diff --git a/open-sse/config/codexClient.ts b/open-sse/config/codexClient.ts
index 0cb1b05772..5c71e931a5 100644
--- a/open-sse/config/codexClient.ts
+++ b/open-sse/config/codexClient.ts
@@ -1,9 +1,13 @@
 import {
+  CODEX_CLI_RS_ORIGINATOR,
   DEFAULT_CODEX_CLIENT_VERSION,
   getCodexCliRsHeaders as buildCodexCliRsHeaders,
 } from "@/shared/constants/codexClient";
 
-export { DEFAULT_CODEX_CLIENT_VERSION } from "@/shared/constants/codexClient";
+export {
+  DEFAULT_CODEX_CLIENT_VERSION,
+  CODEX_CLI_RS_ORIGINATOR,
+} from "@/shared/constants/codexClient";
 const DEFAULT_CODEX_USER_AGENT_PLATFORM = "Windows 10.0.26200";
 const DEFAULT_CODEX_USER_AGENT_ARCH = "x64";
 const CODEX_VERSION_OVERRIDE_ENV = "CODEX_CLIENT_VERSION";
@@ -51,6 +55,35 @@ export function getCodexCliRsHeaders(): Record<string, string> {
   return buildCodexCliRsHeaders(getCodexClientVersion());
 }
 
+/**
+ * Identity for the credential face (auth.openai.com: token exchange / refresh).
+ * The real Codex client sends only `originator` + `User-Agent` on that face
+ * (codex-rs login/default_client.rs default_headers()); the `Version` header
+ * gate exists only on the chatgpt.com/backend-api inference face, so it is
+ * deliberately omitted here. Mirrors sub2api v0.1.178
+ * ApplyCodexCanonicalAuthIdentity.
+ */
+export function getCodexAuthIdentityHeaders(): Record<string, string> {
+  return {
+    "User-Agent": getCodexUserAgent(),
+    originator: CODEX_CLI_RS_ORIGINATOR,
+  };
+}
+
+/**
+ * Canonical Codex CLI identity for server-initiated calls against the
+ * chatgpt.com/backend-api face that are not tied to one end-client request
+ * (usage / quota / models manifest / reset-credits). Same UA/version chain as
+ * inference so these calls do not show up upstream as anonymous half-identities.
+ */
+export function getCodexBackendIdentityHeaders(): Record<string, string> {
+  return {
+    "User-Agent": getCodexUserAgent(),
+    originator: CODEX_CLI_RS_ORIGINATOR,
+    Version: getCodexClientVersion(),
+  };
+}
+
 export function normalizeCodexSessionId(value: unknown): string | null {
   if (typeof value !== "string") return null;
   const normalized = value.trim();
diff --git a/open-sse/config/codexIdentity.ts b/open-sse/config/codexIdentity.ts
index bc45fa1cf6..a081c5402b 100644
--- a/open-sse/config/codexIdentity.ts
+++ b/open-sse/config/codexIdentity.ts
@@ -1,15 +1,30 @@
 import { createHash, randomUUID } from "node:crypto";
 
 import { normalizeCodexSessionId } from "./codexClient.ts";
+import { isCrossAccountCodexTurnState, readCodexTurnStateHeader } from "./codexTurnState.ts";
 
 const CODEX_INSTALLATION_SALT = "omniroute-codex-installation";
 const CODEX_SESSION_SEED_PREFIX = "omniroute:codex-session-id:v1:";
 const CODEX_THREAD_SEED_PREFIX = "omniroute:codex-thread-id:v1:";
+// v2 derivations are keyed by the persisted per-connection random seed
+// (codexFingerprintSeed) instead of the connection-id chain, mirroring
+// sub2api v0.1.178 (#5696): deterministic derivation stays stable, but the
+// seed is generated per connection so identities never collide across
+// deployments and survive connection export/import.
+const CODEX_INSTALLATION_SEED_PREFIX_V2 = "omniroute:codex-installation:v2:";
+const CODEX_SESSION_SEED_PREFIX_V2 = "omniroute:codex-session-id:v2:";
+const CODEX_THREAD_SEED_PREFIX_V2 = "omniroute:codex-thread-id:v2:";
 const UUID_PATTERN = /^[0-9a-f]{8}-[0-9a-f]{4}-[1-5][0-9a-f]{3}-[89ab][0-9a-f]{3}-[0-9a-f]{12}$/i;
 
 export const CODEX_FINGERPRINT_MODES = ["off", "device", "session", "full"] as const;
 export type CodexFingerprintMode = (typeof CODEX_FINGERPRINT_MODES)[number];
 export const CODEX_FINGERPRINT_MODE_KEY = "codexFingerprintMode";
+/**
+ * System-managed per-connection random seed used as the fingerprint
+ * derivation source. Never sent upstream, stripped from API responses, and
+ * preserved across connection updates (sub2api `codex_fingerprint_seed`).
+ */
+export const CODEX_FINGERPRINT_SEED_KEY = "codexFingerprintSeed";
 
 export type CodexClientIdentity = {
   mode: CodexFingerprintMode;
@@ -72,6 +87,61 @@ function accountSeed(
   );
 }
 
+/** The persisted system-managed random seed, when present and a valid UUID. */
+export function getCodexFingerprintSeed(
+  providerSpecificData?: Record<string, unknown> | null
+): string | null {
+  return normalizeUuid(providerSpecificData?.[CODEX_FINGERPRINT_SEED_KEY]);
+}
+
+/** Modes that rewrite account-scoped identifiers and therefore need a stable seed. */
+export function codexFingerprintModeRequiresSeed(mode: CodexFingerprintMode): boolean {
+  return mode === "device" || mode === "session" || mode === "full";
+}
+
+/**
+ * Ensure a Codex OAuth connection carries a persisted fingerprint seed when its
+ * convergence mode derives account-scoped identifiers. Called at connection
+ * create/update time (the persistence layer owns the write); the request path
+ * only ever READS the seed, so an identity never rotates mid-flight.
+ *
+ * Semantics mirror sub2api v0.1.178 `prepareCodexFingerprintExtraFor{Create,Update}`:
+ * - the key is system-managed: any client-supplied value is stripped first;
+ * - an existing valid seed is ALWAYS carried forward (even when the new mode
+ *   is `off` — it stays dormant, ready if convergence is re-enabled later);
+ * - otherwise a fresh seed is created only when the mode requires one
+ *   (device/session/full; the OmniRoute default is session).
+ *
+ * Returns the (possibly new) providerSpecificData, or undefined when there is
+ * nothing to store. Pre-seed connections keep their legacy connection-id
+ * derived identity until the next save — one deliberate rotation, same as
+ * sub2api's migration-225 backfill.
+ */
+export function ensureCodexFingerprintSeed(
+  providerSpecificData?: Record<string, unknown> | null,
+  credentials?: { accessToken?: unknown; refreshToken?: unknown } | null,
+  existingProviderSpecificData?: Record<string, unknown> | null
+): Record<string, unknown> | undefined {
+  const psd: Record<string, unknown> = { ...(providerSpecificData || {}) };
+  // System-managed key: never trust an inbound value, regardless of auth type.
+  delete psd[CODEX_FINGERPRINT_SEED_KEY];
+  if (!isCodexOAuthCredentials(credentials)) {
+    return Object.keys(psd).length > 0 ? psd : undefined;
+  }
+
+  const existingSeed = getCodexFingerprintSeed(existingProviderSpecificData);
+  if (existingSeed) {
+    psd[CODEX_FINGERPRINT_SEED_KEY] = existingSeed;
+    return psd;
+  }
+  const mode = getCodexFingerprintMode(psd, true);
+  if (codexFingerprintModeRequiresSeed(mode)) {
+    psd[CODEX_FINGERPRINT_SEED_KEY] = randomUUID();
+    return psd;
+  }
+  return Object.keys(psd).length > 0 ? psd : undefined;
+}
+
 function readNamedHeader(
   headers: Headers | Record<string, unknown> | null | undefined,
   name: string
@@ -120,6 +190,11 @@ export function getCodexInstallationId(
   const explicit = normalizeUuid(providerSpecificData?.codexInstallationId);
   if (explicit) return explicit;
 
