diff --git a/packages/junior/src/chat/services/auth-pause-response.ts b/packages/junior/src/chat/services/auth-pause-response.ts
index 5243177ed1..c3468fcdec 100644
--- a/packages/junior/src/chat/services/auth-pause-response.ts
+++ b/packages/junior/src/chat/services/auth-pause-response.ts
@@ -26,5 +26,10 @@ export function buildAuthPauseResponse(
   if (!request) {
     return `${mention}I'll need you to authorize ${providerDisplayName}. I sent you a link.`;
   }
-  return `${mention}I need access to ${providerDisplayName} to continue.\n\n*Why:* ${request}\n\nI sent you a link.`;
+  // CommonMark bold (`**...**`), not mrkdwn bold (`*...*`) — this text is
+  // delivered through the same `markdown` block as every other Slack reply,
+  // which Slack renders from standard Markdown. A single-`*` emphasis marker
+  // is CommonMark italics, not bold, so `*Why:*` rendered wrong even before
+  // it contributed to the invalid_blocks failure in JUNIOR-72.
+  return `${mention}I need access to ${providerDisplayName} to continue.\n\n**Why:** ${request}\n\nI sent you a link.`;
 }
diff --git a/packages/junior/src/chat/slack/footer.ts b/packages/junior/src/chat/slack/footer.ts
index 53709d2d3c..ba41e1fdec 100644
--- a/packages/junior/src/chat/slack/footer.ts
+++ b/packages/junior/src/chat/slack/footer.ts
@@ -90,6 +90,17 @@ export function buildSlackReplyFooter(args: {
     : undefined;
 }
 
+/**
+ * Slack's `markdown` block renders standard Markdown and has no user-mention
+ * syntax (see docs.slack.dev/reference/block-kit/blocks/markdown-block). A
+ * literal `<@id>` mention token — valid only in `mrkdwn` text — left inside a
+ * `markdown` block makes Slack's markdown-to-rich_text conversion mis-tag an
+ * inline element and reject the whole message with `invalid_blocks`
+ * (JUNIOR-72). Replies that open with a mention (for example auth-pause
+ * notices) split that mention into its own `mrkdwn` context block instead.
+ */
+const LEADING_SLACK_MENTION_RE = /^<@([UW][A-Z0-9]+)>[ \t]*/;
+
 /** Build Slack blocks for a reply chunk using the Slack-flavored markdown block for the body. */
 export function buildSlackReplyBlocks(
   text: string,
@@ -99,12 +110,26 @@ export function buildSlackReplyBlocks(
     return undefined;
   }
 
-  const blocks: SlackMessageBlock[] = [
-    {
-      type: "markdown",
-      text,
-    },
-  ];
+  const mentionMatch = LEADING_SLACK_MENTION_RE.exec(text);
+  const body = mentionMatch ? text.slice(mentionMatch[0].length) : text;
+
+  const blocks: SlackMessageBlock[] = [];
+  if (mentionMatch) {
+    blocks.push({
+      type: "context",
+      elements: [{ type: "mrkdwn", text: mentionMatch[0].trimEnd() }],
+    });
+  }
+  blocks.push({
+    type: "markdown",
+    text: body,
+  });
+
+  if (mentionMatch && !body.trim()) {
+    // A mention-only reply has nothing left for the markdown block. Drop it
+    // rather than post a Slack-rejected empty `markdown` block.
+    blocks.pop();
+  }
 
