diff --git a/packages/junior/src/chat/providers/slack/resume.ts b/packages/junior/src/chat/providers/slack/resume.ts
index 927fa0d7a8..ab31531aba 100644
--- a/packages/junior/src/chat/providers/slack/resume.ts
+++ b/packages/junior/src/chat/providers/slack/resume.ts
@@ -95,10 +95,12 @@ async function postSlackMessageBestEffort(
   text: string,
   conversationId?: string,
   replyAttribution?: ReplyAttribution,
+  bodyFormat?: import("@/chat/slack/footer").SlackReplyBodyFormat,
 ): Promise<void> {
   try {
     if (conversationId) {
       await sendSlackReply({
+        bodyFormat,
         channelId,
         conversationId,
         replyAttribution,
@@ -904,6 +906,7 @@ async function resumeSlackTurnInContext(
           ),
           runArgs.conversationId,
           runArgs.run?.dispatch?.replyAttribution,
+          "mrkdwn",
         );
       }
       return true;
diff --git a/packages/junior/src/chat/providers/slack/turn.ts b/packages/junior/src/chat/providers/slack/turn.ts
index ea961dcccc..0c2115b92f 100644
--- a/packages/junior/src/chat/providers/slack/turn.ts
+++ b/packages/junior/src/chat/providers/slack/turn.ts
@@ -445,6 +445,7 @@ export function createSlackTurn(deps: SlackTurnDeps) {
             await beforeFirstResponsePost();
             if (channelId && threadTs) {
               await sendSlackReply({
+                bodyFormat: "mrkdwn",
                 channelId,
                 conversationId,
                 replyAttribution: options.execution?.dispatch?.replyAttribution,
diff --git a/packages/junior/src/chat/slack/footer.ts b/packages/junior/src/chat/slack/footer.ts
index 53709d2d3c..46af9eeef6 100644
--- a/packages/junior/src/chat/slack/footer.ts
+++ b/packages/junior/src/chat/slack/footer.ts
@@ -46,6 +46,22 @@ export interface SlackReplyFooter {
   items: SlackReplyFooterItem[];
 }
 
+/**
+ * Controls which Slack block type is used for the reply body.
+ *
+ * - `"commonmark"` (default): wraps the text in a `{ type: "markdown" }` block,
+ *   which Slack renders from standard Markdown (bold, links, tables, headers).
+ *   Use this for normal agent replies where `normalizeSlackReplyMarkdown` has
+ *   already formatted the output for CommonMark delivery.
+ *
+ * - `"mrkdwn"`: wraps the text in a `{ type: "section", text: { type: "mrkdwn" } }`
+ *   block. Use this when the text is already pre-formatted as Slack mrkdwn
+ *   (e.g. `<@user>` mentions, `<url>` angle-bracket links, single-`*` emphasis)
+ *   and must not be passed through the markdown-to-rich_text converter, which
+ *   can mis-tag mrkdwn tokens and trigger `invalid_blocks`.
+ */
+export type SlackReplyBodyFormat = "commonmark" | "mrkdwn";
+
 /** Render compact reply attribution for the Slack footer. */
 export function formatReplyAttribution(attribution: ReplyAttribution): string {
   return attribution.detail
@@ -90,21 +106,29 @@ export function buildSlackReplyFooter(args: {
     : undefined;
 }
 
-/** Build Slack blocks for a reply chunk using the Slack-flavored markdown block for the body. */
+/**
+ * Build Slack blocks for a reply chunk.
+ *
+ * The body block format is controlled by `bodyFormat`:
+ * - `"commonmark"` (default): uses `{ type: "markdown" }` for CommonMark rendering.
+ * - `"mrkdwn"`: uses `{ type: "section", text: { type: "mrkdwn" } }` for
+ *   pre-formatted mrkdwn text (e.g. auth-pause notices with `<@user>` mentions).
+ */
 export function buildSlackReplyBlocks(
   text: string,
   footer: SlackReplyFooter | undefined,
+  bodyFormat: SlackReplyBodyFormat = "commonmark",
 ): SlackMessageBlock[] | undefined {
   if (!text.trim()) {
     return undefined;
   }
 
-  const blocks: SlackMessageBlock[] = [
-    {
-      type: "markdown",
-      text,
-    },
-  ];
+  const bodyBlock: SlackMessageBlock =
+    bodyFormat === "mrkdwn"
+      ? { type: "section", text: { type: "mrkdwn", text } }
+      : { type: "markdown", text };
+
+  const blocks: SlackMessageBlock[] = [bodyBlock];
 
   if (footer && (footer.attribution || footer.items.length > 0)) {
     const attributionElements: SlackPlainTextObject[] = footer.attribution
diff --git a/packages/junior/src/chat/slack/reply.ts b/packages/junior/src/chat/slack/reply.ts
index eaad1b1767..5d5ca26d31 100644
--- a/packages/junior/src/chat/slack/reply.ts
+++ b/packages/junior/src/chat/slack/reply.ts
@@ -8,6 +8,7 @@ import {
   buildSlackReplyBlocks,
   buildSlackReplyFooter,
   formatReplyAttribution,
+  type SlackReplyBodyFormat,
 } from "@/chat/slack/footer";
 import { escapeSlackMrkdwnText } from "@/chat/slack/mrkdwn";
 import { postSlackMessage } from "@/chat/slack/outbound";
@@ -20,6 +21,7 @@ import { splitSlackReplyText } from "@/chat/slack/output";
  * context, and posts through the shared Slack outbound boundary.
  */
 export async function sendSlackReply(args: {
+  bodyFormat?: SlackReplyBodyFormat;
   channelId: string;
   conversationId: string;
   replyAttribution?: ReplyAttribution;
@@ -41,6 +43,7 @@ export async function sendSlackReply(args: {
     const blocks = buildSlackReplyBlocks(
       text,
       isFinalChunk ? footer : undefined,
+      args.bodyFormat,
     );
     const fallbackText =
       isFinalChunk && args.replyAttribution
diff --git a/packages/junior/tests/unit/slack/footer.test.ts b/packages/junior/tests/unit/slack/footer.test.ts
index 6657dd4137..731a2f900b 100644
--- a/packages/junior/tests/unit/slack/footer.test.ts
+++ b/packages/junior/tests/unit/slack/footer.test.ts
@@ -153,6 +153,17 @@ describe("buildSlackReplyBlocks", () => {
     ]);
   });
 
+  it("renders a mrkdwn section block for pre-formatted mrkdwn text (auth-pause path)", () => {
+    const text =
+      "<@U123> I need access to GitHub to continue.\n\n*Why:* check out <https://github.com/foo/bar> and proceed\n\nI sent you a link.";
+    expect(buildSlackReplyBlocks(text, undefined, "mrkdwn")).toEqual([
+      {
+        type: "section",
+        text: { type: "mrkdwn", text },
+      },
+    ]);
+  });
+
   it("does not emit blocks when the reply has no visible text", () => {
     const footer = buildSlackReplyFooter({
       conversationId: "slack:C123:1700000000.000100",