fix(session): emit excerpted tail copies when nothing is summarized

A keep-set that covers every foldable turn still has to ship the
excerpted live copies the tail selector already paid for. Returning
the unexcerpted turns keeps the occupancy that armed the compact.

Skip re-excerpting only on the structured marker so a body that
merely mentions the prefix is still shortened.

Author: TheGreatAxios

src/session/compaction-verify.test.ts

@@ -4,7 +4,7 @@ import os from "node:os";
 import path from "node:path";
 import { createPruningCompactor } from "./compactor.js";
 import { condenseTurns } from "./summarizer.js";
-import { HANDOFF_LATEST_KEY } from "./compaction-handoff.js";
+import { COMPACTED_PREFIX, HANDOFF_LATEST_KEY } from "./compaction-handoff.js";
 import {
   createCompactionArchive,
   wrapCompactorWithCompletenessGate,
@@ -786,6 +786,27 @@ describe("CL-9007 budgeted tail (shared auto+manual pipeline)", () => {
       .join("\n");
   }
 
+  function liveTokenEstimate(turns: ConversationTurn[]): number {
+    let chars = 0;
+    for (const turn of turns) {
+      for (const block of turn.content) {
+        if (block.type === "text") chars += block.text.length;
+        else if (block.type === "tool_call")
+          chars += JSON.stringify(block.arguments).length;
+        else if (block.type === "tool_result") {
+          for (const part of block.content) {
+            if (part.type === "text") chars += part.text.length;
+          }
+        }
+      }
+    }
+    return Math.ceil(chars / 4);
+  }
+
+  function countStructuredTailExcerpts(text: string): number {
+    return text.match(/\[tail-shortened \d+→/g)?.length ?? 0;
+  }
+
   test("large tool outputs in the tail are shortened rather than copied verbatim", async () => {
     const result = await tailCompactor().apply(tailSession(), mockStrategyCtx);
     expect(result.record.reason.startsWith("compacted")).toBe(true);
@@ -840,6 +861,57 @@ describe("CL-9007 budgeted tail (shared auto+manual pipeline)", () => {
       compactionShape: { tailBudgetTokens: 7500, pairSafe: true },
     });
   });
+
+  test("budget-swallow still emits excerpted tail copies; live tokens ≤ budget; shortenedToolOutputs matches live sentinels", async () => {
+    const dump = "z".repeat(12_000);
+    const turns: ConversationTurn[] = [
+      textTurn("user", "do the work"),
+      ...pairTurns("a", "read_file", { path: "src/a.ts" }, dump),
+      ...pairTurns("b", "read_file", { path: "src/b.ts" }, dump),
+      ...pairTurns("c", "read_file", { path: "src/c.ts" }, dump),
+      textTurn("user", "newest ask"),
+      textTurn("assistant", "newest reply"),
+    ];
+    const result = await createPruningCompactor({
+      keepRecentTurns: 2,
+      compactionShape: { tailBudgetTokens: 7500 },
+      summarize: async () => {
+        throw new Error("must not invent a summary on the budget-swallow path");
+      },
+    }).apply(turns, mockStrategyCtx);
+
+    expect(result.record.reason).toBe("no compaction needed");
+    expect(allText(result.output)).not.toContain(COMPACTED_PREFIX);
+    const live = liveResultText(result.output);
+    expect(live).not.toContain(dump);
+    expect(result.record.decisions).toMatchObject({ shortenedToolOutputs: 3 });
+    expect(countStructuredTailExcerpts(live)).toBe(3);
+    expect(liveTokenEstimate(result.output)).toBeLessThanOrEqual(7500);
+  });
+
+  test("a body containing the substring but not the structured marker is still excerpted", async () => {
+    const bait = "[tail-shortened ";
+    const body = `${bait}in the docs\n${"z".repeat(8000)}`;
+    expect(body.includes(bait)).toBe(true);
+    expect(/\[tail-shortened \d+→/.test(body)).toBe(false);
+
+    const turns: ConversationTurn[] = [
+      textTurn("user", "goal"),
+      ...pairTurns("bait", "read_file", { path: "src/bait.ts" }, body),
+      textTurn("user", "newest ask"),
+      textTurn("assistant", "newest reply"),
+    ];
+    const result = await createPruningCompactor({
+      keepRecentTurns: 2,
+      compactionShape: { tailBudgetTokens: 7500 },
+    }).apply(turns, mockStrategyCtx);
+
+    const live = liveResultText(result.output);
+    expect(live).toMatch(/\[tail-shortened \d+→/);
+    expect(live).not.toContain("z".repeat(8000));
+    expect(result.record.decisions).toMatchObject({ shortenedToolOutputs: 1 });
+    expect(countStructuredTailExcerpts(live)).toBe(1);
+  });
 });
 
 describe("continuation facts survive many folds", () => {

src/session/compactor.ts

@@ -946,6 +946,9 @@ function estimateTextTokens(chars: number): number {
 // rides unchanged (summarized from its shortened text, never re-expanded raw
 // and never re-shortened into nested sentinels).
 const TAIL_EXCERPT_SENTINEL = "[tail-shortened ";
+// Match the stamped marker (`[tail-shortened N→`), not a raw prefix: a body
+// that happens to mention the substring must still be excerpted.
+const TAIL_EXCERPT_MARKER = /\[tail-shortened \d+→/;
 
 // Shorten one oversized text part of a tail tool result to a head+tail
 // excerpt. The excerpt carries a sentinel, the original length, and the kept
@@ -957,7 +960,7 @@ function excerptTailText(
 ): { text: string; shortened: boolean } {
   if (
     text.length <= shape.maxTailToolOutputChars ||
-    text.includes(TAIL_EXCERPT_SENTINEL)
+    TAIL_EXCERPT_MARKER.test(text)
   )
     return { text, shortened: false };
   const headChars = shape.excerptHead
@@ -1301,13 +1304,17 @@ export function createPruningCompactor(
         ...carriedSpines,
       ];
 
-      // Keep-set covered everything foldable: nothing to replace. Leave the
-      // input untouched rather than rewriting the head with an empty summary —
-      // but return the image-aged turns (plus their spill blobs), not the raw
-      // input, so eager aging outside the tail is not silently dropped.
+      // Keep-set covered everything foldable: nothing to replace. Do not
+      // invent an empty summary — but still emit the excerpted live copies
+      // selectTail already paid for (plus image-aged turns and their spill
+      // blobs). Returning the unexcerpted aged turns would keep the occupancy
+      // that armed the compact on a short-but-bulky first fold.
       if (summarizedTurns.length === 0) {
         return {
-          output: aged.turns,
+          output:
+            tail.excerpted.size === 0
+              ? aged.turns
+              : aged.turns.map((turn, idx) => tail.excerpted.get(idx) ?? turn),
           record: {
             strategy: this.name,
             version: this.version,