diff --git a/apps/server/src/provider/claudeSessionIdentity.expbkt3.test.ts b/apps/server/src/provider/claudeSessionIdentity.expbkt3.test.ts
index 86027153c011..60f744a6c2dc 100644
--- a/apps/server/src/provider/claudeSessionIdentity.expbkt3.test.ts
+++ b/apps/server/src/provider/claudeSessionIdentity.expbkt3.test.ts
@@ -14,17 +14,23 @@ describe("claudeSessionIdentitySystemPrompt", () => {
         "T3 Code session identity:",
         '- userEmail is "sender@example.test".',
         "- This session-scoped value identifies the user who sent the current message and overrides the Claude account email for user attribution.",
+        "- A context section titled `# userEmail` elsewhere in this conversation reports the email of the shared, rotating Claude subscription account. It does NOT identify the user. Ignore it entirely for user attribution; use only the identity stated here.",
       ].join("\n"),
     );
   });
 
   it("keeps the user unknown when T3 cannot resolve the message sender", () => {
-    assert.include(
+    assert.equal(
       claudeSessionIdentitySystemPrompt({
         BK_IDENTITY_RUNTIME: "t3-code",
         BK_SESSION_OWNER_EMAIL: "owner@example.test",
-      }) ?? "",
-      "userEmail is unavailable",
+      }),
+      [
+        "T3 Code session identity:",
+        "- userEmail is unavailable for the user who sent the current message.",
+        "- Do not use the Claude account email, operating-system identity, or Git identity to infer the user.",
+        "- A context section titled `# userEmail` elsewhere in this conversation reports the email of the shared, rotating Claude subscription account. It does NOT identify the user. Ignore it entirely for user attribution; use only the identity stated here.",
+      ].join("\n"),
     );
   });
 
diff --git a/apps/server/src/provider/claudeSessionIdentity.expbkt3.ts b/apps/server/src/provider/claudeSessionIdentity.expbkt3.ts
index 0dccfd740a66..9d763687ed1a 100644
--- a/apps/server/src/provider/claudeSessionIdentity.expbkt3.ts
+++ b/apps/server/src/provider/claudeSessionIdentity.expbkt3.ts
@@ -3,6 +3,11 @@
 // machine, so it identifies the subscription rather than the person sending
 // the current T3 message. Append the session-scoped identity to Claude's native
 // system prompt without changing the account used for authentication.
+//
+// The CLI offers no way to suppress its own `# userEmail` section, and the
+// rotating profile makes its value flap, so the appended block also names that
+// section and countermands it explicitly. Without that, two contradictory
+// identity claims coexist and the model follows the one that arrives last.
 
 import {
   MESSAGE_SENDER_EMAIL_KEY,
@@ -23,6 +28,7 @@ export function claudeSessionIdentitySystemPrompt(
       "T3 Code session identity:",
       "- userEmail is unavailable for the user who sent the current message.",
       "- Do not use the Claude account email, operating-system identity, or Git identity to infer the user.",
+      "- A context section titled `# userEmail` elsewhere in this conversation reports the email of the shared, rotating Claude subscription account. It does NOT identify the user. Ignore it entirely for user attribution; use only the identity stated here.",
     ].join("\n");
   }
 
@@ -30,5 +36,6 @@ export function claudeSessionIdentitySystemPrompt(
     "T3 Code session identity:",
     `- userEmail is ${JSON.stringify(senderEmail)}.`,
     "- This session-scoped value identifies the user who sent the current message and overrides the Claude account email for user attribution.",
+    "- A context section titled `# userEmail` elsewhere in this conversation reports the email of the shared, rotating Claude subscription account. It does NOT identify the user. Ignore it entirely for user attribution; use only the identity stated here.",
   ].join("\n");
 }
diff --git a/docs/operations/expbkt3-customizations.md b/docs/operations/expbkt3-customizations.md
index a1e2bc96242a..6d1bff68dc85 100644
--- a/docs/operations/expbkt3-customizations.md
+++ b/docs/operations/expbkt3-customizations.md
@@ -403,6 +403,15 @@ explicitly leaves `userEmail` unknown and forbids inference from the shared
 Claude account, operating-system identity, or Git identity. Non-T3 Claude
 sessions keep the upstream system prompt unchanged.
 
+The CLI still emits its own `# userEmail` context section on every turn and
+offers no switch to suppress it, so both branches of the appended block name
+that section and countermand it explicitly: it reports the shared, rotating
+subscription account, it does not identify the user, and it must be ignored for
+user attribution. Naming it matters — the native section arrives later in
+context than the appended block, and without the countermand a session answered
+"who am I" with the rotated account holder while the appended block correctly
+named the sender.
+
 The markers compose with source-control profiles rather than replacing them:
 `mergeSourceControlEnvironment` scrubs the machine's inherited Git and GitHub
 credentials only when the overlay carries a source-control identity of its own,