diff --git a/packages/junior-dashboard/e2e/conversation-send.spec.ts b/packages/junior-dashboard/e2e/conversation-send.spec.ts
index 83a39b3e9b..5a2d279e40 100644
--- a/packages/junior-dashboard/e2e/conversation-send.spec.ts
+++ b/packages/junior-dashboard/e2e/conversation-send.spec.ts
@@ -1,4 +1,7 @@
-import { webMessageId } from "@sentry/junior/api/schema";
+import {
+  CONVERSATION_DETAIL_VERSION_HEADER,
+  webMessageId,
+} from "@sentry/junior/api/schema";
 import type {
   ConversationPendingMessage,
   InputImage,
@@ -178,7 +181,10 @@ test("starts and continues conversations from the dashboard", async ({
       if (holdDetailRefresh) await detailRefreshHeld;
       // Request the full body because this test changes it.
       const response = await route.fetch({
-        headers: { ...route.request().headers(), "if-none-match": "" },
+        headers: {
+          ...route.request().headers(),
+          [CONVERSATION_DETAIL_VERSION_HEADER]: "",
+        },
       });
       await route.fulfill({
         response,
@@ -331,7 +337,10 @@ test("hands web and external messages from queue to history without gaps", async
     await historyHeld;
     // Request the full body because this test changes it.
     const response = await route.fetch({
-      headers: { ...route.request().headers(), "if-none-match": "" },
+      headers: {
+        ...route.request().headers(),
+        [CONVERSATION_DETAIL_VERSION_HEADER]: "",
+      },
     });
     const detail = await response.json();
     await route.fulfill({
@@ -678,7 +687,10 @@ test("stops the active Turn and returns queued text to the composer", async ({
   });
   await page.route(path, async (route) => {
     const response = await route.fetch({
-      headers: { ...route.request().headers(), "if-none-match": "" },
+      headers: {
+        ...route.request().headers(),
+        [CONVERSATION_DETAIL_VERSION_HEADER]: "",
+      },
     });
     const detail = await response.json();
     await route.fulfill({
@@ -795,7 +807,10 @@ test("edits a queued message back into the composer", async ({
   });
   await page.route(path, async (route) => {
     const response = await route.fetch({
-      headers: { ...route.request().headers(), "if-none-match": "" },
+      headers: {
+        ...route.request().headers(),
+        [CONVERSATION_DETAIL_VERSION_HEADER]: "",
+      },
     });
     const detail = await response.json();
     await route.fulfill({
diff --git a/packages/junior-dashboard/e2e/review-desk.spec.ts b/packages/junior-dashboard/e2e/review-desk.spec.ts
index d9141cbed2..bcd5d51d86 100644
--- a/packages/junior-dashboard/e2e/review-desk.spec.ts
+++ b/packages/junior-dashboard/e2e/review-desk.spec.ts
@@ -1,3 +1,4 @@
+import { CONVERSATION_DETAIL_VERSION_HEADER } from "@sentry/junior/api/schema";
 import type { Page } from "@playwright/test";
 import { screenshot } from "./screenshot";
 import { expect, test } from "./test";
@@ -279,7 +280,10 @@ test("review item changes show as rows, not as context for other items", async (
     `**/api/conversations/${encodeURIComponent(conversationId)}`,
     async (route) => {
       const response = await route.fetch({
-        headers: { ...route.request().headers(), "if-none-match": "" },
+        headers: {
+          ...route.request().headers(),
+          [CONVERSATION_DETAIL_VERSION_HEADER]: "",
+        },
       });
       const detail = await response.json();
       const askedAt = Date.parse(rollout.createdAt);
@@ -455,7 +459,10 @@ test("a message asks before it drops unsent picks", async ({
   // Show the tool call that added the item, so its card is in the transcript.
   await page.route(path, async (route) => {
     const response = await route.fetch({
-      headers: { ...route.request().headers(), "if-none-match": "" },
+      headers: {
+        ...route.request().headers(),
+        [CONVERSATION_DETAIL_VERSION_HEADER]: "",
+      },
     });
     const detail = await response.json();
     const last = detail.events.at(-1);
diff --git a/packages/junior-dashboard/src/app.ts b/packages/junior-dashboard/src/app.ts
index 7b76f6c677..42d1207fc1 100644
--- a/packages/junior-dashboard/src/app.ts
+++ b/packages/junior-dashboard/src/app.ts
@@ -537,7 +537,7 @@ export function createDashboardApp(
   };
 
