diff --git a/changelog.d/fixes/pending-opencode-empty-rejection-rotation.md b/changelog.d/fixes/pending-opencode-empty-rejection-rotation.md
new file mode 100644
index 0000000000..82a88c5905
--- /dev/null
+++ b/changelog.d/fixes/pending-opencode-empty-rejection-rotation.md
@@ -0,0 +1 @@
+- **fix(executors):** OpencodeExecutor rotates (or retries once on a single-account direct path) on upstream 400 empty-body rejections — malformed completion envelopes with no error field were propagated as success and killed client sessions. Bounded +1 attempt per request; body reads are conditioned on status 400 so successful/streaming responses are never buffered. 400s carrying an error field keep propagating immediately.
diff --git a/open-sse/executors/accountRotation.ts b/open-sse/executors/accountRotation.ts
index a64321e83d..67115bdd8d 100644
--- a/open-sse/executors/accountRotation.ts
+++ b/open-sse/executors/accountRotation.ts
@@ -1,7 +1,7 @@
 /**
  * Shared multi-account rotation mechanics for noauth executors that round-robin
  * across several "accounts" (fingerprints), each with an optional dedicated
- * proxy — currently `OpencodeExecutor` and `MimocodeExecutor`.
+ * proxy — currently `OpencodeExecutor`.
  *
  * Extracted after both executors independently implemented the same
  * pickAccount/markCooldown/markSuccess skeleton with the same exponential
@@ -120,3 +120,58 @@ export function maskAccountId(fingerprint: string): string {
 export function isNetworkErrorRotatable(account: RotatableAccount): boolean {
   return account.proxy !== null;
 }
+
+/**
+ * Detect an *empty* upstream rejection: a 400 whose body carries no usable
+ * completion — the kind `OpencodeExecutor` must rotate/retry on instead of
+ * propagating as a fatal success.
+ *
+ * Signature is deliberately strict and scoped to the observed malformed
+ * envelope (`choices[0].message` with no `error`, no real `content`,
+ * `finish_reason: null`):
+ *  - status must be exactly 400 (anything else → false);
+ *  - body must parse and contain a `choices` array with at least one entry
+ *    holding a `message` object;
+ *  - an `error` field (present or empty) → false, so genuine 400s keep
+ *    propagating immediately (#10460 precedent: classify by signature before
+ *    rotating);
+ *  - `tool_calls` / `reasoning_content` → false (real content);
+ *  - `message.content` absent / null / "" → eligible; any other value
+ *    (non-empty text, number, block array…) → false (conservative);
+ *  - a literal `finish_reason` (not null) → false (a completed, if empty, turn).
+ *
+ * Does NOT reuse `detectMalformedNonStream` (diagnostics.ts): that classifier
+ * also flags `{error:{…}}` bodies as `empty_choices`, which would rotate on
+ * real errors — a false-positive class with a history here.
