestDMThreadIsolationCrossPlatform:
+    """Verify thread isolation is consistent across all platforms."""
 
     @pytest.mark.parametrize("platform", [Platform.SLACK, Platform.TELEGRAM, Platform.DISCORD])
-    def test_seeding_works_across_platforms(self, store, platform):
-        """DM thread seeding should work for any platform that uses thread_id."""
+    def test_thread_starts_empty_across_platforms(self, store, platform):
+        """DM thread sessions start empty regardless of platform."""
         parent_source = _dm_source(platform=platform)
         parent_entry = store.get_or_create_session(parent_source)
         for msg in PARENT_HISTORY:
@@ -217,5 +189,4 @@ class TestDMThreadSeedingCrossPlatform:
         thread_entry = store.get_or_create_session(thread_source)
 
         thread_transcript = store.load_transcript(thread_entry.session_id)
-        assert len(thread_transcript) == 2
-        assert thread_transcript[0]["content"] == "What's the weather?"
+        assert len(thread_transcript) == 0