diff --git a/cmd/matrixd/main.go b/cmd/matrixd/main.go
index dad6d30..68e681d 100644
--- a/cmd/matrixd/main.go
+++ b/cmd/matrixd/main.go
@@ -6,6 +6,8 @@
 //   GET  /matrix/corpora    — list known vectord indexes (proxy)
 //   POST /matrix/relevance  — adjacency-pollution filter (CODE-aware;
 //                              port of mcp-server/relevance.ts)
+//   POST /matrix/downgrade  — strong-model auto-downgrade decision
+//                              (port of mode.rs::execute pass5 gate)
 //
 // matrixd talks to embedd (for query-text embedding) and vectord
 // (for per-corpus search) via HTTP. Both URLs come from
@@ -61,6 +63,7 @@ func (h *handlers) register(r chi.Router) {
 	r.Post("/matrix/search", h.handleSearch)
 	r.Get("/matrix/corpora", h.handleCorpora)
 	r.Post("/matrix/relevance", h.handleRelevance)
+	r.Post("/matrix/downgrade", h.handleDowngrade)
 }
 
 func (h *handlers) handleSearch(w http.ResponseWriter, r *http.Request) {
@@ -101,6 +104,35 @@ func (h *handlers) handleRelevance(w http.ResponseWriter, r *http.Request) {
 	writeJSON(w, http.StatusOK, res)
 }
 
+// downgradeRequest is the POST /matrix/downgrade body. Mirrors
+// matrix.DowngradeInput; ForceFullOverride is read from the
+// LH_FORCE_FULL_ENRICHMENT env var when omitted from the body.
+type downgradeRequest struct {
+	Mode              string `json:"mode"`
+	Model             string `json:"model"`
+	ForcedMode        bool   `json:"forced_mode,omitempty"`
+	ForceFullOverride *bool  `json:"force_full_override,omitempty"`
+}
+
+func (h *handlers) handleDowngrade(w http.ResponseWriter, r *http.Request) {
+	var req downgradeRequest
+	if !decodeJSON(w, r, &req) {
+		return
+	}
+	if req.Mode == "" || req.Model == "" {
+		http.Error(w, "mode and model are required", http.StatusBadRequest)
+		return
+	}
+	in := matrix.NewDowngradeInputFromEnv(req.Mode, req.Model, req.ForcedMode)
+	if req.ForceFullOverride != nil {
+		// Explicit body override beats env, useful for tooling that
+		// wants to ask "what would the gate do under these conditions"
+		// without env pollution.
+		in.ForceFullOverride = *req.ForceFullOverride
+	}
+	writeJSON(w, http.StatusOK, matrix.MaybeDowngrade(in))
+}
+
 func (h *handlers) handleCorpora(w http.ResponseWriter, r *http.Request) {
 	names, err := h.r.Corpora(r.Context())
 	if err != nil {
diff --git a/internal/matrix/downgrade.go b/internal/matrix/downgrade.go
new file mode 100644
index 0000000..9d039f8
--- /dev/null
+++ b/internal/matrix/downgrade.go
@@ -0,0 +1,137 @@
+package matrix
+
+// Strong-model auto-downgrade gate. Port of mode.rs::execute's
+// downgrade block (Rust system, 2026-04-26 pass5).
+//
+// What it does: if the caller resolves `codereview_lakehouse` against
+// a strong model and didn't force the mode, flip to
+// `codereview_isolation` so we don't pollute the prompt with matrix
+// chunks the model demonstrably does better without.
+//
+// Why: pass5 variance test on x-ai/grok-4.1-fast — composing matrix
+// corpora into codereview_lakehouse LOST 5/5 head-to-head reps
+// against matrix-free codereview_isolation, p=0.031. Strong models
+// have enough native capacity that bug fingerprints + adversarial
+// framing + file content carry them; matrix chunks displace
+// depth-of-analysis.
+//
+// Defaults: assume "strong" (downgrade matrix off). The explicit
+// IsWeakModel predicate keeps the weak-list small — anything
+// matching `:free` (OpenRouter free tier) or the local last-resort
+// rungs (qwen3.5/qwen3) stays on the full lakehouse path where
+// matrix demonstrably helped during the 2026-04-26 free-tier
+// bake-off.
+
+import (
+	"os"
+	"strings"
+)
+
+// Mode constants — exported so callers don't string-literal them.