   app.use("*", requireAuth);
-  // Core detail reads set a version ETag before they build the report. Mock
+  // Core detail reads set a version token before they build the report. Mock
   // reporting has no stored version, so it revalidates on content instead.
   if (options.mockConversations) {
     app.use("/api/conversations/:conversationId", revalidateConversation);
diff --git a/packages/junior-dashboard/src/client/conversations/queries.ts b/packages/junior-dashboard/src/client/conversations/queries.ts
index e44c9bbd36..e5de666444 100644
--- a/packages/junior-dashboard/src/client/conversations/queries.ts
+++ b/packages/junior-dashboard/src/client/conversations/queries.ts
@@ -16,6 +16,7 @@ import type {
 } from "@sentry/junior/api/schema";
 import {
   acceptedConversationMessageSchema,
+  CONVERSATION_DETAIL_VERSION_HEADER,
   webMessageId,
   cancelConversationPendingMessagesResponseSchema,
   conversationDetailReportSchema,
@@ -748,11 +749,17 @@ export async function readConversationData(
   const response = await readDashboardResponse(
     `/api/conversations/${encodeURIComponent(conversationId)}`,
     signal,
-    previous?.detailEtag,
+    previous?.detailEtag
+      ? { [CONVERSATION_DETAIL_VERSION_HEADER]: previous.detailEtag }
+      : undefined,
   );
   if (response.status === 304 && previous) return previous;
   const report = conversationDetailReportSchema.parse(await response.json());
-  return { ...report, detailEtag: response.headers.get("etag") ?? undefined };
+  return {
+    ...report,
+    detailEtag:
+      response.headers.get(CONVERSATION_DETAIL_VERSION_HEADER) ?? undefined,
+  };
 }
 
 /** Read accepted mailbox messages that have not reached durable history yet. */
diff --git a/packages/junior-dashboard/src/client/http.ts b/packages/junior-dashboard/src/client/http.ts
index 72c761cdaa..fb002a198b 100644
--- a/packages/junior-dashboard/src/client/http.ts
+++ b/packages/junior-dashboard/src/client/http.ts
@@ -174,21 +174,21 @@ export async function fetchDashboardJson<T>(
   return schema.parse(await response.json());
 }
 
-/** Read an authenticated response, allowing 304 only for an explicit validator. */
+/** Read an authenticated response, allowing 304 only for explicit validator headers. */
 export async function readDashboardResponse(
   path: string,
   signal?: AbortSignal,
-  etag?: string,
+  validator?: Record<string, string>,
 ): Promise<Response> {
   const response = await fetchDashboard(path, {
     credentials: "same-origin",
     ...(signal ? { signal } : undefined),
-    ...(etag
-      ? { cache: "no-store" as const, headers: { "if-none-match": etag } }
+    ...(validator
+      ? { cache: "no-store" as const, headers: validator }
       : undefined),
   });
   if (response.status === 401) restartDashboardSignIn();
-  if (!response.ok && !(etag && response.status === 304)) {
+  if (!response.ok && !(validator && response.status === 304)) {
     await throwDashboardApiError(path, response);
   }
   return response;
diff --git a/packages/junior-dashboard/src/conversation-cache.ts b/packages/junior-dashboard/src/conversation-cache.ts
index 8852f52160..96fba0b60e 100644
--- a/packages/junior-dashboard/src/conversation-cache.ts
+++ b/packages/junior-dashboard/src/conversation-cache.ts
@@ -1,4 +1,5 @@
 import { createHash } from "node:crypto";
+import { CONVERSATION_DETAIL_VERSION_HEADER } from "@sentry/junior/api/schema";
 import type { MiddlewareHandler } from "hono";
 
