get_messages() only deserializes content and tool_calls; the structured
reasoning columns (reasoning_details, codex_reasoning_items,
codex_message_items) come back as the raw TEXT they were stored as.
Feeding those rows straight back into a write, which is exactly what
the POST /api/sessions/{id}/fork handler does by piping get_messages()
into replace_messages(), hit an unguarded json.dumps() and stored the
already-serialized string encoded a second time. On replay of the fork,
json.loads() then yields the inner string instead of a list, and every
consumer's isinstance(..., list) gate silently drops it: preserved
Anthropic thinking blocks, Codex encrypted-reasoning/message-item
replay, and OpenRouter multi-turn reasoning context are all lost after
a fork, with one more encoding layer added per fork.
The /branch copy loop had the same defect from the other side: it
forwarded reasoning but none of the structured columns, and both TUI
branch writers persisted role/content alone, dropping reasoning and
reasoning_content along with them.
Route the six dumps sites in append_message and _insert_message_rows
through a shared guard that keeps already-serialized strings as-is;
structured values from the live runtime are dumped exactly as before.
Forward the reasoning fields in all three branch writers, matching the
set gateway/slash_commands.py already forwards on its own /branch path.
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