diff --git a/sessions/0020/SESSION-0020.0-TRANSCRIPT-2026-06-11T08-19--INPROGRESS.md b/sessions/0020/SESSION-0020.0-TRANSCRIPT-2026-06-11T08-19--INPROGRESS.md
new file mode 100644
index 0000000..d4af033
--- /dev/null
+++ b/sessions/0020/SESSION-0020.0-TRANSCRIPT-2026-06-11T08-19--INPROGRESS.md
@@ -0,0 +1,23 @@
+# Session 0020.0 — Transcript
+
+> App: vscode-cowriting-plugin
+> Start: 2026-06-11T08-19 (PST)
+> Type: brainstorming
+> Status: **PLACEHOLDER — claimed at session start; finalized at session end.**
+>
+> This file reserves session ID 0020 for vscode-cowriting-plugin. The driver replaces this
+> body with the full transcript and renames the file to its final
+> SESSION-0020.0-TRANSCRIPT-2026-06-11T08-19--<end>.md form at session end.
+
+## Launch prompt
+
+```
+Brainstorming: explore F7 (rendered track-changes markdown preview, baseline-aware, full mermaid) and write its Solution Design. Anchor Feature #21; deferred mermaid intra-diagram diffing #22. Builds on F6 baseline (#17/#19). Locked: custom webview; markdown-it + mermaid + jsdiff deps; block-level diff + atomic mermaid whole-diagram changed badge.
+
+```
+
+## Deferred decisions
+
+_Autonomous-mode low-confidence calls the driver made and would have
+liked operator input on. Appended as the session runs; surfaced at
+finalize. Empty if none._
diff --git a/sessions/sessions.json b/sessions/sessions.json
index 0b38e5e..3c3a163 100644
--- a/sessions/sessions.json
+++ b/sessions/sessions.json
@@ -55,5 +55,8 @@
   },
   "0019": {
     "title": ""
+  },
+  "0020": {
+    "title": ""
   }
 }