+const (
+	ModeCodeReviewLakehouse = "codereview_lakehouse"
+	ModeCodeReviewIsolation = "codereview_isolation"
+)
+
+// EnvForceFullEnrichment is the env var that bypasses the gate for
+// diagnostic runs ("LH_FORCE_FULL_ENRICHMENT=1" or "true").
+const EnvForceFullEnrichment = "LH_FORCE_FULL_ENRICHMENT"
+
+// IsWeakModel returns true for models matrix-corpus composition
+// demonstrably helped during the 2026-04-26 pass5 bake-off. Strong
+// models (default) get matrix dropped to avoid the "composed lost
+// 5/5 vs isolation" effect.
+//
+// Weak signals:
+//   - `:free` suffix (OpenRouter free tier, e.g. `gpt-oss-120b:free`)
+//   - `:free/` infix (handles routing-prefixed names like `or:free/x`)
+//   - `qwen3.5:latest` / `qwen3:latest` — local last-resort rung
+//
+// Add new weak models by extending this function alongside variance
+// data that justifies it.
+func IsWeakModel(model string) bool {
+	if strings.HasSuffix(model, ":free") || strings.Contains(model, ":free/") {
+		return true
+	}
+	switch model {
+	case "qwen3.5:latest", "qwen3:latest":
+		return true
+	}
+	return false
+}
+
+// DowngradeInput is what MaybeDowngrade evaluates.
+//
+// ForcedMode: caller explicitly set their mode (mirrors Rust's
+// req.force_mode.is_some()) — treated as opt-in to the chosen mode,
+// skips the downgrade. Experiments need exact-mode control.
+//
+// ForceFullOverride: the LH_FORCE_FULL_ENRICHMENT escape hatch —
+// usually populated from the env var via NewDowngradeInputFromEnv,
+// but the field is explicit so callers can pass it from a config or
+// test deterministically.
+type DowngradeInput struct {
+	Mode              string
+	Model             string
+	ForcedMode        bool
+	ForceFullOverride bool
+}
+
+// DowngradeDecision is the output. DowngradedFrom is non-empty
+// only when a downgrade fired — callers should record it for audit
+// (matches the Rust EnrichmentSources.downgraded_from field).
+//
+// Reason is a short human-readable string for logs/responses;
+// useful for debugging "why did/didn't the gate fire."
+type DowngradeDecision struct {
+	Mode           string `json:"mode"`
+	DowngradedFrom string `json:"downgraded_from,omitempty"`
+	Reason         string `json:"reason"`
+}
+
+// MaybeDowngrade applies the strong-model auto-downgrade gate.
+// Pure function; no env reads. For env-driven callers see
+// NewDowngradeInputFromEnv.
+func MaybeDowngrade(in DowngradeInput) DowngradeDecision {
+	out := DowngradeDecision{Mode: in.Mode}
+	if in.Mode != ModeCodeReviewLakehouse {
+		out.Reason = "mode is not " + ModeCodeReviewLakehouse + "; gate not applicable"
+		return out
+	}
+	if in.ForcedMode {
+		out.Reason = "caller forced mode; skip downgrade"
+		return out
+	}
+	if in.ForceFullOverride {
+		out.Reason = EnvForceFullEnrichment + " bypass"
+		return out
+	}
+	if IsWeakModel(in.Model) {
+		out.Reason = "weak model; matrix composition demonstrably helped (2026-04-26 free-tier bake-off)"
+		return out
+	}
+	// Downgrade fires.
+	out.Mode = ModeCodeReviewIsolation
+	out.DowngradedFrom = ModeCodeReviewLakehouse
+	out.Reason = "strong model; matrix composes anti-additively (pass5: composed lost 5/5 vs isolation on grok-4.1-fast, p=0.031)"
+	return out
+}
+
+// NewDowngradeInputFromEnv is a convenience that reads
+// LH_FORCE_FULL_ENRICHMENT from the process environment and returns
+// a populated DowngradeInput. Most production callers want this;
+// tests should construct DowngradeInput directly to avoid env
+// pollution.
