fix(sse): shallow per-target copy for the combo attempt body (#7847) (#8553)

* fix(sse): shallow per-target copy for the combo attempt body (#7847)

combo.ts deep-cloned the request body for every target. On a 3.05 MiB agent request that
is 9.53 MiB at 3 targets, and it scales linearly:

  3 targets    9.53 MiB (3.12x wire)  ->  ~0.001 MiB
  5 targets   15.89 MiB (5.19x wire)  ->  ~0.000 MiB
 10 targets   31.78 MiB (10.39x wire) ->  ~0.001 MiB

The isolation it bought only ever needed to contain TOP-LEVEL SCALAR writes. The full
mutation surface on this path is two assignments:

  combo.ts     bodyRecord.max_tokens = ...   (reasoning buffer)
  chatCore.ts  body.model = model            (Background Task Redirection T41)

Nothing mutates the nested payload; applyCompression and injectUniversalHandoffBody both
return new objects (verified empirically -- neither touches its input, and both tolerate a
frozen one). So a fresh top-level object per target gives identical isolation while sharing
the expensive messages/tools arrays.

Also fixes a REAL cross-target leak in handleRoundRobinCombo. It already used a shallow
copy, but took it only when the reasoning buffer actually changed max_tokens -- every other
attempt shared the caller's object outright. The new test reproduces it on the unmodified
code: target 2 received model "mutated-by-openai/gpt-4o-mini". In production that is a
Background Task Redirection on one round-robin target rewriting body.model for the next.
The copy is now unconditional.

The invariant is pinned by tests rather than by a comment listing mutation sites, so the
clone strategy can change again without anyone re-deriving them by hand:
 - a target's in-place write must not leak into the next (priority / fill-first /
   round-robin; the stub reproduces chatCore's body.model write)
 - the caller's body is never mutated
 - the per-target copy stays shallow (targets share one messages array)
 - freeze probe: combo's own body handling performs no in-place writes

* test(sse): register the combo attempt-body isolation test in stryker tap.testFiles

The new test resets the circuit breaker in beforeEach, so it counts as a covering test
for src/shared/utils/circuitBreaker.ts. Without registering it,
check:mutation-test-coverage --strict reported a 4th drift entry that was not there on the
pristine tip -- new drift introduced by this PR. Registered in sorted position; the gate is
back to the 3 pre-existing entries (accountFallback.ts, error.ts, comboPredicates.ts) that
#8538 addresses.

changelog.d/fixes/7847-combo-attempt-body-cow.md

@@ -0,0 +1 @@
+- fix(sse): copy the combo attempt body shallowly instead of deep-cloning it per target (#7847) — the deep clone cost 9.53 MiB at 3 targets and scaled linearly with the target count (31.78 MiB at 10) on a 3.05 MiB agent request, while the isolation it provided only ever needed to contain top-level scalar writes. Also fixes a real cross-target leak in round-robin, which copied the body only when the reasoning buffer changed `max_tokens` and otherwise shared the caller's object, so a Background Task Redirection on one target rewrote `body.model` for the next

open-sse/services/combo.ts

@@ -1718,12 +1718,9 @@ export async function handleComboChat({
           // QA P0 diagnostics: capture the attempt order (provider/model ids only).
           comboAttemptOrder.push({ provider: provider ?? "unknown", model: modelStr });
 
-          // Deep clone the body to ensure context preservation and prevent mutations
-          // from affecting other targets in the combo. structuredClone avoids the
-          // full intermediate JSON string that JSON.parse(JSON.stringify(...)) builds
-          // (a second multi-hundred-KB allocation per target on large agent payloads),
-          // halving the per-target transient heap on the hot path (#5152).
-          let attemptBody = structuredClone(body);
+          // Copy-on-write, not a deep clone (#7847 — 9.53 MiB at 3 targets). Writes here are
+          // top-level scalars. Invariant: tests/unit/combo-attempt-body-isolation-7847.test.ts.
+          let attemptBody = { ...(body as Record<string, unknown>) } as typeof body;
 
