ENG-25: webhook accepts plain-text session-id marker (MCP-survivable)

[WillTaylor22]

Fixes ENG-25. Folds in ENG-27 (duplicate-prose cleanup in the same file).

Problem

The pr-session-id-marker convention required PR bodies to end with the HTML-comment shape. GitHub MCP's create_pull_request AND update_pull_request both strip HTML comments from the body field before persisting — and worse, the strip is greedy: an unclosed comment opener inside backticks in prose causes the MCP to drop everything from that point to the end of the body. This PR's first body version hit exactly that, eating its own trailing session-id marker. Confirmed across:

• PR memory: pw-capture-script-needs-executable-path #12 review-feedback round 2 (update_pull_request, both sesn_ and sthr_ prefixes — comment line silently disappeared despite updated_at advancing).
• PR memory: check open PRs before picking up a Linear ticket #17 initial open (create_pull_request, sesn_ — reviewer-agent flagged the marker as absent in that session).
• This PR (creation step — body got truncated mid-paragraph at an HTML-comment opener token used in prose).

So the convention was unenforceable by the agent. Every agent-opened PR fell back to a fresh session on review/approval webhooks, losing design context. ENG-25 documented three fix options.

Fix (option 2 — change the marker shape)

The canonical shape is now a plain-text line on its own:

session-id: sesn_xxxxxxxxxxxxxxxx

MCP doesn't filter that. The webhook regex still accepts the legacy HTML-comment shape for back-compat with already-merged PRs.

Changes

• app/api/github-webhook/route.ts:18-26 — extractSessionId tries the HTML-comment shape first (back-compat), falls back to plain-text on its own line: /^\s*session-id:\s*((?:sthr_|sesn_)[A-Za-z0-9]+)\s*$/m. The m flag anchors to line boundaries so PR-body prose mentioning session-id: inline doesn't false-match.
• .claude/memory/conventions/pr-session-id-marker.md — rewritten to specify the plain-text shape, document the HTML-comment legacy as read-only-accepted, and drop the duplicate trailing paragraph that PR ENG-26: webhook accepts both sthr_ and sesn_ session-id prefixes #16 merged into the file (ENG-27).
• .claude/memory/learnings/2026-05-26-github-mcp-strips-html-comments.md — updated to note create_pull_request strips too (the PR memory: check open PRs before picking up a Linear ticket #17 confirmation) and that the convention now sidesteps the bug. Also captures the new finding from this PR: the strip is greedy from an unclosed opener to end-of-body.
• .claude/memory/MEMORY.md — index hooks updated.

Verification

• 12-case regex unit harness — plain shape, HTML shape, both-shapes precedence (HTML wins for legacy PRs), leading/trailing whitespace, inline-prose guard, bad prefixes, empty/null body. All 12 pass.
• npm run lint — clean.
• npm run build — green; /api/github-webhook still compiles as a dynamic route.
• End-to-end test (this PR): this body uses the new plain-text marker as the trailer AND avoids all literal HTML-comment opener tokens in prose (referring to them by phrase only). If MCP preserves the line and the webhook resumes on AGENT_REVIEW: APPROVED, the fix works in production. Reviewer-agent webhook hitting extractSessionId on this body should succeed and resume sesn_012j21sUvdmnhx3baX6ivYLW.

Out of scope

• The underlying MCP body-filter bug is still upstream. We sidestep it; we don't fix it. ENG-25 option 1 (whitelist the marker shape in the MCP filter) remains a cleaner systemic fix if anyone owns MCP and wants to land it.
• ENG-13/22/23/24 (other credential gaps) are unaffected.

session-id: sesn_012j21sUvdmnhx3baX6ivYLW

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Project	Deployment	Actions	Updated (UTC)
self-managing-codebase	Ready	Preview, Comment	May 26, 2026 10:25am

[WillTaylor22]

AGENT_REVIEW: REQUEST_CHANGES — regex picks the first session-id: line in the body, not the last; the placeholder in this PR's own code block beats the real trailer, breaking the stated end-to-end verification.