 /** Revalidate mock detail reads on content, after the route checks viewer access. */
@@ -15,15 +16,12 @@ export const revalidateConversation: MiddlewareHandler = async (
   // generatedAt is the read time, not a change to the Conversation. The mailbox
   // has its own live watermark and is never handled by this middleware.
   const { generatedAt: _generatedAt, ...content } = report;
-  const etag = `W/"${createHash("sha256").update(JSON.stringify(content)).digest("hex")}"`;
+  const version = createHash("sha256")
+    .update(JSON.stringify(content))
+    .digest("hex");
   context.header("cache-control", "private, no-store");
-  context.header("etag", etag);
-  const validators = context.req.header("if-none-match")?.split(",");
-  if (
-    validators?.some(
-      (value) => value.trim().replace(/^W\//, "") === etag.slice(2),
-    )
-  ) {
+  context.header(CONVERSATION_DETAIL_VERSION_HEADER, version);
+  if (context.req.header(CONVERSATION_DETAIL_VERSION_HEADER) === version) {
     context.res = new Response(null, {
       status: 304,
       headers: context.res.headers,
diff --git a/packages/junior-dashboard/tests/client-api.test.ts b/packages/junior-dashboard/tests/client-api.test.ts
index 7c6cc1f3f1..85430da41f 100644
--- a/packages/junior-dashboard/tests/client-api.test.ts
+++ b/packages/junior-dashboard/tests/client-api.test.ts
@@ -1,3 +1,4 @@
+import { CONVERSATION_DETAIL_VERSION_HEADER } from "@sentry/junior/api/schema";
 import { createHash } from "node:crypto";
 import { QueryClient } from "@tanstack/react-query";
 import { webMessageId } from "@sentry/junior/api/schema";
@@ -167,7 +168,9 @@ describe("dashboard client API", () => {
       signedIn = false;
       const denied = await app.fetch(
         new Request(`http://localhost${detailPath}`, {
-          headers: { "if-none-match": changed.detailEtag! },
+          headers: {
+            [CONVERSATION_DETAIL_VERSION_HEADER]: changed.detailEtag!,
+          },
         }),
       );
       expect(denied.status).toBe(401);
diff --git a/packages/junior/src/api/conversations/detail-etag.ts b/packages/junior/src/api/conversations/detail-etag.ts
index 2e1f08ab69..52de29a144 100644
--- a/packages/junior/src/api/conversations/detail-etag.ts
+++ b/packages/junior/src/api/conversations/detail-etag.ts
@@ -82,15 +82,5 @@ export async function readConversationDetailEtag(
     subtree,
     viewerMarks: viewerMarks ?? null,
   });
-  return `W/"${createHash("sha256").update(basis).digest("hex")}"`;
-}
-
-/** Report whether an `If-None-Match` header names the current ETag. */
-export function etagMatches(header: string | undefined, etag: string): boolean {
-  const opaque = etag.replace(/^W\//, "");
-  return Boolean(
-    header
-      ?.split(",")
-      .some((value) => value.trim().replace(/^W\//, "") === opaque),
-  );
+  return createHash("sha256").update(basis).digest("hex");
 }
diff --git a/packages/junior/src/api/conversations/routes.ts b/packages/junior/src/api/conversations/routes.ts
index f7cd2cf936..259eda70e1 100644
--- a/packages/junior/src/api/conversations/routes.ts
+++ b/packages/junior/src/api/conversations/routes.ts
@@ -12,6 +12,7 @@ import {
   steerConversationPendingMessagesResponseSchema,