           // Proactive Context Compression for fallbacks (Zero-Latency optimization)
           if (
@@ -2838,7 +2835,11 @@ async function handleRoundRobinCombo({
   // BEFORE availability is known; if every compat-kept target then turns out to be
   // runtime-unavailable, we must reconsider these before returning 503, instead of
   // permanently dropping a compat-rejected-but-healthy provider.
-  const compatRejectedTargets = computeCompatRejectedTargets(evalRankedTargets, filteredTargets, body);
+  const compatRejectedTargets = computeCompatRejectedTargets(
+    evalRankedTargets,
+    filteredTargets,
+    body
+  );
   let modelCount = filteredTargets.length;
   if (modelCount === 0) {
     return comboModelNotFoundResponse("Round-robin combo has no executable targets");
@@ -3096,9 +3097,11 @@ async function handleRoundRobinCombo({
         // Issue #3587: Reasoning models can spend the whole output budget on
         // reasoning. Apply any safe buffer to a per-attempt copy so round-robin
         // retries never compound across models.
-        let attemptBody = body;
+        // #7847: UNCONDITIONAL — copying only when the buffer changed max_tokens left every
+        // other attempt sharing the caller's object, leaking chatCore's `body.model` forward.
+        let attemptBody = { ...(body as Record<string, unknown>) } as typeof body;
         {
-          const bodyRecord = body as Record<string, unknown>;
+          const bodyRecord = attemptBody as Record<string, unknown>;
           const currentMaxTokens = toPositiveInteger(bodyRecord.max_tokens);
           const bufferedMaxTokens = resolveReasoningBufferedMaxTokens(
             modelStr,
@@ -3110,10 +3113,8 @@ async function handleRoundRobinCombo({
             bufferedMaxTokens !== null &&
             bufferedMaxTokens !== currentMaxTokens
           ) {
-            attemptBody = {
-              ...bodyRecord,
-              max_tokens: bufferedMaxTokens,
-            } as typeof body;
+            // Safe to write in place: bodyRecord is the per-attempt copy above, not the caller's.
+            bodyRecord.max_tokens = bufferedMaxTokens;
             log.info(
               "COMBO-RR",
               `Reasoning model ${modelStr}: adjusted max_tokens ${currentMaxTokens} -> ${bufferedMaxTokens}`

stryker.conf.json

@@ -39,9 +39,7 @@
   "incremental": true,
   "incrementalFile": "reports/mutation/stryker-incremental.json",
   "testRunner": "tap",
-  "plugins": [
-    "@stryker-mutator/tap-runner"
-  ],
+  "plugins": ["@stryker-mutator/tap-runner"],
   "tap": {
     "testFiles": [
       "tests/unit/7993-noauth-proxy-routing.test.ts",
@@ -136,6 +134,7 @@
       "tests/unit/collect-metrics-module-coverage.test.ts",
       "tests/unit/combo-499-abort.test.ts",
       "tests/unit/combo-account-allowlist-3266.test.ts",
+      "tests/unit/combo-attempt-body-isolation-7847.test.ts",
       "tests/unit/combo-auto-candidate-expansion.test.ts",
       "tests/unit/combo-breaker-429.test.ts",
       "tests/unit/combo-cache-invalidation.test.ts",
@@ -406,11 +405,7 @@
     ".worktrees",
     ".stryker-tmp"
   ],
-  "reporters": [
-    "progress",
-    "html",
-    "json"
-  ],
+  "reporters": ["progress", "html", "json"],
   "htmlReporter": {
     "fileName": "reports/mutation/mutation.html"
   },

