diff --git a/skill-evals/cases/create-message-diagram.yml b/skill-evals/cases/create-message-diagram.yml
new file mode 100644
index 000000000..be61a7d69
--- /dev/null
+++ b/skill-evals/cases/create-message-diagram.yml
@@ -0,0 +1,11 @@
+task: Post a message titled "Release flow" in project 55555 with a diagram showing that a release goes from merge to CI to staging to production
+tags: [domain, create]
+accept:
+  - 'message(s create)? .*Release flow'
+  - '--(in|project) 55555'
+  - '```mermaid'
+  - '(?mi)merge.*?ci.*?staging.*?production'
+reject:
+  - '--subject'
+  - '--body'
+  - '--content'
diff --git a/skills/basecamp/SKILL.md b/skills/basecamp/SKILL.md
index 8905c85e7..2ccf0d1bb 100644
--- a/skills/basecamp/SKILL.md
+++ b/skills/basecamp/SKILL.md
@@ -207,6 +207,24 @@ A pipe is never consumed implicitly. Only one input may read stdin in an
 invocation. Do not use shell-specific ANSI-C quoting such as `$'line 1\nline 2'`
 in portable commands.
 
+Rich-text fields support [Mermaid](https://mermaid.js.org) diagrams. Put the
+source in a fenced code block with the language `mermaid`, and Basecamp shows
+the diagram to people reading the post. Previews, emails, and exports show the
+source, and so does a post whose source does not parse. Use a diagram for a
+flow, sequence, timeline, or structure that is easier to follow as a picture
+than as prose. Diagram source spans several lines, so pass it on stdin:
+
+````bash
+basecamp messages create "Release flow" - --in "<project>" --json <<'EOF'
+How a release reaches production:
+
+```mermaid
+flowchart LR
+  Merge --> CI --> Staging --> Production
+```
+EOF
+````
+
 For deterministic mentions, prefer the machine-provided mention syntax from
 people or comment results, for example `[@Name](mention:SGID)`. Mention support
 varies by field; leaf help is authoritative.