+ */
+export function isEmptyUpstreamRejection(status: number, bodyText: string): boolean {
+  if (status !== 400) return false;
+  let parsed: unknown;
+  try {
+    parsed = JSON.parse(bodyText);
+  } catch {
+    return false;
+  }
+  const choices = (parsed as { choices?: unknown })?.choices;
+  if (!Array.isArray(choices) || choices.length === 0) return false;
+  const first = choices[0] as { message?: unknown; finish_reason?: unknown };
+  if (typeof first !== "object" || first === null) return false;
+  const rawMessage = (first as { message?: unknown }).message;
+  if (typeof rawMessage === "undefined" || rawMessage === null) return false;
+  if (typeof parsed !== "object" || parsed === null) return false;
+  if ("error" in (parsed as Record<string, unknown>)) return false;
+  const msg = rawMessage as Record<string, unknown>;
+  if ("tool_calls" in msg) return false;
+  if ("reasoning_content" in msg) return false;
+  const content = msg.content;
+  if (content !== undefined && content !== null && content !== "") return false;
+  if (first.finish_reason !== null && first.finish_reason !== undefined) return false;
+  return true;
+}
+
+/** Best-effort extraction of the upstream `chatcmpl_*` id from a response body,
+ * for observability logging. Returns `"unknown"` when absent or unparseable. */
+export function extractChatcmplId(bodyText: string): string {
+  const match = /"id"\s*:\s*"(chatcmpl_[^"]+)"/.exec(bodyText);
+  return match ? match[1] : "unknown";
+}
diff --git a/open-sse/executors/opencode.ts b/open-sse/executors/opencode.ts
index c00ae258a3..0829bfa871 100644
--- a/open-sse/executors/opencode.ts
+++ b/open-sse/executors/opencode.ts
@@ -15,6 +15,8 @@ import {
   markSuccess as markAccountSuccess,
   maskAccountId,
   isNetworkErrorRotatable,
+  isEmptyUpstreamRejection,
+  extractChatcmplId,
 } from "./accountRotation.ts";
 import { isNetworkRotationSharedEgressGuardEnabled } from "@/shared/utils/featureFlags";
 
@@ -253,14 +255,41 @@ export class OpencodeExecutor extends BaseExecutor {
 
     try {
       this.syncAccountsFromCredentials(input.credentials);
+      const { log } = input;
 
       const hasProxies = this.accounts.some((a) => a.proxy !== null);
-      // Fast path: no multi-account proxy wiring configured → original behavior.
+      // Fast path: no multi-account proxy wiring configured → original behavior,
+      // plus exactly ONE bounded retry when the upstream answers a 400 empty
+      // rejection (same predicate and logging as the rotation loop). Everything
+      // else passes untouched: this path deliberately preserves BaseExecutor's
+      // intra-URL 429 retries (no skipUpstreamRetry here).
       if (this.accounts.length === 1 && !hasProxies) {
-        return await super.execute(input);
+        const single = (await super.execute(input)) as HttpExecuteResult;
+        if (single.response.status === 400) {
+          let bodyText: string | null = null;
+          try {
+            bodyText = await single.response.clone().text();
+          } catch {
+            log?.debug?.("OPENCODE", "body read failed on direct account");
+          }
+          if (bodyText !== null) {
+            if (isEmptyUpstreamRejection(400, bodyText)) {
+              const chatcmplId = extractChatcmplId(bodyText);
+              log?.warn?.(
+                "OPENCODE",
+                `upstream empty rejection on direct account (${chatcmplId}), retrying once…`
+              );
+              return await super.execute(input);
+            }
+            log?.debug?.(
+              "OPENCODE",
+              "400 without error field, signature not matched on direct account — observing"
+            );
+          }
+        }
+        return single;
       }
 
-      const { log } = input;
       // This loop only ever dispatches through super.execute() (the HTTP request
       // path), which always resolves the object-shaped arm of ExecutorExecuteResult
       // — the bare-Response arm belongs to web/scraping executors only (base.ts:290).
@@ -277,8 +306,13 @@ export class OpencodeExecutor extends BaseExecutor {
       // network call, but proxied accounts (independent egress) are still
       // tried normally.
       let sharedEgressDown = false;
+      // Bounded extra attempts for empty upstream rejections: +1 for a single
+      // account (retry the same one), none for a multi-account fleet (rotation
+      // through the accounts is the retry). Avoids an unbounded loop on a
+      // persistently malformed upstream.
+      const emptyRejectionBudget = this.accounts.length === 1 ? 1 : 0;
 
-      for (let attempt = 0; attempt < this.accounts.length; attempt++) {
+      for (let attempt = 0; attempt < this.accounts.length + emptyRejectionBudget; attempt++) {
         const account = this.pickAccount();
         const masked = maskAccountId(account.fingerprint);
 
@@ -354,6 +388,34 @@ export class OpencodeExecutor extends BaseExecutor {
           continue;
         }
 
+        // Empty upstream rejection (malformed 400: no error field, no real
+        // content, finish_reason null — see isEmptyUpstreamRejection). Rotate/
+        // retry instead of propagating it as a fatal success: the observed
+        // envelope was marking subagent sessions as failed. Read the body ONLY
+        // for a 400 (never a 200/streaming — that would buffer the good path);
+        // classify, log, and continue. Neitheries markCooldown nor markSuccess:
+        // the failure is upstream's, not this account's.