+  const persistedSeed = getCodexFingerprintSeed(providerSpecificData);
+  if (persistedSeed) {
+    return deriveStableUUIDv4(`${CODEX_INSTALLATION_SEED_PREFIX_V2}${persistedSeed}`);
+  }
+
   const legacyStableSource =
     nonEmptyString(providerSpecificData?.workspaceId) ||
     nonEmptyString(providerSpecificData?.accountId) ||
@@ -137,6 +212,10 @@ export function getCodexConvergedSessionId(
   providerSpecificData?: Record<string, unknown> | null,
   accountKey?: string | null
 ): string {
+  const persistedSeed = getCodexFingerprintSeed(providerSpecificData);
+  if (persistedSeed) {
+    return deriveStableUUIDv4(`${CODEX_SESSION_SEED_PREFIX_V2}${persistedSeed}`);
+  }
   return deriveStableUUIDv4(
     `${CODEX_SESSION_SEED_PREFIX}${accountSeed(providerSpecificData, accountKey)}`
   );
@@ -148,6 +227,10 @@ export function getCodexConvergedThreadId(
   accountKey?: string | null
 ): string {
   if (!nonEmptyString(clientSessionId)) return "";
+  const persistedSeed = getCodexFingerprintSeed(providerSpecificData);
+  if (persistedSeed) {
+    return deriveStableUUIDv4(`${CODEX_THREAD_SEED_PREFIX_V2}${persistedSeed}:${clientSessionId}`);
+  }
   return deriveStableUUIDv4(
     `${CODEX_THREAD_SEED_PREFIX}${accountSeed(providerSpecificData, accountKey)}:${clientSessionId}`
   );
@@ -163,6 +246,26 @@ export function getCodexClientSessionId(
   );
 }
 
+/**
+ * Decide what to do with the client's `x-codex-turn-state` echo for the
+ * account about to serve this request. The blob is minted per account by the
+ * upstream; replaying another account's blob after failover is a proxy-only
+ * contradiction, so a known cross-account echo is stripped. Same-account or
+ * unknown provenance passes through unchanged (strip only, never inject).
+ * Independent of the fingerprint-convergence mode — account consistency also
+ * applies to explicit `off` / passthrough.
+ */
+export function resolveCodexTurnStateEcho(
+  clientHeaders?: Headers | Record<string, unknown> | null,
+  accountKey?: string | null
+): string | null {
+  const value = readCodexTurnStateHeader(clientHeaders);
+  if (!value) return null;
+  const sessionId = getCodexClientSessionId(clientHeaders);
+  if (sessionId && isCrossAccountCodexTurnState(sessionId, accountKey)) return null;
+  return value;
+}
+
 /**
  * One identity object for every carrier in one upstream turn.
  * accountKey may be the OmniRoute connection id; it is never sent upstream.
@@ -284,13 +387,19 @@ export function withCodexFingerprintCredentials<T extends CodexCredentialIdentit
 ): T {
   const identity = resolveCodexFingerprintIdentity({ credentials, clientHeaders, body });
   const original = resolveCodexOriginalIdentityHeaders({ credentials, clientHeaders });
-  if (!identity && !original) return credentials;
+  // The turn-state echo guard runs for every Codex request (including compact
+  // and explicit-off), unlike the convergence identity above.
+  const turnStateEcho = credentials
+    ? resolveCodexTurnStateEcho(clientHeaders, credentials.connectionId ?? null)
+    : null;
+  if (!identity && !original && !turnStateEcho) return credentials;
   return {
     ...credentials,
     providerSpecificData: {
       ...(credentials.providerSpecificData || {}),
       ...(identity ? { codexClientIdentity: identity } : {}),
       ...(original ? { codexOriginalIdentityHeaders: original } : {}),
+      ...(turnStateEcho ? { codexTurnStateEcho: turnStateEcho } : {}),
     },
   };
 }
diff --git a/open-sse/config/codexTurnState.ts b/open-sse/config/codexTurnState.ts
new file mode 100644
index 0000000000..04fc0ff005
--- /dev/null
+++ b/open-sse/config/codexTurnState.ts
@@ -0,0 +1,144 @@
+/**
+ * codexTurnState.ts — `x-codex-turn-state` relay bookkeeping and the
+ * cross-account echo guard.
+ *
+ * The upstream mints the opaque turn-state blob under the outbound identity
+ * (including the fingerprint-converged installation/session/thread ids), and
+ * the real Codex client echoes it back on later requests of the same turn —
+ * codex-rs captures it from the /responses SSE, the /responses/compact JSON,
+ * and the WS handshake (codex-api/src/sse/responses.rs, endpoint/compact.rs).
+ *
+ * Replaying a blob to the SAME account is self-consistent. Replaying it to a
+ * DIFFERENT account (failover rotated the connection while the client still
+ * echoes the old account's blob) is a contradiction only a proxy chain can
+ * produce — a real Codex client never emits it. The provenance table records
+ * which connection minted the blob a downstream session last received, and
+ * the outbound guard strips echoes known to come from another account.
+ *
+ * Mirrors sub2api v0.1.177 `openai_codex_turn_state.go` (commit 8219dcfc8).
+ * OmniRoute keys the table by the client's original session id only — the
+ * executor pipeline does not carry the API key id, and a real Codex session
+ * id is a random UUID, so accidental cross-key collisions are not a
+ * practical concern.
+ */
+
+const CODEX_TURN_STATE_HEADER = "x-codex-turn-state";
+
+/**
+ * How long a provenance record lives. The blob is echoed within one turn,
+ * but clients may hold it across a whole session; 2h covers the standard
+ * 5-hour quota window's early turns without letting the map grow stale
+ * entries for days.
+ */
+const CODEX_TURN_STATE_TTL_MS = 2 * 60 * 60 * 1000;
+
+/** Opportunistic full sweep every N writes (the read side also lazily expires). */
+const CODEX_TURN_STATE_SWEEP_EVERY_WRITES = 256;
+
+type CodexTurnStateOrigin = {
+  accountKey: string;
+  expiresAt: number;
+};
+
+const turnStateOrigins = new Map<string, CodexTurnStateOrigin>();
+let turnStateWrites = 0;
+
+function normalizeAccountKey(accountKey: unknown): string | null {
+  if (typeof accountKey !== "string") return null;
+  const trimmed = accountKey.trim();
+  return trimmed || null;
+}
+
+/**
+ * Read the turn-state blob from a headers bag (Headers instance or a plain
+ * record with arbitrary casing). Returns null when absent/blank.
+ */
+export function readCodexTurnStateHeader(
+  headers: Headers | Record<string, unknown> | null | undefined
+): string | null {
+  if (!headers) return null;
+  if (headers instanceof Headers) {
+    const value = headers.get(CODEX_TURN_STATE_HEADER);
+    return typeof value === "string" && value.trim() ? value.trim() : null;
+  }
+  if (typeof headers === "object") {
+    for (const [key, value] of Object.entries(headers)) {
+      if (
+        key.toLowerCase() === CODEX_TURN_STATE_HEADER &&
+        typeof value === "string" &&
+        value.trim()
+      ) {
+        return value.trim();
+      }
+    }
+  }
+  return null;
+}
+
+function sweepExpiredTurnStateOrigins(now: number): void {
+  for (const [key, origin] of turnStateOrigins) {
+    if (origin.expiresAt <= now) {
+      turnStateOrigins.delete(key);
+    }
+  }
+}
+
+/**
+ * Record that `accountKey` minted the turn-state blob this downstream session
+ * just received. Must only be called at the response commit point — when the
+ * header is actually written to the client. Recording earlier (e.g. for an
+ * attempt later discarded by failover) would poison the table and make the
+ * guard strip the NEXT account's legitimate echo.
+ */
+export function noteCodexTurnStateProvenance(
+  clientSessionId: string | null | undefined,
+  accountKey: unknown,
+  nowMs?: number
+): void {
+  const sessionId = typeof clientSessionId === "string" ? clientSessionId.trim() : "";
+  const account = normalizeAccountKey(accountKey);
+  if (!sessionId || !account) return;
+
+  const now = typeof nowMs === "number" ? nowMs : Date.now();
+  turnStateOrigins.set(sessionId, {
+    accountKey: account,
+    expiresAt: now + CODEX_TURN_STATE_TTL_MS,
+  });
+
+  turnStateWrites += 1;
+  if (turnStateWrites % CODEX_TURN_STATE_SWEEP_EVERY_WRITES === 0) {
+    sweepExpiredTurnStateOrigins(now);
+  }
+}
+
+/**
+ * Outbound guard: true when the echoed blob is KNOWN to have been minted by a
+ * different account and must be stripped before going upstream. Same-account
+ * or unknown provenance passes through unchanged — stripping only, never
+ * injection (clients that cannot echo are the Claude bridge's concern, not
+ * this module's).
+ */
+export function isCrossAccountCodexTurnState(
+  clientSessionId: string | null | undefined,
+  accountKey: unknown,
+  nowMs?: number
+): boolean {
+  const sessionId = typeof clientSessionId === "string" ? clientSessionId.trim() : "";
+  const account = normalizeAccountKey(accountKey);
+  if (!sessionId || !account) return false;
+
+  const origin = turnStateOrigins.get(sessionId);
+  if (!origin) return false;
+  const now = typeof nowMs === "number" ? nowMs : Date.now();
+  if (origin.expiresAt <= now) {
+    turnStateOrigins.delete(sessionId);
+    return false;
+  }
+  return origin.accountKey !== account;
+}
+
+/** Test hook: forget all provenance records and reset the sweep counter. */
+export function __resetCodexTurnStateOriginsForTesting(): void {
+  turnStateOrigins.clear();
+  turnStateWrites = 0;
+}
diff --git a/open-sse/executors/codex.ts b/open-sse/executors/codex.ts
index 93e5a2fe16..d73d520650 100644
--- a/open-sse/executors/codex.ts
+++ b/open-sse/executors/codex.ts
@@ -20,6 +20,7 @@ import {
 import { FETCH_BODY_TIMEOUT_MS, HTTP_STATUS, PROVIDERS } from "../config/constants.ts";
 import { readCodexPeekChunk, buildCodexTimeoutSafePassthroughBody } from "./codex/bodyTimeout.ts";
 import {
+  CODEX_CLI_RS_ORIGINATOR,
   getCodexClientVersion,
   getCodexUserAgent,
   normalizeCodexSessionId,
@@ -225,7 +226,6 @@ function convertSystemToDeveloperRole(body: Record<string, unknown>): void {
   }
 }
 
