fix(compact): count the logged session prefix toward token pressure

ds-review-bot critical: compactIfNeeded estimated pressure from the
derived history + system prompt only, but every loop-built request also
carries EpochHeader.messagePrefix in front of the history — a
deployment at the window edge would under-estimate by exactly the
prefix, skip compaction, and ship an over-window request.

BasicCompactService now gates on estimatePressure(): the session prefix
read from the log's folded header + the derived history + the system
prompt. The fold is exact from the instance's second request on (and
from a resumed instance's first — the previous instance logged its
prefix); it is absent only before a fresh session's first request,
where the history is a single prompt and compaction is moot. Compaction
itself still shrinks history only — a prefix that alone approaches the
window is a configuration error no compactor fixes, same as the
documented single-unit-overflow stance.

packages/compact/compact-basic/README.md

@@ -8,7 +8,7 @@ This is the implementation tier of the compaction capability — see the [interf
 
 The abstract contract states only WHAT compaction does; this backend owns every HOW decision:
 
-- **Token estimation** — `estimateContentTokens()`: chars divided by the `charsPerToken` config (default 4) with per-block structural overhead (`text`/`reasoning` = `ceil(len/charsPerToken) + 4`, `tool-call` from name + arguments, `tool-result` recursive, unknown blocks via JSON length).
+- **Token estimation** — `estimateContentTokens()`: chars divided by the `charsPerToken` config (default 4) with per-block structural overhead (`text`/`reasoning` = `ceil(len/charsPerToken) + 4`, `tool-call` from name + arguments, `tool-result` recursive, unknown blocks via JSON length). The pressure gate estimates the NEXT request via `estimatePressure()`: the logged session prefix (`EpochHeader.messagePrefix` from the header fold — the `agent/session-prefix` product rides every request in front of the history, so omitting it would under-estimate pressure by exactly the prefix) + the derived history + the system prompt.
 - **Retention policy** — `compactIfNeeded()` walks the surface nodes tail→head summing per-node token estimates, and retains the smallest tail-run of WHOLE units (a closed step, or a single no-step node such as a pre-step `user/message` or inter-step `steering/message`) whose total reaches `retainTokens`; everything older is compacted. Retention is **turn-agnostic** — turn boundaries play no role, so a single runaway turn that alone exceeds the window compacts its OWN early closed steps rather than being retained verbatim (the failure mode that motivated dropping turn-protection: a tool-heavy turn must stay compactable or the harness dies exactly when compaction is needed). The only structural guard is **tool-pairing balance**: the compacted region's edges are balanced cuts on the surface (no unanswered tool-call crosses either edge), so it never splits a step's `assistant/message` tool-calls from their `tool/result`s. When the only compactable content left is an un-splittable open tail step, it declines (returns `null`) and retries once an older step closes. **Single-unit overflow is out of scope, by design**: if one retained unit (a single closed step, or a large pasted `user/message`) ALONE exceeds the budget, compaction cannot help and the call may go out over-budget — bounding an individual unit's size is a separate concern. `compactRegion()` enforces tool-pairing balance strictly, throwing on a boundary that would split a step. `dsh-session` exports `isToolPairingBalanced` for the check.
 - **Dynamic convergence** — no static summary-length config pretends to bound what the model will write. If framing/estimator/system overhead leaves the compacted surface above threshold, `compactIfNeeded()` re-compacts the head checkpoint up to `compactionRetries` extra times; if it still cannot get below threshold, it throws. A summary whose estimated stored size is not smaller than the shadowed content fails closed before it mutates the surface.
 - **Summarization** — `summarize()`: a `GenerateOptions` request assembled via `BlockAssembler` with a fixed system prompt that asks for a structured checkpoint (Primary Request and Intent · Key Technical Concepts · Files and Code · Errors and Fixes · Pending Tasks · Current Work · Next Step · Critical Context), every section mandatory, exact paths/commands/identifiers preserved. The request is a direct one-shot `ctx.llm.stream()` call — NOT a loop step, so it does not run `agent/request` (that seam shapes the loop's conversation requests); the model comes from `summarizationModel` falling back to the agent's own, and per-call routing happens at `llm/stream` like any other direct call. `maxTokens` is the provider-side generation cap; only text blocks from the model's reply are kept before the checkpoint is stored (reasoning is dropped so private chain-of-thought never leaks into the durable summary, and a stray `tool-call` is dropped so the synthesized `user/message` summary cannot land an orphaned call with no matching `tool-result`). The compacted region is flattened to a plain-text transcript first: text and reasoning contribute their text, and every non-text block (tool-call, tool-result, plugin-added types) contributes a type-tagged placeholder (`[tool-call: name(args)]`, `[tool-result: …]`, …) so the summarizer is told what existed rather than silently dropping it.

packages/compact/compact-basic/src/index.ts

@@ -187,7 +187,7 @@ export class BasicCompactService extends CompactService {
         try {
           const result = await this.compactIfNeeded(agent, fullSystemPrompt, signal)
           if (result) {
-            const after = this.estimateTokens(agent.session.deriveMessages(), fullSystemPrompt)
+            const after = this.estimatePressure(agent.session, fullSystemPrompt)
             ctx.logger.info(
               `compaction: shadowed ${result.shadowedSeqs.length} surface nodes ` +
               `(seqs ${result.shadowedRange.start}-${result.shadowedRange.end}, ` +
@@ -359,11 +359,19 @@ export class BasicCompactService extends CompactService {
   // ---- Core API (implements the abstract contract) ----
 
