refactor(compression): flatten the structured-clone gate to clear the new-code complexity ratchet (#13154)

isStrictlySerializable rejected any undefined value and any non-plain object
(Date/Map/Set/RegExp) before ever checking whether structuredClone (what
worker.postMessage actually uses) would accept it. strategySelector.ts's
runCompressionAsync always builds the 9-key workerOptions object with every
key explicitly present, so provider: undefined alone (the common case when
no provider is resolved yet) rejected almost every real stacked/rtk/standard
compression call, forcing it onto the main event loop instead of a worker
thread.

Widen isStrictlySerializable to accept undefined and the structured-clone-
native Date/Map/Set/RegExp types (copied, not walked, by structuredClone),
while still rejecting functions and Symbols.

Companion fix: compressionWorker.ts's stacked-mode branch called
applyStackedCompression directly on the raw job body, skipping the
adaptBodyForCompression/restore step that the sync in-process path
(strategySelector.ts's runCompression) always applies for Responses
input[] and Kiro conversationState envelopes. Because the over-strict gate
above meant essentially no real stacked call ever reached the worker, this
was dead code until this fix; without also fixing it, widening the gate
would have shipped a live regression that miscompresses Responses/Kiro
bodies whenever they are now correctly routed to the worker.

changelog.d/fixes/13154-compression-worker-clone-gate.md

@@ -0,0 +1 @@
+- fix(sse): widen compression worker eligibility gate to accept structured-clone-safe `undefined`/Date/Map/Set/RegExp values, restoring worker offload for real requests (#13154)

open-sse/services/compression/compressionWorker.ts

@@ -4,10 +4,36 @@ import {
   applyStackedCompression,
   type StackedCompressionStep,
 } from "./strategySelector.ts";
+import { adaptBodyForCompression } from "./bodyAdapter.ts";
 import type {
   CompressionWorkerJob,
   CompressionWorkerMessage,
 } from "./compressionWorkerProtocol.ts";
+import type { CompressionResult } from "./types.ts";
+
+// #13154 follow-up: `applyCompression`'s sync/in-process "stacked" branch runs every body
+// through `adaptBodyForCompression` first (Responses `input[]` and Kiro `conversationState`
+// envelopes get flattened to `messages[]`, then restored after compression) — see
+// strategySelector.ts's `runCompression`. Before the worker-eligibility gate widening in this
+// same fix, essentially no real "stacked" call ever reached the worker (any `undefined`
+// option key rejected it), so this branch calling `applyStackedCompression` directly on the
+// raw body was dead code. Now that eligible calls are actually routed here, it must mirror
+// that same adapt/restore step or Responses/Kiro bodies get miscompressed (wrong shape, and
+// hard-budget post-pass warnings silently lost) only when the worker happens to run them.
+function runStackedJob(
+  job: CompressionWorkerJob,
+  onEngineStep: (step: StackedCompressionStep) => void
+): CompressionResult {
+  const adapter = adaptBodyForCompression(
+    job.body,
+    job.options?.config?.codexResponsesConfig?.preserveToolNames
+  );
+  const result = applyStackedCompression(adapter.body, job.options?.config?.stackedPipeline, {
+    ...job.options,
+    onEngineStep,
+  });
+  return adapter.adapted ? { ...result, body: adapter.restore(result.body) } : result;
+}
 
 if (!parentPort) throw new Error("compressionWorker must run in a worker thread");
 parentPort.on("message", (job: CompressionWorkerJob) => {
@@ -20,10 +46,7 @@ parentPort.on("message", (job: CompressionWorkerJob) => {
       } satisfies CompressionWorkerMessage);
     const result =
       job.mode === "stacked"
-        ? applyStackedCompression(job.body, job.options?.config?.stackedPipeline, {
-            ...job.options,
-            onEngineStep,
-          })
+        ? runStackedJob(job, onEngineStep)
         : applyCompression(job.body, job.mode, job.options);
     parentPort.postMessage({
       id: job.id,

open-sse/services/compression/compressionWorkerProtocol.ts

@@ -33,24 +33,34 @@ function isPlainObject(value: object): value is Record<string, unknown> {
   return prototype === Object.prototype || prototype === null;
 }
 
+// Anything that is not a (non-null) object is either a structured-clone-safe primitive
+// or an unsupported value (e.g. a non-finite number, a function, a symbol). Isolated
+// from `isStrictlySerializable` so the recursive walk below stays flat.
+function isClonablePrimitive(value: unknown): boolean {
+  if (value === null || value === undefined) return true;
+  if (typeof value === "string" || typeof value === "boolean") return true;
+  if (typeof value === "number") return Number.isFinite(value);
+  return false;
+}
+
+// Date/Map/Set/RegExp are copied natively by structuredClone (not walked as plain
+// objects), so they are always structured-clone-safe regardless of their contents.
+const NATIVELY_CLONABLE_CTORS = [Date, Map, Set, RegExp] as const;
+function isNativelyClonable(value: object): boolean {
+  return NATIVELY_CLONABLE_CTORS.some((ctor) => value instanceof ctor);
+}
+
 // `seen` tracks only the current recursion PATH (ancestors), not every node ever visited:
 // add before descending, remove after returning. That way a real cycle (a node reachable
 // from itself) is still rejected, but two sibling branches that happen to reference the
 // SAME non-cyclic sub-object (a false positive with a globally-shared `seen` set) are not.
 export function isStrictlySerializable(value: unknown, seen = new Set<object>()): boolean {
-  if (
-    value === null ||
-    typeof value === "string" ||
-    typeof value === "boolean" ||
-    typeof value === "number"
-  ) {
-    return typeof value !== "number" || Number.isFinite(value);
-  }
-  if (typeof value !== "object") return false;
+  if (value === null || typeof value !== "object") return isClonablePrimitive(value);
   if (seen.has(value)) return false;
   seen.add(value);
   try {
     if (Array.isArray(value)) return value.every((entry) => isStrictlySerializable(entry, seen));
+    if (isNativelyClonable(value)) return true;
     if (!isPlainObject(value)) return false;
     return Object.values(value).every((entry) => isStrictlySerializable(entry, seen));
   } finally {

