diff --git a/plugins/voyage/commands/trekbrief.md b/plugins/voyage/commands/trekbrief.md
index 0036747..edafd94 100644
--- a/plugins/voyage/commands/trekbrief.md
+++ b/plugins/voyage/commands/trekbrief.md
@@ -288,6 +288,118 @@ Phase 3 complete: {N} questions asked across {M} sections.
 Proceeding to draft and review.
 ```
 
+## Phase 3.5 — Per-phase effort dialog
+
+Phase 3.5 is the v5.1 entry-point for **adaptive-depth execution**. After
+Phase 3 has gathered intent / goal / success criteria / research plan, the
+operator commits an effort level and (optional) model per downstream phase
+(`research`, `plan`, `execute`, `review`). The committed signals are written
+to brief frontmatter as a `phase_signals:` list that the four downstream
+commands read via their `## Composition rule (v5.1)` section.
+
+### State requirements
+
+Before entering Phase 3.5 the following must be populated:
+
+- `state.intent` — the Phase 3 Intent answer (1+ paragraph)
+- `state.goal` — the Phase 3 Goal answer
+- `state.success_criteria` — at least one falsifiable SC
+- `state.research_plan.topics` — list (may be empty)
+
+If any are absent: skip Phase 3.5 entirely and write `phase_signals_partial:
+true` to the draft frontmatter. Do not block.
+
+### --quick mode
+
+If the operator launched with `--quick`: skip Phase 3.5 entirely and
+auto-write `phase_signals_partial: true` to draft frontmatter. The brief
+will satisfy the v5.1 sequencing gate without going through the dialog.
+
+### Default-derivation heuristic (LLM judgment, not algorithmic)
+
+Before each phase question, propose a default tier marked `(default)`. Use
+these signals — they are weak heuristics, not rules:
+
+- `research_topics_count` → high (`high`), low (`low`), absent (`low`)
+- `sc_count` (count of falsifiable SCs) → high (≥6 ⇒ `high`), low (≤2 ⇒ `low`)
+- Goal complexity: keywords like "rewrite", "migration", "refactor across",
+  "new platform" ⇒ `high`; "typo", "small bugfix", "docs touch-up" ⇒ `low`
+- Otherwise: `standard`
+
+Mix these into one proposed default per phase. Document the proposed tier
+in the question body so the operator sees why it was picked.
+
+### The loop — 4 tier-coupled AskUserQuestion calls
+
+Loop over `[research, plan, execute, review]` in order. For each phase,
+issue one `AskUserQuestion` with 3 options:
+
+| Option | Maps to phase_signals entry |
+|--------|----------------------------|
+| **Low effort** | `{phase: <name>, effort: low, model: sonnet}` |
+| **Standard (default)** | `{phase: <name>, effort: standard}` *(model omitted — composition falls through to profile)* |
+| **High effort** | `{phase: <name>, effort: high, model: opus}` |
+
+The proposed tier per phase (from the default-derivation heuristic) MUST be
+labelled `(default)` in the option list so the operator can one-click
+accept. Commit the chosen tier immediately to an in-memory `effort_state`
+dict — no bulk summary-before-commit. The loop is interruptible.
+
+The mapping table is canonical:
+- `low → {effort: low, model: sonnet}` (force sonnet for the low-cost path)
+- `standard → {effort: standard}` (model omitted; composition rule resolves via profile)
+- `high → {effort: high, model: opus}` (force opus for the high-confidence path)
+
+### Force-stop handling
+
+If during any of the four `AskUserQuestion` calls the operator says "stop",
+"skip", "enough", "just write it", or similar, do NOT exit silently — apply
+the Phase 4f force-stop pattern verbatim:
+
+```
+You stopped before committing per-phase signals. Remaining phases:
+  - {list of phases not yet answered}
+
+The brief will still be valid (v5.1 supports `phase_signals_partial: true`
+as a force-stop record). Downstream commands will fall back to the profile
+resolver for the un-committed phases.
+
+Continue anyway?
+```
+
+Then `AskUserQuestion`:
+
+| Option | Action |
+|--------|--------|
+| **Answer one more phase** | Return to the next un-answered phase question. |
+| **Stop now (record partial)** | Drop any in-progress `effort_state` and set `phase_signals_partial: true` in draft frontmatter. Mutually exclusive with `phase_signals`. Break Phase 3.5. |
+
+This pattern matches Step 4f (line 436-458) so the force-stop UX is
+identical across both surfaces.
+
+### Hand-off to Phase 4a
+
+If `effort_state` is fully populated (4 commits, no force-stop): write a
+`phase_signals:` block to draft frontmatter — one entry per phase,
+preserving the canonical-mapping form above. Omit `model:` for standard
+tier (composition falls through to profile).
+
+If `phase_signals_partial: true` was set: write that single line to draft
+frontmatter and skip the `phase_signals:` block (mutually exclusive per
+validator).
+
+Phase 4a (Step 4a — Draft in memory) reads from `effort_state` /
+`phase_signals_partial` and incorporates the appropriate frontmatter block
+into the draft brief.
+
+### Sequencing gate (downstream)
+
+`brief_version: 2.1` activates the validator's sequencing gate. If the
+final brief reaches `/trekplan`, `/trekresearch`, `/trekexecute`, or
+`/trekreview` WITHOUT `phase_signals` and WITHOUT `phase_signals_partial:
+true`, the validator emits `BRIEF_V51_MISSING_SIGNALS` and the command
+halts with a friendly hint pointing back to `/trekbrief`.
+
 ## Phase 4 — Draft, review, and revise
 
 Phase 4 runs a **draft → brief-reviewer → revise** loop. The draft is