diff --git a/extensions/workflows/artifacts.ts b/extensions/workflows/artifacts.ts
index a19fb6cd..0fa06a95 100644
--- a/extensions/workflows/artifacts.ts
+++ b/extensions/workflows/artifacts.ts
@@ -24,6 +24,7 @@ import {
 import {
   encodeCompleteJson,
   safeStringify,
+  toSerializable,
   truncateUtf8,
   writeFileAtomic,
 } from "./serialization.ts";
@@ -379,7 +380,7 @@ function boundEntry(entry: TranscriptEntry, maxBytes: number) {
 
 /** Keep the initial prompt plus the newest useful context within the artifact cap. */
 export function boundedArtifactTranscript(
-  transcript: TranscriptEntry[],
+  transcript: readonly TranscriptEntry[],
   options: { maxBytes?: number; entryMaxBytes?: number } = {},
 ) {
   if (transcript.length === 0) return [];
@@ -440,12 +441,149 @@ export interface BoundedTranscriptsArtifact {
   omitted: { agents: number; entries: number };
 }
 
+/**
+ * One agent's bounded transcript, encoded once for `transcripts.json`. Live
+ * progress replaces an agent's transcript array instead of mutating it, so
+ * unchanged agents keep the same array and reuse this between checkpoints.
+ */
+interface EncodedAgentTranscript {
+  /** The entries after `toSerializable`, nested one level deep. */
+  readonly serialized: string;
+  /** `toSerializable` nodes; undefined when the shape is not plain data. */
+  readonly nodes: number | undefined;
+}
+
+// Omitted agents need only sizing metadata. Never retain their measured JSON:
+// live workflows can keep thousands of transcript arrays reachable.
+const transcriptMeasurements = new WeakMap<
+  readonly TranscriptEntry[],
+  {
+    readonly length: number;
+    readonly last: TranscriptEntry | undefined;
+    readonly entryCount: number;
+    readonly measuredBytes: number;
+  }
+>();
+
+// A process-wide LRU bounds both encoded text and bookkeeping, even across
+// concurrent workflows or caller-supplied artifact budgets. Charge UTF-16
+// storage conservatively using twice the UTF-8 byte length.
+const ENCODED_TRANSCRIPTS_MAX_BYTES = 4 * 1024 * 1024;
+const ENCODED_TRANSCRIPTS_MAX_ENTRIES = 128;
+const encodedTranscripts = new WeakMap<
+  readonly TranscriptEntry[],
+  WeakRef<EncodedAgentTranscript>
+>();
+const retainedTranscripts = new Map<EncodedAgentTranscript, number>();
+let encodedTranscriptBytes = 0;
+
+function forgetEncodedTranscript(transcript: readonly TranscriptEntry[]) {
+  const cached = encodedTranscripts.get(transcript)?.deref();
+  if (cached) {
+    encodedTranscriptBytes -= retainedTranscripts.get(cached) ?? 0;
+    retainedTranscripts.delete(cached);
+  }
+  encodedTranscripts.delete(transcript);
+}
+
+const nestOneLevel = (json: string) => json.replaceAll("\n", "\n  ");
+
+/** Nodes `toSerializable` would visit, or undefined for non-plain values. */
+function plainNodes(value: unknown, depth = 0): number | undefined {
+  if (value === null || typeof value !== "object") return 1;
+  if (depth > 8) return undefined;
+  const prototype = Object.getPrototypeOf(value);
+  if (
+    !Array.isArray(value) &&
+    prototype !== Object.prototype &&
+    prototype !== null
+  )
+    return undefined;
+  let nodes = 1;
+  for (const child of Array.isArray(value) ? value : Object.values(value)) {
+    const childNodes = plainNodes(child, depth + 1);
+    if (childNodes === undefined) return undefined;
+    nodes += childNodes;
+  }
+  return nodes;
+}
+
+function measureAgentTranscript(transcript: readonly TranscriptEntry[]) {
+  const last = transcript.at(-1);
+  const cached = transcriptMeasurements.get(transcript);
+  // Guard the identity cache against an in-place append or replacement.