+        if (status === 400) {
+          let bodyText: string | null = null;
+          try {
+            bodyText = await result.response.clone().text();
+          } catch {
+            log?.debug?.("OPENCODE", "body read failed on empty rejection check");
+          }
+          if (bodyText !== null && isEmptyUpstreamRejection(400, bodyText)) {
+            const chatcmplId = extractChatcmplId(bodyText);
+            log?.warn?.(
+              "OPENCODE",
+              `upstream empty rejection on account ${masked} (${chatcmplId}), rotating to next…`
+            );
+            continue;
+          }
+          // A 400 carrying a real error (or non-empty content): propagate
+          // immediately, untouched — same as before this change.
+          this.markSuccess(account);
+          return result;
+        }
+
         this.markSuccess(account);
         return result;
       }
diff --git a/tests/unit/account-rotation.test.ts b/tests/unit/account-rotation.test.ts
index f4864ee2dd..a682f02a0d 100644
--- a/tests/unit/account-rotation.test.ts
+++ b/tests/unit/account-rotation.test.ts
@@ -7,6 +7,8 @@ import {
   markSuccess,
   maskAccountId,
   isNetworkErrorRotatable,
+  isEmptyUpstreamRejection,
+  extractChatcmplId,
   type RotatableAccount,
 } from "../../open-sse/executors/accountRotation.ts";
 
@@ -114,3 +116,100 @@ describe("accountRotation", () => {
     assert.strictEqual(isNetworkErrorRotatable(withoutProxy), false);
   });
 });
+
+describe("isEmptyUpstreamRejection", () => {
+  it("matches the observed malformed completion envelope (no error field, empty content, null finish_reason)", () => {
+    const observed =
+      '{"id":"chatcmpl_44fn2g6e7kk","object":"chat.completion","created":1787419957,"model":"muse-spark-1.2-contributor-free","choices":[{"index":0,"message":{"role":"assistant"},"finish_reason":null}]}';
+    assert.strictEqual(isEmptyUpstreamRejection(400, observed), true);
+  });
+
+  it("does not match a non-400 status", () => {
+    const observed =
+      '{"id":"chatcmpl_44fn2g6e7kk","object":"chat.completion","created":1787419957,"model":"muse-spark-1.2-contributor-free","choices":[{"index":0,"message":{"role":"assistant"},"finish_reason":null}]}';
+    assert.strictEqual(isEmptyUpstreamRejection(200, observed), false);
+    assert.strictEqual(isEmptyUpstreamRejection(429, observed), false);
+    assert.strictEqual(isEmptyUpstreamRejection(502, observed), false);
+  });
+
+  it("does not match when an error field is present", () => {
+    const withError = JSON.stringify({
+      error: { message: "bad request", type: "invalid_request_error" },
+    });
+    assert.strictEqual(isEmptyUpstreamRejection(400, withError), false);
+    const emptyError = JSON.stringify({ error: {} });
+    assert.strictEqual(isEmptyUpstreamRejection(400, emptyError), false);
+  });
+
+  it("does not match when content is non-empty or tool_calls present", () => {
+    const nonEmpty = JSON.stringify({
+      choices: [{ message: { role: "assistant", content: "hi" }, finish_reason: "stop" }],
+    });
+    assert.strictEqual(isEmptyUpstreamRejection(400, nonEmpty), false);
+
+    const toolCalls = JSON.stringify({
+      choices: [
+        { message: { role: "assistant", tool_calls: [{ id: "x" }] }, finish_reason: "tool_calls" },
+      ],
+    });
+    assert.strictEqual(isEmptyUpstreamRejection(400, toolCalls), false);
+  });
+