   conversationAttachmentParamsSchema,
   conversationDetailQuerySchema,
+  CONVERSATION_DETAIL_VERSION_HEADER,
   conversationDetailReportSchema,
   conversationEventPageSchema,
   conversationEventsQuerySchema,
@@ -56,7 +57,7 @@ import {
   type WebInputServices,
 } from "./create";
 import { readConversationDetail } from "./detail";
-import { etagMatches, readConversationDetailEtag } from "./detail-etag";
+import { readConversationDetailEtag } from "./detail-etag";
 import { forkConversationForViewer, keepQuickQuestionForViewer } from "./fork";
 import { readConversationEvents } from "./event-list";
 import { readConversationFeed } from "./list";
@@ -494,17 +495,23 @@ export function createConversationRoutes(
       const { conversationId } = context.req.valid("param");
       const query = context.req.valid("query");
       const viewer = context.get("viewer");
-      // Polls send the last ETag. Compare it before the report is built,
-      // because building the report is the expensive part of this route.
+      // Polls send the last version token. Compare it before the report is
+      // built, because building the report is the expensive part of this route.
       const etag = await readConversationDetailEtag(conversationId, {
         limit: query.limit,
         ...(viewer ? { viewer } : undefined),
       });
-      if (etag && etagMatches(context.req.header("if-none-match"), etag)) {
-        return new Response(null, {
-          status: 304,
-          headers: { "cache-control": "private, no-store", etag },
-        });
+      const headers = etag
+        ? {
+            "cache-control": "private, no-store",
+            [CONVERSATION_DETAIL_VERSION_HEADER]: etag,
+          }
+        : undefined;
+      if (
+        headers &&
+        context.req.header(CONVERSATION_DETAIL_VERSION_HEADER) === etag
+      ) {
+        return new Response(null, { status: 304, headers });
       }
       const report = await readConversationDetail(conversationId, {
         ...query,
@@ -514,9 +521,7 @@ export function createConversationRoutes(
       return jsonResponse(
         conversationDetailReportSchema,
         report,
-        etag
-          ? { headers: { "cache-control": "private, no-store", etag } }
-          : undefined,
+        headers ? { headers } : undefined,
       );
     },
   );
diff --git a/packages/junior/src/api/schema.ts b/packages/junior/src/api/schema.ts
index 67e08dc110..cdd8695d48 100644
--- a/packages/junior/src/api/schema.ts
+++ b/packages/junior/src/api/schema.ts
@@ -55,6 +55,7 @@ export {
   steerConversationPendingMessagesResponseSchema,
   conversationAuxiliaryCostsSchema,
   conversationDetailQuerySchema,
+  CONVERSATION_DETAIL_VERSION_HEADER,
   conversationDetailReportSchema,
   conversationEventPageSchema,
   conversationEventHistorySchema,
diff --git a/packages/junior/src/api/schema/conversation.ts b/packages/junior/src/api/schema/conversation.ts
index 030e65d63d..497340eb7d 100644
--- a/packages/junior/src/api/schema/conversation.ts
+++ b/packages/junior/src/api/schema/conversation.ts
@@ -847,6 +847,13 @@ export type ConversationAgentTreeNode = z.infer<
   typeof conversationAgentTreeNodeSchema
 >;
 
