aider session format

Store and files

aider appends Markdown to .aider.chat.history.md at the git root it runs in (the launch directory outside a repo), so ~/.aider.chat.history.md exists only for a launch from home; AIDER_CHAT_HISTORY_FILE relocates it. deja reads the home file, AIDER_CHAT_HISTORY_FILE, every project deja aider has started aider in (listed in ~/.config/deja/aider-projects), and each directory in the platform path-list variable DEJA_AIDER_ROOTS, to two levels below each root.

One history file contains multiple sessions. A session begins with:

# aider chat started at 2026-07-17 09:00:00

#### Show me the failing query

The query misses the tenant predicate.

> Applied edit to query.go

Message mapping

Outside fenced code blocks, #### starts or continues a user message. Plain Markdown is assistant output. Lines beginning with > are tool or system output and are not indexed as messages; neither are unprefixed lines directly under one (the rest of that output block), nor anything before the session's first #### line (the banner and the --verbose dump). Of the output, Applied edit to X, Added X to the chat and Added X to read-only files become a files record for X (relative to the history's directory), and Running X a command record. A #### line that is one of aider's own commands (/add, /undo, /clear, …) is dropped; /run X, !X and /test X become a command record for X and /git X one for git X. /ask, /code and /context are dropped too: aider logs their question again as its own #### line. Blank lines remain part of the current message.

The header timestamp uses local time with layout YYYY-MM-DD HH:MM:SS. aider does not store message timestamps, so every message receives the session start. It does not store a session ID; deja derives a stable ID from the history path and the session's start time (aider-<path hash>-<YYYYMMDDTHHMMSS>, with -2 and on for a second launch in the same second, and the ordinal in the file when the header does not parse). The ordinal alone was the ID until #4332: a new history at the path of a deleted one restarted it and overwrote the sessions deja kept.

Skills

None, and there is nothing to build on. aider loads instruction files rather

than offering a catalogue an agent picks from: --read (and a CONVENTIONS.md by

habit) puts a file in every message, which is exactly what deja's context file

already is. A skill here would be the same always-on text under another name.

Known quirks and drift

  • The file is append-only and can contain many launches.
  • Markdown fences may contain lines beginning with #### or >; these are content, not role markers. The parser tracks triple-backtick fences.
  • <blank> after the user prefix represents an empty line.
  • Tool output terminates an assistant block but does not become a message.
  • Moving a history file changes deja's derived session IDs because the path is part of the ID.
  • aider loads none of the history back into a new chat unless started with --restore-chat-history, and that loads the whole file, every session in it. deja resume refuses the session and names that flag in its message rather than claiming to reopen one session.
  • A session that leaves the file — the file was deleted and aider started a new one, or someone cut it by hand — stays in the index, as a deleted transcript does; deja forget <id> drops it.

Last verified: 2026-07-27

deja reads this format and thirty-seven others, and turns what it finds into memory your agents can search. See the harness matrix for what is wired where, or install it and search your own history.

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