fix(conversation_store): splice assistant text in place, keeping thinking blocks

src/beaver_gateway/core/conversation_store.py

@@ -526,6 +526,75 @@ def _assemble_tail(
 
 def _splice_assistant_group(
     *, stored_group: _StoredDisplayTurn, incoming: ParsedTurn
+) -> list[dict[str, Any]] | None:
+    """Apply the file's prose to a stored assistant turn.
+
+    Prefers substituting text in place, which keeps everything the
+    markdown can't express. Falls back to rebuilding from the file's
+    structure when the text blocks don't line up one-to-one.
+    """
+    preserved = _splice_in_place(stored_group=stored_group, incoming=incoming)
+    if preserved is not None:
+        return preserved
+    return _splice_by_rebuild(stored_group=stored_group, incoming=incoming)
+
+
+def _splice_in_place(
+    *, stored_group: _StoredDisplayTurn, incoming: ParsedTurn
+) -> list[dict[str, Any]] | None:
+    """Copy the stored messages, swapping only their text block contents.
+
+    Rebuilding a turn from the file loses everything the markdown never
+    carried — thinking blocks, and the message boundaries claude chose.
+    Both matter: an ``assistant[thinking] + assistant[text]`` pair (what
+    claude emits for a reasoning turn) collapses into a single message,
+    so the history is one message shorter than the one the backend
+    pooled its live session under, and the next turn misses the cache
+    and respawns. Substituting in place keeps the message count and the
+    invisible blocks exactly as stored.
+
+    Returns ``None`` when stored text blocks and incoming text segments
+    aren't one-to-one — consecutive text blocks merge into a single
+    rendered segment, so there'd be no way to know how to split the
+    edited prose back apart. The caller then rebuilds instead.
+    """
+    # See ``diff_and_fork`` for why the parser-type import is deferred.
+    from beaver_gateway.frontends.markdown.parser import TextSegment
+
+    new_texts = [
+        s.text for s in incoming.structure if isinstance(s, TextSegment) and s.text
+    ]
+    positions: list[tuple[int, int]] = []
+    for mi, msg in enumerate(stored_group.messages):
+        content = msg.get("content")
+        if msg["role"] != "assistant" or not isinstance(content, list):
+            continue
+        positions.extend(
+            (mi, bi)
+            for bi, blk in enumerate(content)
+            if isinstance(blk, dict)
+            and blk.get("type") == "text"
+            and str(blk.get("text", "")).strip()
+        )
+    if len(positions) != len(new_texts):
+        return None
+
+    out: list[dict[str, Any]] = [
+        {
+            "role": m["role"],
+            "content": list(m["content"])
+            if isinstance(m.get("content"), list)
+            else m.get("content"),
+        }
+        for m in stored_group.messages
+    ]
+    for (mi, bi), text in zip(positions, new_texts, strict=True):
+        out[mi]["content"][bi] = {**out[mi]["content"][bi], "text": text}
+    return out
+
+
+def _splice_by_rebuild(
+    *, stored_group: _StoredDisplayTurn, incoming: ParsedTurn
 ) -> list[dict[str, Any]] | None:
     """Rebuild an assistant display turn with new text + stored tool_use blocks.