diff --git a/src/server/responses/core-opaque-recovery.ts b/src/server/responses/core-opaque-recovery.ts
index bc7bbe40a18..03df9766863 100644
--- a/src/server/responses/core-opaque-recovery.ts
+++ b/src/server/responses/core-opaque-recovery.ts
@@ -88,6 +88,7 @@ export function isEncryptedFunctionOutputRejection(bodyText: string): boolean {
   try {
     const payload = JSON.parse(bodyText) as unknown;
     if (!payload || typeof payload !== "object" || Array.isArray(payload)) return false;
+    if (upstreamErrorMessageFromPayload(payload) === ENCRYPTED_FUNCTION_OUTPUT_REJECTION) return true;
     const record = payload as { detail?: unknown; message?: unknown; error?: unknown };
     if (record.detail === ENCRYPTED_FUNCTION_OUTPUT_REJECTION) return true;
     if (record.message === ENCRYPTED_FUNCTION_OUTPUT_REJECTION) return true;
diff --git a/src/server/responses/encrypted-payload.ts b/src/server/responses/encrypted-payload.ts
index 0e9efddf999..9835e851cca 100644
--- a/src/server/responses/encrypted-payload.ts
+++ b/src/server/responses/encrypted-payload.ts
@@ -98,6 +98,19 @@ import type { EffectiveSubagentRoster, SpawnAgentSurface } from "../../codex/cat
 
 
 export function looksLikeBackendCiphertext(payload: string): boolean {
+  // Unknown replay history has no authenticity proof. Require the key-independent Fernet wire
+  // structure instead of granting ciphertext authority to any long base64-like model output.
+  // Proven backend bytes still remain opaque and byte-identical; malformed/plaintext slots are
+  // lowered by the compatibility path below rather than poisoning every later native replay.
+  return isStructurallyValidFernetToken(payload);
+}
+
+/**
+ * Pre-route compatibility keeps an encoded-looking unknown slot opaque until the destination is
+ * known. A routed destination strips that slot instead of exposing possible truncated ciphertext;
+ * the canonical backend later applies the stricter structural classifier.
+ */
+function looksLikeUnknownOpaqueSlot(payload: string): boolean {
   return payload.length >= 64 && /^[A-Za-z0-9+/=_-]+$/.test(payload);
 }
 
@@ -480,7 +493,10 @@ function contentWithoutCiphertext(content: unknown[]): unknown[] {
 
 
 