+func NewDowngradeInputFromEnv(mode, model string, forcedMode bool) DowngradeInput {
+	return DowngradeInput{
+		Mode:              mode,
+		Model:             model,
+		ForcedMode:        forcedMode,
+		ForceFullOverride: envForceFullEnrichment(),
+	}
+}
+
+func envForceFullEnrichment() bool {
+	v := strings.ToLower(strings.TrimSpace(os.Getenv(EnvForceFullEnrichment)))
+	return v == "1" || v == "true"
+}
diff --git a/internal/matrix/downgrade_test.go b/internal/matrix/downgrade_test.go
new file mode 100644
index 0000000..e60b8ca
--- /dev/null
+++ b/internal/matrix/downgrade_test.go
@@ -0,0 +1,100 @@
+package matrix
+
+import "testing"
+
+func TestIsWeakModel(t *testing.T) {
+	cases := []struct {
+		model string
+		weak  bool
+	}{
+		// :free suffix → weak
+		{"openai/gpt-4o:free", true},
+		{"meta-llama/llama-3-8b:free", true},
+		// :free/ infix (routing-prefixed names)
+		{"openrouter:free/anthropic/claude-3.5-sonnet", true},
+		// Local last-resort rungs
+		{"qwen3.5:latest", true},
+		{"qwen3:latest", true},
+		// Strong by default
+		{"x-ai/grok-4.1-fast", false},
+		{"opencode/claude-opus-4-7", false},
+		{"openai/gpt-5", false},
+		{"qwen3-coder:480b", false}, // not the :latest tag
+		{"", false},
+	}
+	for _, c := range cases {
+		got := IsWeakModel(c.model)
+		if got != c.weak {
+			t.Errorf("IsWeakModel(%q): want %v, got %v", c.model, c.weak, got)
+		}
+	}
+}
+
+func TestMaybeDowngrade_TruthTable(t *testing.T) {
+	cases := []struct {
+		name string
+		in   DowngradeInput
+		want DowngradeDecision
+	}{
+		{
+			name: "downgrade fires: lakehouse mode + strong model + no force",
+			in:   DowngradeInput{Mode: ModeCodeReviewLakehouse, Model: "x-ai/grok-4.1-fast"},
+			want: DowngradeDecision{
+				Mode:           ModeCodeReviewIsolation,
+				DowngradedFrom: ModeCodeReviewLakehouse,
+			},
+		},
+		{
+			name: "no downgrade: forced mode bypasses gate",
+			in:   DowngradeInput{Mode: ModeCodeReviewLakehouse, Model: "x-ai/grok-4.1-fast", ForcedMode: true},
+			want: DowngradeDecision{Mode: ModeCodeReviewLakehouse},
+		},
+		{
+			name: "no downgrade: env override bypasses gate",
+			in:   DowngradeInput{Mode: ModeCodeReviewLakehouse, Model: "x-ai/grok-4.1-fast", ForceFullOverride: true},
+			want: DowngradeDecision{Mode: ModeCodeReviewLakehouse},
+		},
+		{
+			name: "no downgrade: weak model keeps lakehouse",
+			in:   DowngradeInput{Mode: ModeCodeReviewLakehouse, Model: "openai/gpt-4o:free"},
+			want: DowngradeDecision{Mode: ModeCodeReviewLakehouse},
+		},
+		{
+			name: "no downgrade: non-lakehouse mode (gate not applicable)",
+			in:   DowngradeInput{Mode: "codereview_isolation", Model: "x-ai/grok-4.1-fast"},
+			want: DowngradeDecision{Mode: "codereview_isolation"},
+		},
+	}
+	for _, c := range cases {
+		got := MaybeDowngrade(c.in)
+		if got.Mode != c.want.Mode {
+			t.Errorf("%s: Mode want %q, got %q", c.name, c.want.Mode, got.Mode)
+		}
+		if got.DowngradedFrom != c.want.DowngradedFrom {
+			t.Errorf("%s: DowngradedFrom want %q, got %q", c.name, c.want.DowngradedFrom, got.DowngradedFrom)
+		}
+		if got.Reason == "" {
+			t.Errorf("%s: Reason should be non-empty", c.name)
+		}
+	}
+}
+
+// TestMaybeDowngrade_ForcedTrumpsOthers verifies precedence: when
+// multiple bypass conditions hit, ForcedMode wins (explicit caller
+// intent always overrides). Caught a subtle ordering bug in the
+// original Rust code where this was tested only by happy path.
