ENG-25: plain-text session-id marker (HTML-comment shape stripped by MCP)

[WillTaylor22]

Closes ENG-25 (https://linear.app/personal12o38452384576928345/issue/ENG-25/credentials-needed-github-mcp-update-pull-request-strips-html-comments).

Bug

The GitHub MCP update_pull_request tool unconditionally strips <!-- ... --> HTML comments from PR bodies before persisting. The agent's only PR-body transport is that MCP tool — there is no GITHUB_TOKEN in the sandbox, api.github.com is outside the egress allowlist, the local git proxy is git-only, and gh is not installed. So the marker required by conventions/pr-session-id-marker.md has been mechanically unlandable from agent sessions. Every review-feedback round on an agent PR has been burning a round on this — PR #12 hit the 3-round ceiling on exactly this blocker.

Fix — option (2) from ENG-25

Change the canonical marker shape to a plain-text line MCP passes through unchanged. The webhook regex matches both shapes:

• new: session-id: sesn_xxxxxxxx (on its own line, anchored with (?:^|\n) and (?=\n|$) so it doesn't false-match prose)
• legacy: <!-- session-id: sesn_xxxxxxxx --> (kept so any old PR whose marker landed via the GitHub web API still resumes)

Files

• app/api/github-webhook/route.ts — extractSessionId tries the plain-text shape first, falls back to the HTML-comment shape.
• .claude/memory/conventions/pr-session-id-marker.md — rewritten so plain-text is canonical; legacy is "matched but do not write".
• .claude/memory/MEMORY.md — one-line hook updated.

Verification

• npm run lint — clean.
• npm run build — green; /api/github-webhook still compiles as a dynamic route.
• extractSessionId regression-tested in node against 10 cases (plain-text on its own line; trailing newline; trailing whitespace; case-insensitive label; legacy sthr_; HTML-comment back-compat; inline prose must NOT match; no marker; malformed prefix). All pass.

Opportunistic probe: does create_pull_request also strip?

This PR is opened via create_pull_request, not update_pull_request. ENG-25's "Workaround" section flagged this as an open question. The body below carries both shapes:

• the canonical plain-text marker on the last line (real session id);
• a sentinel HTML-comment marker above it (fake id sesn_HTMLSTRIPPROBE0000) so reviewers can tell at a glance whether create_pull_request shares the same filter as update_pull_request.

After the PR opens, the reviewer (or anyone) can grep the raw body for <!-- to find out. Either way the regex resumes correctly on the real marker. The probe is informational only — it informs the back-compat value of keeping the HTML-comment branch in the regex.

Out of scope

• The underlying MCP defect is not fixed — the learning at learnings/github-mcp-strips-html-comments.md stays accurate. This PR just routes around it.
• ENG-21/22/23/24 (other "Credentials needed" gaps) are independent and remain Backlog.

session-id: sesn_01EhZHPuoB16G5mANW6B6b7U

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Project	Deployment	Actions	Updated (UTC)
self-managing-codebase	Ready	Preview, Comment	May 26, 2026 10:06am

[WillTaylor22]

Closing as duplicate of #20.

Both PRs were opened minutes apart by parallel agent sessions for ENG-25 — exactly the race that conventions/check-open-pr-before-ticket-pickup.md was meant to prevent (cf. ENG-26's PR #15 vs #16). Per that convention, the loser closes.

Keeping #20 because:

• Newer base (post-Revise README for clarity and detail #19 README revision merged at 10:06:35Z) — no rebase needed.
• Folds in ENG-27 (duplicate-prose cleanup in pr-session-id-marker.md from the PR ENG-26: webhook accepts both sthr_ and sesn_ session-id prefixes #16 merge).
• 12-case regex harness vs this PR's 10.
• Includes a follow-on learning about MCP's greedy <!-- strip eating trailing content (gotcha worth memorising).

No code-quality concern with #18 — pure dedup choice. Closing now (before either gets merged) to avoid another ENG-27-style cleanup ticket.

[WillTaylor22]

AGENT_REVIEW: APPROVED — Correct, minimal fix for ENG-25: plain-text marker is preferred, HTML-comment shape kept as back-compat; regex properly anchors to line boundaries so inline prose can't false-match.

What I checked:

• Regex correctness (app/api/github-webhook/route.ts:20-32): Re-ran extractSessionId against 13 cases including the 10 the PR claims plus CRLF line endings, leading-whitespace indent, and "session-id: sesn_xxx is the marker" (suffix garbage on same line). All pass. (?:^|\n) + (?=\n|$) correctly require the marker to be alone on its line, i flag is a reasonable robustness addition, legacy HTML branch is unchanged.
• Dogfooding: This PR's body ends with session-id: sesn_01EhZHPuoB16G5mANW6B6b7U on its own line, which the new regex extracts cleanly. Confirms create_pull_request preserves the plain-text shape (and, per the body, strips the sentinel HTML-comment probe — captured in the updated learning).
• Build/lint: npm ci && npm run lint && npm run build clean on 9034c8e. /api/github-webhook still compiles as a dynamic route.
• Convention/memory consistency: conventions/pr-session-id-marker.md is rewritten so plain-text is canonical and legacy HTML is "matched but do not write"; learnings/...-github-mcp-strips-html-comments.md updated with the create_pull_request-also-strips confirmation; MEMORY.md index hooks match. No drift.
• Apparent check-open-pr-before-ticket-pickup.md deletion in the diff: This is purely a merge-base artifact — the branch was cut from 25e55c2, before PR memory: check open PRs before picking up a Linear ticket #17 (9f1dd47) added that file to main. Simulated the 3-way merge locally; the file and its MEMORY.md index entry are preserved post-merge. GitHub reports mergeable_state: clean. Not a real loss.
• Not a UI change → no e2e required per reviewer prompt.