-export function sanitizeEncryptedContentInPlace(input: unknown): number {
+export function sanitizeEncryptedContentInPlace(
+  input: unknown,
+  options: { preserveUnknownOpaqueSlots?: boolean } = {},
+): number {
   if (!Array.isArray(input)) return 0;
   let rewritten = 0;
   const protectedFragments = new WeakSet<object>();
@@ -512,7 +528,9 @@ export function sanitizeEncryptedContentInPlace(input: unknown): number {
         && typeof (child as { encrypted_content?: unknown }).encrypted_content === "string"
       ) {
         const payload = (child as { encrypted_content: string }).encrypted_content;
-        if (!protectedFragments.has(child) && !looksLikeBackendCiphertext(payload)) {
+        const preserveUnknown = options.preserveUnknownOpaqueSlots === true
+          && looksLikeUnknownOpaqueSlot(payload);
+        if (!protectedFragments.has(child) && !looksLikeBackendCiphertext(payload) && !preserveUnknown) {
           const parts = encryptedSlotParts(payload);
           frame.node.splice(frame.index, 1, ...parts);
           rewritten += 1;
diff --git a/src/server/responses/passthrough-dispatch.ts b/src/server/responses/passthrough-dispatch.ts
index e4ca80ccb22..6676bbd1d40 100644
--- a/src/server/responses/passthrough-dispatch.ts
+++ b/src/server/responses/passthrough-dispatch.ts
@@ -143,6 +143,7 @@ import { captureCodexAffinityDiagnostic } from "../../codex/affinity-debug";
 import { ACCOUNT_GATED_NATIVE_OPENAI_MODELS } from "../../codex/catalog/native-models";
 import {
   attemptOpaqueBlobRecovery,
+  isEncryptedFunctionOutputRejection,
   outboundResponsesBodyCarriesEncryptedFunctionOutput,
   resetStreamedOpaqueBlobLogContext,
   consoleGoUploadRejectionBody,
@@ -1474,8 +1475,13 @@ export async function preparePassthroughExchange(
         payload => {
           if (!payload || typeof payload !== "object" || Array.isArray(payload)) return false;
           const type = (payload as { type?: unknown }).type;
-          return (type === "error" || type === "response.failed" || type === "response.incomplete")
-            && upstreamErrorMessageFromPayload(payload) === ENCRYPTED_FUNCTION_OUTPUT_REJECTION;
+          const decryptRejection = (type === "error" || type === "response.failed" || type === "response.incomplete")
+            && isEncryptedFunctionOutputRejection(JSON.stringify(payload));
+          // `detail` is a real WebSocket error shape but the generic preflight failure projector
+          // intentionally understands only Responses error/message fields. Preserve only this
+          // exact identity so the projected 502 can enter the existing single-shot recovery.
+          if (decryptRejection) preflightLog.upstreamError = ENCRYPTED_FUNCTION_OUTPUT_REJECTION;
+          return decryptRejection;
         }, {
           allowMissingContentType: !recoveryContentType && parsed.stream,
           replayReadErrors: true,
diff --git a/src/server/responses/request-prepare.ts b/src/server/responses/request-prepare.ts
index 3c35bd3a806..4d94d6a71ad 100644
--- a/src/server/responses/request-prepare.ts
+++ b/src/server/responses/request-prepare.ts
@@ -273,10 +273,14 @@ export async function prepareResponsesRequest(
   // encrypted_content slots as plaintext. Rewrite them to input_text on the RAW body BEFORE
   // parsing so every consumer sees the payload: parseRequest (routed/translated providers read
   // the parsed messages) and the native passthrough (_rawBody is this same object, serialized
-  // verbatim). Genuine backend ciphertext is left byte-identical (looksLikeBackendCiphertext).
+  // verbatim). Structurally valid backend ciphertext stays byte-identical; encoded-looking unknown
+  // slots remain opaque only until final-route handling can preserve or strip them safely.
   {
     const rewritten = sanitizeEncryptedContentInPlace(
       (body as { input?: unknown } | undefined)?.input,
+      // The final destination is not known yet. Keep ambiguous encoded slots opaque until route
+      // selection can either strip them for a third party or apply strict native classification.
+      { preserveUnknownOpaqueSlots: true },
     );
     if (rewritten > 0)
       console.warn(
@@ -891,6 +895,17 @@ export async function prepareResponsesRequest(
 
   if (options.abortSignal?.aborted) return clientCancelledResponse();
 
+  if (inboundWire === "responses" && isCanonicalOpenAiForwardProvider(route.provider)) {
+    const rewritten = sanitizeEncryptedContentInPlace(
+      (body as { input?: unknown } | undefined)?.input,
+    );
+    if (rewritten > 0) {
+      console.warn(
+        `[opencodex] rewrote ${rewritten} non-Fernet encrypted_content part(s) before canonical native replay`,
+      );
+    }
+  }
+
   // Encrypted child tasks may reach the canonical native backend or an explicitly trusted
   // direct Responses route. This runs against the FINAL route so native-only fallback can
   // rescue an incompatible primary without weakening combo behavior.
diff --git a/structure/decisions/ADR-5236-responses-http-sse.md b/structure/decisions/ADR-5236-responses-http-sse.md
new file mode 100644
index 00000000000..68bf4243e79
--- /dev/null
+++ b/structure/decisions/ADR-5236-responses-http-sse.md
@@ -0,0 +1,12 @@
+# ADR-5236 — decision recorded under "Responses HTTP/SSE"
+
+- Contract owner: [transports/responses.md](../transports/responses.md#responses-httpsse)
+
+## Decision record
+
+- 목적과 의도: Agent-generated text must not poison later native replay as trusted ciphertext.
+- 기존 구현 및 제약 조건: The proxy cannot authenticate backend Fernet tokens after full-history replay, but must preserve genuine opaque bytes byte-for-byte.
+- 검토한 주요 대안: Trust every encoded-looking string; strip every encrypted slot; require canonical Fernet structure and retain bounded rejection recovery.
+- 선택한 방식: Use Fernet structure as the unknown-history floor and one guarded recovery for the exact backend rejection.
+- 다른 대안 대신 이 방식을 선택한 이유: Loose shape grants authority to plaintext, while unconditional stripping destroys valid backend state.
+- 장점, 단점 및 영향: False positives stop before dispatch and poisoned history self-recovers once; structurally valid unauthenticated text may still need the bounded backend rejection path.
diff --git a/structure/transports/responses.md b/structure/transports/responses.md
index 031f1d5417f..09bf0d1ccb7 100644
--- a/structure/transports/responses.md
+++ b/structure/transports/responses.md
@@ -333,6 +333,18 @@ including the compaction turn the proxy itself drives. With `store: false`, requ
 strips ids from every input item, including compact-wire items, matching codex-rs
 (`core/src/client.rs:918-925`). Compact-wire items remain exempt from response-side field backfill.
 
