hermes-agent/docs/micro-compaction.md

Micro-compaction

A way to amortize the cost of compression.

Long conversations eventually outgrow the model's context window, and something has to be thrown away or summarized. Hermes has always done this in one batch: when the transcript crosses a threshold, the session stops, a large chunk of the middle is summarized in a single call, and the conversation resumes. That works, but the whole bill comes due at once — one visible pause, one big summarization request, at whatever moment you happened to cross the line.

Micro-compaction pays the same bill in instalments. After each completed turn, Hermes folds the single oldest un-absorbed exchange into a running summary. The work is the same work; it just happens continuously, in small pieces, during the idle moment after a response instead of all at once in the middle of your session.

The tradeoff is that knowledge gets a little earlier than you may be used to. Because compaction is always running, older parts of the conversation become summaries sooner than they would under batch compaction — which leaves everything verbatim until the window actually fills. Detail from earlier in the session turns second-hand faster. You trade some of that fidelity for never eating one long stall, and for a context window that stays consistently smaller rather than sawtoothing up to the threshold and back.

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What it does

After every turn that finishes normally, finalize_turn asks the context compressor to absorb one exchange:

1. Find the oldest exchange that hasn't been summarized yet.
2. Send just that exchange, plus the current running summary, to the auxiliary summarization model.
3. Replace those messages in the transcript with a single summary marker carrying the updated running summary.

One exchange per turn. The per-turn cost stays bounded no matter how long the conversation gets.

An exchange is an assistant message together with any tool results that followed it. In tool-heavy work that's where the bulk of the tokens live — a file read or a command's output dwarfs the surrounding prose — which is why absorbing one exchange at a time is worth doing at all.

Your messages are never compacted

An exchange deliberately starts at the assistant message. Micro-compaction walks straight past user messages to get there, so what you typed is never summarized — your prompts stay verbatim for the entire session, no matter how long it runs or how many times compaction fires.

This is the most useful property of the whole design, and it's worth being explicit about why. What the assistant produces is largely an account of what it did: it read this file, it ran that command, it got this result. That kind of narration survives summarising with very little loss — "it did it this way" is about as informative compressed as it was in full. Your instructions are a different kind of thing. They're the intent everything else is derived from, and they cannot be reconstructed from the work that followed. Paraphrasing "use the existing retry helper, don't add a new one" into a summary is exactly how an agent ends up confidently doing the thing you told it not to, six turns later.

So the asymmetry is on purpose: compact the derived material, keep the source of truth. The cost is a floor on how small the middle can get, since user turns accumulate and are never absorbed. In practice that floor is low — a prompt is normally a tiny fraction of what a single tool result costs — but it is a real floor. If you routinely paste 10–20K-token prompts, that weight stays in context by design.

What it never touches

Two more regions are protected and stay verbatim:

• The head — the system prompt and the opening messages, so the session's founding instructions are never paraphrased.
• The tail — a token-budgeted window of the most recent messages, so everything that's immediately relevant is still there in full.

Micro-compaction only ever works in the middle, between those two.

How it works

The cursor

The compressor keeps a cursor: the index of the first message not yet absorbed. Each successful pass advances it past the exchange it just summarized.

If that in-memory cursor is missing or out of range — a fresh process, a resumed session — it's recovered by scanning the transcript for the last summary marker and resuming just after it. The transcript itself is the source of truth, so resuming a session doesn't re-summarize work already done.

The rolling summary

Rather than keeping a pile of per-exchange summaries, there is exactly one running summary that each new exchange is merged into. The summarizer is asked to fold in the new material's decisions, requirements, file paths and open questions, drop details that are no longer relevant, and preserve the existing structure. It's also explicitly instructed to replace any credentials it encounters with [REDACTED].

Because that summary is cumulative, only the newest marker is kept in the transcript. Earlier markers are strictly redundant — the current summary already contains everything they held — so they're dropped as they're superseded. This matters more than it sounds: leaving them in place stacks near-duplicate copies of the same text, each with its own heading and end-marker scaffolding, and the transcript grows on every turn instead of shrinking.

Defrag

Merge into a summary often enough and it gets baggy — repetitive, and larger than the material justifies. When the running summary crosses a token threshold (2000 by default), the next pass defrags: it re-summarizes the summary and whatever middle remains in one shot, replacing it with a fresh compact version and advancing the cursor to the tail.

This is still much cheaper than full batch compaction. It only ever processes the summary plus the un-absorbed middle, never the whole transcript.

Staying in step with the session database

The in-memory splice alone isn't enough. Hermes's normal session flush is append-only, so the original rows would stay marked active and a resume would load both the summary and the messages it replaced — putting the session straight over the context limit.

So each pass also calls archive_and_compact, which atomically soft-archives the active rows and inserts the compacted set. The messages are then stamped as already-persisted so the append-only flush that follows skips them. If that database step fails, it's logged and the session continues; the resume would double-load until the next batch compression cleans up.

When the summarizer fails

A summarization call can fail — the auxiliary model is unreachable, out of quota, or the exchange itself is somehow unsummarizable. The transcript is left untouched and the failure is counted.

If the same exchange fails three times in a row, the cursor is advanced past it anyway. Without that, one bad exchange would be retried on every single turn forever. Those skipped messages stay in the transcript and get picked up by the next defrag or batch compaction.

Interaction with batch compaction

Micro-compaction doesn't replace batch compaction — it defers it. Threshold-based compaction is still there and still fires if the window fills anyway, and its summary markers are the same format, so the two interoperate. In practice micro-compaction keeps the transcript far enough below the threshold that the batch path fires much less often.

Configuration

compression:
  micro_compact: true   # default

Set it to false to disable micro-compaction and return to batch-only compaction. Everything else about compression is unchanged.

What you'll see in the logs

Micro-compaction: 37 -> 36 messages
Micro-compaction defrag: rolling summary re-summarized (1843 chars)
Micro-compaction: skipping exchange at cursor 12 after 3 consecutive failures

Message counts move by small amounts — that's the point. The token count is where the effect shows: absorbing one tool-heavy exchange can drop hundreds of tokens while changing the message count by one or two.

Failure behaviour

Micro-compaction is best-effort throughout. The call in finalize_turn is wrapped so that any exception is logged and swallowed — a failure returns the conversation unchanged and the turn completes normally. It can degrade, but it shouldn't be able to break a session.