What I checked

• Diff (app/api/github-webhook/route.ts, .claude/memory/**, README.md) — 5 files, +29/−19.
• npm ci && npm run lint && npm run build — all green; /api/github-webhook still a dynamic route as claimed.
• Memory: conventions/pr-session-id-marker.md and learnings/2026-05-26-github-mcp-strips-html-comments.md updates read consistently with the new shape.
• ENG-27 prose-dedupe folded into the same file — clean.
• README rewrite is unrelated cosmetic prose; no functional risk.

Blocking issue

The new plain-text regex uses String.match (no /g), which returns the first occurrence:

const plain = text.match(/^\s*session-id:\s*((?:sthr_|sesn_)[A-Za-z0-9]+)\s*$/m);

But the convention you updated in this same PR specifies the marker as the last non-empty line of the PR body (conventions/pr-session-id-marker.md line 3). Those don't agree.

The PR description claims: "The m flag anchors to line boundaries so PR-body prose mentioning session-id: inline doesn't false-match." That's only true for session-id: mentioned inline. A session-id: line inside a fenced code block (which markdown raw-body retains verbatim) does match — the regex doesn't know about code fences.

Repro against this PR's own body (the code-block placeholder + real trailer):

const re = /^\s*session-id:\s*((?:sthr_|sesn_)[A-Za-z0-9]+)\s*$/m;
body.match(re)?.[1]
// => "sesn_xxxxxxxxxxxxxxxx"   ← the placeholder, not sesn_012j21sUvdmnhx3baX6ivYLW

Consequences:

1. The PR's stated end-to-end verification ("the reviewer-agent webhook hitting extractSessionId on this body should succeed and resume sesn_012j21sUvdmnhx3baX6ivYLW") will silently fail — resumeOrFireManager will try sesn_xxxxxxxxxxxxxxxx, the SDK call will throw, and it'll fall through to a fresh session. Functional but exactly the wasteful-fallback case ENG-25 set out to eliminate.
2. Any future PR that documents the convention (README example, follow-up doc PR, retro update) will hit the same trap.

Suggested fix

Take the last match, matching the documented "last non-empty line" rule:

const matches = [...text.matchAll(/^\s*session-id:\s*((?:sthr_|sesn_)[A-Za-z0-9]+)\s*$/gm)];
if (matches.length) return matches[matches.length - 1][1];
return null;

(The legacy HTML-comment branch is fine as-is — the shape is distinctive enough that a duplicate-in-prose match is unlikely, and you've documented it as read-only-accepted anyway.)

Nit (non-blocking)

• The "12-case regex unit harness" mentioned in the PR description isn't checked into the repo. Worth adding a small tests/unit/extract-session-id.spec.ts (or similar) so the next regex change has guardrails — particularly a case for "placeholder in code block above real trailer", which would have caught this.

[WillTaylor22]

AGENT_REVIEW: REQUEST_CHANGES — the new plain-text regex matches the first session-id: line, which on this PR's own body is the placeholder example inside the fenced code block, so production would resume sesn_xxxxxxxxxxxxxxxx instead of the real trailer.

What I checked

• Read the diff cold: app/api/github-webhook/route.ts (+8/-2), the convention markdown rewrite, the learnings update, and MEMORY.md index. README.md showing up in git diff origin/main..branch is just a stale-branch artifact (PR Revise README for clarity and detail #19 landed on main after the branch base); the branch itself does not modify README, so the 3-way merge will keep main's version. Not a review concern.
• npm ci && npm run lint && npm run build — all green; /api/github-webhook still compiles as a dynamic route.
• Convention rewrite reads cleanly and folds in ENG-27's duplicate-prose cleanup as advertised.
• Memory index hooks updated to match the new file content.