   if (footer && (footer.attribution || footer.items.length > 0)) {
     const attributionElements: SlackPlainTextObject[] = footer.attribution
diff --git a/packages/junior/tests/component/auth/mcp-auth-runtime-slack.test.ts b/packages/junior/tests/component/auth/mcp-auth-runtime-slack.test.ts
index 6eb3425995..544346f185 100644
--- a/packages/junior/tests/component/auth/mcp-auth-runtime-slack.test.ts
+++ b/packages/junior/tests/component/auth/mcp-auth-runtime-slack.test.ts
@@ -506,7 +506,7 @@ describe("mcp auth runtime slack integration", () => {
         params: expect.objectContaining({
           channel: "C123",
           thread_ts: "1700000000.001",
-          text: "<@U123> I need access to Eval Auth to continue.\n\n*Why:* what did i say about the budget?\n\nI sent you a link.",
+          text: "<@U123> I need access to Eval Auth to continue.\n\n**Why:** what did i say about the budget?\n\nI sent you a link.",
         }),
       }),
     ]);
@@ -666,7 +666,7 @@ describe("mcp auth runtime slack integration", () => {
         params: expect.objectContaining({
           channel: "C123",
           thread_ts: "1700000000.001",
-          text: "<@U123> I need access to Eval Auth to continue.\n\n*Why:* what did i say about the budget?\n\nI sent you a link.",
+          text: "<@U123> I need access to Eval Auth to continue.\n\n**Why:** what did i say about the budget?\n\nI sent you a link.",
         }),
       }),
       expect.objectContaining({
@@ -764,7 +764,7 @@ describe("mcp auth runtime slack integration", () => {
         params: expect.objectContaining({
           channel: "C124",
           thread_ts: "1700000000.002",
-          text: "<@U123> I need access to Eval Auth to continue.\n\n*Why:* what did i say about the budget?\n\nI sent you a link.",
+          text: "<@U123> I need access to Eval Auth to continue.\n\n**Why:** what did i say about the budget?\n\nI sent you a link.",
         }),
       }),
     ]);
@@ -1017,7 +1017,7 @@ describe("mcp auth runtime slack integration", () => {
         params: expect.objectContaining({
           channel: "C125",
           thread_ts: "1700000000.003",
-          text: "<@U123> I need access to Eval Auth to continue.\n\n*Why:* use eval-auth directly for the budget answer\n\nI sent you a link.",
+          text: "<@U123> I need access to Eval Auth to continue.\n\n**Why:** use eval-auth directly for the budget answer\n\nI sent you a link.",
         }),
       }),
       expect.objectContaining({
diff --git a/packages/junior/tests/unit/services/auth-pause-response.test.ts b/packages/junior/tests/unit/services/auth-pause-response.test.ts
index 4c4694c98f..3309082678 100644
--- a/packages/junior/tests/unit/services/auth-pause-response.test.ts
+++ b/packages/junior/tests/unit/services/auth-pause-response.test.ts
@@ -10,7 +10,7 @@ describe("buildAuthPauseResponse", () => {
         "  Update <roadmap> & notify the team  ",
       ),
     ).toBe(
-      "<@U123> I need access to GitHub to continue.\n\n*Why:* Update &lt;roadmap&gt; &amp; notify the team\n\nI sent you a link.",
+      "<@U123> I need access to GitHub to continue.\n\n**Why:** Update &lt;roadmap&gt; &amp; notify the team\n\nI sent you a link.",
     );
   });
 
diff --git a/packages/junior/tests/unit/slack/footer.test.ts b/packages/junior/tests/unit/slack/footer.test.ts
index 6657dd4137..404bc4c02d 100644
--- a/packages/junior/tests/unit/slack/footer.test.ts
+++ b/packages/junior/tests/unit/slack/footer.test.ts
@@ -160,6 +160,55 @@ describe("buildSlackReplyBlocks", () => {
 
     expect(buildSlackReplyBlocks("   ", footer)).toBeUndefined();
   });
+
+  it("renders auth-pause-style text (leading mention, CommonMark bold, embedded URL) as a leading mention context block plus a markdown body", async () => {
+    // Regression test for JUNIOR-72: Slack's `markdown` block has no
+    // user-mention syntax (docs.slack.dev/reference/block-kit/blocks/markdown-block).
+    // A literal `<@id>` mention left inside that block made Slack's
+    // markdown-to-rich_text conversion reject the auth-pause notice as
+    // invalid_blocks. buildAuthPauseResponse's leading mention must be split
+    // into its own `mrkdwn` context block instead.
+    const { buildAuthPauseResponse } = await import(
+      "@/chat/services/auth-pause-response"
+    );
+    const text = buildAuthPauseResponse(
+      "U123",
+      "GitHub",
+      "check out https://github.com/foo/bar and proceed",
+    );
+
+    expect(text).toBe(
+      "<@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)).toEqual([
+      {
+        type: "context",
+        elements: [{ type: "mrkdwn", text: "<@U123>" }],
+      },
+      {
+        type: "markdown",
+        text: "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.",
+      },
+    ]);
+  });
+
+  it("drops the markdown block for a mention-only reply", () => {
+    expect(buildSlackReplyBlocks("<@U123> ", undefined)).toEqual([
+      {
+        type: "context",
+        elements: [{ type: "mrkdwn", text: "<@U123>" }],
+      },
+    ]);
+  });
+
+  it("does not treat a mention elsewhere in the text as a leading mention", () => {
+    expect(buildSlackReplyBlocks("cc <@U123> please review", undefined)).toEqual([
+      {
+        type: "markdown",
+        text: "cc <@U123> please review",
+      },
+    ]);
+  });
 });
 
 describe("getDashboardTaskLink", () => {