diff --git a/.github/workflows/ci.yaml b/.github/workflows/ci.yaml
index c501d33..66c78fc 100644
--- a/.github/workflows/ci.yaml
+++ b/.github/workflows/ci.yaml
@@ -90,3 +90,23 @@ jobs:
             exit 0
           fi
           pnpm run test:e2e
+
+  structural-gate:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          fetch-depth: 2
+
+      - name: Install sentrux
+        env:
+          SENTRUX_VERSION: v0.5.7
+        run: |
+          curl -fsSL \
+            "https://github.com/sentrux/sentrux/releases/download/${SENTRUX_VERSION}/sentrux-linux-x86_64" \
+            -o /usr/local/bin/sentrux
+          chmod +x /usr/local/bin/sentrux
+          sentrux --version
+
+      - name: Structural regression gate
+        run: sentrux gate
diff --git a/.mcp.json b/.mcp.json
new file mode 100644
index 0000000..7257c1f
--- /dev/null
+++ b/.mcp.json
@@ -0,0 +1,8 @@
+{
+  "mcpServers": {
+    "sentrux": {
+      "command": "sentrux",
+      "args": ["mcp"]
+    }
+  }
+}
diff --git a/.sentrux/baseline.json b/.sentrux/baseline.json
new file mode 100644
index 0000000..cff7b1c
--- /dev/null
+++ b/.sentrux/baseline.json
@@ -0,0 +1,12 @@
+{
+  "timestamp": 1778026327.988928,
+  "quality_signal": 0.5194591550342765,
+  "coupling_score": 0.41732283464566927,
+  "cycle_count": 1,
+  "god_file_count": 0,
+  "hotspot_count": 0,
+  "complex_fn_count": 9,
+  "max_depth": 7,
+  "total_import_edges": 127,
+  "cross_module_edges": 55
+}
\ No newline at end of file
diff --git a/.sentrux/rules.toml b/.sentrux/rules.toml
new file mode 100644
index 0000000..9e261c1
--- /dev/null
+++ b/.sentrux/rules.toml
@@ -0,0 +1,45 @@
+# Architectural rules enforced by `sentrux check` and `sentrux gate`.
+# Run from repo root: `sentrux check`
+
+[constraints]
+max_cycles = 0
+max_coupling = "B"
+max_cc = 15
+no_god_files = true
+
+# Layers: lower `order` = higher in the stack (may depend on higher-order
+# layers but not the other way around). Verified empirically — sentrux
+# flags "order N must not depend on order N+M" regardless of docs.
+
+[[layers]]
+name = "inner-loop"
+paths = ["packages/agent/src/inner-loop/*"]
+order = 0
+
+[[layers]]
+name = "lib"
+paths = ["packages/agent/src/lib/*"]
+order = 1
+
+[[layers]]
+name = "api-shape-helpers"
+paths = ["packages/agent/src/api-shape-helpers/*"]
+order = 2
+
+# Boundaries: explicit bans backing up the layer ordering.
+# `from` must not depend on `to`.
+
+[[boundaries]]
+from = "packages/agent/src/api-shape-helpers/*"
+to = "packages/agent/src/lib/*"
+reason = "api-shape-helpers holds leaf type definitions; it must not depend on lib."
+
+[[boundaries]]
+from = "packages/agent/src/api-shape-helpers/*"
+to = "packages/agent/src/inner-loop/*"
+reason = "api-shape-helpers is a leaf layer; it must not depend on anything above it."
+
+[[boundaries]]
+from = "packages/agent/src/lib/*"
+to = "packages/agent/src/inner-loop/*"
+reason = "lib is a utility layer; it must not depend on the inner-loop entry point."