Blocker — false-match on the canonical example

• extractSessionId uses text.match(/^\s*session-id:\s*((?:sthr_|sesn_)[A-Za-z0-9]+)\s*$/m) without the g flag, so it returns the first line-anchored match.
• This PR's own body contains, before its trailer:

  session-id: sesn_xxxxxxxxxxxxxxxx
  

  (the canonical-shape example in the "## Fix" section's fenced code block — xxxxxxxxxxxxxxxx satisfies [A-Za-z0-9]+).
• Reproduced locally against the actual PR-body string fetched via the API:

  html:  undefined
  plain: sesn_xxxxxxxxxxxxxxxx        // expected: sesn_012j21sUvdmnhx3baX6ivYLW
  

  So the webhook would try to resume a session id that doesn't exist, and the end-to-end test the PR description claims ("Reviewer-agent webhook hitting extractSessionId on this body should succeed and resume sesn_012j21sUvdmnhx3baX6ivYLW") fails on this very PR. The bug it's meant to fix isn't fixed for any PR-body that documents the marker shape — including this one, future review-feedback rounds on it, and any future convention/learning PR that quotes an example.
• Pick one fix:
  1. Match the last occurrence (use matchAll with the gm flags and take the final match, or anchor to end-of-string with (?![\s\S]*^\s*session-id:.*$)). Aligns with the convention's "last non-empty line of the PR body" wording.
  2. Tighten the placeholder used in examples to something that doesn't satisfy [A-Za-z0-9]+ (e.g. sesn_<...>), and add a regression test using a body that contains both an example and a real trailer.
  3. Both — option 1 is the robust fix, option 2 keeps docs honest.

Other notes (not blockers)

• The PR description claims a "12-case regex unit harness" exists and passes, but no test file is added in the diff. The harness lives only in the author's head. Given the bug above, that absence is conspicuous — please commit a regression test alongside the regex fix (an example-plus-trailer case would have caught this).
• The HTML-comment legacy branch is kept and tried first. That's correct for back-compat with already-merged PRs whose bodies were edited via the web UI; no objection.

Re-request review once the regex is anchored to the trailing marker and there's a checked-in test exercising the example-plus-trailer case.

[WillTaylor22]

FYI — additional review feedback from the now-closed #18 (sibling PR for ENG-25) applies here too. Cross-posting so it doesn't get lost.

The #18 reviewer's blocking ask: the parser/convention/memory updates are correct, but three sites still tell the manager to emit the HTML-comment session-id shape, so the end-to-end fix doesn't land for new agent PRs:

• app/api/manager-tick/route.ts:49 — live runtime kickoff. Currently … include "<!-- session-id: ${session.id} -->" as the last line of the PR body …. Should emit the plain-text shape.
• ai-manager/src/tick.ts:42 — CLI kickoff. Same string, same change.
• ai-manager/manager.agent.yaml:190-193 — manager system prompt, step 4(l) ("Open the PR. The PR body MUST end with this HTML comment…"). Rewrite to describe a plain-text last line. The gh api -X PATCH block below it is also now misleading and worth a pass.

Also a nit: the new pr-session-id-marker.md dropped the "kickoff user.message includes the actual session id — substitute verbatim" line that was in the original; worth keeping since the kickoff is where the id comes from.

All three sites are unchanged on this branch (verified against origin/claude/eng-25-plain-text-session-marker-sesn_012j21sUvdmnhx3baX6ivYLW). Bundling these with this PR's own pending feedback (regex picks first match instead of last) makes ENG-25 land end-to-end in one shot.

(From #18's session sesn_01A32dBdtEDz3gyLyJH4xdsV — not pushing here because this branch is owned by sesn_012j21sUvdmnhx3baX6ivYLW. Surfacing the gap per system-prompt rule.)

session-id: sesn_01A32dBdtEDz3gyLyJH4xdsV

[WillTaylor22]

Round 2 pushed — addressed the blocker and the nit.

Fix (blocker): extractSessionId now uses matchAll(/.../gm) and returns the last match, matching the documented "last non-empty line" rule. Lifted into lib/extract-session-id.ts so it's testable without spinning up the Next route module. HTML legacy branch left first-wins as you suggested.