+  it("does not match when content is a non-string non-null value (number, block array)", () => {
+    const numericContent = JSON.stringify({
+      choices: [{ message: { role: "assistant", content: 123 }, finish_reason: null }],
+    });
+    assert.strictEqual(
+      isEmptyUpstreamRejection(400, numericContent),
+      false,
+      "non-string non-null content is not eligible"
+    );
+
+    const reasoningContent = JSON.stringify({
+      choices: [
+        { message: { role: "assistant", reasoning_content: "thinking" }, finish_reason: null },
+      ],
+    });
+    assert.strictEqual(isEmptyUpstreamRejection(400, reasoningContent), false);
+  });
+
+  it("does not match when choices or message are absent", () => {
+    const noChoices = JSON.stringify({ id: "chatcmpl_x", model: "muse" });
+    assert.strictEqual(isEmptyUpstreamRejection(400, noChoices), false);
+    const noMessage = JSON.stringify({ choices: [{ finish_reason: null }] });
+    assert.strictEqual(isEmptyUpstreamRejection(400, noMessage), false);
+  });
+
+  it("does not match when finish_reason is a literal value (not null)", () => {
+    const stopReason = JSON.stringify({
+      choices: [{ message: { role: "assistant" }, finish_reason: "stop" }],
+    });
+    assert.strictEqual(isEmptyUpstreamRejection(400, stopReason), false);
+  });
+
+  it("matches an empty string content (treated as eligible)", () => {
+    const emptyContent = JSON.stringify({
+      choices: [{ message: { role: "assistant", content: "" }, finish_reason: null }],
+    });
+    assert.strictEqual(isEmptyUpstreamRejection(400, emptyContent), true);
+  });
+
+  it("returns false for unparseable JSON rather than throwing", () => {
+    assert.strictEqual(isEmptyUpstreamRejection(400, "not json"), false);
+    assert.strictEqual(isEmptyUpstreamRejection(400, ""), false);
+  });
+});
+
+describe("extractChatcmplId", () => {
+  it("extracts the chatcmpl id from an observed envelope", () => {
+    const observed =
+      '{"id":"chatcmpl_44fn2g6e7kk","object":"chat.completion","created":1787419957,"model":"muse-spark-1.2-contributor-free","choices":[{"index":0,"message":{"role":"assistant"},"finish_reason":null}]}';
+    assert.strictEqual(extractChatcmplId(observed), "chatcmpl_44fn2g6e7kk");
+  });
+
+  it("falls back to 'unknown' when no id is present", () => {
+    assert.strictEqual(extractChatcmplId("{choices:[]}"), "unknown");
+    assert.strictEqual(extractChatcmplId(""), "unknown");
+    assert.strictEqual(extractChatcmplId("not json"), "unknown");
+  });
+});
diff --git a/tests/unit/opencode-empty-rejection-rotation.test.ts b/tests/unit/opencode-empty-rejection-rotation.test.ts
new file mode 100644
index 0000000000..784a4b83a7
--- /dev/null
+++ b/tests/unit/opencode-empty-rejection-rotation.test.ts
@@ -0,0 +1,416 @@
+import { describe, it, beforeEach, afterEach, before, after } from "node:test";
+import assert from "node:assert";
+import net from "node:net";
+import { OpencodeExecutor } from "../../open-sse/executors/opencode.ts";
+import type { ExecutorLog, ProviderCredentials } from "../../open-sse/executors/base.ts";
+import { resolveProxyForRequest } from "../../open-sse/utils/proxyFetch.ts";
+import {
+  isEmptyUpstreamRejection,
+  extractChatcmplId,
+} from "../../open-sse/executors/accountRotation.ts";
+
+/**
+ * Empty-upstream-rejection rotation (#design opencode-empty-rejection-rotation).
