diff --git a/plugins/voyage/templates/trekbrief-template.md b/plugins/voyage/templates/trekbrief-template.md
index b35d893..ff72ac4 100644
--- a/plugins/voyage/templates/trekbrief-template.md
+++ b/plugins/voyage/templates/trekbrief-template.md
@@ -1,6 +1,6 @@
 ---
 type: trekbrief
-brief_version: 2.0
+brief_version: 2.1
 created: {YYYY-MM-DD}
 task: "{one-line task description}"
 slug: {slug}
@@ -10,6 +10,20 @@ research_status: pending         # pending | in_progress | complete | skipped
 auto_research: false             # true if user opted into Claude-managed research
 interview_turns: {N}
 source: {interview | manual}
+# v5.1 — per-phase effort + model signal (Phase 3.5).
+# `effort` ∈ {low, standard, high}. Omit `model:` for `standard` so composition
+# falls through to profile resolver. Force-stop alternative is the commented
+# `phase_signals_partial: true` below (mutually exclusive with `phase_signals`).
+phase_signals:
+  - phase: research
+    effort: standard
+  - phase: plan
+    effort: standard
+  - phase: execute
+    effort: standard
+  - phase: review
+    effort: standard
+# phase_signals_partial: true   # uncomment to record force-stop instead of phase_signals
 ---
 
 # Task: {title}
diff --git a/plugins/voyage/tests/fixtures/brief-with-phase-signals.md b/plugins/voyage/tests/fixtures/brief-with-phase-signals.md
new file mode 100644
index 0000000..c68e37c
--- /dev/null
+++ b/plugins/voyage/tests/fixtures/brief-with-phase-signals.md
@@ -0,0 +1,42 @@
+---
+type: trekbrief
+brief_version: "2.1"
+created: 2026-05-13
+task: "Add per-phase effort dialog to /trekbrief"
+slug: phase-signals-example
+project_dir: .claude/projects/2026-05-13-phase-signals-example/
+research_topics: 2
+research_status: complete
+auto_research: false
+interview_turns: 6
+source: interview
+phase_signals:
+  - phase: research
+    effort: low
+    model: sonnet
+  - phase: plan
+    effort: standard
+  - phase: execute
+    effort: high
+    model: opus
+  - phase: review
+    effort: standard
+---
+
+# Task: Phase-signals example
+
+## Intent
+
+A minimal brief that exercises the v5.1 phase_signals additive field with a
+mix of effort levels and model overrides. Used by tests/validators to confirm
+the validator accepts well-formed signals across the supported tier matrix.
+
+## Goal
+
+Validator returns valid: true. annotate.mjs strips phase_signals from the
+rendered HTML body (frontmatter stays in source).
+
+## Success Criteria
+
+- Validator passes.
+- annotate.mjs determinism: re-run produces byte-identical HTML.
diff --git a/plugins/voyage/tests/fixtures/brief-without-phase-signals.md b/plugins/voyage/tests/fixtures/brief-without-phase-signals.md
new file mode 100644
index 0000000..8bec99e
--- /dev/null
+++ b/plugins/voyage/tests/fixtures/brief-without-phase-signals.md
@@ -0,0 +1,31 @@
+---
+type: trekbrief
+brief_version: "2.0"
+created: 2026-05-13
+task: "Backward-compat fixture for v5.0-style brief"
+slug: legacy-brief-example
+project_dir: .claude/projects/2026-05-13-legacy-brief-example/
+research_topics: 0
+research_status: complete
+auto_research: false
+interview_turns: 3
+source: interview
+---
+
+# Task: Legacy brief example
+
+## Intent
+
+A pre-v5.1 brief that pre-dates the phase_signals field. Used by
+tests/validators to confirm backward-compatibility: the brief is accepted
+without phase_signals as long as brief_version is < 2.1.
+
+## Goal
+
+Validator returns valid: true. The sequencing gate
+(BRIEF_V51_MISSING_SIGNALS) does NOT fire for brief_version 2.0.
+
+## Success Criteria
+
+- Validator passes.
+- No BRIEF_V51_MISSING_SIGNALS error in r.errors.