-
 function stripOrphanedCodexFunctionCallOutputs(body: Record<string, unknown>): void {
   if (!Array.isArray(body.input)) return;
   const input = body.input;
@@ -1045,10 +1045,11 @@ export class CodexExecutor extends BaseExecutor {
       CodexClientIdentity | null | undefined;
     const originalIdentityHeaders = credentials?.providerSpecificData
       ?.codexOriginalIdentityHeaders as Record<string, string> | null | undefined;
+    const turnStateEcho = credentials?.providerSpecificData?.codexTurnStateEcho;
 
     // Originator header — identifies the client type to the Codex backend.
     // Ref: openai/codex login/src/auth/default_client.rs DEFAULT_ORIGINATOR = "codex_cli_rs"
-    headers["originator"] = "codex_cli_rs";
+    headers["originator"] = CODEX_CLI_RS_ORIGINATOR;
 
     // session_id header — enables prompt cache affinity on the Codex backend.
     // The official Codex client sets this to conversation_id (a stable UUID per session).
@@ -1060,6 +1061,13 @@ export class CodexExecutor extends BaseExecutor {
     applyCodexOriginalIdentityHeaders(headers, originalIdentityHeaders);
     applyCodexClientIdentityHeaders(headers, clientIdentity);
 
+    // x-codex-turn-state: forward the client's echo when the provenance guard
+    // (in withCodexFingerprintCredentials) cleared it as same-account. The
+    // blob is account-bound; a stripped (absent) value must stay absent.
+    if (typeof turnStateEcho === "string" && turnStateEcho) {
+      headers["x-codex-turn-state"] = turnStateEcho;
+    }
+
     return headers;
   }
 
diff --git a/open-sse/handlers/chatCore.ts b/open-sse/handlers/chatCore.ts
index d54689472c..e8d0c7835b 100644
--- a/open-sse/handlers/chatCore.ts
+++ b/open-sse/handlers/chatCore.ts
@@ -41,7 +41,11 @@ import {
   isStripReasoningRequested,
 } from "./chatCore/headers.ts";
 import { markCodexScopeRateLimited } from "./chatCore/codexFailover.ts";
-import { isCodexOriginatedHeaders } from "../config/codexIdentity.ts";
+import { getCodexClientSessionId, isCodexOriginatedHeaders } from "../config/codexIdentity.ts";
+import {
+  noteCodexTurnStateProvenance,
+  readCodexTurnStateHeader,
+} from "../config/codexTurnState.ts";
 import { trackDevice, extractIpFromHeaders } from "../services/deviceTracker.ts";
 import { getCombosCached } from "./chatCore/comboContextCache.ts";
 export { clearCombosCache, clearUpstreamProxyConfigCache } from "./chatCore/comboContextCache.ts";
@@ -722,12 +726,13 @@ export async function handleChatCore({
     copilotCompatibleReasoning,
     clientResponseFormat,
   } = resolveChatCoreRequestFormat({ clientRawRequest, body, provider, userAgent });
-  const nativeOpenAICompatibleResponsesPassthrough = shouldUseNativeOpenAICompatibleResponsesPassthrough({
-    provider,
-    sourceFormat,
-    endpointPath,
-    providerSpecificData: credentials?.providerSpecificData,
-  });
+  const nativeOpenAICompatibleResponsesPassthrough =
+    shouldUseNativeOpenAICompatibleResponsesPassthrough({
+      provider,
+      sourceFormat,
+      endpointPath,
+      providerSpecificData: credentials?.providerSpecificData,
+    });
   const responsesInputItems = Array.isArray(body?.input) ? body.input : [];
   const customToolNames = collectCustomToolNamesForSourceFormat(
     sourceFormat,
@@ -3387,6 +3392,15 @@ export async function handleChatCore({
         const responseHeaders = new Headers(headersObj);
         stripStaleForwardingHeaders(responseHeaders);
         stripNextMiddlewareControlHeaders(responseHeaders);
+        // The upstream headers (turn-state included) are about to be committed
+        // to the client — record which connection minted the blob so a later
+        // cross-account echo can be stripped (Codex failover guard).
+        if (provider === "codex" && readCodexTurnStateHeader(responseHeaders)) {
+          noteCodexTurnStateProvenance(
+            getCodexClientSessionId(clientRawRequest?.headers),
+            rawResult._executionCredentials?.connectionId ?? credentials?.connectionId
+          );
+        }
         const contentType = (responseHeaders.get("content-type") || "").toLowerCase();
         const payload = await readNonStreamingResponseBody(
           rawResult.response,
@@ -5106,6 +5120,17 @@ export async function handleChatCore({
     comboStrategy,
   });
 
