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Compaction and Branch Summaries ​

Compaction and branch summaries are the two mechanisms that keep long sessions usable without losing prior work context.

  • Compaction rewrites old history into a summary on the current branch.
  • Branch summary captures abandoned branch context during /tree navigation.

Both are persisted as session entries and converted back into user-context messages when rebuilding LLM input.

Key implementation files ​

  • src/session/compaction/compaction.ts
  • src/session/compaction/branch-summarization.ts
  • src/session/compaction/pruning.ts
  • src/session/compaction/utils.ts
  • src/session/session-manager.ts
  • src/session/agent-session.ts
  • src/session/messages.ts
  • src/extensibility/hooks/types.ts
  • src/config/settings-schema.ts

Session entry model ​

Compaction and branch summaries are first-class session entries, not plain assistant/user messages.

  • CompactionEntry
    • type: "compaction"
    • summary, optional shortSummary
    • firstKeptEntryId (compaction boundary)
    • tokensBefore
    • optional details, preserveData, fromExtension
  • BranchSummaryEntry
    • type: "branch_summary"
    • fromId, summary
    • optional details, fromExtension

When context is rebuilt (buildSessionContext):

  1. Latest compaction on the active path is converted to one compactionSummary message.
  2. Kept entries from firstKeptEntryId to the compaction point are re-included.
  3. Later entries on the path are appended.
  4. branch_summary entries are converted to branchSummary messages.
  5. custom_message entries are converted to custom messages.

Those custom roles are then transformed into LLM-facing user messages in convertToLlm() using the static templates:

  • prompts/compaction/compaction-summary-context.md
  • prompts/compaction/branch-summary-context.md

Compaction pipeline ​

Triggers ​

Compaction can run in three ways:

  1. Manual: /compact [instructions] calls AgentSession.compact(...).
  2. Automatic overflow recovery: after an assistant error that matches context overflow.
  3. Automatic threshold compaction: after a successful turn when context exceeds threshold.

Compaction shape (visual) ​

text
Before compaction:

  entry:  0     1     2     3      4     5     6      7      8     9
        ┌─────┬─────┬─────┬──────┬─────┬─────┬──────┬──────┬─────┬──────┐
        │ hdr │ usr │ ass │ tool │ usr │ ass │ tool │ tool │ ass │ tool │
        └─────┴─────┴─────┴──────┴─────┴─────┴──────┴──────┴─────┴──────┘
                └────────┬───────┘ └──────────────┬──────────────┘
               messagesToSummarize            kept messages
                                   ↑
                          firstKeptEntryId (entry 4)

After compaction (new entry appended):

  entry:  0     1     2     3      4     5     6      7      8     9      10
        ┌─────┬─────┬─────┬──────┬─────┬─────┬──────┬──────┬─────┬──────┬─────┐
        │ hdr │ usr │ ass │ tool │ usr │ ass │ tool │ tool │ ass │ tool │ cmp │
        └─────┴─────┴─────┴──────┴─────┴─────┴──────┴──────┴─────┴──────┴─────┘
               └──────────┬──────┘ └──────────────────────┬───────────────────┘
                 not sent to LLM                    sent to LLM
                                                         ↑
                                              starts from firstKeptEntryId

What the LLM sees:

  ┌────────┬─────────┬─────┬─────┬──────┬──────┬─────┬──────┐
  │ system │ summary │ usr │ ass │ tool │ tool │ ass │ tool │
  └────────┴─────────┴─────┴─────┴──────┴──────┴─────┴──────┘
       ↑         ↑      └─────────────────┬────────────────┘
    prompt   from cmp          messages from firstKeptEntryId

Overflow-retry vs threshold compaction ​

The two automatic paths are intentionally different:

  • Overflow-retry compaction

    • Trigger: current-model assistant error is detected as context overflow.
    • The failing assistant error message is removed from active agent state before retry.
    • Auto compaction runs with reason: "overflow" and willRetry: true.
    • On success, agent auto-continues (agent.continue()) after compaction.

  • Threshold compaction

    • Trigger: contextTokens > contextWindow - compaction.reserveTokens.
    • Runs with reason: "threshold" and willRetry: false.
    • On success, if compaction.autoContinue !== false, injects a synthetic prompt:
      • "Continue if you have next steps."

Pre-compaction pruning ​

Before compaction checks, tool-result pruning may run (pruneToolOutputs).

Default prune policy:

  • Protect newest 40_000 tool-output tokens.
  • Require at least 20_000 total estimated savings.
  • Never prune tool results from skill or read.

Pruned tool results are replaced with:

  • [Output truncated - N tokens]

If pruning changes entries, session storage is rewritten and agent message state is refreshed before compaction decisions.

Boundary and cut-point logic ​

prepareCompaction() only considers entries since the last compaction entry (if any).

  1. Find previous compaction index.
  2. Compute boundaryStart = prevCompactionIndex + 1.
  3. Adapt keepRecentTokens using measured usage ratio when available.
  4. Run findCutPoint() over the boundary window.

Valid cut points include:

  • message entries with roles: user, assistant, bashExecution, hookMessage, branchSummary, compactionSummary
  • custom_message entries
  • branch_summary entries

Hard rule: never cut at toolResult.

If there are non-message metadata entries immediately before the cut point (model_change, thinking_level_change, labels, etc.), they are pulled into the kept region by moving cut index backward until a message or compaction boundary is hit.

Split-turn handling ​

If cut point is not at a user-turn start, compaction treats it as a split turn.

