diff --git a/.claude/memory/MEMORY.md b/.claude/memory/MEMORY.md
index 17e87ea..115b917 100644
--- a/.claude/memory/MEMORY.md
+++ b/.claude/memory/MEMORY.md
@@ -7,6 +7,7 @@ Keep this file under 200 lines — anything longer is content bloat, not memory.
 - [learnings/sentry-mcp-no-comment-tool](learnings/2026-05-26-sentry-mcp-no-comment-tool.md) — Sentry MCP can't comment on issues; back-link from Linear only
 - [learnings/vercel-blocks-unknown-author-email](learnings/2026-05-26-vercel-blocks-unknown-author-email.md) — Vercel preview deploys block when commit author email has no GitHub account; use the noreply alias
 - [learnings/sandbox-cant-clone-private-repo](learnings/2026-05-26-sandbox-cant-clone-private-repo.md) — Don't `git clone` from sandbox bash; the `github_repository` resource is auth'd, raw `git clone` is not
+- [learnings/github-mcp-strips-html-comments](learnings/2026-05-26-github-mcp-strips-html-comments.md) — `update_pull_request` silently strips `<!-- ... -->` from PR bodies; session-id marker can't be set by agent (ENG-25)
 
 ## Decisions
 - [decisions/mcp-for-small-writes-checkout-for-big](decisions/2026-05-26-mcp-for-small-writes-checkout-for-big.md) — Single-file writes go through GitHub MCP; multi-file or test-needing changes use the mounted checkout + `git push`
diff --git a/.claude/memory/learnings/2026-05-26-github-mcp-strips-html-comments.md b/.claude/memory/learnings/2026-05-26-github-mcp-strips-html-comments.md
new file mode 100644
index 0000000..5e8fde0
--- /dev/null
+++ b/.claude/memory/learnings/2026-05-26-github-mcp-strips-html-comments.md
@@ -0,0 +1 @@
+The GitHub MCP `update_pull_request` tool unconditionally strips `<!-- ... -->` HTML comments from the `body` field before persisting — the PATCH fires (other prose changes land, `updated_at` advances), but any HTML comment line silently disappears. Confirmed twice on PR #12 review-feedback round 2 with both `sesn_...` and `sthr_...` prefixes; the filter isn't gated on content. This makes the `pr-session-id-marker` convention unenforceable by the agent via the standard transport — there's no `GITHUB_TOKEN` in the sandbox env and `api.github.com` is outside the egress allowlist, so you can't route around MCP with a direct PATCH. Don't waste review rounds on it — see ENG-25 for the fix proposal and the open question of whether `create_pull_request` shares the same filter (worth opportunistic testing whenever you open a new PR).