+  // The streaming headers (turn-state included, when present) are committed to
+  // the client from here on — record which connection minted the blob so a
+  // later cross-account echo can be stripped (Codex failover guard). The
+  // in-place failover update means `credentials` is the winning account.
+  if (provider === "codex" && readCodexTurnStateHeader(providerResponse.headers)) {
+    noteCodexTurnStateProvenance(
+      getCodexClientSessionId(clientRawRequest?.headers),
+      credentials?.connectionId
+    );
+  }
+
   // Create transform stream with logger for streaming response
   let transformStream;
   const responseToolNameMap = mergeResponseToolNameMap(
diff --git a/open-sse/handlers/chatCore/responseHeaders.ts b/open-sse/handlers/chatCore/responseHeaders.ts
index 59c45ba829..8206304544 100644
--- a/open-sse/handlers/chatCore/responseHeaders.ts
+++ b/open-sse/handlers/chatCore/responseHeaders.ts
@@ -28,11 +28,18 @@ const STREAMING_RESPONSE_HEADER_DENYLIST = new Set([
   "x-amz-security-token",
   "x-auth-token",
   "x-accel-buffering",
-  // 314-byte Codex session blob. It is not a client rate-limit signal and
-  // alone ate ~40% of the old 768-byte budget, evicting x-codex-*-used-percent.
-  "x-codex-turn-state",
 ]);
 
+/**
+ * `x-codex-turn-state` is forwarded verbatim and EXEMPT from the forwarding
+ * budget. The real Codex client captures this ~314-byte blob from /responses
+ * (and echoes it back within the same turn), so dropping it breaks the
+ * protocol chain — but naively counting it against the budget used to evict
+ * the x-codex-*-used-percent quota headers (the reason it was denylisted
+ * under #10315-era budgeting). Carving it out keeps both.
+ */
+const CODEX_TURN_STATE_RESPONSE_HEADER = "x-codex-turn-state";
+
 const DEFAULT_FORWARDED_HEADER_BUDGET_BYTES = 768;
 
 /**
@@ -206,7 +213,9 @@ export function buildStreamingResponseHeaders(
       STREAMING_RESPONSE_HEADER_DENYLIST.has(normalized) ||
       connectionScopedHeaders.has(normalized) ||
       isNextMiddlewareControlHeader(normalized) ||
-      isOmniRouteInternalHeader(normalized)
+      isOmniRouteInternalHeader(normalized) ||
+      // Forwarded separately below, outside the byte budget.
+      normalized === CODEX_TURN_STATE_RESPONSE_HEADER
     ) {
       return;
     }
@@ -269,6 +278,10 @@ export function buildStreamingResponseHeaders(
     "X-Accel-Buffering": "no",
     [OMNIROUTE_RESPONSE_HEADERS.cache]: "MISS",
   };
+  const codexTurnState = providerHeaders.get(CODEX_TURN_STATE_RESPONSE_HEADER)?.trim();
+  if (codexTurnState) {
+    responseHeaders[CODEX_TURN_STATE_RESPONSE_HEADER] = codexTurnState;
+  }
   attachOmniRouteMetaHeaders(responseHeaders, meta);
   return responseHeaders;
 }
diff --git a/open-sse/services/codexQuotaFetcher.ts b/open-sse/services/codexQuotaFetcher.ts
index eb588ac3ba..12f955906d 100644
--- a/open-sse/services/codexQuotaFetcher.ts
+++ b/open-sse/services/codexQuotaFetcher.ts
@@ -25,6 +25,7 @@ import {
 import { registerQuotaFetcher, registerQuotaWindows, type QuotaInfo } from "./quotaPreflight.ts";
 import { registerMonitorFetcher } from "./quotaMonitor.ts";
 import { throttleQuotaFetch } from "./quotaFetchThrottle.ts";
+import { getCodexBackendIdentityHeaders } from "../config/codexClient.ts";
 
 /**
  * Stable identifiers for Codex's quota windows. These match the quota keys
@@ -222,6 +223,9 @@ export async function fetchCodexQuota(
       Authorization: `Bearer ${meta.accessToken}`,
       "Content-Type": "application/json",
       Accept: "application/json",
+      // Canonical Codex backend identity (UA + originator + version), same
+      // chain as inference — see getCodexUsage.
+      ...getCodexBackendIdentityHeaders(),
     };
 
     if (meta.workspaceId) {
diff --git a/open-sse/services/tokenRefresh.ts b/open-sse/services/tokenRefresh.ts
index 6b8b018a04..2ca3c5df62 100755
--- a/open-sse/services/tokenRefresh.ts
+++ b/open-sse/services/tokenRefresh.ts
@@ -12,6 +12,7 @@
 // tests) keep a stable surface.
 import { AsyncLocalStorage } from "node:async_hooks";
 import { PROVIDERS } from "../config/constants.ts";
+import { getCodexAuthIdentityHeaders } from "../config/codexClient.ts";
 import { runWithProxyContext } from "../utils/proxyFetch.ts";
 import { serializeRefresh } from "./refreshSerializer.ts";
 import {
@@ -254,6 +255,12 @@ export async function refreshAccessToken(
         headers: {
           "Content-Type": "application/x-www-form-urlencoded",
           Accept: "application/json",
+          // Credential face (auth.openai.com): the real Codex client sends only
+          // originator + User-Agent here — no version header (that gate exists
+          // only on the /backend-api/codex inference face). Refreshing with a
+          // bare/anonymous identity is a half-identity no real client emits.
+          // Mirrors sub2api v0.1.178 ApplyCodexCanonicalAuthIdentity.
+          ...(provider === "codex" ? getCodexAuthIdentityHeaders() : null),
         },
         body: params,
       })
diff --git a/open-sse/services/usage/codex.ts b/open-sse/services/usage/codex.ts
index 564cbdad9d..64b37c0183 100644
--- a/open-sse/services/usage/codex.ts
+++ b/open-sse/services/usage/codex.ts
@@ -9,6 +9,7 @@
  */
 
 import { buildCodexUsageQuotas } from "../codexUsageQuotas.ts";
+import { getCodexBackendIdentityHeaders } from "../../config/codexClient.ts";
 import { getFieldValue } from "./scalars.ts";
 