+For replayed `encrypted_content` slots whose minting provenance is unavailable after a restart or
+full-history resend, the plaintext-compatibility boundary requires canonical key-independent Fernet
+structure (version byte, timestamp, IV, block-aligned ciphertext and HMAC layout). A long
+base64-like agent message does not gain ciphertext authority from its spelling. Structure is not
+authentication: it is only the minimum legacy fallback needed to avoid corrupting genuine opaque
+history. If the canonical backend still rejects an encrypted function or agent output, the exact
+decrypt/decode identity enters one request-budgeted sanitize-and-rebuild attempt for HTTP and
+pre-commit SSE/WebSocket terminal envelopes; the single-shot guard remains armed on the rebuilt
+send.
+
+> Decision record: [ADR-5236](../decisions/ADR-5236-responses-http-sse.md)
+
 Codex pool account changes are a separate portability question from destination serving identity.
 `src/codex/routing.ts` remembers, in process memory and keyed like thread affinity, which pool
 account minted a conversation's carried state (`previous_response_id`, encrypted reasoning, and
diff --git a/tests/responses/responses-compaction-routing.test.ts b/tests/responses/responses-compaction-routing.test.ts
index 45e174abedd..f111aafe71b 100644
--- a/tests/responses/responses-compaction-routing.test.ts
+++ b/tests/responses/responses-compaction-routing.test.ts
@@ -2316,9 +2316,13 @@ describe("computer screenshot output translation boundary", () => {
 
 describe("external task-input envelopes (#3735)", () => {
   beforeEach(takeSpendHome);
-  // Synthetic charset/length fixture: short plaintext in this slot is deliberately
-  // normalized to input_text before parsing, so it cannot exercise opaque rejection.
-  const opaqueOutput = `g${"A".repeat(127)}`;
+  const opaqueOutput = `${Buffer.concat([
+    Buffer.from([0x80]),
+    Buffer.alloc(8),
+    Buffer.alloc(16),
+    Buffer.alloc(16),
+    Buffer.alloc(32),
+  ]).toString("base64url")}==`;
   const external = (output: unknown = "external task input") => ({
     type: "function_call_output", id: "external-fixture", name: "handoff_input", namespace: "task_inbox", output,
   });
@@ -2326,8 +2330,9 @@ describe("external task-input envelopes (#3735)", () => {
     model: "gw/model", stream: false, input: [item],
   });
 
-  test("opaque negative fixtures survive the plaintext-slot classifier", () => {
+  test("only canonical Fernet structure survives the plaintext-slot classifier", () => {
     expect(looksLikeBackendCiphertext(opaqueOutput)).toBe(true);
+    expect(looksLikeBackendCiphertext(`gAAAAA${"A".repeat(189)}`)).toBe(false);
   });
 
   test("sends a complete envelope as user text without an orphan-tool marker", async () => {
diff --git a/tests/responses/responses-opaque-blob-recovery.test.ts b/tests/responses/responses-opaque-blob-recovery.test.ts
index 48290dad1ba..1fcf62d7ab4 100644
--- a/tests/responses/responses-opaque-blob-recovery.test.ts
+++ b/tests/responses/responses-opaque-blob-recovery.test.ts
@@ -21,8 +21,15 @@ import { markBodyNonPersistable, rememberResponseState, previousResponseProvider
 const originalFetch = globalThis.fetch;
 const originalOpenCodexHome = process.env.OPENCODEX_HOME;
 const BLOB = "provider-minted-opaque-state";
-// Synthetic Fernet-shaped data must survive the outbound ciphertext shape gate.
-const FUNCTION_OUTPUT_BLOB = `g${"A".repeat(127)}`;
+// Canonical key-independent Fernet structure; authenticity is deliberately not needed in tests.
+const FUNCTION_OUTPUT_BLOB = `${Buffer.concat([
+  Buffer.from([0x80]),
+  Buffer.alloc(8),
+  Buffer.alloc(16),
+  Buffer.alloc(16),
+  Buffer.alloc(32),
+]).toString("base64url")}==`;
+const FERNET_SHAPED_PLAINTEXT = `gAAAAA${"A".repeat(189)}`;
 const FUNCTION_OUTPUT_DECRYPT_MESSAGE = "Encrypted function output content could not be decrypted or decoded.";
 const OPENAI_BLOB_ERROR = JSON.stringify({
   error: {
@@ -370,6 +377,13 @@ function streamedFunctionOutputDecryptErrorEvent(
   );
 }
 