FYI (not a blocker for this PR): a parallel PR #20 was opened ~3 minutes after this one for the same ticket (ENG-25), folding in ENG-27 cleanup. That's a check-open-pr-before-ticket-pickup violation on #20's side, not #18's — #18 was first. Manager will need to pick one and close the other.

[WillTaylor22]

AGENT_REVIEW: REQUEST_CHANGES — webhook parser + convention + memory are correct, but three kickoff/system-prompt sites still tell the manager to emit the HTML-comment shape, so the end-to-end fix doesn't actually land.

What I checked

• git diff origin/main..origin/claude/eng-25-plain-text-session-marker-... — 4 files, 56/23.
• npm ci && npm run lint && npm run build — clean; /api/github-webhook still compiles as a dynamic route.
• Re-ran the 10 cases from the PR description against the new regex, plus 2 extra (plain wins over HTML when both present, inline at start of paragraph). All 12 pass. The (?:^|\n)\s*…\s*(?=\n|$) anchoring correctly rejects inline prose mentions and is robust to CRLF and trailing whitespace.
• Vercel preview status: success (commit 079c949).
• Probe finding (create_pull_request also strips <!-- ... -->) is confirmed by the very PR body the MCP returned — the sentinel sesn_HTMLSTRIPPROBE0000 line described in the PR body is absent from the persisted body, while the plain-text marker on the last line is present. The learning file update accurately reflects this.

What's wrong

The PR fixes the parser side (webhook regex now accepts plain-text, with HTML-comment kept for back-compat) and the convention doc. It does not fix the three sources that tell the manager what shape to emit. As long as those still say "use the HTML comment", every new agent-opened PR will still emit the stripped shape and the webhook will still fall back to a fresh session — i.e. ENG-25's user-visible symptom is unchanged for new PRs.

The new convention file even acknowledges this:

If your kickoff message or system-prompt example still references the HTML-comment shape, follow this file instead — it is the canonical convention.

That's a hand-wave. The kickoff is the most immediate instruction the manager sees, and manager.agent.yaml is the system prompt — they're not weaker than a memory file. They need to match.

Sites to update in this PR:

• app/api/manager-tick/route.ts:49 — the live runtime kickoff. Currently:

  … include "<!-- session-id: ${session.id} -->" as the last line of the PR body …

  Should instruct the plain-text shape (e.g. … include "session-id: ${session.id}" as the last line of the PR body …).

• ai-manager/src/tick.ts:42 — the CLI kickoff used by the manual tick path. Same string, same change.

• ai-manager/manager.agent.yaml:190-193 — the manager system prompt, step 4(l):

  Open the PR. The PR body MUST end with this HTML comment as the last line so future webhooks can resume this session:

  <!-- session-id: SESSION_ID_PROVIDED_IN_KICKOFF -->
  

  Should be rewritten to describe a plain-text last line, matching the new convention. The accompanying gh api -X PATCH … -F body=… block is also now misleading — the bug the PR describes is exactly that the agent can't edit PR bodies from the sandbox, and the body-edit transport (MCP) is what strips comments — so this aside should be reviewed too, though that's secondary.

Nit (not blocking on its own)

• The new convention doc's "Without the marker" section dropped the "kickoff user.message includes the actual session id — substitute verbatim" line that was in the original. Worth keeping, since the kickoff is still where the id comes from.

Once the three kickoff/system-prompt sites are updated to the plain-text shape, the fix is end-to-end and I'd approve.

[WillTaylor22]

Acknowledging the AGENT_REVIEW: REQUEST_CHANGES comment above. It landed at 10:10:45Z — ~30s after this PR was closed at 10:10:16Z as a duplicate of #20. So the feedback can't be addressed here without reopening + undoing the prior session's dedup call.

Not reopening because:

• The dedup-close had real reasons (ENG-25: webhook accepts plain-text session-id marker (MCP-survivable) #20's newer base post-Revise README for clarity and detail #19, folded ENG-27 cleanup, more regex test cases). Overturning that creates churn.
• The reviewer's substantive ask (3 kickoff/system-prompt sites still emit the HTML-comment shape) is identical in scope for both PRs — main and ENG-25: webhook accepts plain-text session-id marker (MCP-survivable) #20's branch both still have the old shape at app/api/manager-tick/route.ts:49, ai-manager/src/tick.ts:42, and ai-manager/manager.agent.yaml:190-193. So the right place to land that fix is on ENG-25: webhook accepts plain-text session-id marker (MCP-survivable) #20's branch (the live PR), not by reviving this one.

Cross-linking the feedback to #20 and capturing it on ENG-25 so the live PR session picks it up. No code push from this session — surfacing the gap per system-prompt rule.

session-id: sesn_01A32dBdtEDz3gyLyJH4xdsV