 // Codex (OpenAI) API config
@@ -36,6 +37,10 @@ export async function getCodexUsage(
       Authorization: `Bearer ${accessToken}`,
       "Content-Type": "application/json",
       Accept: "application/json",
+      // Same UA/version identity chain as Codex inference (sub2api v0.1.178
+      // unified-outbound-identity): usage probes must not show up upstream as
+      // an anonymous half-identity next to the converged inference traffic.
+      ...getCodexBackendIdentityHeaders(),
     };
     if (accountId) {
       headers["chatgpt-account-id"] = accountId;
diff --git a/src/app/api/providers/[id]/models/discovery/codex.ts b/src/app/api/providers/[id]/models/discovery/codex.ts
index 6e71592a3f..4d113863f3 100644
--- a/src/app/api/providers/[id]/models/discovery/codex.ts
+++ b/src/app/api/providers/[id]/models/discovery/codex.ts
@@ -1,4 +1,5 @@
 import {
+  CODEX_CLI_RS_ORIGINATOR,
   getCodexClientVersion,
   getCodexDefaultHeaders,
 } from "@omniroute/open-sse/config/codexClient.ts";
@@ -464,7 +465,7 @@ export async function fetchCodexDiscoveryModels({
       Accept: "application/json",
       "Content-Type": "application/json",
       Authorization: `Bearer ${accessToken}`,
-      originator: "codex_cli_rs",
+      originator: CODEX_CLI_RS_ORIGINATOR,
     };
     if (workspaceId) headers["chatgpt-account-id"] = workspaceId;
 
diff --git a/src/lib/db/providers.ts b/src/lib/db/providers.ts
index 46d9742e61..a800ee7852 100644
--- a/src/lib/db/providers.ts
+++ b/src/lib/db/providers.ts
@@ -19,6 +19,7 @@ import {
 } from "@omniroute/open-sse/services/apiKeyRotator.ts";
 import { invalidateReasoningRoutingRuleCache } from "./reasoningRoutingRules";
 import { normalizeProviderSpecificData } from "@/lib/providers/requestDefaults";
+import { ensureCodexFingerprintSeed } from "@omniroute/open-sse/config/codexIdentity.ts";
 import { bumpProxyConfigGeneration, getSettings } from "./settings";
 import {
   getStoredManagementPassword,
@@ -28,6 +29,32 @@ import {
 import { webSessionCredentialKey, parseProviderSpecificData } from "./webSessionDedup";
 import { pickCodexConnectionForUser } from "@/lib/oauth/utils/codexConnectionSelection";
 import { reconcileCodexUsageHistory } from "./providers/usageIdentityReconciliation";
+
+/**
+ * normalizeProviderSpecificData + the Codex fingerprint-seed invariant: Codex
+ * OAuth connections whose convergence mode derives account-scoped identities
+ * (device/session/full — the default session included) carry a persisted
+ * random seed (`codexFingerprintSeed`) as the derivation source. Created here
+ * at the persistence choke point so every write path (manual create, OAuth
+ * persist, edit, import) is covered; the seed is never regenerated once valid,
+ * so identities stay put across saves. Pre-seed connections rotate from the
+ * legacy connection-id derivation exactly once on their next write — the
+ * OmniRoute analog of sub2api's migration-225 backfill (v0.1.178, #5696).
+ */
+function normalizeConnectionProviderSpecificData(
+  provider: string | null,
+  providerSpecificData: unknown,
+  credentials: { accessToken?: unknown; refreshToken?: unknown },
+  existingProviderSpecificData?: unknown
+) {
+  const normalized = normalizeProviderSpecificData(provider, providerSpecificData);
+  if (provider !== "codex") return normalized;
+  return ensureCodexFingerprintSeed(
+    normalized,
+    credentials,
+    (existingProviderSpecificData as Record<string, unknown> | null) ?? null
+  );
+}
 import {
   withNullableMaxConcurrent,
   withNullableQuotaWindowThresholds,
@@ -353,9 +380,10 @@ export async function createProviderConnection(data: JsonRecord) {
   await assertApiKeyIsNotManagementPassword(data.apiKey);
   const db = getDbInstance() as unknown as DbLike;
   const now = new Date().toISOString();
-  const normalizedProviderSpecificData = normalizeProviderSpecificData(
+  const normalizedProviderSpecificData = normalizeConnectionProviderSpecificData(
     toStringOrNull(data.provider),
-    data.providerSpecificData
+    data.providerSpecificData,
+    data
   );
 
   let existing: JsonRecord | null = null;
@@ -483,9 +511,11 @@ export async function createProviderConnection(data: JsonRecord) {
     const rawExisting = toRecord(rowToCamel(existing));
     const decryptedExisting = decryptConnectionFields({ ...rawExisting });
     const merged: JsonRecord = { ...decryptedExisting, ...data, updatedAt: now };
-    merged.providerSpecificData = normalizeProviderSpecificData(
+    merged.providerSpecificData = normalizeConnectionProviderSpecificData(
       toStringOrNull(merged.provider),
-      merged.providerSpecificData
+      merged.providerSpecificData,
+      merged,
+      decryptedExisting.providerSpecificData
     );
     const persistence: JsonRecord = { ...merged };
     for (const field of CONNECTION_CREDENTIAL_FIELDS) {
@@ -805,14 +835,17 @@ export async function updateProviderConnection(id: string, data: JsonRecord) {
   // on every unrelated field edit.
   await assertApiKeyIsNotManagementPassword(data.apiKey);
 
+  const existingCamel = toRecord(rowToCamel(existing));
   const merged: JsonRecord = {
-    ...toRecord(rowToCamel(existing)),
+    ...existingCamel,
     ...data,
     updatedAt: new Date().toISOString(),
   };
-  merged.providerSpecificData = normalizeProviderSpecificData(
+  merged.providerSpecificData = normalizeConnectionProviderSpecificData(
     toStringOrNull(merged.provider),
-    merged.providerSpecificData
+    merged.providerSpecificData,
+    merged,
+    existingCamel.providerSpecificData
   );
   // Mirror the sanitization the create path applies — keep the returned
   // object in lockstep with what we persist.
diff --git a/src/lib/providers/requestDefaults.ts b/src/lib/providers/requestDefaults.ts
index 1fb62c0bdc..015df460f7 100644
--- a/src/lib/providers/requestDefaults.ts
+++ b/src/lib/providers/requestDefaults.ts
@@ -316,6 +316,15 @@ export function sanitizeProviderSpecificDataForResponse(value: unknown): JsonRec
   delete sanitized.usageCookie;
   delete sanitized.runtimeKey;
   delete sanitized.validationId;
+  // System-managed Codex fingerprint seed: never exposed through the API
+  // (mirrors sub2api stripping `codex_fingerprint_seed`); the server-side
+  // partial-update merge keeps it alive without the client round-tripping it.
+  delete sanitized.codexFingerprintSeed;
+  // Runtime-only Codex identity carriers (in-memory per request, never
+  // persisted) — strip defensively if they ever leak into a response payload.
+  delete sanitized.codexClientIdentity;
+  delete sanitized.codexOriginalIdentityHeaders;
+  delete sanitized.codexTurnStateEcho;
   if (sanitized.browserCdpEndpoint) sanitized.browserCdpEndpoint = "configured";
   return sanitized;
 }
diff --git a/src/lib/usage/codexResetCredits.ts b/src/lib/usage/codexResetCredits.ts
index a0097173d0..ab54b40a9d 100644
--- a/src/lib/usage/codexResetCredits.ts
+++ b/src/lib/usage/codexResetCredits.ts
@@ -5,6 +5,7 @@ import {
   refreshAndUpdateCredentials,
 } from "@/lib/usage/providerLimits";
 import { invalidateCodexQuotaCache } from "@omniroute/open-sse/services/codexQuotaFetcher.ts";
+import { getCodexBackendIdentityHeaders } from "@omniroute/open-sse/config/codexClient.ts";
 import { runWithProxyContext } from "@omniroute/open-sse/utils/proxyFetch.ts";
 import { sanitizeErrorMessage } from "@omniroute/open-sse/utils/error.ts";
 