+function streamedFunctionOutputDecryptDetailEvent(): Response {
+  return decryptStreamResponse(
+    `event: error\ndata: ${JSON.stringify({ type: "error", detail: FUNCTION_OUTPUT_DECRYPT_MESSAGE })}\n\n`,
+    "text/event-stream",
+  );
+}
+
 function streamedSuccess(id: string): Response {
   const completed = {
     type: "response.completed",
@@ -544,6 +558,44 @@ describe("opaque blob recovery trigger", () => {
 });
 
 describe("opaque blob recovery through /v1/responses", () => {
+  test("lowers Fernet-shaped agent plaintext before native dispatch", async () => {
+    const outbound: Array<Record<string, unknown>> = [];
+    globalThis.fetch = (async (_input: RequestInfo | URL, init?: RequestInit) => {
+      outbound.push(JSON.parse(String(init?.body)) as Record<string, unknown>);
+      return success("resp-agent-plaintext");
+    }) as typeof fetch;
+
+    const request = new Request("http://localhost/v1/responses", {
+      method: "POST",
+      headers: {
+        "content-type": "application/json",
+        "x-codex-parent-thread-id": "thread-agent-plaintext",
+        authorization: "Bearer caller-codex-token",
+      },
+      body: JSON.stringify({
+        model: "gpt-5.5",
+        stream: false,
+        store: false,
+        input: [{
+          type: "agent_message",
+          author: "/root/child_task",
+          recipient: "/root",
+          content: [{ type: "encrypted_content", encrypted_content: FERNET_SHAPED_PLAINTEXT }],
+        }],
+      }),
+    });
+
+    const response = await handleResponses(request, nativeConfig(), { model: "", provider: "" });
+    expect(response.status).toBe(200);
+    await response.text();
+    expect(outbound).toHaveLength(1);
+    expect(outbound[0]?.input).toEqual([{
+      type: "message",
+      role: "user",
+      content: [{ type: "input_text", text: FERNET_SHAPED_PLAINTEXT }],
+    }]);
+  });
+
   test("recovers a zero-output streamed function-output decrypt failure before client relay", async () => {
     const outbound: Array<Record<string, unknown>> = [];
     globalThis.fetch = (async (_input: RequestInfo | URL, init?: RequestInit) => {
@@ -789,6 +841,27 @@ describe("opaque blob recovery through /v1/responses", () => {
     expect(retriedInput?.at(1)).toEqual(recoveredFunctionOutput());
   });
 
+  test("recovers a WebSocket-style detail decrypt error before client relay", async () => {
+    const outbound: Array<Record<string, unknown>> = [];
+    globalThis.fetch = (async (_input: RequestInfo | URL, init?: RequestInit) => {
+      outbound.push(JSON.parse(String(init?.body)) as Record<string, unknown>);
+      return outbound.length === 1
+        ? streamedFunctionOutputDecryptDetailEvent()
+        : streamedSuccess("resp-stream-detail-recovered");
+    }) as typeof fetch;
+
+    const logCtx: RequestLogContext = { model: "", provider: "" };
+    const response = await handleResponses(functionOutputRequest(true), config(), logCtx);
+    const body = await response.text();
+
+    expect(response.status).toBe(200);
+    expect(body).toContain("response.completed");
+    expect(body).not.toContain(FUNCTION_OUTPUT_DECRYPT_MESSAGE);
+    expect(outbound).toHaveLength(2);
+    expect(JSON.stringify(outbound[1])).not.toContain(FUNCTION_OUTPUT_BLOB);
+    expect(logCtx.activeAttempt?.recoveryKinds).toEqual(["opaque-blob-rejection"]);
+  });
+
   for (const streamMode of ["legacy-tee", "eager-relay"] as const) {
     test(`preserves non-decrypt failed SSE with encrypted history (${streamMode})`, async () => {
       const failed = { type: "response.failed", response: {