tests/unit/compression-worker-gate-13154-repro.test.ts

@@ -0,0 +1,69 @@
+import assert from "node:assert/strict";
+import { describe, it } from "node:test";
+import {
+  isCompressionWorkerEligible,
+  isStrictlySerializable,
+} from "../../open-sse/services/compression/compressionWorkerProtocol.ts";
+import type { CompressionConfig } from "../../open-sse/services/compression/types.ts";
+
+const body = {
+  model: "gpt-test",
+  messages: [{ role: "user", content: "hi" }],
+};
+const config = {
+  enabled: true,
+  defaultMode: "stacked",
+  autoTriggerTokens: 1,
+  cacheMinutes: 0,
+  preserveSystemPrompt: true,
+  stackedPipeline: [{ engine: "rtk" }, { engine: "caveman" }],
+} as CompressionConfig;
+
+describe("#13154: compression worker gate rejects structured-cloneable bodies", () => {
+  it("structuredClone accepts a workerOptions shape with an explicit `undefined` key", () => {
+    const workerOptions = {
+      model: "gpt-test",
+      supportsVision: true,
+      providerTransport: "direct" as const,
+      provider: undefined,
+      imageTransportFidelity: "unknown" as const,
+      sourceFormat: "chat" as const,
+      targetFormat: "chat" as const,
+      compressionStage: "pre-translation" as const,
+      config,
+    };
+    assert.doesNotThrow(() => structuredClone({ body, mode: "stacked", options: workerOptions }));
+  });
+
+  it("the gate should accept that same structured-cloneable shape", () => {
+    const workerOptions = {
+      model: "gpt-test",
+      supportsVision: true,
+      providerTransport: "direct" as const,
+      provider: undefined,
+      imageTransportFidelity: "unknown" as const,
+      sourceFormat: "chat" as const,
+      targetFormat: "chat" as const,
+      compressionStage: "pre-translation" as const,
+      config,
+    };
+    assert.equal(isStrictlySerializable({ body, mode: "stacked", options: workerOptions }), true);
+    assert.equal(isCompressionWorkerEligible(body, "stacked", workerOptions), true);
+  });
+
+  it("realistic runCompressionAsync-shaped call (only `provider` unset) should be eligible", () => {
+    const workerOptions = {
+      model: "gpt-test",
+      supportsVision: undefined,
+      providerTransport: undefined,
+      provider: undefined,
+      imageTransportFidelity: undefined,
+      sourceFormat: undefined,
+      targetFormat: undefined,
+      compressionStage: undefined,
+      config,
+    };
+    assert.doesNotThrow(() => structuredClone({ body, mode: "stacked", options: workerOptions }));
+    assert.equal(isCompressionWorkerEligible(body, "stacked", workerOptions), true);
+  });
+});

tests/unit/compression/compression-worker.test.ts

@@ -79,17 +79,8 @@ describe("compression worker eligibility", () => {
     }
   });
 
-  it("rejects functions, symbols, classes, special objects, cycles, and non-finite numbers", () => {
-    for (const value of [
-      () => undefined,
-      Symbol("x"),
-      new Date(),
-      new Map(),
-      new Set(),
-      /x/,
-      NaN,
-      Infinity,
-    ]) {
+  it("rejects functions, symbols, cycles, and non-finite numbers", () => {
+    for (const value of [() => undefined, Symbol("x"), NaN, Infinity]) {
       assert.equal(isStrictlySerializable(value), false);
     }
     const cyclic: Record<string, unknown> = {};
@@ -97,6 +88,34 @@ describe("compression worker eligibility", () => {
     assert.equal(isStrictlySerializable(cyclic), false);
   });
 
+  it("#13154: accepts structured-clone-native Date/Map/Set/RegExp values", () => {
+    for (const value of [new Date(), new Map(), new Set(), /x/]) {
+      assert.equal(isStrictlySerializable(value), true);
+    }
+  });
+
+  it("#13154: accepts `undefined` values instead of rejecting the whole tree", () => {
+    assert.equal(isStrictlySerializable(undefined), true);
+    assert.equal(isStrictlySerializable({ provider: undefined, model: "gpt-test" }), true);
+  });
+
+  it("#13154: accepts strategySelector.ts's exact 9-key workerOptions shape with `provider` unset", () => {
+    // Mirrors runCompressionAsync's workerOptions object: all 9 keys always present,
+    // `provider` commonly unresolved (undefined) at call time.
+    const workerOptions = {
+      model: "gpt-test",
+      supportsVision: undefined,
+      providerTransport: undefined,
+      provider: undefined,
+      imageTransportFidelity: undefined,
+      sourceFormat: undefined,
+      targetFormat: undefined,
+      compressionStage: undefined,
+      config,
+    };
+    assert.equal(isCompressionWorkerEligible(body, "stacked", workerOptions), true);
+  });
+
   it("#13154: does not misread a shared (non-cyclic) sub-object referenced by two sibling branches as a cycle", () => {
     // Original bug: a single `seen` set shared across the whole recursion tree (never
     // backtracked) meant visiting the SAME object twice via two different, non-cyclic