@@ -291,6 +292,9 @@ function buildCodexResetCreditHeaders(connection: CodexConnectionLike): Record<s
     Authorization: `Bearer ${connection.accessToken}`,
     "Content-Type": "application/json",
     Accept: "application/json",
+    // Canonical Codex backend identity (UA + originator + version), same
+    // chain as inference — see getCodexUsage.
+    ...getCodexBackendIdentityHeaders(),
   };
 
   const workspaceId = getWorkspaceId(connection);
diff --git a/tests/unit/codex-fingerprint-convergence.test.ts b/tests/unit/codex-fingerprint-convergence.test.ts
index 11c186776a..5e44d350b9 100644
--- a/tests/unit/codex-fingerprint-convergence.test.ts
+++ b/tests/unit/codex-fingerprint-convergence.test.ts
@@ -6,6 +6,7 @@ import {
   applyCodexClientMetadata,
   applyCodexOriginalIdentityHeaders,
   createCodexClientIdentity,
+  ensureCodexFingerprintSeed,
   getCodexClientSessionId,
   getCodexConvergedSessionId,
   getCodexConvergedThreadId,
@@ -371,3 +372,84 @@ test("Codex websocket headers and payload share one fingerprint identity", async
   assert.equal(wsHeaders["x-codex-window-id"], metadata["x-codex-window-id"]);
   assert.equal(payload.type, "response.create");
 });
+
+test("Codex fingerprint seed: persisted seed drives v2 derivation deterministically", () => {
+  const seed = "11111111-2222-4233-8444-555555555555";
+  const seeded = { workspaceId: "workspace-42", codexFingerprintSeed: seed };
+
+  const installationId = getCodexInstallationId(seeded, "connection-42");
+  const sessionId = getCodexConvergedSessionId(seeded, "connection-42");
+  const threadId = getCodexConvergedThreadId("client-session", seeded, "connection-42");
+
+  // Deterministic: same seed → same ids, regardless of the connection key.
+  assert.equal(getCodexInstallationId(seeded, "another-connection"), installationId);
+  assert.equal(getCodexConvergedSessionId(seeded, "another-connection"), sessionId);
+  assert.equal(getCodexConvergedThreadId("client-session", seeded, null), threadId);
+
+  // v2 derivation deliberately rotates away from the legacy workspace-derived ids.
+  const legacy = { workspaceId: "workspace-42" };
+  assert.notEqual(installationId, getCodexInstallationId(legacy, "connection-42"));
+  assert.notEqual(sessionId, getCodexConvergedSessionId(legacy, "connection-42"));
+
+  // Two connections never share an identity once seeded (sub2api #5696).
+  const otherSeed = { codexFingerprintSeed: "aaaaaaaa-bbbb-4ccc-8ddd-eeeeeeeeeeee" };
+  assert.notEqual(getCodexInstallationId(otherSeed, "connection-42"), installationId);
+  assert.notEqual(getCodexConvergedSessionId(otherSeed, "connection-42"), sessionId);
+
+  // Admin-configured explicit installation id still wins over the seed.
+  assert.equal(
+    getCodexInstallationId(
+      { ...seeded, codexInstallationId: "99999999-8888-4777-8666-555555555555" },
+      "connection-42"
+    ),
+    "99999999-8888-4777-8666-555555555555"
+  );
+});
+
+test("ensureCodexFingerprintSeed creates once, preserves, and skips non-converged", () => {
+  const oauth = { accessToken: "oauth-token" };
+
+  // Default mode (session) on OAuth → seed created.
+  const created = ensureCodexFingerprintSeed(undefined, oauth);
+  assert.match(created?.codexFingerprintSeed as string, /^[0-9a-f-]{36}$/);
+
+  // The stored seed ALWAYS wins over an incoming payload that omits it
+  // (partial update) or carries a client-forged one (system-managed key).
+  const stored = { codexFingerprintSeed: "11111111-2222-4233-8444-555555555555" };
+  assert.deepEqual(ensureCodexFingerprintSeed(undefined, oauth, stored), stored);
+  assert.deepEqual(
+    ensureCodexFingerprintSeed(
+      { codexFingerprintSeed: "99999999-8888-4777-8666-555555555555" },
+      oauth,
+      stored
+    ),
+    stored
+  );
+  // The stored seed stays dormant when the mode is switched off.
+  assert.deepEqual(ensureCodexFingerprintSeed({ codexFingerprintMode: "off" }, oauth, stored), {
+    codexFingerprintMode: "off",
+    codexFingerprintSeed: "11111111-2222-4233-8444-555555555555",
+  });
+
+  // Explicit off without a stored seed stays seedless; a client-supplied seed
+  // on create is stripped and replaced by a system-generated one.
+  assert.deepEqual(ensureCodexFingerprintSeed({ codexFingerprintMode: "off" }, oauth), {
+    codexFingerprintMode: "off",
+  });
+  const forgedOnCreate = ensureCodexFingerprintSeed(
+    { codexFingerprintSeed: "99999999-8888-4777-8666-555555555555" },
+    oauth
+  );
+  assert.match(forgedOnCreate?.codexFingerprintSeed as string, /^[0-9a-f-]{36}$/);
+  assert.notEqual(forgedOnCreate?.codexFingerprintSeed, "99999999-8888-4777-8666-555555555555");
+
+  // Non-OAuth (API key) connections are never seeded.
+  assert.equal(ensureCodexFingerprintSeed(undefined, { apiKey: "sk-x" }), undefined);
+  assert.equal(ensureCodexFingerprintSeed(undefined, undefined), undefined);
+
+  // device/full modes require the seed as well.
+  for (const mode of ["device", "full"]) {
+    const result = ensureCodexFingerprintSeed({ codexFingerprintMode: mode }, oauth);
+    assert.match(result?.codexFingerprintSeed as string, /^[0-9a-f-]{36}$/);
+  }
+});
diff --git a/tests/unit/codex-fingerprint-seed-persistence.test.ts b/tests/unit/codex-fingerprint-seed-persistence.test.ts
new file mode 100644