+ *
+ * An upstream 400 whose body carries no usable completion (the observed malformed
+ * envelope: `choices[0].message` with no error field, no real content,
+ * `finish_reason: null`) must be rotated/retried instead of propagated as a fatal
+ * success — that was killing subagent sessions. These tests pin the wiring:
+ *
+ *   1. A 400 empty rejection rotates to the next account (and its proxy).
+ *   2. The retry budget is bounded: +1 attempt for a single account, exactly N
+ *      for an N-account all-empty run (propagate the last 400, never loop forever).
+ *   3. A 400 carrying a real error field (or non-empty content) still propagates
+ *      immediately — no cooldown, no success, no rotation.
+ *   4. The 200/success path is never cloned or read (anti-bufferisation).
+ *
+ * The dispatch layer is mocked by stubbing globalThis.fetch (exactly what the
+ * #4954 proxy integration test does). Three throwaway TCP listeners stand in for
+ * the per-account proxies so runWithProxyContext's reachability probe passes.
+ */
+
+const log: ExecutorLog = { debug() {}, info() {}, warn() {}, error() {} };
+
+const ACCOUNT_A = "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa";
+const ACCOUNT_B = "bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb";
+const ACCOUNT_C = "cccccccccccccccccccccccccccccccc";
+
+const EMPTY_BODY =
+  '{"id":"chatcmpl_44fn2g6e7kk","object":"chat.completion","created":1787419957,"model":"muse-spark-1.2-contributor-free","choices":[{"index":0,"message":{"role":"assistant"},"finish_reason":null}]}';
+const ERROR_BODY = JSON.stringify({
+  error: { message: "bad request", type: "invalid_request_error" },
+});
+
+let serverA: net.Server;
+let serverB: net.Server;
+let serverC: net.Server;
+let portA = 0;
+let portB = 0;
+let portC = 0;
+
+function listen(server: net.Server): Promise<number> {
+  return new Promise((resolve) => {
+    server.listen(0, "127.0.0.1", () => {
+      resolve((server.address() as net.AddressInfo).port);
+    });
+  });
+}
+
+before(async () => {
+  serverA = net.createServer((s) => s.destroy());
+  serverB = net.createServer((s) => s.destroy());
+  serverC = net.createServer((s) => s.destroy());
+  portA = await listen(serverA);
+  portB = await listen(serverB);
+  portC = await listen(serverC);
+});
+
+after(() => {
+  serverA?.close();
+  serverB?.close();
+  serverC?.close();
+});
+
+function portFor(fp: string): number {
+  if (fp === ACCOUNT_A) return portA;
+  if (fp === ACCOUNT_B) return portB;
+  return portC;
+}
+
+/** `fingerprints` accounts; `proxied` is the subset that get a dedicated proxy
+ * (defaults to all). A proxy-less account shares the default egress. */
+function credentialsFor(
+  fingerprints: string[],
+  proxied: string[] = [...fingerprints]
+): ProviderCredentials {
+  return {
+    apiKey: null,
+    accessToken: null,
+    connectionId: "noauth",
+    providerSpecificData: {
+      fingerprints,
+      ...(proxied.length > 0 && {
+        accountProxies: proxied.map((fp) => ({
+          fingerprint: fp,
+          proxy: { type: "http", host: "127.0.0.1", port: portFor(fp) },
+        })),
+      }),
+    },
+  };
+}
+
+/** A Response subclass that counts clone() so we can assert the executor never
+ * buffers a 200/streaming response. Note: `clone()` returns a plain Response, so
+ * only `clone()` is reliably counted (a read on the clone hits the native
+ * method, not this override) — counting clones is the meaningful invariant. */
+class SpyResponse extends Response {
+  static clones = 0;
+  clone(): Response {
+    SpyResponse.clones++;
+    return super.clone();
+  }
+}
+
+interface PlanStep {
+  status: number;
+  body?: string;
+  throw?: Error;
+}
+