+  if (cached?.length === transcript.length && cached.last === last)
+    return cached;
+  forgetEncodedTranscript(transcript);
+  const entries = boundedArtifactTranscript(transcript);
+  const measurement = {
+    length: transcript.length,
+    last,
+    entryCount: entries.length,
+    measuredBytes: textBytes(nestOneLevel(JSON.stringify(entries, null, 2))),
+  };
+  transcriptMeasurements.set(transcript, measurement);
+  return measurement;
+}
+
+function encodeAgentTranscript(transcript: readonly TranscriptEntry[]) {
+  const cached = encodedTranscripts.get(transcript)?.deref();
+  const cachedBytes = cached && retainedTranscripts.get(cached);
+  if (cached && cachedBytes !== undefined) {
+    retainedTranscripts.delete(cached);
+    retainedTranscripts.set(cached, cachedBytes);
+    return cached;
+  }
+  const entries = boundedArtifactTranscript(transcript);
+  const encoded: EncodedAgentTranscript = {
+    serialized: nestOneLevel(
+      JSON.stringify(
+        toSerializable(entries, { maxNodes: Number.POSITIVE_INFINITY }),
+        null,
+        2,
+      ),
+    ),
+    nodes: plainNodes(entries),
+  };
+  const retainedBytes = 2 * textBytes(encoded.serialized);
+  if (retainedBytes <= ENCODED_TRANSCRIPTS_MAX_BYTES) {
+    while (
+      encodedTranscriptBytes + retainedBytes > ENCODED_TRANSCRIPTS_MAX_BYTES ||
+      retainedTranscripts.size >= ENCODED_TRANSCRIPTS_MAX_ENTRIES
+    ) {
+      const oldest = retainedTranscripts.keys().next().value!;
+      encodedTranscriptBytes -= retainedTranscripts.get(oldest)!;
+      retainedTranscripts.delete(oldest);
+    }
+    encodedTranscripts.set(transcript, new WeakRef(encoded));
+    retainedTranscripts.set(encoded, retainedBytes);
+    encodedTranscriptBytes += retainedBytes;
+  }
+  return encoded;
+}
+
+/** `JSON.stringify({ [index]: entries }, null, 2)` built from encoded agents. */
+function assembleTranscripts(
+  agents: readonly { readonly index: number; readonly json: string }[],
+) {
+  if (agents.length === 0) return "{}";
+  // Integer-like keys serialize in ascending numeric order.
+  const ordered = [...agents].sort((a, b) => a.index - b.index);
+  return `{\n${ordered.map((agent) => `  "${agent.index}": ${agent.json}`).join(",\n")}\n}`;
+}
+
 /**
  * Assemble `transcripts.json`, keeping whole agents in index order until the
  * next one would exceed the byte budget. The dropped tail is counted and
  * reported (`transcriptsOmitted`) rather than silently discarded by the
  * serializer's node budget, which previously stopped at 20_000 nodes — roughly
  * 11 full agents — while the file sat at a quarter of its byte cap (issue #558).
+ *
+ * Agent measurements are weakly cached; only the kept prefix is encoded, with
+ * a bounded cache for serialized fragments. A checkpoint
+ * where one child changed does not re-serialize every finished child. The
+ * result is byte-identical to serializing the whole object; any shape the
+ * fragment assembly cannot reproduce exactly takes the whole-object path.
  */
 export function boundedTranscriptsArtifact(
   agents: readonly { index: number; transcript: TranscriptEntry[] }[],
@@ -456,6 +594,65 @@ export function boundedTranscriptsArtifact(
     options.maxBytes ?? WORKFLOW_TRANSCRIPTS_MAX_BYTES,
   );
   const maxNodes = options.maxNodes ?? WORKFLOW_TRANSCRIPTS_MAX_NODES;
+  const indexes = new Set(agents.map((agent) => agent.index));
+  const integerKeys =
+    indexes.size === agents.length &&
+    agents.every(
+      (agent) =>
+        Number.isInteger(agent.index) &&
+        agent.index >= 0 &&
+        agent.index < 2 ** 32 - 1,
+    );
+  if (!integerKeys) return wholeTranscriptsArtifact(agents, maxBytes, maxNodes);
+
+  const measured = agents.map((agent) => ({
+    index: agent.index,
+    transcript: agent.transcript,
+    ...measureAgentTranscript(agent.transcript),
+  }));
+  // Byte size of the pretty object holding the first `count` agents.
+  let kept = 0;
+  let bytes = 2;
+  for (const agent of measured) {
+    // "{}" grows to "{\n<line>\n}", then by ",\n<line>": 2 bytes either way.
+    const next =
+      bytes + 2 + textBytes(`  "${agent.index}": `) + agent.measuredBytes;
+    if (next > maxBytes) break;
+    bytes = next;
+    kept++;
+  }
+  let omittedEntries = 0;
+  for (let i = kept; i < measured.length; i++)
+    omittedEntries += measured[i]!.entryCount;
+
+  const keptAgents = measured.slice(0, kept).map((agent) => ({
+    index: agent.index,
+    ...encodeAgentTranscript(agent.transcript),
+  }));
+  const nodes = keptAgents.reduce<number | undefined>(
+    (total, agent) =>
+      total === undefined || agent.nodes === undefined
+        ? undefined
+        : total + agent.nodes,
+    1,
+  );
+  const content = assembleTranscripts(
+    keptAgents.map((agent) => ({ index: agent.index, json: agent.serialized })),
+  );
+  if (nodes === undefined || nodes > maxNodes || textBytes(content) > maxBytes)
+    return wholeTranscriptsArtifact(agents, maxBytes, maxNodes);
+  return {
+    content,
+    omitted: { agents: measured.length - kept, entries: omittedEntries },
+  };
+}
+
+/** The original whole-object assembly, kept for shapes fragments cannot match. */
+function wholeTranscriptsArtifact(
+  agents: readonly { index: number; transcript: TranscriptEntry[] }[],
+  maxBytes: number,
+  maxNodes: number,
+): BoundedTranscriptsArtifact {
   // Bound each agent first; a single agent is capped well under maxBytes by
   // boundedArtifactTranscript, so at least one agent always fits.