index 0000000000..d65b7197b4
--- /dev/null
+++ b/tests/unit/codex-fingerprint-seed-persistence.test.ts
@@ -0,0 +1,111 @@
+import { after, beforeEach, test } from "node:test";
+import assert from "node:assert/strict";
+import fs from "node:fs";
+import os from "node:os";
+import path from "node:path";
+
+const TEST_DATA_DIR = fs.mkdtempSync(path.join(os.tmpdir(), "omniroute-codex-seed-"));
+process.env.DATA_DIR = TEST_DATA_DIR;
+
+const core = await import("../../src/lib/db/core.ts");
+const providersDb = await import("../../src/lib/db/providers.ts");
+
+const UUID_V4_PATTERN = /^[0-9a-f]{8}-[0-9a-f]{4}-4[0-9a-f]{3}-[89ab][0-9a-f]{3}-[0-9a-f]{12}$/;
+
+async function resetStorage() {
+  core.resetDbInstance();
+  fs.rmSync(TEST_DATA_DIR, { recursive: true, force: true });
+  fs.mkdirSync(TEST_DATA_DIR, { recursive: true });
+}
+
+beforeEach(resetStorage);
+after(() => {
+  fs.rmSync(TEST_DATA_DIR, { recursive: true, force: true });
+});
+
+async function createCodexOAuthConnection(providerSpecificData?: Record<string, unknown>) {
+  const suffix = Math.random().toString(16).slice(2, 10);
+  const connection = await providersDb.createProviderConnection({
+    provider: "codex",
+    authType: "oauth",
+    name: `codex-${suffix}`,
+    accessToken: `access-${suffix}`,
+    refreshToken: `refresh-${suffix}`,
+    providerSpecificData,
+  });
+  assert.ok(connection && typeof connection.id === "string");
+  return connection;
+}
+
+test("codex OAuth connections persist a fingerprint seed at creation (default session mode)", async () => {
+  const connection = await createCodexOAuthConnection();
+  const psd = connection.providerSpecificData as Record<string, unknown>;
+  assert.match(String(psd.codexFingerprintSeed), UUID_V4_PATTERN);
+});
+
+test("the seed survives unrelated edits and explicit re-saves (identity stability)", async () => {
+  const connection = await createCodexOAuthConnection({ workspaceId: "ws-1" });
+  const seed = (connection.providerSpecificData as Record<string, unknown>).codexFingerprintSeed;
+
+  const renamed = await providersDb.updateProviderConnection(connection.id, { name: "renamed" });
+  assert.equal(
+    (renamed?.providerSpecificData as Record<string, unknown>).codexFingerprintSeed,
+    seed
+  );
+
+  const resaved = await providersDb.updateProviderConnection(connection.id, {
+    providerSpecificData: { workspaceId: "ws-1", codexFingerprintMode: "full" },
+  });
+  assert.equal(
+    (resaved?.providerSpecificData as Record<string, unknown>).codexFingerprintSeed,
+    seed
+  );
+  assert.equal(
+    (resaved?.providerSpecificData as Record<string, unknown>).codexFingerprintMode,
+    "full"
+  );
+});
+
+test("explicit off is not seeded; switching to full seeds once and keeps it", async () => {
+  const connection = await createCodexOAuthConnection({ codexFingerprintMode: "off" });
+  assert.equal(
+    (connection.providerSpecificData as Record<string, unknown>).codexFingerprintSeed,
+    undefined
+  );
+
+  const switched = await providersDb.updateProviderConnection(connection.id, {
+    providerSpecificData: { codexFingerprintMode: "full" },
+  });
+  const psd = switched?.providerSpecificData as Record<string, unknown>;
+  assert.match(String(psd.codexFingerprintSeed), UUID_V4_PATTERN);
+
+  const again = await providersDb.updateProviderConnection(connection.id, { name: "again" });
+  assert.equal(
+    (again?.providerSpecificData as Record<string, unknown>).codexFingerprintSeed,
+    psd.codexFingerprintSeed
+  );
+});
+
+test("a client-supplied seed is replaced by a system-managed one", async () => {
+  const connection = await createCodexOAuthConnection({
+    codexFingerprintSeed: "client-supplied-not-a-uuid",
+  });
+  const seed = String(
+    (connection.providerSpecificData as Record<string, unknown>).codexFingerprintSeed
+  );
+  assert.match(seed, UUID_V4_PATTERN);
+  assert.notEqual(seed, "client-supplied-not-a-uuid");
+});
+
+test("non-OAuth codex connections are never seeded", async () => {
+  const suffix = Math.random().toString(16).slice(2, 10);
+  const connection = await providersDb.createProviderConnection({
+    provider: "codex",
+    authType: "apikey",
+    name: `codex-key-${suffix}`,
+    apiKey: `sk-codex-${suffix}`,
+  });
+  assert.ok(connection);
+  const psd = (connection.providerSpecificData ?? {}) as Record<string, unknown>;
+  assert.equal(psd.codexFingerprintSeed, undefined);
+});
diff --git a/tests/unit/codex-import-token-route.test.ts b/tests/unit/codex-import-token-route.test.ts
index bff8c49b83..f9bbbc4ec2 100644
--- a/tests/unit/codex-import-token-route.test.ts
+++ b/tests/unit/codex-import-token-route.test.ts
@@ -75,7 +75,14 @@ test("import-token: decodes email + workspace claims from the access token and c
   assert.deepEqual(created?.providerSpecificData, {
     chatgptAccountId: "acct-bare",
     chatgptPlanType: "plus",
+    // Convergence is on by default (session mode), so the connection persists
+    // its system-managed fingerprint seed at creation time (v178 parity).
+    codexFingerprintSeed: created?.providerSpecificData?.codexFingerprintSeed,
   });
+  assert.match(
+    String(created?.providerSpecificData?.codexFingerprintSeed),
+    /^[0-9a-f]{8}-[0-9a-f]{4}-4[0-9a-f]{3}-[89ab][0-9a-f]{3}-[0-9a-f]{12}$/
+  );
 });
 