+describe("OpencodeExecutor empty-rejection rotation", () => {
+  let originalFetch: typeof globalThis.fetch;
+  let observed: Array<{ source: string; host: string | null; port: string | null }>;
+  const GUARD_FLAG = "NETWORK_ROTATION_SHARED_EGRESS_GUARD";
+  let savedGuardFlag: string | undefined;
+
+  beforeEach(() => {
+    originalFetch = globalThis.fetch;
+    observed = [];
+    SpyResponse.clones = 0;
+    savedGuardFlag = process.env[GUARD_FLAG];
+    delete process.env[GUARD_FLAG];
+  });
+
+  afterEach(() => {
+    globalThis.fetch = originalFetch;
+    if (savedGuardFlag === undefined) delete process.env[GUARD_FLAG];
+    else process.env[GUARD_FLAG] = savedGuardFlag;
+  });
+
+  function installFetch(plan: PlanStep[]) {
+    let call = 0;
+    globalThis.fetch = (async (input: RequestInfo | URL) => {
+      const url =
+        typeof input === "string" ? input : input instanceof URL ? input.toString() : input.url;
+      const resolved = resolveProxyForRequest(url);
+      observed.push({
+        source: resolved.source,
+        host: resolved.proxyUrl ? new URL(resolved.proxyUrl).hostname : null,
+        port: resolved.proxyUrl ? new URL(resolved.proxyUrl).port : null,
+      });
+      const step = plan[Math.min(call, plan.length - 1)];
+      call++;
+      if (step.throw) throw step.throw;
+      return new SpyResponse(step.body ?? JSON.stringify({ ok: step.status === 200 }), {
+        status: step.status,
+        headers: { "Content-Type": "application/json" },
+      });
+    }) as typeof globalThis.fetch;
+  }
+
+  /**
+   * Launches the executor. Asserts the predicate itself behaves (regression guard
+   * for the design's signature — the wiring tests below depend on it).
+   */
+  it("predicate matches the observed envelope and rejects real errors", () => {
+    assert.strictEqual(isEmptyUpstreamRejection(400, EMPTY_BODY), true);
+    assert.strictEqual(isEmptyUpstreamRejection(200, EMPTY_BODY), false);
+    assert.strictEqual(isEmptyUpstreamRejection(400, ERROR_BODY), false);
+    assert.strictEqual(extractChatcmplId(EMPTY_BODY), "chatcmpl_44fn2g6e7kk");
+  });
+
+  it("rotates to the next account on an empty 400 rejection (loop)", async () => {
+    const exec = new OpencodeExecutor("opencode-zen");
+    installFetch([{ status: 400, body: EMPTY_BODY }, { status: 200 }]);
+
+    const result = await exec.execute({
+      model: "deepseek-v4-flash-free",
+      body: { messages: [{ role: "user", content: "hi" }], stream: false },
+      stream: false,
+      signal: null,
+      credentials: credentialsFor([ACCOUNT_A, ACCOUNT_B]),
+      log,
+    });
+
+    assert.strictEqual(
+      (result as { response: Response }).response.status,
+      200,
+      "must rotate past the empty 400"
+    );
+    assert.ok(observed.length >= 2, "should have dispatched on a second account");
+    assert.ok(
+      observed.some((o) => o.port === String(portA)),
+      "first attempt on account A"
+    );
+    assert.ok(
+      observed.some((o) => o.port === String(portB)),
+      "rotated attempt on account B"
+    );
+  });
+
+  it("caps an all-empty N-account run at N attempts and propagates the last 400 intact", async () => {
+    const exec = new OpencodeExecutor("opencode-zen");
+    installFetch([
+      { status: 400, body: EMPTY_BODY },
+      { status: 400, body: EMPTY_BODY },
+      { status: 400, body: EMPTY_BODY },
+      { status: 200 },
+    ]);
+
+    const result = await exec.execute({
+      model: "deepseek-v4-flash-free",
+      body: { messages: [{ role: "user", content: "hi" }], stream: false },
+      stream: false,
+      signal: null,
+      credentials: credentialsFor([ACCOUNT_A, ACCOUNT_B, ACCOUNT_C]),
+      log,
+    });
+
+    assert.strictEqual(