Turn start detection treats these as user-turn boundaries:

  • message.role === "user"
  • message.role === "bashExecution"
  • custom_message entry
  • branch_summary entry

Split-turn compaction generates two summaries:

  1. History summary (messagesToSummarize)
  2. Turn-prefix summary (turnPrefixMessages)

Final stored summary is merged as:

markdown
<history summary>

---

**Turn Context (split turn):**

<turn prefix summary>

Summary generation ​

compact(...) builds summaries from serialized conversation text:

  1. Convert messages via convertToLlm().
  2. Serialize with serializeConversation().
  3. Wrap in <conversation>...</conversation>.
  4. Optionally include <previous-summary>...</previous-summary>.
  5. Optionally inject hook context as <additional-context> list.
  6. Execute summarization prompt with SUMMARIZATION_SYSTEM_PROMPT.

Prompt selection:

  • first compaction: compaction-summary.md
  • iterative compaction with prior summary: compaction-update-summary.md
  • split-turn second pass: compaction-turn-prefix.md
  • short UI summary: compaction-short-summary.md

Remote summarization mode:

  • If compaction.remoteEndpoint is set, compaction POSTs:
    • { systemPrompt, prompt }
  • Expects JSON containing at least { summary }.

File-operation context in summaries ​

Compaction tracks cumulative file activity using assistant tool calls:

  • read(path) → read set
  • write(path) → modified set
  • edit(path) → modified set

Cumulative behavior:

  • Includes prior compaction details only when prior entry is pi-generated (fromExtension !== true).
  • In split turns, includes turn-prefix file ops too.
  • readFiles excludes files also modified.

Summary text gets file tags appended via prompt template:

xml
<read-files>
...
</read-files>
<modified-files>
...
</modified-files>

Persist and reload ​

After summary generation (or hook-provided summary), agent session:

  1. Appends CompactionEntry with appendCompaction(...).
  2. Rebuilds context via buildSessionContext().
  3. Replaces live agent messages with rebuilt context.
  4. Emits session_compact hook event.

Branch summarization pipeline ​

Branch summarization is tied to tree navigation, not token overflow.

Trigger ​

During navigateTree(...):

  1. Compute abandoned entries from old leaf to common ancestor using collectEntriesForBranchSummary(...).
  2. If caller requested summary (options.summarize), generate summary before switching leaf.
  3. If summary exists, attach it at the navigation target using branchWithSummary(...).

Operationally this is commonly driven by /tree flow when branchSummary.enabled is enabled.

Branch switch shape (visual) ​

text
Tree before navigation:

         ┌─ B ─ C ─ D (old leaf, being abandoned)
    A ───┤
         └─ E ─ F (target)

Common ancestor: A
Entries to summarize: B, C, D

After navigation with summary:

         ┌─ B ─ C ─ D ─ [summary of B,C,D]
    A ───┤
         └─ E ─ F (new leaf)

Preparation and token budget ​

generateBranchSummary(...) computes budget as:

  • tokenBudget = model.contextWindow - branchSummary.reserveTokens

prepareBranchEntries(...) then:

  1. First pass: collect cumulative file ops from all summarized entries, including prior pi-generated branch_summary details.
  2. Second pass: walk newest → oldest, adding messages until token budget is reached.
  3. Prefer preserving recent context.
  4. May still include large summary entries near budget edge for continuity.

Compaction entries are included as messages (compactionSummary) during branch summarization input.

Summary generation and persistence ​

Branch summarization:

  1. Converts and serializes selected messages.
  2. Wraps in <conversation>.
  3. Uses custom instructions if supplied, otherwise branch-summary.md.
  4. Calls summarization model with SUMMARIZATION_SYSTEM_PROMPT.
  5. Prepends branch-summary-preamble.md.
  6. Appends file-operation tags.

Result is stored as BranchSummaryEntry with optional details (readFiles, modifiedFiles).

Extension and hook touchpoints ​

session_before_compact ​

Pre-compaction hook.

Can:

  • cancel compaction ({ cancel: true })
  • provide full custom compaction payload ({ compaction: CompactionResult })

session.compacting ​

Prompt/context customization hook for default compaction.

Can return:

  • prompt (override base summary prompt)
  • context (extra context lines injected into <additional-context>)
  • preserveData (stored on compaction entry)

session_compact ​

Post-compaction notification with saved compactionEntry and fromExtension flag.

session_before_tree ​

Runs on tree navigation before default branch summary generation.

Can:

  • cancel navigation
  • provide custom { summary: { summary, details } } used when user requested summarization

session_tree ​

Post-navigation event exposing new/old leaf and optional summary entry.

Runtime behavior and failure semantics ​

  • Manual compaction aborts current agent operation first.
  • abortCompaction() cancels both manual and auto-compaction controllers.
  • Auto compaction emits start/end session events for UI/state updates.
  • Auto compaction can try multiple model candidates and retry transient failures.
  • Overflow errors are excluded from generic retry path because they are handled by compaction.
  • If auto-compaction fails:
    • overflow path emits Context overflow recovery failed: ...
    • threshold path emits Auto-compaction failed: ...
  • Branch summarization can be cancelled via abort signal (e.g., Escape), returning canceled/aborted navigation result.

Settings and defaults ​

From settings-schema.ts:

  • compaction.enabled = true
  • compaction.reserveTokens = 16384
  • compaction.keepRecentTokens = 20000
  • compaction.autoContinue = true
  • compaction.remoteEndpoint = undefined
  • branchSummary.enabled = false
  • branchSummary.reserveTokens = 16384

These values are consumed at runtime by AgentSession and compaction/branch summarization modules.