 test("import-token: falls back to the explicit `name` when the JWT carries no email", async () => {
diff --git a/tests/unit/codex-turn-state.test.ts b/tests/unit/codex-turn-state.test.ts
new file mode 100644
index 0000000000..bbe9b67fd2
--- /dev/null
+++ b/tests/unit/codex-turn-state.test.ts
@@ -0,0 +1,147 @@
+import test from "node:test";
+import assert from "node:assert/strict";
+
+import {
+  __resetCodexTurnStateOriginsForTesting,
+  isCrossAccountCodexTurnState,
+  noteCodexTurnStateProvenance,
+  readCodexTurnStateHeader,
+} from "../../open-sse/config/codexTurnState.ts";
+import {
+  resolveCodexTurnStateEcho,
+  withCodexFingerprintCredentials,
+} from "../../open-sse/config/codexIdentity.ts";
+import { buildStreamingResponseHeaders } from "../../open-sse/handlers/chatCore/responseHeaders.ts";
+
+const TURN_STATE = "ts-blob-0123456789";
+
+function reset() {
+  __resetCodexTurnStateOriginsForTesting();
+}
+
+test("readCodexTurnStateHeader reads Headers and plain records case-insensitively", () => {
+  reset();
+  assert.equal(readCodexTurnStateHeader(null), null);
+  assert.equal(readCodexTurnStateHeader({}), null);
+  assert.equal(readCodexTurnStateHeader({ "x-codex-turn-state": "  " }), null);
+  assert.equal(
+    readCodexTurnStateHeader(new Headers({ "x-codex-turn-state": TURN_STATE })),
+    TURN_STATE
+  );
+  assert.equal(readCodexTurnStateHeader({ "X-Codex-Turn-State": TURN_STATE }), TURN_STATE);
+});
+
+test("provenance: same account passes, cross account is flagged, unknown session passes", () => {
+  reset();
+  noteCodexTurnStateProvenance("session-1", "conn-a");
+  assert.equal(isCrossAccountCodexTurnState("session-1", "conn-a"), false);
+  assert.equal(isCrossAccountCodexTurnState("session-1", "conn-b"), true);
+  assert.equal(isCrossAccountCodexTurnState("session-unknown", "conn-b"), false);
+});
+
+test("provenance: missing session or account does not track", () => {
+  reset();
+  noteCodexTurnStateProvenance("", "conn-a");
+  noteCodexTurnStateProvenance("session-1", "");
+  noteCodexTurnStateProvenance(null, "conn-a");
+  assert.equal(isCrossAccountCodexTurnState("session-1", "conn-b"), false);
+});
+
+test("provenance: expired records stop guarding", () => {
+  reset();
+  const t0 = 1_000_000;
+  noteCodexTurnStateProvenance("session-1", "conn-a", t0);
+  assert.equal(isCrossAccountCodexTurnState("session-1", "conn-b", t0 + 1000), true);
+  // 2h TTL — after it lapses the record is lazily dropped and the echo passes.
+  assert.equal(
+    isCrossAccountCodexTurnState("session-1", "conn-b", t0 + 2 * 60 * 60 * 1000 + 1),
+    false
+  );
+});
+
+test("provenance: newest commit wins for a re-minted session blob", () => {
+  reset();
+  noteCodexTurnStateProvenance("session-1", "conn-a");
+  // Failover committed a response from conn-b — the client now holds b's blob.
+  noteCodexTurnStateProvenance("session-1", "conn-b");
+  assert.equal(isCrossAccountCodexTurnState("session-1", "conn-b"), false);
+  assert.equal(isCrossAccountCodexTurnState("session-1", "conn-a"), true);
+});
+
+test("resolveCodexTurnStateEcho strips only known cross-account echoes", () => {
+  reset();
+  const clientHeaders = {
+    "session-id": "session-1",
+    "x-codex-turn-state": TURN_STATE,
+  };
+  // No provenance yet → pass through unchanged.
+  assert.equal(resolveCodexTurnStateEcho(clientHeaders, "conn-a"), TURN_STATE);
+
+  // Blob minted by conn-a and client now served by conn-a again → pass.
+  noteCodexTurnStateProvenance("session-1", "conn-a");
+  assert.equal(resolveCodexTurnStateEcho(clientHeaders, "conn-a"), TURN_STATE);
+
+  // Failover to conn-b while the client echoes conn-a's blob → strip.
+  assert.equal(resolveCodexTurnStateEcho(clientHeaders, "conn-b"), null);
+
+  // No echo header → nothing to forward.
+  assert.equal(resolveCodexTurnStateEcho({ "session-id": "session-1" }, "conn-a"), null);
+
+  // Echo without a session id cannot be provenance-checked → pass through
+  // (same as sub2api: no tracking key, keep passthrough behavior).
+  assert.equal(
+    resolveCodexTurnStateEcho({ "x-codex-turn-state": TURN_STATE }, "conn-b"),
+    TURN_STATE
+  );
+});
+
+test("withCodexFingerprintCredentials stashes the allowed echo independent of mode", () => {
+  reset();
+  noteCodexTurnStateProvenance("session-1", "conn-a");
+
+  const baseCredentials = {
+    accessToken: "oauth-token",
+    connectionId: "conn-a",
+    providerSpecificData: { codexFingerprintMode: "off" as const },
+  };
+  const clientHeaders = { "session-id": "session-1", "x-codex-turn-state": TURN_STATE };
+
+  // Same account, explicit off: echo survives alongside original identity passthrough.
+  const sameAccount = withCodexFingerprintCredentials(baseCredentials, clientHeaders, {});
+  assert.equal(sameAccount.providerSpecificData?.codexTurnStateEcho, TURN_STATE);
+  assert.ok(sameAccount.providerSpecificData?.codexOriginalIdentityHeaders);
+  assert.equal(sameAccount.providerSpecificData?.codexClientIdentity, undefined);
+
+  // Cross account: echo stripped, original client identity still preserved.
+  const crossAccount = withCodexFingerprintCredentials(
+    { ...baseCredentials, connectionId: "conn-b" },
+    clientHeaders,
+    {}
+  );
+  assert.equal(crossAccount.providerSpecificData?.codexTurnStateEcho, undefined);
+
+  // Compact endpoint: convergence identity is skipped but the echo guard still runs.
+  const compact = withCodexFingerprintCredentials(
+    { ...baseCredentials, requestEndpointPath: "/responses/compact" },
+    clientHeaders,
+    {}
+  );
+  assert.equal(compact.providerSpecificData?.codexClientIdentity, undefined);
+  assert.equal(compact.providerSpecificData?.codexTurnStateEcho, TURN_STATE);
+});
+
+test("streaming response headers forward x-codex-turn-state outside the byte budget", () => {
+  reset();
+  const upstream = new Headers();
+  upstream.set("x-codex-turn-state", "s".repeat(300));
+  // Fill the budget with low-priority noise the blob would otherwise evict into.
+  for (let index = 0; index < 12; index += 1) {
+    upstream.set(`x-noise-${index}`, "n".repeat(60));
+  }
+  upstream.set("x-codex-primary-used-percent", "41");
+
+  const out = buildStreamingResponseHeaders(upstream, {}, null);
+  const record = out as Record<string, string>;
+  assert.equal(record["x-codex-turn-state"], "s".repeat(300));
+  assert.equal(record["x-codex-primary-used-percent"], "41");
+});
diff --git a/tests/unit/middleware-header-strip-5849.test.ts b/tests/unit/middleware-header-strip-5849.test.ts
index ed9ea6be4a..0d6eaf4634 100644
--- a/tests/unit/middleware-header-strip-5849.test.ts
+++ b/tests/unit/middleware-header-strip-5849.test.ts
@@ -100,7 +100,7 @@ test("streaming path bounds the aggregate size of many small upstream response h
   assert.equal(getHeaderValue(out, "x-request-id"), "req-many-small-headers");
 });
 
-test("streaming path keeps Codex quota headers and drops x-codex-turn-state", () => {
+test("streaming path keeps Codex quota headers and forwards x-codex-turn-state budget-exempt", () => {
   const upstream = new Headers();
   upstream.set("x-codex-turn-state", "s".repeat(300));
   upstream.set("content-security-policy", "default-src 'none'");
@@ -116,7 +116,10 @@ test("streaming path keeps Codex quota headers and drops x-codex-turn-state", ()
   assert.equal(getHeaderValue(out, "x-codex-primary-used-percent"), "41");
   assert.equal(getHeaderValue(out, "x-codex-primary-reset-after-seconds"), "120");
   assert.equal(getHeaderValue(out, "x-codex-credits-has-credits"), "true");
-  assert.equal(getHeaderValue(out, "x-codex-turn-state"), undefined);
+  // The turn-state blob rides along without evicting quota headers from the
+  // budget — the real Codex client echoes it back within the turn, so
+  // dropping it would break the protocol chain (sub2api v0.1.177 parity).
+  assert.equal(getHeaderValue(out, "x-codex-turn-state"), "s".repeat(300));
 });
 
 test("streaming path prioritizes request and rate-limit headers over diagnostics", () => {