diff --git a/plugins/voyage/commands/trekexecute.md b/plugins/voyage/commands/trekexecute.md
index 27f1514..888ba5f 100644
--- a/plugins/voyage/commands/trekexecute.md
+++ b/plugins/voyage/commands/trekexecute.md
@@ -1519,6 +1519,37 @@ VOYAGE_PROFILE=balanced /trekexecute --project ...
 Stats records emit `profile`, `phase_models`, and `profile_source` per
 Phase 9 record.
 
+## Composition rule (v5.1)
+
+Independent of the profile system. When `brief.md` carries
+`phase_signals` (brief_version ≥ 2.1), each downstream phase resolves
+effort + model as:
+
+```
+effort_for_phase = brief.phase_signals[<phase>]?.effort ?? 'standard'
+model_for_phase  = brief.phase_signals[<phase>]?.model  ?? profile.phase_models[<phase>]
+```
+
+The brief signal wins per-phase when present; the profile fills any
+gaps. There is no helper module — composition is documented prose in
+each downstream command.
+
+For `/trekexecute` specifically: `effort == 'low'` activates `--gates open`
++ sequential-only execution (no worktree-isolated parallel waves — runs
+all sessions in a single foreground loop). `effort == 'standard'` (or
+absent) → no change (default execution strategy applies). High-effort
+behavior is deferred to v5.1.1 per brief Non-Goal.
+
+### Sequencing gate surface
+
+When `/trekexecute --project <dir>` is invoked, optionally run
+`brief-validator.mjs --soft --json` against `{dir}/brief.md`. If
+`BRIEF_V51_MISSING_SIGNALS` appears in `errors` (brief_version ≥ 2.1
+without `phase_signals` or `phase_signals_partial: true`), halt with:
+`Brief is brief_version 2.1 but does not carry phase_signals — re-run
+/trekbrief to commit them (Phase 3.5).` Enforcement is validator-only;
+commands surface, don't re-enforce.
+
 ## Hard rules
 
 1. **No AskUserQuestion for execution decisions.** All execution decisions come
diff --git a/plugins/voyage/commands/trekplan.md b/plugins/voyage/commands/trekplan.md
index ddf7bc9..68d3d1b 100644
--- a/plugins/voyage/commands/trekplan.md
+++ b/plugins/voyage/commands/trekplan.md
@@ -894,6 +894,36 @@ VOYAGE_PROFILE=balanced /trekplan --project ...
 Stats records emit `profile`, `phase_models`, `parallel_agents`, and
 `profile_source` so operators can audit which profile drove which session.
 
+## Composition rule (v5.1)
+
+Independent of the profile system. When `brief.md` carries
+`phase_signals` (brief_version ≥ 2.1), each downstream phase resolves
+effort + model as:
+
+```
+effort_for_phase = brief.phase_signals[<phase>]?.effort ?? 'standard'
+model_for_phase  = brief.phase_signals[<phase>]?.model  ?? profile.phase_models[<phase>]
+```
+
+The brief signal wins per-phase when present; the profile fills any
+gaps. There is no helper module — composition is documented prose in
+each downstream command.
+
+For `/trekplan` specifically: `effort == 'low'` activates the existing
+`--quick`-equivalent code-path (skip Phase 5 agent swarm — plan directly
+without exploration agents). `effort == 'standard'` (or absent) → no
+change. High-effort behavior is deferred to v5.1.1 per brief Non-Goal
+("No complete per-phase effort dictionary").
+
+### Sequencing gate surface
+
+Phase 1 already calls `brief-validator.mjs --soft`. If the validator
+returns `BRIEF_V51_MISSING_SIGNALS` in `errors` (brief_version ≥ 2.1
+without `phase_signals` or `phase_signals_partial: true`), halt with a
+one-line message: `Brief is brief_version 2.1 but does not carry phase_signals
+— re-run /trekbrief to commit them (Phase 3.5).` Enforcement is
+validator-only; this surface just makes the friendly hint readable.
+
 ## Hard rules
 
 - **Brief-driven**: Every plan decision must trace back to a section of the
diff --git a/plugins/voyage/commands/trekresearch.md b/plugins/voyage/commands/trekresearch.md
index d39c4c3..ea1d84a 100644
--- a/plugins/voyage/commands/trekresearch.md
+++ b/plugins/voyage/commands/trekresearch.md
@@ -435,6 +435,36 @@ Stats records emit `profile`, `phase_models`, `parallel_agents`,
 `external_research_enabled`, and `profile_source` so operators can audit
 which profile drove which session.
 
