diff --git a/.changeset/clever-ravens-romp.md b/.changeset/clever-ravens-romp.md
new file mode 100644
index 000000000..3c806c7fc
--- /dev/null
+++ b/.changeset/clever-ravens-romp.md
@@ -0,0 +1,5 @@
+---
+type: Fixed
+pr: 3130
+---
+**`phase complete` no longer advances `next_phase` into 999.x backlog headings** — the roadmap heading scan (stage 2 of the next-phase cascade) accepted any higher-numbered heading without checking the sentinel convention, so a `Phase 999.1: Backlog Item` heading was treated as the next real phase. Sentinel phase ids (999.x backlog, 0.x drafts) are now skipped. (#2786)
diff --git a/src/phase.cts b/src/phase.cts
index e49d11bc5..6f6466c48 100644
--- a/src/phase.cts
+++ b/src/phase.cts
@@ -2576,6 +2576,11 @@ function cmdPhaseComplete(cwd: string, phaseNum: string, raw: boolean): void {
           );
           let pm: RegExpExecArray | null;
           while ((pm = phasePattern.exec(roadmapForPhases)) !== null) {
+            // #2786: skip sentinel phase ids (999.x backlog, 0.x drafts) — stage 1
+            // already skips 999 dirs on disk; stage 2's heading scan must not
+            // advance into backlog headings. Mirrors the /^999(?:\.|$)/ guard
+            // stage 1 uses at line 2536, but via isSentinelPhaseId for both ranges.
+            if (isSentinelPhaseId(pm[1])) continue;
             if (comparePhaseNum(pm[1], phaseNum) > 0) {
               nextPhaseNum = pm[1];
               nextPhaseName = pm[2]