fix(gateway): seed DM thread sessions with parent transcript to preserve context

tests/gateway/test_session_dm_thread_seeding.py

@@ -0,0 +1,221 @@
+"""Tests for DM thread session seeding.
+
+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.
+
+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)
+"""
+
+import pytest
+from unittest.mock import patch
+
+from gateway.config import Platform, GatewayConfig
+from gateway.session import SessionSource, SessionStore, build_session_key
+
+
+@pytest.fixture()
+def store(tmp_path):
+    """SessionStore with no SQLite, for fast unit tests."""
+    config = GatewayConfig()
+    with patch("gateway.session.SessionStore._ensure_loaded"):
+        s = SessionStore(sessions_dir=tmp_path, config=config)
+    s._db = None
+    s._loaded = True
+    return s
+
+
+def _dm_source(platform=Platform.SLACK, chat_id="D123", thread_id=None, user_id="U1"):
+    return SessionSource(
+        platform=platform,
+        chat_id=chat_id,
+        chat_type="dm",
+        user_id=user_id,
+        thread_id=thread_id,
+    )
+
+
+def _group_source(platform=Platform.SLACK, chat_id="C456", thread_id=None, user_id="U1"):
+    return SessionSource(
+        platform=platform,
+        chat_id=chat_id,
+        chat_type="group",
+        user_id=user_id,
+        thread_id=thread_id,
+    )
+
+
+PARENT_HISTORY = [
+    {"role": "user", "content": "What's the weather?"},
+    {"role": "assistant", "content": "It's sunny and 72°F."},
+]
+
+
+class TestDMThreadSeeding:
+    """Core seeding behavior."""
+
+    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
+        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."
+
+    def test_parent_transcript_not_mutated(self, store):
+        """Seeding should not alter the parent session'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."""
+        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)
+
+        # Thread A
+        thread_a_source = _dm_source(thread_id="1111.000001")
+        thread_a_entry = store.get_or_create_session(thread_a_source)
+        store.append_to_transcript(thread_a_entry.session_id, {
+            "role": "user", "content": "thread A message"
+        })
+
+        # Thread B
+        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_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)
+
+        # Thread A should have parent history + its own message
+        thread_a_transcript = store.load_transcript(thread_a_entry.session_id)
+        assert len(thread_a_transcript) == 3
+
+    def test_existing_thread_session_not_reseeded(self, store):
+        """Returning to an existing thread session should not re-copy parent history."""
+        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)
+        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
+        thread_transcript = store.load_transcript(thread_entry_again.session_id)
+        assert len(thread_transcript) == 3
+        assert thread_transcript[2]["content"] == "follow-up"
+
+
+class TestDMThreadSeedingEdgeCases:
+    """Edge cases and conditions where seeding should NOT happen."""
+
+    def test_no_seeding_for_group_threads(self, store):
+        """Group/channel threads should not trigger seeding."""
+        parent_source = _group_source()
+        parent_entry = store.get_or_create_session(parent_source)
+        for msg in PARENT_HISTORY:
+            store.append_to_transcript(parent_entry.session_id, msg)
+
+        thread_source = _group_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_without_parent_session(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)
+
+        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."""
+        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."""
+
+    @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."""
+        parent_source = _dm_source(platform=platform)
+        parent_entry = store.get_or_create_session(parent_source)
+        for msg in PARENT_HISTORY:
+            store.append_to_transcript(parent_entry.session_id, msg)
+
+        thread_source = _dm_source(platform=platform, thread_id="thread_123")
+        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?"