+## Composition rule (v5.1)
+
+Independent of the profile system. When `brief.md` carries
+`phase_signals` (brief_version ≥ 2.1), each downstream phase resolves
+effort + model as:
+
+```
+effort_for_phase = brief.phase_signals[<phase>]?.effort ?? 'standard'
+model_for_phase  = brief.phase_signals[<phase>]?.model  ?? profile.phase_models[<phase>]
+```
+
+The brief signal wins per-phase when present; the profile fills any
+gaps. There is no helper module — composition is documented prose in
+each downstream command.
+
+For `/trekresearch` specifically: `effort == 'low'` activates the
+existing `--quick`-equivalent code-path (inline research, no agent swarm).
+`effort == 'standard'` (or absent) → no change. High-effort behavior is
+deferred to v5.1.1 per brief Non-Goal.
+
+### Sequencing gate surface
+
+When `/trekresearch --project <dir>` is invoked and `{dir}/brief.md`
+exists, optionally run `brief-validator.mjs --soft --json` against it.
+If `BRIEF_V51_MISSING_SIGNALS` appears in `errors` (brief_version ≥ 2.1
+without `phase_signals` or `phase_signals_partial: true`), halt with:
+`Brief is brief_version 2.1 but does not carry phase_signals — re-run
+/trekbrief to commit them (Phase 3.5).` Enforcement is validator-only;
+commands surface, don't re-enforce.
+
 ## Hard rules
 
 - **No planning:** This command produces research briefs, not implementation plans.
diff --git a/plugins/voyage/commands/trekreview.md b/plugins/voyage/commands/trekreview.md
index 73bf18d..73bdcbb 100644
--- a/plugins/voyage/commands/trekreview.md
+++ b/plugins/voyage/commands/trekreview.md
@@ -357,6 +357,36 @@ VOYAGE_PROFILE=premium /trekreview --project ...
 
 Stats records emit `profile` and `profile_source`.
 
+## Composition rule (v5.1)
+
+Independent of the profile system. When `brief.md` carries
+`phase_signals` (brief_version ≥ 2.1), each downstream phase resolves
+effort + model as:
+
+```
+effort_for_phase = brief.phase_signals[<phase>]?.effort ?? 'standard'
+model_for_phase  = brief.phase_signals[<phase>]?.model  ?? profile.phase_models[<phase>]
+```
+
+The brief signal wins per-phase when present; the profile fills any
+gaps. There is no helper module — composition is documented prose in
+each downstream command.
+
+For `/trekreview` specifically: `effort == 'low'` activates the existing
+`--quick`-equivalent code-path (skip the brief-conformance reviewer; run
+correctness-only). `effort == 'standard'` (or absent) → no change.
+High-effort behavior is deferred to v5.1.1 per brief Non-Goal.
+
+### Sequencing gate surface
+
+Phase 1 already calls `brief-validator.mjs --soft` against `{brief_path}`.
+If the validator returns `BRIEF_V51_MISSING_SIGNALS` in `errors`
+(brief_version ≥ 2.1 without `phase_signals` or `phase_signals_partial:
+true`), halt with: `Brief is brief_version 2.1 but does not carry
+phase_signals — re-run /trekbrief to commit them (Phase 3.5).`
+Enforcement is validator-only; this surface just makes the friendly hint
+readable.
+
 ## Hard rules
 
 - **Brief is the contract.** Every finding in the review traces to a