diff --git a/docs/cookbook/07-slack.md b/docs/cookbook/07-slack.md
index 6955754..bd6eb6b 100644
--- a/docs/cookbook/07-slack.md
+++ b/docs/cookbook/07-slack.md
@@ -33,6 +33,11 @@ Output is the CLI's piped-mode bytes in a code fence. stdout in the bot is a
 pipe, so by the CLI's own contract there is no colour to strip and the bytes
 match what `grapharc … | cat` prints on the host.
 
+A successful `viz` reply carries one extra line: a *render this diagram* link.
+The whole diagram is zlib-compressed into the URL fragment, which a browser
+never sends to any server — mermaid.live's JavaScript renders it locally, so
+following the link ships the diagram to no one.
+
 ## Slack app setup (once, ~5 minutes)
 
 1. <https://api.slack.com/apps> → **Create New App** → *From a manifest*, pick
diff --git a/grapharc/slack/format.py b/grapharc/slack/format.py
index 7290c93..37c47ad 100644
--- a/grapharc/slack/format.py
+++ b/grapharc/slack/format.py
@@ -13,7 +13,10 @@
 
 from __future__ import annotations
 
+import base64
+import json
 import shlex
+import zlib
 
 from grapharc.slack.runner import CommandResult
 
@@ -39,6 +42,19 @@ def _truncate(body: str) -> tuple[str, int]:
     return body[:MAX_FENCE_CHARS], len(body) - MAX_FENCE_CHARS
 
 
+def mermaid_live_url(code: str) -> str:
+    """A mermaid.live link with the whole diagram compressed into the fragment.
+
+    The editor's `#pako:` form: zlib-deflated JSON, base64url. Everything after
+    the `#` is a URL fragment, which a browser never sends to the server — the
+    site's JavaScript renders the diagram locally, so following the link ships
+    the diagram to no one.
+    """
+    payload = json.dumps({"code": code, "mermaid": {"theme": "default"}})
+    packed = base64.urlsafe_b64encode(zlib.compress(payload.encode())).decode()
+    return f"https://mermaid.live/view#pako:{packed}"
+
+
 def format_result(result: CommandResult) -> str:
     shown = shlex.join(["grapharc", *result.argv])
     if result.exit_code is None:
@@ -64,4 +80,8 @@ def format_result(result: CommandResult) -> str:
             parts.append(f"_…{cut} more characters not shown._")
     if len(parts) == 1:
         parts.append("_(no output)_")
+    # `viz` prints raw Mermaid, which Slack shows as text. One extra line makes
+    # it a diagram: a link that renders it in the browser, locally.
+    if result.argv[:1] == ["viz"] and result.exit_code == 0 and result.stdout.strip():
+        parts.append(f"<{mermaid_live_url(result.stdout.strip())}|render this diagram>")
     return "\n".join(parts)
diff --git a/tests/test_slack_gateway.py b/tests/test_slack_gateway.py
index 6fedb61..1fd41a6 100644
--- a/tests/test_slack_gateway.py
+++ b/tests/test_slack_gateway.py
@@ -15,7 +15,7 @@
 
 from grapharc.slack.command import SlackCommandError, parse_command, usage_text
 from grapharc.slack.config import SlackBotConfig, SlackConfigError
-from grapharc.slack.format import MAX_FENCE_CHARS, format_result
+from grapharc.slack.format import MAX_FENCE_CHARS, format_result, mermaid_live_url
 from grapharc.slack.runner import CommandResult, run_command
 
 # ---------------------------------------------------------------------------
@@ -147,6 +147,41 @@ def test_a_fence_in_the_output_cannot_break_out():
     assert "\n```\n" not in body.rsplit("```", 1)[0]
 
 
+def test_a_successful_viz_gets_a_render_link_and_the_url_round_trips():
+    import base64
+    import json
+    import zlib
+
+    mermaid = 'flowchart TD\n  start((start)) --> n0["triage"]'
+    result = CommandResult(
+        argv=["viz", "t.jsonl", "r1"],
+        exit_code=0,
+        stdout=mermaid + "\n",
+        stderr="",
+        duration_seconds=0.1,
+        timeout_seconds=60,
+    )
+    message = format_result(result)
+    assert "mermaid.live/view#pako:" in message
+
+    packed = mermaid_live_url(mermaid).split("#pako:", 1)[1]
+    decoded = json.loads(zlib.decompress(base64.urlsafe_b64decode(packed)))
+    assert decoded["code"] == mermaid
+
+
+def test_a_failed_or_non_viz_command_gets_no_render_link():
+    for argv, code in ((["viz", "t.jsonl", "r1"], 1), (["trace", "t.jsonl"], 0)):
+        result = CommandResult(
+            argv=argv,
+            exit_code=code,
+            stdout="flowchart TD",
+            stderr="",
+            duration_seconds=0.1,
+            timeout_seconds=60,
+        )
+        assert "mermaid.live" not in format_result(result)
+
+
 # ---------------------------------------------------------------------------
 # Config and the bot's import posture.
 # ---------------------------------------------------------------------------