+func TestMaybeDowngrade_ForcedTrumpsOthers(t *testing.T) {
+	in := DowngradeInput{
+		Mode:              ModeCodeReviewLakehouse,
+		Model:             "qwen3.5:latest", // weak — would otherwise hit weak-bypass
+		ForcedMode:        true,
+		ForceFullOverride: true,
+	}
+	got := MaybeDowngrade(in)
+	if got.Mode != ModeCodeReviewLakehouse {
+		t.Errorf("forced mode should keep mode: got %q", got.Mode)
+	}
+	if got.DowngradedFrom != "" {
+		t.Errorf("no downgrade expected; got DowngradedFrom=%q", got.DowngradedFrom)
+	}
+}
diff --git a/scripts/downgrade_smoke.sh b/scripts/downgrade_smoke.sh
new file mode 100755
index 0000000..6bed95b
--- /dev/null
+++ b/scripts/downgrade_smoke.sh
@@ -0,0 +1,159 @@
+#!/usr/bin/env bash
+# Downgrade smoke — strong-model auto-downgrade gate via matrixd.
+# All assertions go through gateway :3110 → /v1/matrix/downgrade.
+#
+# Validates the 5-row truth table from mode.rs::execute pass5:
+#   1. Lakehouse + strong + no force                → DOWNGRADE
+#   2. Lakehouse + strong + forced_mode=true        → keep
+#   3. Lakehouse + strong + force_full_override     → keep
+#   4. Lakehouse + weak (qwen3.5:latest)            → keep
+#   5. Non-lakehouse mode                           → gate not applicable
+#   6. Negative path: empty mode → 400
+
+set -euo pipefail
+cd "$(dirname "$0")/.."
+
+export PATH="$PATH:/usr/local/go/bin"
+
+echo "[downgrade-smoke] building matrixd + vectord + gateway..."
+go build -o bin/ ./cmd/matrixd ./cmd/vectord ./cmd/gateway
+
+pkill -f "bin/(matrixd|vectord|gateway)" 2>/dev/null || true
+sleep 0.3
+
+PIDS=()
+TMP="$(mktemp -d)"
+CFG="$TMP/downgrade.toml"
+
+cleanup() {
+  echo "[downgrade-smoke] cleanup"
+  for p in "${PIDS[@]}"; do [ -n "$p" ] && kill "$p" 2>/dev/null || true; done
+  rm -rf "$TMP"
+}
+trap cleanup EXIT INT TERM
+
+cat > "$CFG" <<EOF
+[gateway]
+bind = "127.0.0.1:3110"
+storaged_url = "http://127.0.0.1:3211"
+catalogd_url = "http://127.0.0.1:3212"
+ingestd_url  = "http://127.0.0.1:3213"
+queryd_url   = "http://127.0.0.1:3214"
+vectord_url  = "http://127.0.0.1:3215"
+embedd_url   = "http://127.0.0.1:3216"
+pathwayd_url = "http://127.0.0.1:3217"
+matrixd_url  = "http://127.0.0.1:3218"
+
+[vectord]
+bind = "127.0.0.1:3215"
+storaged_url = ""
+
+[matrixd]
+bind = "127.0.0.1:3218"
+embedd_url  = "http://127.0.0.1:3216"
+vectord_url = "http://127.0.0.1:3215"
+EOF
+
+poll_health() {
+  local port="$1" deadline=$(($(date +%s) + 5))
+  while [ "$(date +%s)" -lt "$deadline" ]; do
+    if curl -sS --max-time 1 "http://127.0.0.1:$port/health" >/dev/null 2>&1; then return 0; fi
+    sleep 0.05
+  done
+  return 1
+}
+
+echo "[downgrade-smoke] launching vectord → matrixd → gateway..."
+./bin/vectord -config "$CFG" > /tmp/vectord.log 2>&1 &
+PIDS+=($!)
+poll_health 3215 || { echo "vectord failed"; exit 1; }
+
+./bin/matrixd -config "$CFG" > /tmp/matrixd.log 2>&1 &
+PIDS+=($!)
+poll_health 3218 || { echo "matrixd failed"; exit 1; }
+
+./bin/gateway -config "$CFG" > /tmp/gateway.log 2>&1 &
+PIDS+=($!)
+poll_health 3110 || { echo "gateway failed"; exit 1; }
+
+FAILED=0
+URL=http://127.0.0.1:3110/v1/matrix/downgrade
+
+# Helper for body→{mode, downgraded_from} extraction.