+/**
+ * Carries the detail version token in both directions. A custom header, not
+ * `ETag` / `If-None-Match`: in production the Vercel CDN answered standard
+ * validators itself, after the function had already built the full report.
+ */
+export const CONVERSATION_DETAIL_VERSION_HEADER = "x-junior-detail-version";
+
 export const conversationDetailReportSchema = conversationSummaryReportSchema
   .extend({
     /**
diff --git a/packages/junior/tests/integration/api/conversations/archive.test.ts b/packages/junior/tests/integration/api/conversations/archive.test.ts
index 49894ef323..e1eb8ba9b1 100644
--- a/packages/junior/tests/integration/api/conversations/archive.test.ts
+++ b/packages/junior/tests/integration/api/conversations/archive.test.ts
@@ -1,3 +1,4 @@
+import { CONVERSATION_DETAIL_VERSION_HEADER } from "@/api/schema";
 import { Hono } from "hono";
 import { afterEach, describe, expect, it } from "vitest";
 import { defineJuniorPlugin } from "@sentry/junior-plugin-api";
@@ -346,7 +347,9 @@ describe("conversation archive API", () => {
 
     const { app } = await authenticatedApi(viewer.email);
     const detailUrl = `http://localhost/api/conversations/${optedOutId}`;
-    const beforeEtag = (await app.request(detailUrl)).headers.get("etag")!;
+    const beforeEtag = (await app.request(detailUrl)).headers.get(
+      CONVERSATION_DETAIL_VERSION_HEADER,
+    )!;
     const optOut = await app.request(
       `http://localhost/api/conversations/${optedOutId}/auto-archive`,
       {
@@ -363,7 +366,7 @@ describe("conversation archive API", () => {
     expect(
       (
         await app.request(detailUrl, {
-          headers: { "if-none-match": beforeEtag },
+          headers: { [CONVERSATION_DETAIL_VERSION_HEADER]: beforeEtag },
         })
       ).status,
     ).toBe(200);
diff --git a/packages/junior/tests/integration/api/conversations/detail.test.ts b/packages/junior/tests/integration/api/conversations/detail.test.ts
index e556451bfb..c3db9803fd 100644
--- a/packages/junior/tests/integration/api/conversations/detail.test.ts
+++ b/packages/junior/tests/integration/api/conversations/detail.test.ts
@@ -1,3 +1,4 @@
+import { CONVERSATION_DETAIL_VERSION_HEADER } from "@/api/schema";
 import { afterEach, describe, expect, it } from "vitest";
 import { avatarPath } from "../../../fixtures/avatar";
 import { testViewer } from "../../../fixtures/user";
@@ -134,13 +135,12 @@ describe("conversation detail API", () => {
     const app = createJuniorApi();
     const poll = (etag?: string) =>
       app.request(`http://localhost/api/conversations/${conversationId}`, {
-        headers: etag ? { "if-none-match": etag } : {},
+        headers: etag ? { [CONVERSATION_DETAIL_VERSION_HEADER]: etag } : {},
       });
 
     const first = await poll();
-    const firstEtag = first.headers.get("etag")!;
+    const firstEtag = first.headers.get(CONVERSATION_DETAIL_VERSION_HEADER)!;
     expect(first.status).toBe(200);
-    expect(firstEtag).toMatch(/^W\/"/);
     const unchanged = await poll(firstEtag);
     expect(unchanged.status).toBe(304);
     expect(await unchanged.text()).toBe("");
@@ -150,7 +150,9 @@ describe("conversation detail API", () => {
       parentConversationId: conversationId,
     });
     const withChild = await poll(firstEtag);
-    const childEtag = withChild.headers.get("etag")!;
+    const childEtag = withChild.headers.get(
+      CONVERSATION_DETAIL_VERSION_HEADER,
+    )!;
     expect(withChild.status).toBe(200);
     expect(childEtag).not.toBe(firstEtag);
 
@@ -167,7 +169,9 @@ describe("conversation detail API", () => {
       },
     ]);
     const childChanged = await poll(childEtag);
-    const childChangedEtag = childChanged.headers.get("etag")!;
+    const childChangedEtag = childChanged.headers.get(
+      CONVERSATION_DETAIL_VERSION_HEADER,
+    )!;
     expect(childChanged.status).toBe(200);
 
     await createPluginAnnotations({
diff --git a/packages/junior/tests/integration/api/conversations/pin.test.ts b/packages/junior/tests/integration/api/conversations/pin.test.ts
index 292b033f0b..960792eda6 100644
--- a/packages/junior/tests/integration/api/conversations/pin.test.ts
+++ b/packages/junior/tests/integration/api/conversations/pin.test.ts
@@ -1,3 +1,4 @@
+import { CONVERSATION_DETAIL_VERSION_HEADER } from "@/api/schema";
 import { Hono } from "hono";
 import { afterEach, describe, expect, it } from "vitest";
 import { createJuniorApi } from "@/api";
@@ -98,7 +99,9 @@ describe("conversation pin API", () => {
     }
 
     const detailPath = `http://localhost/api/conversations/${older}`;
-    const unpinnedEtag = (await app.request(detailPath)).headers.get("etag")!;
+    const unpinnedEtag = (await app.request(detailPath)).headers.get(
+      CONVERSATION_DETAIL_VERSION_HEADER,
+    )!;
 
     const pin = await setPin(app, older, true);
     expect(pin.status).toBe(200);
@@ -117,7 +120,7 @@ describe("conversation pin API", () => {
     ]);
     // A pin is viewer state, so an open detail page revalidates.
     const pinnedDetail = await app.request(detailPath, {
-      headers: { "if-none-match": unpinnedEtag },
+      headers: { [CONVERSATION_DETAIL_VERSION_HEADER]: unpinnedEtag },
     });
     expect(pinnedDetail.status).toBe(200);
     expect(