fix(markdown): a rewritten assistant reply reseeds the session instead of resuming it

src/beaver_gateway/backends/claude_sdk.py

@@ -296,6 +296,7 @@ class ClaudeSdkBackend:
         system: str | None = None,  # noqa: ARG002 - the agent owns its prompt
         conversation_id: str | None = None,
         session_id: str | None = None,
+        reseed: bool = False,
         capture: TurnCapture | None = None,
         kind: str = "deep",
         pinned: bool = False,
@@ -315,7 +316,9 @@ class ClaudeSdkBackend:
         prior = history[:-1]
         key = conversation_id or fingerprint(prior)
         spec = _SessionSpec(kind=kind, pinned=pinned, tools=tools)
-        live = await self._acquire(key, session_id=session_id, history=prior, spec=spec)
+        live = await self._acquire(
+            key, session_id=session_id, history=prior, spec=spec, reseed=reseed
+        )
         message_id = f"msg_{uuid.uuid4().hex}"
         yield build_message_start(message_id=message_id, model=self._agent.model)
         turn = _Turn()
@@ -484,10 +487,13 @@ class ClaudeSdkBackend:
         session_id: str | None,
         history: list[dict[str, Any]],
         spec: _SessionSpec,
+        reseed: bool = False,
     ) -> Session:
         live = self._pool.get(key)
         if live is not None:
-            return live
+            if not reseed:
+                return live
+            await self._pool.close(key)
         resume = session_id
         if resume is not None:
             await self.repair_session(resume)

src/beaver_gateway/core/conversation_store.py

@@ -87,6 +87,12 @@ class ForkOutcome:
     messages: list[MessageParam]
     persist_messages: list[dict[str, Any]]
     divergence_index: int | None
+    edited: bool = False
+    """An earlier assistant turn's prose was rewritten in the file."""
+
+    @property
+    def reuse_session(self) -> bool:
+        return self.divergence_index is None and not self.edited
 
 
 # ---- public store API ---------------------------------------------------
@@ -407,7 +413,7 @@ def diff_and_fork(
     new_user_turn = incoming[-1]
     prior_incoming = incoming[:-1]
 
-    spliced_groups, divergence = _walk_prefix(prior_incoming, stored_groups)
+    spliced_groups, divergence, edited = _walk_prefix(prior_incoming, stored_groups)
 
     if divergence is None and len(prior_incoming) < len(stored_groups):
         if _file_lags_store(stored_groups, len(prior_incoming), new_user_turn):
@@ -433,6 +439,7 @@ def diff_and_fork(
         messages=backend_msgs,
         persist_messages=persist_msgs,
         divergence_index=divergence,
+        edited=edited,
     )
 
 
@@ -470,26 +477,29 @@ def _file_lags_store(
 
 def _walk_prefix(
     prior_incoming: list[ParsedTurn], stored_groups: list[_StoredDisplayTurn]
-) -> tuple[list[list[dict[str, Any]]], int | None]:
+) -> tuple[list[list[dict[str, Any]]], int | None, bool]:
     """Walk incoming vs stored side-by-side until first divergence.
 
     Returns the spliced/matched group list (one entry per matched
     display turn, each carrying the raw messages we'll feed to the
-    backend for that turn) and the divergence index (``None`` if all
-    of ``prior_incoming`` matched).
+    backend for that turn), the divergence index (``None`` if all
+    of ``prior_incoming`` matched) and whether any assistant prose
+    was spliced in from the file - a rewritten reply keeps the
+    structure but must not resume the session that said otherwise.
     """
     from beaver_gateway.frontends.markdown.parser import TextSegment, ToolSegment
 
     spliced_groups: list[list[dict[str, Any]]] = []
+    edited = False
     for i, inc in enumerate(prior_incoming):
         if i >= len(stored_groups):
-            return spliced_groups, i
+            return spliced_groups, i, edited
         st = stored_groups[i]
         if inc.role != st.role:
-            return spliced_groups, i
+            return spliced_groups, i, edited
         if inc.role == "user":
             if inc.text != st.spoken_text:
-                return spliced_groups, i
+                return spliced_groups, i, edited
             spliced_groups.append(list(st.messages))
             continue
         inc_skeleton = tuple(
@@ -499,17 +509,18 @@ def _walk_prefix(
         # Files rendered without tool callouts (§3.10) carry no skeleton:
         # prose alone decides whether the turn matched.
         if inc_skeleton and inc_skeleton != st.skeleton:
-            return spliced_groups, i
+            return spliced_groups, i, edited
         if inc.text == st.spoken_text:
             spliced_groups.append(list(st.messages))
             continue
         if inc_skeleton and inc_text_count != st.text_segment_count:
-            return spliced_groups, i
+            return spliced_groups, i, edited
         spliced = _splice_assistant_group(stored_group=st, incoming=inc)
         if spliced is None:
-            return spliced_groups, i
+            return spliced_groups, i, edited
         spliced_groups.append(spliced)
-    return spliced_groups, None
+        edited = True
+    return spliced_groups, None, edited
 
 
 def _assemble_tail(

src/beaver_gateway/core/conversations.py

@@ -877,6 +877,7 @@ class Conversations:
                     messages=messages,
                     conversation_id=conv.external_id,
                     session_id=resume if use_session else None,
+                    reseed=not use_session,
                     capture=capture,
                     kind=conv.kind,
                     pinned=conv.kind == "master",

src/beaver_gateway/frontends/markdown/frontend.py

@@ -400,7 +400,7 @@ class MarkdownFrontend(Frontend):
             messages=outcome.messages,
             origin="user",
             capture=capture,
-            use_session=outcome.divergence_index is None,
+            use_session=outcome.reuse_session,
         )
         try:
             message = await self._stream_to_file(
@@ -623,7 +623,7 @@ class MarkdownFrontend(Frontend):
             messages=outcome.messages,
             origin="user",
             capture=capture,
-            use_session=outcome.divergence_index is None,
+            use_session=outcome.reuse_session,
         )
 
         acc = StreamAccumulator()

tests/test_routing.py

@@ -228,3 +228,30 @@ async def test_anthropic_turns_become_one_deep_conversation(stack: Stack) -> Non
             headers=AUTH,
         )
         assert r.status_code == 400 and "runs on 'd'" in r.json()["detail"]
+
+
+async def test_markdown_edited_reply_reseeds_the_session(stack: Stack) -> None:
+    async with stack.client(stack.markdown) as c:
+        r = await c.post(
+            "/chat",
+            json={"filename": "e.md", "agent": "d", "content": "### User:\n\nanimal\n"},
+            headers=AUTH,
+        )
+        assert r.status_code == 200, r.text
+        content = r.json()["new_content"]
+        assert "ok:animal" in content and len(ScriptedClient.instances) == 1
+        edited = content.replace("ok:animal", "penguin") + "\nwhich one?\n"
+        r = await c.post(
+            "/chat",
+            json={"filename": "e.md", "agent": "d", "content": edited},
+            headers=AUTH,
+        )
+        assert r.status_code == 200, r.text
+        assert len(ScriptedClient.instances) == 2
+        first, second = ScriptedClient.instances
+        assert not first.connected and second.prompts == ["which one?"]
+        assert second.options.resume not in (None, first.session_id)
+        assert "penguin" in repr(vars(stack.world.store))
+        async with stack.world.db.session() as session:
+            stored = await load_messages(session, conversation_id=1)
+        assert stored[1]["content"] == [{"type": "text", "text": "penguin"}]