+      (result as { response: Response }).response.status,
+      400,
+      "must propagate the last empty 400"
+    );
+    assert.strictEqual(observed.length, 3, "must NOT exceed N attempts (no infinite loop)");
+    assert.ok(SpyResponse.clones >= 1, "the empty 400 path must read the body to classify it");
+    const propagated = await (result as { response: Response }).response.clone().text();
+    assert.strictEqual(propagated, EMPTY_BODY, "propagated 400 body must stay intact");
+    for (const p of observed) {
+      assert.strictEqual(p.source, "context", "every dispatch must egress through a proxy context");
+    }
+  });
+
+  it("retries the same proxied account once when it is the only account", async () => {
+    const exec = new OpencodeExecutor("opencode-zen");
+    installFetch([
+      { status: 400, body: EMPTY_BODY },
+      { status: 400, body: EMPTY_BODY },
+    ]);
+
+    const result = await exec.execute({
+      model: "deepseek-v4-flash-free",
+      body: { messages: [{ role: "user", content: "hi" }], stream: false },
+      stream: false,
+      signal: null,
+      credentials: credentialsFor([ACCOUNT_A]),
+      log,
+    });
+
+    assert.strictEqual((result as { response: Response }).response.status, 400);
+    assert.strictEqual(observed.length, 2, "exactly one bounded retry on the sole account");
+    assert.ok(
+      observed.every((o) => o.port === String(portA)),
+      "both attempts egress through the single account's proxy"
+    );
+  });
+
+  it("coexists with 429 rotation and 200 success in the same request", async () => {
+    const exec = new OpencodeExecutor("opencode-zen");
+    installFetch([{ status: 429 }, { status: 400, body: EMPTY_BODY }, { status: 200 }]);
+
+    const result = await exec.execute({
+      model: "deepseek-v4-flash-free",
+      body: { messages: [{ role: "user", content: "hi" }], stream: false },
+      stream: false,
+      signal: null,
+      credentials: credentialsFor([ACCOUNT_A, ACCOUNT_B, ACCOUNT_C]),
+      log,
+    });
+
+    assert.strictEqual(
+      (result as { response: Response }).response.status,
+      200,
+      "final response should succeed"
+    );
+    assert.strictEqual(observed.length, 3, "429 + empty-400 + success across three accounts");
+    assert.ok(
+      observed.some((o) => o.port === String(portA)),
+      "account A (429)"
+    );
+    assert.ok(
+      observed.some((o) => o.port === String(portB)),
+      "account B (empty 400)"
+    );
+    assert.ok(
+      observed.some((o) => o.port === String(portC)),
+      "account C (200)"
+    );
+  });
+
+  it("propagates a 400 carrying an error field immediately (no rotation)", async () => {
+    const exec = new OpencodeExecutor("opencode-zen");
+    installFetch([{ status: 400, body: ERROR_BODY }]);
+
+    const result = await exec.execute({
+      model: "deepseek-v4-flash-free",
+      body: { messages: [{ role: "user", content: "hi" }], stream: false },
+      stream: false,
+      signal: null,
+      credentials: credentialsFor([ACCOUNT_A, ACCOUNT_B]),
+      log,
+    });
+
+    assert.strictEqual(
+      (result as { response: Response }).response.status,
+      400,
+      "real error 400 must propagate"
+    );
+    assert.strictEqual(observed.length, 1, "must NOT rotate on a genuine error 400");
+  });
+
+  it("never clones or reads the body of a 200 via the loop", async () => {
+    const exec = new OpencodeExecutor("opencode-zen");
+    installFetch([{ status: 200 }, { status: 200 }, { status: 200 }]);
+
+    const result = await exec.execute({
+      model: "deepseek-v4-flash-free",
+      body: { messages: [{ role: "user", content: "hi" }], stream: false },
+      stream: false,
