diff --git a/scripts/test-layout/layout.json b/scripts/test-layout/layout.json
index b4d27b9425e..d910dc0ae31 100644
--- a/scripts/test-layout/layout.json
+++ b/scripts/test-layout/layout.json
@@ -1502,6 +1502,7 @@
     "responses-spill-inspection.test.ts": "responses",
     "responses-spill-orphan-sweep.test.ts": "responses",
     "responses-spill-shutdown-clock.test.ts": "responses",
+    "responses-state-snapshot-select.test.ts": "responses",
     "responses-state-snapshot-stubs.test.ts": "responses",
     "responses-state-write-amplification.test.ts": "responses",
     "responses-state.test.ts": "responses",
diff --git a/src/responses/state.ts b/src/responses/state.ts
index 21241e9faef..892ba556e09 100644
--- a/src/responses/state.ts
+++ b/src/responses/state.ts
@@ -19,7 +19,7 @@ import {
   writeResponseSpillDurably,
 } from "./spill-store";
 import { collectReferencedSpillFileNames, snapshotReferencedSpillFileNames } from "./state/spill-inspect";
-import { selectSnapshotEntries } from "./state/snapshot-select";
+import { selectSnapshotEntries, snapshotPayload } from "./state/snapshot-select";
 import { clientCarriedPrefixLength, providerIssuedIdentity } from "./state/replay-fingerprint";
 export type { ResponseStateTempRecoveryResult, ResponseStateTempRecoveryOptions } from "./state/temp-recovery";
 export type { ResponseSpillDirInspection } from "./spill-store";
@@ -705,8 +705,7 @@ async function writeBoundedSnapshot(path: string, attemptLimit: number): Promise
   try {
     for (let attempt = 0; attempt < attemptLimit; attempt += 1) {
       const revision = stateRevision;
-      const entries = selectSnapshotEntries(states, snapshotTotalBytes(), SNAPSHOT_ENTRY_MAX_BYTES);
-      const payload = JSON.stringify({ version: 2, states: entries });
+      const payload = snapshotPayload(selectSnapshotEntries(states, snapshotTotalBytes(), SNAPSHOT_ENTRY_MAX_BYTES));
       const payloadBytes = Buffer.byteLength(payload, "utf8");
       const payloadDigest = Bun.hash(payload).toString(36);
       // A mutation does not always change what gets persisted: entries past the
diff --git a/src/responses/state/snapshot-select.ts b/src/responses/state/snapshot-select.ts
index 51bcc5a2c84..73f503ec3af 100644
--- a/src/responses/state/snapshot-select.ts
+++ b/src/responses/state/snapshot-select.ts
@@ -1,30 +1,34 @@
 import type { StoredResponseState } from "../state";
 
 /**
- * Pick the snapshot entries that fit the byte budgets, in `states` order.
+ * Pick the snapshot entries that fit the byte budgets, in `states` order, and
+ * return them already serialized.
  *
  * Bounded stubs and tombstones are selected before residents: they are the
  * only durable references a spill file has, and demotion is oldest-first, so a
  * single newest-first pass would let resident payloads consume the whole
  * budget ahead of them. Residents then fill what remains, newest-first so the
  * most recent chains survive both legacy snapshot caps.
+ *
+ * Each entry has to be stringified once anyway to measure it, so the kept
+ * strings are returned instead of the entries: serializing the whole selection
+ * again would walk up to the full snapshot budget a second time per write.
  */
 export function selectSnapshotEntries(
   states: ReadonlyMap<string, StoredResponseState>,
   totalMaxBytes: number,
   residentEntryMaxBytes: number,
-): Array<[string, unknown]> {
+): string[] {
   const ordered = [...states].reverse();
-  const persisted = new Map<string, [string, unknown]>();
+  const persisted = new Map<string, string>();
   let total = 0;
   // UTF-8 bytes, not UTF-16 code units: multibyte items otherwise slip past
   // both snapshot caps at up to 2x the intended size.
-  const sizeOf = (entry: [string, unknown]): number => Buffer.byteLength(JSON.stringify(entry), "utf8");
   for (const [id, state] of ordered) {
     if (state.kind === "resident") continue;
     const { sizeBytes: _sizeBytes, ...smallState } = state;
-    const entry: [string, unknown] = [id, smallState];
-    const size = sizeOf(entry);
+    const entry = JSON.stringify([id, smallState]);
+    const size = Buffer.byteLength(entry, "utf8");
     if (total + size > totalMaxBytes) continue;
     total += size;
     persisted.set(id, entry);
@@ -32,18 +36,27 @@ export function selectSnapshotEntries(
   for (const [id, state] of ordered) {
     if (state.kind !== "resident") continue;
     const { sizeBytes: _sizeBytes, kind: _kind, ...resident } = state;
-    const entry: [string, unknown] = [id, resident];
-    const size = sizeOf(entry);
+    const entry = JSON.stringify([id, resident]);
+    const size = Buffer.byteLength(entry, "utf8");
     if (size > residentEntryMaxBytes) continue;
     if (total + size > totalMaxBytes) break;
     total += size;
     persisted.set(id, entry);
   }
   // Emit in map order so reload and count eviction keep the same relative order as `states`.
-  const entries: Array<[string, unknown]> = [];
+  const entries: string[] = [];
   for (const [id] of states) {
     const kept = persisted.get(id);
     if (kept) entries.push(kept);
   }
   return entries;
 }
+
+/**
+ * The snapshot file body for serialized entries. Byte-identical to
+ * `JSON.stringify({ version: 2, states: entries.map(e => JSON.parse(e)) })`: compact
+ * JSON.stringify renders an array element exactly as it renders that value alone.