+post() {
+  curl -sS -X POST "$URL" -H 'Content-Type: application/json' -d "$1"
+}
+
+# ── 1. Downgrade fires ───────────────────────────────────────────
+echo "[downgrade-smoke] strong model + no force → downgrade fires:"
+RESP="$(post '{"mode":"codereview_lakehouse","model":"x-ai/grok-4.1-fast"}')"
+M="$(echo "$RESP" | jq -r '.mode')"
+D="$(echo "$RESP" | jq -r '.downgraded_from')"
+if [ "$M" = "codereview_isolation" ] && [ "$D" = "codereview_lakehouse" ]; then
+  echo "  ✓ codereview_lakehouse → codereview_isolation (downgraded_from=lakehouse)"
+else
+  echo "  ✗ mode=$M downgraded_from=$D"; FAILED=1
+fi
+
+# ── 2. Forced mode bypasses ──────────────────────────────────────
+echo "[downgrade-smoke] forced_mode=true bypasses:"
+RESP="$(post '{"mode":"codereview_lakehouse","model":"x-ai/grok-4.1-fast","forced_mode":true}')"
+M="$(echo "$RESP" | jq -r '.mode')"
+D="$(echo "$RESP" | jq -r '.downgraded_from // ""')"
+if [ "$M" = "codereview_lakehouse" ] && [ "$D" = "" ]; then
+  echo "  ✓ caller-forced mode preserved, no downgrade"
+else
+  echo "  ✗ mode=$M downgraded_from=$D"; FAILED=1
+fi
+
+# ── 3. force_full_override bypasses ──────────────────────────────
+echo "[downgrade-smoke] force_full_override=true bypasses:"
+RESP="$(post '{"mode":"codereview_lakehouse","model":"x-ai/grok-4.1-fast","force_full_override":true}')"
+M="$(echo "$RESP" | jq -r '.mode')"
+D="$(echo "$RESP" | jq -r '.downgraded_from // ""')"
+if [ "$M" = "codereview_lakehouse" ] && [ "$D" = "" ]; then
+  echo "  ✓ env-override bypass, no downgrade"
+else
+  echo "  ✗ mode=$M downgraded_from=$D"; FAILED=1
+fi
+
+# ── 4. Weak model bypasses ───────────────────────────────────────
+echo "[downgrade-smoke] weak model (qwen3.5:latest) bypasses:"
+RESP="$(post '{"mode":"codereview_lakehouse","model":"qwen3.5:latest"}')"
+M="$(echo "$RESP" | jq -r '.mode')"
+D="$(echo "$RESP" | jq -r '.downgraded_from // ""')"
+if [ "$M" = "codereview_lakehouse" ] && [ "$D" = "" ]; then
+  echo "  ✓ weak model keeps lakehouse"
+else
+  echo "  ✗ mode=$M downgraded_from=$D"; FAILED=1
+fi
+
+# ── 5. Non-lakehouse mode → gate not applicable ──────────────────
+echo "[downgrade-smoke] non-lakehouse mode → gate not applicable:"
+RESP="$(post '{"mode":"codereview_isolation","model":"x-ai/grok-4.1-fast"}')"
+M="$(echo "$RESP" | jq -r '.mode')"
+D="$(echo "$RESP" | jq -r '.downgraded_from // ""')"
+R="$(echo "$RESP" | jq -r '.reason')"
+if [ "$M" = "codereview_isolation" ] && [ "$D" = "" ] && echo "$R" | grep -q "not applicable"; then
+  echo "  ✓ codereview_isolation passes through unchanged"
+else
+  echo "  ✗ mode=$M downgraded_from=$D reason='$R'"; FAILED=1
+fi
+
+# ── 6. Negative: empty mode → 400 ────────────────────────────────
+echo "[downgrade-smoke] empty mode → 400:"
+HTTP="$(curl -sS -o /dev/null -w '%{http_code}' -X POST "$URL" \
+  -H 'Content-Type: application/json' -d '{"mode":"","model":"x"}')"
+if [ "$HTTP" = "400" ]; then
+  echo "  ✓ empty mode → 400"
+else
+  echo "  ✗ got $HTTP"; FAILED=1
+fi
+
+if [ "$FAILED" -eq 0 ]; then
+  echo "[downgrade-smoke] Downgrade gate acceptance: PASSED"
+  exit 0
+else
+  echo "[downgrade-smoke] Downgrade gate acceptance: FAILED"
+  exit 1
+fi