tests/unit/combo-attempt-body-isolation-7847.test.ts

@@ -0,0 +1,243 @@
+// Target isolation for the combo attempt body (#7847).
+//
+// combo.ts deep-clones the request body per target (`attemptBody = structuredClone(body)`) so one
+// target's mutations cannot reach the next. On a 3.05 MiB agent request that costs 9.53 MiB at
+// only 3 targets — 3x the wire size — and it scales with the target count.
+//
+// A shallow per-target copy would give the same isolation for ~nothing, because every known
+// in-place mutation on this path is a TOP-LEVEL SCALAR:
+//   combo.ts     bodyRecord.max_tokens = ...   (reasoning buffer)
+//   chatCore.ts  body.model = ...              (Background Task Redirection T41)
+//
+// These tests lock the isolation invariant itself, so the clone strategy can be changed
+// underneath them without anyone having to trust a grep for mutation sites.
+import test from "node:test";
+import assert from "node:assert/strict";
+import fs from "node:fs";
+import os from "node:os";
+import path from "node:path";
+
+const TEST_DATA_DIR = fs.mkdtempSync(path.join(os.tmpdir(), "omniroute-combo-cow-"));
+const ORIGINAL_DATA_DIR = process.env.DATA_DIR;
+process.env.DATA_DIR = TEST_DATA_DIR;
+
+const { handleComboChat } = await import("../../open-sse/services/combo.ts");
+const core = await import("../../src/lib/db/core.ts");
+const { resetAllComboMetrics } = await import("../../open-sse/services/comboMetrics.ts");
+const { resetAllCircuitBreakers } = await import("../../src/shared/utils/circuitBreaker.ts");
+const { resetAll: resetAllSemaphores } =
+  await import("../../open-sse/services/rateLimitSemaphore.ts");
+const { _resetAllDecks } = await import("../../src/shared/utils/shuffleDeck.ts");
+const { clearSessions } = await import("../../open-sse/services/sessionManager.ts");
+
+function createLog() {
+  const entries: unknown[] = [];
+  const push = (level: string) => (tag: unknown, msg: unknown) => entries.push({ level, tag, msg });
+  return {
+    info: push("info"),
+    warn: push("warn"),
+    error: push("error"),
+    debug: push("debug"),
+    entries,
+  };
+}
+
+const okResponse = () =>
+  new Response(JSON.stringify({ choices: [{ message: { content: "ok" } }] }), {
+    status: 200,
+    headers: { "content-type": "application/json" },
+  });
+const errResponse = (status: number) =>
+  new Response(JSON.stringify({ error: { message: `Error ${status}` } }), {
+    status,
+    headers: { "content-type": "application/json" },
+  });
+
+/** A body with nested structure, so a shallow copy is visibly different from a deep clone. */
+function agentBody() {
+  return {
+    model: "openai/gpt-4o-mini",
+    max_tokens: 100,
+    messages: [
+      { role: "user", content: "first" },
+      { role: "assistant", content: "second" },
+    ],
+    tools: [{ type: "function", function: { name: "t", parameters: { type: "object" } } }],
+  };
+}
+
+function deepFreeze<T>(value: T): T {
+  if (value && typeof value === "object") {
+    Object.getOwnPropertyNames(value).forEach((k) => deepFreeze((value as never)[k]));
+    Object.freeze(value);
+  }
+  return value;
+}
+
+const MODELS = ["openai/gpt-4o-mini", "claude/sonnet", "gemini/flash"];
+
+function comboOf(strategy: string, name: string) {
+  return {
+    name,
+    strategy,
+    models: MODELS,
+    config: { maxRetries: 0, retryDelayMs: 0, fallbackDelayMs: 0 },
+  };
+}
+
+test.beforeEach(() => {
+  resetAllComboMetrics();
+  resetAllCircuitBreakers();
+  resetAllSemaphores();
+  _resetAllDecks();
+  clearSessions();
+});
+
+test.after(() => {
+  core.resetDbInstance();
+  fs.rmSync(TEST_DATA_DIR, { recursive: true, force: true });
+  if (ORIGINAL_DATA_DIR === undefined) delete process.env.DATA_DIR;
+  else process.env.DATA_DIR = ORIGINAL_DATA_DIR;
+});
+
+// ── The invariant: one target's writes must never reach the next ──────────────
+// The stub reproduces chatCore.ts's `body.model = model` (Background Task Redirection),
+// which is the real downstream mutation this isolation exists to contain.
+for (const strategy of ["priority", "fill-first", "round-robin"]) {
+  test(`${strategy}: a target's in-place write must not leak into the next target`, async () => {
+    const seen: { model: string; maxTokens: number }[] = [];
+
+    await handleComboChat({
+      body: agentBody(),
+      combo: comboOf(strategy, `cow-isolation-${strategy}`),
+      handleSingleModel: async (received: Record<string, unknown>, modelStr: string) => {
+        seen.push({
+          model: received.model as string,
+          maxTokens: received.max_tokens as number,