   const bounded = agents.map((agent) => ({
diff --git a/tests/extensions/workflows/artifact-node-budget.test.ts b/tests/extensions/workflows/artifact-node-budget.test.ts
index efb80134..ac7a6a2f 100644
--- a/tests/extensions/workflows/artifact-node-budget.test.ts
+++ b/tests/extensions/workflows/artifact-node-budget.test.ts
@@ -1,4 +1,5 @@
 import assert from "node:assert/strict";
+import { spawnSync } from "node:child_process";
 import { mkdtempSync, readFileSync, rmSync } from "node:fs";
 import { tmpdir } from "node:os";
 import { join } from "node:path";
@@ -291,3 +292,104 @@ test("transcriptsOmitted surfaces through the completion display projection", ()
   );
   assert.match(entry.expanded, /omitted from transcripts\.json/);
 });
+
+test("FAIL: rebuilding transcripts.json does not re-encode agents whose transcript is unchanged", () => {
+  let reads = 0;
+  const counted = (entries: TranscriptEntry[]) =>
+    entries.map(
+      (entry) =>
+        new Proxy(entry, {
+          get(target, property, receiver) {
+            if (property === "text") reads++;
+            return Reflect.get(target, property, receiver);
+          },
+        }),
+    );
+  const agents = Array.from({ length: 40 }, (_, index) =>
+    agent(index, counted(transcript(8))),
+  );
+  const first = boundedTranscriptsArtifact(agents);
+  assert.ok(reads > 0);
+
+  // Live progress replaces one child's array; the others keep theirs.
+  reads = 0;
+  agents[0]!.transcript = [
+    ...transcript(8),
+    { role: "assistant", text: "next step" },
+  ];
+  const second = boundedTranscriptsArtifact(agents);
+  assert.equal(reads, 0, "finished agents were not re-read");
+  assert.notEqual(second.content, first.content);
+
+  // The cached result is byte-identical to encoding fresh copies.
+  const fresh = boundedTranscriptsArtifact(
+    agents.map((item) => ({
+      index: item.index,
+      transcript: item.transcript.map((entry) => ({ ...entry })),
+    })),
+  );
+  assert.equal(second.content, fresh.content);
+  assert.deepEqual(second.omitted, fresh.omitted);
+});
+
+test("transcript encoding retains bounded memory while workflow agents remain live", () => {
+  // Isolate GC and heap accounting from the test runner. Keep source arrays live
+  // so a WeakMap alone cannot hide retained encodings. The old cache retained
+  // ~73 MiB for this fixture; allow ample headroom above a few MiB of output.
+  const result = spawnSync(
+    process.execPath,
+    [
+      "--experimental-strip-types",
+      "--expose-gc",
+      "--input-type=module",
+      "--eval",
+      `
+        import { boundedTranscriptsArtifact } from ${JSON.stringify(
+          new URL("../../../extensions/workflows/artifacts.ts", import.meta.url)
+            .href,
+        )};
+        const agents = Array.from({ length: 256 }, (_, index) => ({
+          index,
+          transcript: Array.from({ length: 202 }, (_, i) => ({
+            role: i % 2 ? "toolResult" : "tool",
+            text: "Read src/module-" + i + ".ts",
+            name: "Read",
+            toolCallId: "toolu_" + i.toString(36).padStart(8, "0"),
+            timestamp: 1700000000000 + i,
+            startedAt: 1700000000000 + i,
+            finishedAt: 1700000000120 + i,
+            durationMs: 120,
+          })),
+        }));
+        global.gc();
+        const before = process.memoryUsage().heapUsed;
+        const first = boundedTranscriptsArtifact(agents);
+        // Separate workflows/prefixes must share a bound, rather than each
+        // keeping a full cache forever. Exercise warm reads and eviction too.
+        for (let start = 0; start < agents.length; start += 32) {
+          const batch = agents.slice(start, start + 32);
+          const cold = boundedTranscriptsArtifact(batch);
+          const warm = boundedTranscriptsArtifact(batch);
+          if (cold.content !== warm.content) throw new Error("cache changed bytes");
+        }
+        const again = boundedTranscriptsArtifact(agents);
+        if (again.content !== first.content) throw new Error("eviction changed bytes");
+        global.gc();
+        console.log(JSON.stringify({
+          retained: process.memoryUsage().heapUsed - before,
+          omitted: again.omitted.agents,
+          liveAgents: agents.length,
+        }));
+      `,
+    ],
+    { encoding: "utf8", timeout: 30_000 },
+  );
+  assert.equal(result.status, 0, result.stderr || String(result.error));
+  const memory = JSON.parse(result.stdout);
+  assert.equal(memory.liveAgents, 256);
+  assert.ok(memory.omitted > 0, "fixture must exceed the artifact byte budget");
+  assert.ok(
+    memory.retained < 24 * 1024 * 1024,
+    `encoding retained ${(memory.retained / 1024 / 1024).toFixed(1)} MiB`,
+  );
+});