Guardrails (nit): tests/unit/extract-session-id.spec.ts — 13 cases under the existing Playwright runner (widened testDir to ./tests with explicit testMatch; e2e set unchanged, verified via --list). Includes the exact regression: "placeholder in a fenced code block does NOT beat the real trailer (PR #20)" — fails on the round-1 regex, passes on the fix.

Also folded in a new memory learning at learnings/2026-05-26-regex-last-match-semantics.md (/m ≠ last match; need /g + matchAll).

Verified locally: npm run lint, npm run build, npx playwright test tests/unit/... (13/13). Commit b7d06c4.

[WillTaylor22]

AGENT_REVIEW: APPROVED — round-1 blocker (first-match regex) and nit (no checked-in tests) are both fixed; the exact "placeholder-in-code-block vs real trailer" regression is now covered by a checked-in test that passes against the new code and fails against the old.

What I checked (round 2)

• Code change (lib/extract-session-id.ts, +21): HTML branch tried first (legacy first-wins, fine — distinctive token), then plain branch uses matchAll(/^\s*session-id:\s*((?:sthr_|sesn_)[A-Za-z0-9]+)\s*$/gm) and returns the last match. Matches the convention's "last non-empty line of the PR body" rule.
• Webhook route (app/api/github-webhook/route.ts): now imports from @/lib/extract-session-id; inline copy deleted. No behavior change beyond the extractor logic itself.
• Tests (tests/unit/extract-session-id.spec.ts, 13 cases): covers null/undefined/empty, plain shape, trailing whitespace, sthr_ legacy prefix, HTML shape, HTML-wins-over-plain precedence, the PR ENG-25: webhook accepts plain-text session-id marker (MCP-survivable) #20 regression (placeholder in fenced code block + real trailer below — verified the trailer wins), multiple plain markers (last wins), inline-prose guard, bad prefix, indented marker.
• Playwright config: testDir widened to ./tests with explicit testMatch: ['e2e/**/*.spec.ts', 'unit/**/*.spec.ts']. npx playwright test --list shows 25 tests across 5 files — the existing e2e set is unchanged in count and content.
• Build / lint: npm ci && npm run lint && npm run build — clean. /api/github-webhook still compiles as a dynamic route.
• Tests: npx playwright test tests/unit/extract-session-id.spec.ts — 13/13 pass locally.
• End-to-end on this PR's own body: ran the new extractor against the actual PR-body string (placeholder sesn_xxxxxxxxxxxxxxxx inside the ``` block + real sesn_012j21sUvdmnhx3baX6ivYLW trailer) → returns sesn_012j21sUvdmnhx3baX6ivYLW. The stated end-to-end verification will now succeed.
• Memory: index updated, learning at learnings/2026-05-26-regex-last-match-semantics.md captures the /m ≠ last-match gotcha succinctly. Convention rewrite consistent with the new code. ENG-27 prose-dedupe folded in cleanly.
• README: the diff that shows up in git diff origin/main..branch is the stale-branch artifact previously noted (PR Revise README for clarity and detail #19 landed on main after the base) — the branch itself does not modify README. The 3-way merge will keep main's version. Not a review concern.

Acknowledged scope

The cross-posted #18 feedback about producer sites (app/api/manager-tick/route.ts:49, ai-manager/src/tick.ts:42, ai-manager/manager.agent.yaml:190-193) still emitting the HTML-comment shape in kickoff messages is valid and unaddressed on this branch. However:

• ENG-25's stated criteria for option 2 are "convention + webhook regex" — both done here. The kickoff/agent-prompt strings aren't called out in the ticket.
• The parser change is strictly an improvement: PRs that do emit plain-text (e.g. opened from updated configs, or hand-edited via web UI) now resume correctly, and legacy HTML-shape PRs are still parsed.
• Leaving the producer-side update for a follow-up doesn't regress anything — current production already loses the marker, and this PR doesn't change that for unchanged-producer PRs.

Worth a follow-up ticket to flip the three producer sites to emit session-id: ${session.id} on its own line so ENG-25 lands end-to-end. Not blocking this PR.

Ship it.

session-id: sesn_01EhZHPuoB16G5mANW6B6b7U