   /**
-   * The sole token-pressure gate: estimate the current surface-derived history,
+   * The sole token-pressure gate: estimate the NEXT request's pressure — the
-   * and if it exceeds the threshold (`contextWindow * thresholdRatio`), compact
+   * logged session prefix + the surface-derived history + the system prompt
+   * ({@link estimatePressure}) — and if it exceeds the threshold
+   * (`contextWindow * thresholdRatio`), compact
    * the oldest surface nodes outside the `retainTokens` budget. The auto-
    * compaction listener delegates here rather than pre-checking, so this is the
-   * only place the decision lives.
+   * only place the decision lives. The prefix counts because every request
+   * carries it in front of the history (`EpochHeader.messagePrefix`) even
+   * though it is not derived history — omitting it would under-estimate by
+   * exactly the prefix and let a deployment at the window edge skip
+   * compaction, then ship an over-window request. Compaction itself can only
+   * shrink HISTORY: a prefix that alone approaches the window is a
+   * configuration error no compactor fixes.
    *
    * Retention is a UNIFORM tail→head walk over the whole surface — turn
    * boundaries play NO role. Walking node-by-node from the tail and summing
@@ -393,7 +401,7 @@ export class BasicCompactService extends CompactService {
     const threshold = Math.floor(this.config.contextWindow * this.config.thresholdRatio)
     let result: CompactionResult | null = null
     for (let attempt = 0; attempt <= this.config.compactionRetries; attempt++) {
-      const totalTokens = this.estimateTokens(session.deriveMessages(), fullSystemPrompt)
+      const totalTokens = this.estimatePressure(session, fullSystemPrompt)
       if (totalTokens < threshold) return result
 
       const range = this._compactableRange(session)
@@ -407,7 +415,7 @@ export class BasicCompactService extends CompactService {
       result = await this.compactRegion(session, range.start, range.end, agent, signal)
     }
 
-    const totalTokens = this.estimateTokens(session.deriveMessages(), fullSystemPrompt)
+    const totalTokens = this.estimatePressure(session, fullSystemPrompt)
     if (totalTokens < threshold) return result
 
     throw new Error(
@@ -416,6 +424,23 @@ export class BasicCompactService extends CompactService {
     )
   }
 
+  /**
+   * Estimated token pressure of the NEXT request: the logged session prefix
+   * (`EpochHeader.messagePrefix` from the header fold — request-only messages
+   * the loop sends in front of the derived history), the derived history, and
+   * the system prompt. The fold is exact from the loop instance's second
+   * request on (and from a resumed instance's first — the previous instance
+   * logged its prefix); it is absent only before a fresh session's first
+   * request, where the history is a single prompt and compaction is moot.
+   * @param session - the session whose next request is being estimated.
+   * @param fullSystemPrompt - the assembled system prompt (counts toward pressure).
+   * @returns the estimated token total the next request will carry.
+   */
+  estimatePressure(session: Session, fullSystemPrompt: string): number {
+    const sessionPrefix = session.requestHeader()?.messagePrefix ?? []
+    return this.estimateTokens([...sessionPrefix, ...session.deriveMessages()], fullSystemPrompt)
+  }
+
   override async compactRegion(
     session: Session,
     start: number,

packages/compact/compact-basic/tests/compact-basic.spec.ts

@@ -557,6 +557,29 @@ describe('BasicCompactService.compactIfNeeded', () => {
     expect(result!.shadowedSeqs.length).toBeGreaterThan(0)
   })
 
+  it('counts the logged session prefix toward pressure (every request carries it in front of the history)', async () => {
+    const svc = createTestService({ contextWindow: 200, thresholdRatio: 0.5, retainTokens: 10 })
+    const session = multiTurnSession(3, 1) // 6 derived messages ≈ 84 estimated tokens — under the 100 threshold alone
+    expect(await compactIfNeeded(svc, session, '', 'm', SIGNAL)).toBeNull()
+
+    // The loop records the composed agent/session-prefix product on the
+    // request header; it rides every request, so pressure must include it.
+    session.append('request/header', {
+      header: {
+        config: { model: 'm' },
+        messagePrefix: [
+          { role: 'user', content: [{ type: 'text', text: `opener one.${LONG_FIXTURE_TEXT}` }] },
+          { role: 'user', content: [{ type: 'text', text: `opener two.${LONG_FIXTURE_TEXT}` }] },
+        ],
+      },
+      reason: 'initial',
+    })
+    const result = await compactIfNeeded(svc, session, '', 'm', SIGNAL)
+    expect(result).not.toBeNull()
+    // The prefix itself is NOT history: compaction shadowed surface nodes only.
+    expect(session.requestHeader()?.messagePrefix).toHaveLength(2)
+  })
+
   it('returns the first compaction result when a zero-retry pass converges after the loop', async () => {
     // With compactionRetries=0 there is no next-loop threshold check after the
     // first mutation, so the success path is the post-loop `return result`.