+        });
+        // Simulate chatCore.ts:693 (`body.model = model`) and the reasoning buffer write.
+        received.model = `mutated-by-${modelStr}`;
+        received.max_tokens = 999;
+        return seen.length < MODELS.length ? errResponse(503) : okResponse();
+      },
+      isModelAvailable: async () => true,
+      log: createLog(),
+      settings: null,
+      allCombos: null,
+    });
+
+    assert.ok(seen.length >= 2, `expected at least 2 targets to run, got ${seen.length}`);
+    for (const [i, s] of seen.entries()) {
+      assert.equal(
+        s.model,
+        "openai/gpt-4o-mini",
+        `target ${i} received model "${s.model}" — a previous target's write leaked through`
+      );
+      assert.equal(
+        s.maxTokens,
+        100,
+        `target ${i} received max_tokens ${s.maxTokens} — a previous target's write leaked through`
+      );
+    }
+  });
+}
+
+// ── The caller's body is an input, not scratch space ──────────────────────────
+test("the caller's body object is never mutated by the combo loop", async () => {
+  const body = agentBody();
+  const before = JSON.stringify(body);
+
+  await handleComboChat({
+    body,
+    combo: comboOf("priority", "cow-caller-body"),
+    handleSingleModel: async (received: Record<string, unknown>) => {
+      received.model = "mutated";
+      received.max_tokens = 1;
+      return okResponse();
+    },
+    isModelAvailable: async () => true,
+    log: createLog(),
+    settings: null,
+    allCombos: null,
+  });
+
+  assert.equal(JSON.stringify(body), before, "handleComboChat must treat `body` as read-only");
+});
+
+// ── The copy must stay shallow — that is the whole point ─────────────────────
+test("the per-target copy shares the nested payload instead of deep-cloning it", async () => {
+  const body = agentBody();
+  const received: Record<string, unknown>[] = [];
+
+  await handleComboChat({
+    body,
+    combo: comboOf("priority", "cow-shallow"),
+    handleSingleModel: async (b: Record<string, unknown>) => {
+      received.push(b);
+      return received.length < 2 ? errResponse(503) : okResponse();
+    },
+    isModelAvailable: async () => true,
+    log: createLog(),
+    settings: null,
+    allCombos: null,
+  });
+
+  assert.ok(received.length >= 2, "need at least two targets to compare");
+  for (const [i, b] of received.entries()) {
+    assert.notEqual(
+      b,
+      body,
+      `target ${i} received the caller's object — isolation requires a copy`
+    );
+  }
+  assert.notEqual(received[0], received[1], "each target needs its own top-level object");
+
+  // The memory property (#7847): `messages` is the expensive part of an agent request, and
+  // duplicating it per target is what cost 9.53 MiB at 3 targets. Every target must therefore
+  // point at the SAME array. (handleComboChat rebuilds the messages container once during
+  // setup, before the per-target loop, so this is not necessarily the caller's own array —
+  // what matters is that it is not rebuilt per target.)
+  assert.equal(
+    received[0].messages,
+    received[1].messages,
+    "targets got different messages arrays — the per-target copy must stay shallow (#7847)"
+  );
+  assert.equal(received[0].tools, body.tools, "tools must be shared, not copied");
+
+  // And nothing deep-clones: the message objects themselves are still the caller's.
+  const first = (body.messages as unknown[])[0];
+  assert.equal(
+    (received[0].messages as unknown[])[0],
+    first,
+    "message objects were cloned — nothing on this path mutates them, so they must be shared"
+  );
+});
+
+// ── Freeze probe: nothing on the combo-owned path may write in place ──────────
+// Scope note: handleSingleModel is stubbed, so this covers combo.ts's own handling of the body
+// (compression, handoff injection, the reasoning-buffer write) — NOT chatCore or the executors.
+// The isolation tests above are what cover a mutating downstream.
+test("freeze probe: combo's own body handling performs no in-place writes", async () => {
+  const frozen = deepFreeze(agentBody());
+  let threw: unknown = null;
+
+  try {
+    await handleComboChat({
+      body: frozen,
+      combo: comboOf("priority", "cow-freeze-probe"),
+      handleSingleModel: async () => okResponse(),
+      isModelAvailable: async () => true,
+      log: createLog(),
+      settings: null,
+      allCombos: null,
+    });
+  } catch (err) {
+    threw = err;
+  }
+
+  assert.equal(
+    threw,
+    null,
+    `combo wrote to a frozen body: ${threw instanceof Error ? threw.message : String(threw)}`
+  );
+});