From 203c734340ffc60be1352f968004440586c5deb6 Mon Sep 17 00:00:00 2001
From: gsxdsm <gsxdsm@users.noreply.github.com>
Date: Tue, 7 Jul 2026 10:25:11 -0700
Subject: [PATCH] fix(engine): require exact trailer line, not substring, for
 proven landed commit (Greptile P1)

findProvenLandedCommit now keeps --grep as a prefilter but verifies each
candidate carries an actual 'Fusion-Task-Id: <taskId>' trailer line via
git show -s --format=%B, so a later commit that merely mentions the trailer
text in its body cannot be selected. Regression covers a body-mention
intervening commit.
---
 .../workspace-merger-idempotency.test.ts      | 18 ++++++++++-----
 .../engine/src/workspace-land-predicate.ts    | 22 +++++++++++++++----
 2 files changed, 30 insertions(+), 10 deletions(-)

diff --git a/packages/engine/src/__tests__/workspace-merger-idempotency.test.ts b/packages/engine/src/__tests__/workspace-merger-idempotency.test.ts
index 12021975a8..ee6e066b02 100644
--- a/packages/engine/src/__tests__/workspace-merger-idempotency.test.ts
+++ b/packages/engine/src/__tests__/workspace-merger-idempotency.test.ts
@@ -374,14 +374,20 @@ describeIfGit("landWorkspaceTask — DB-failure resilience (Phase C review A1/A4
     expect(store.task.status ?? null).toBeNull();
 
     /*
-    FNXC:Workspace 2026-07-07-10:30 (Phase C A1 precision regression — Greptile P1):
-    Simulate an intervening sub-repo land: advance repo-a's integration tip with an UNRELATED
-    commit AFTER the task's squash landed (tipAfterFirst) but BEFORE the lost-persist retry. The
-    recovered landedSha must be the task's OWN landing commit (tipAfterFirst), NOT the later
-    unrelated tip — otherwise finalize would attribute a wrong commit to this repo.
+    FNXC:Workspace 2026-07-07-10:55 (Phase C A1 precision regression — Greptile P1, two surfaces):
+    Advance repo-a's integration tip with an intervening commit whose MESSAGE BODY mentions the
+    trailer text "Fusion-Task-Id: FN-2002" (a changelog/diagnostic-style mention, NOT a real trailer
+    line) AFTER the task's squash landed (tipAfterFirst) but BEFORE the lost-persist retry. This
+    covers both precision surfaces: (1) the recovered landedSha must be the task's OWN landing commit
+    (tipAfterFirst), not the later tip; (2) the substring --grep prefilter must NOT select the
+    body-mention commit — findProvenLandedCommit requires an actual trailer line, so it skips the
+    mention and returns the real squash commit.
     */
     configureIdentity(fx.repoPath("repo-a"));
-    fx.git("repo-a", 'git commit --allow-empty -m "unrelated intervening land (no Fusion-Task-Id trailer)"');
+    fx.git(
+      "repo-a",
+      'git commit --allow-empty -m "unrelated intervening land" -m "changelog: relates to Fusion-Task-Id: FN-2002 (body mention, not a trailer line)"',
+    );
     const tipAfterIntervening = fx.git("repo-a", "git rev-parse refs/heads/main");
     expect(tipAfterIntervening).not.toBe(tipAfterFirst);
 
diff --git a/packages/engine/src/workspace-land-predicate.ts b/packages/engine/src/workspace-land-predicate.ts
index 64ffcd31cb..769e037c7c 100644
--- a/packages/engine/src/workspace-land-predicate.ts
+++ b/packages/engine/src/workspace-land-predicate.ts
@@ -116,13 +116,27 @@ export async function findProvenLandedCommit(
       if (base) range = `${base.trim()}..${intRef}`;
     }
     const trailer = `${FUSION_TASK_ID_TRAILER_KEY}: ${taskId}`;
-    const found = await gitCapture(
+    /*
+    FNXC:Workspace 2026-07-07-10:50 (Phase C A1 precision — Greptile P1, trailer-line verification):
+    `git log --grep=<trailer> --fixed-strings` is a substring search over the WHOLE commit message,
+    so a later changelog/diagnostic commit that merely mentions the trailer text in its body would
+    be selected over the actual squash commit. Use --grep only as a prefilter, then require an actual
+    trailer LINE (a line whose trimmed text is exactly the trailer) via `git show -s --format=%B`.
+    Candidates are reverse-chronological, so the first one with an exact trailer line is the task's
+    own landing commit.
+    */
+    const candidates = await gitCapture(
       ["log", "--format=%H", `--grep=${trailer}`, "--fixed-strings", range],
       repoRootDir,
     );
-    if (found) {
-      const firstSha = found.trim().split("\n")[0];
-      if (firstSha) return firstSha;
+    if (candidates) {
+      for (const sha of candidates.trim().split("\n")) {
+        if (!sha) continue;
+        const body = await gitCapture(["show", "-s", "--format=%B", sha], repoRootDir);
+        if (body && body.split("\n").some((line) => line.trim() === trailer)) {
+          return sha;
+        }
+      }
     }
   }
   return undefined;