+ */
+export function snapshotPayload(serializedEntries: readonly string[]): string {
+  return '{"version":2,"states":[' + serializedEntries.join(",") + "]}";
+}
diff --git a/tests/fixtures/test-layout-expected.json b/tests/fixtures/test-layout-expected.json
index 83170e64b6b..c66dbf5ece8 100644
--- a/tests/fixtures/test-layout-expected.json
+++ b/tests/fixtures/test-layout-expected.json
@@ -1515,6 +1515,7 @@
   "responses-spill-inspection.test.ts": "responses",
   "responses-spill-orphan-sweep.test.ts": "responses",
   "responses-spill-shutdown-clock.test.ts": "responses",
+  "responses-state-snapshot-select.test.ts": "responses",
   "responses-state-snapshot-stubs.test.ts": "responses",
   "responses-state-write-amplification.test.ts": "responses",
   "responses-state.test.ts": "responses",
diff --git a/tests/responses/responses-state-snapshot-select.test.ts b/tests/responses/responses-state-snapshot-select.test.ts
new file mode 100644
index 00000000000..0cd6ca86e85
--- /dev/null
+++ b/tests/responses/responses-state-snapshot-select.test.ts
@@ -0,0 +1,66 @@
+/**
+ * The snapshot writer reuses the per-entry strings it already produced to measure each
+ * entry, instead of serializing the whole selection a second time. That is only safe if
+ * the file bytes stay exactly what \`JSON.stringify({ version: 2, states })\` wrote before:
+ * the unchanged-payload digest skip and every reader depend on them. The expected
+ * strings below were produced by the previous implementation.
+ */
+import { describe, expect, test } from "bun:test";
+import type { StoredResponseState } from "../../src/responses/state";
+import { selectSnapshotEntries, snapshotPayload } from "../../src/responses/state/snapshot-select";
+
+const RESIDENT_ENTRY_MAX_BYTES = 200;
+
+function fixtureStates(): Map<string, StoredResponseState> {
+  const rows: Array<[string, unknown]> = [
+    ["resp_old_resident", {
+      kind: "resident",
+      createdAt: 1,
+      items: [{ type: "message", content: 'quote " backslash \\ newline \n tab \t' }],
+      providerOutputStart: 1,
+      sizeBytes: 999,
+    }],
+    ["resp_spill", { kind: "spill", createdAt: 2, clientThreadId: "thr_1", spill: { file: "a.json", bytes: 10 }, sizeBytes: 5000 }],
+    ["resp_failed", { kind: "spill-failed", createdAt: 3, sizeBytes: 7 }],
+    ["resp_multibyte", {
+      kind: "resident",
+      createdAt: 4,
+      items: ["한글 ✓ 🚀", { nested: { a: undefined, b: null, c: [1.5, -0, 1e21] } }],
+      sizeBytes: 1,
+    }],
+    ["resp_oversized", { kind: "resident", createdAt: 5, items: ["x".repeat(300)], sizeBytes: 1 }],
+    ["resp_newest", { kind: "resident", createdAt: 6, items: ["newest"], sizeBytes: 1 }],
+  ];
+  return new Map(rows as Array<[string, StoredResponseState]>);
+}
+
+function payload(states: Map<string, StoredResponseState>, totalMaxBytes: number): string {
+  return snapshotPayload(selectSnapshotEntries(states, totalMaxBytes, RESIDENT_ENTRY_MAX_BYTES));
+}
+
+describe("responses-state snapshot payload bytes", () => {
+  test("matches the previous whole-tree serialization when everything fits", () => {
+    expect(payload(fixtureStates(), 10_000)).toBe(
+      '{"version":2,"states":[["resp_old_resident",{"createdAt":1,"items":[{"type":"message","content":"quote \\" backslash \\\\ newline \\n tab \\t"}],"providerOutputStart":1}],["resp_spill",{"kind":"spill","createdAt":2,"clientThreadId":"thr_1","spill":{"file":"a.json","bytes":10}}],["resp_failed",{"kind":"spill-failed","createdAt":3}],["resp_multibyte",{"createdAt":4,"items":["한글 ✓ 🚀",{"nested":{"b":null,"c":[1.5,0,1e+21]}}]}],["resp_newest",{"createdAt":6,"items":["newest"]}]]}',
+    );
+  });
+
+  test("matches the previous serialization when the byte budget cuts residents off", () => {
+    // Stubs take 160 bytes, the newest resident 50; the multibyte resident's 102 UTF-8
+    // bytes then overflow 300, which ends the newest-first resident pass.
+    expect(payload(fixtureStates(), 300)).toBe(
+      '{"version":2,"states":[["resp_spill",{"kind":"spill","createdAt":2,"clientThreadId":"thr_1","spill":{"file":"a.json","bytes":10}}],["resp_failed",{"kind":"spill-failed","createdAt":3}],["resp_newest",{"createdAt":6,"items":["newest"]}]]}',
+    );
+  });
+
+  test("an empty store still writes the version envelope", () => {
+    expect(payload(new Map(), 10_000)).toBe('{"version":2,"states":[]}');
+  });
+
+  test("joining the kept strings equals serializing the parsed entries as one tree", () => {
+    for (const budget of [0, 120, 300, 10_000]) {
+      const kept = selectSnapshotEntries(fixtureStates(), budget, RESIDENT_ENTRY_MAX_BYTES);
+      expect(snapshotPayload(kept)).toBe(JSON.stringify({ version: 2, states: kept.map(entry => JSON.parse(entry)) }));
+    }
+  });
+});