+      signal: null,
+      credentials: credentialsFor([ACCOUNT_A, ACCOUNT_B, ACCOUNT_C]),
+      log,
+    });
+
+    assert.strictEqual((result as { response: Response }).response.status, 200);
+    assert.strictEqual(SpyResponse.clones, 0, "loop 200 must never be cloned");
+  });
+
+  it("retries once via the fast path when a direct account answers an empty 400", async () => {
+    const exec = new OpencodeExecutor("opencode-zen");
+    installFetch([
+      { status: 400, body: EMPTY_BODY },
+      { status: 400, body: EMPTY_BODY },
+    ]);
+
+    const result = await exec.execute({
+      model: "deepseek-v4-flash-free",
+      body: { messages: [{ role: "user", content: "hi" }], stream: false },
+      stream: false,
+      signal: null,
+      credentials: credentialsFor([ACCOUNT_A], []),
+      log,
+    });
+
+    assert.strictEqual((result as { response: Response }).response.status, 400);
+    assert.strictEqual(observed.length, 2, "fast path must retry the direct account exactly once");
+  });
+
+  it("propagates the second 400 intact when the fast path retries and empty-rejects again", async () => {
+    const exec = new OpencodeExecutor("opencode-zen");
+    installFetch([
+      { status: 400, body: EMPTY_BODY },
+      { status: 400, body: EMPTY_BODY },
+    ]);
+
+    const result = await exec.execute({
+      model: "deepseek-v4-flash-free",
+      body: { messages: [{ role: "user", content: "hi" }], stream: false },
+      stream: false,
+      signal: null,
+      credentials: credentialsFor([ACCOUNT_A], []),
+      log,
+    });
+
+    assert.strictEqual((result as { response: Response }).response.status, 400);
+    const propagated = await (result as { response: Response }).response.clone().text();
+    assert.strictEqual(propagated, EMPTY_BODY, "second rejection propagates with intact body");
+    assert.strictEqual(observed.length, 2, "exactly one retry, no loop");
+  });
+
+  it("never clones or reads the body of a 200 via the fast path", async () => {
+    const exec = new OpencodeExecutor("opencode-zen");
+    installFetch([{ status: 200 }]);
+
+    const result = await exec.execute({
+      model: "deepseek-v4-flash-free",
+      body: { messages: [{ role: "user", content: "hi" }], stream: false },
+      stream: false,
+      signal: null,
+      credentials: credentialsFor([ACCOUNT_A], []),
+      log,
+    });
+
+    assert.strictEqual((result as { response: Response }).response.status, 200);
+    assert.strictEqual(SpyResponse.clones, 0, "fast path 200 must never be cloned");
+  });
+
+  it("rotates to a proxied account after a proxy-less account empty-rejects (shared-egress guard on by default)", async () => {
+    const exec = new OpencodeExecutor("opencode-zen");
+    installFetch([{ status: 400, body: EMPTY_BODY }, { status: 200 }]);
+
+    const result = await exec.execute({
+      model: "deepseek-v4-flash-free",
+      body: { messages: [{ role: "user", content: "hi" }], stream: false },
+      stream: false,
+      signal: null,
+      // A proxy-less, B proxied: B must still be tried and succeed.
+      credentials: credentialsFor([ACCOUNT_A, ACCOUNT_B], [ACCOUNT_B]),
+      log,
+    });
+
+    assert.strictEqual(
+      (result as { response: { status: number } }).response.status,
+      200,
+      "the proxied account (B) must still be tried and must succeed"
+    );
+    assert.strictEqual(observed.length, 2, "exactly one empty rejection (A) then one success (B)");
+    assert.ok(
+      observed.some((o) => o.source === "direct"),
+      "first dispatch on the proxy-less account"
+    );
+    assert.ok(
+      observed.some((o) => o.port === String(portB)),
+      "rotated dispatch on the proxied account"
+    );
+  });
+});