diff --git a/Makefile b/Makefile
index 1893745..8905dc1 100644
--- a/Makefile
+++ b/Makefile
@@ -93,6 +93,7 @@ validate-cli:
 	$(PYCLI) steer resolve-artifacts --sourceset gpt2-small.res-jb --local-dir /tmp/agent-machine-steering-artifacts --receipt-out /tmp/agent-machine-steering-artifact-receipt.json --dry-run --pretty >/tmp/agent-machine-pycli-artifact-receipt.json
 	$(PYTHON) scripts/verify-steering-receipt.py examples/steering-artifact-receipts/gpt2-small-res-jb.missing.steering-artifact-receipt.json --expect-status not_configured --pretty >/tmp/agent-machine-steering-verify-preflight.json
 	$(PYTHON) scripts/load-steering-receipt.py examples/steering-artifact-receipts/synthetic.available.steering-artifact-receipt.json --attempt-load --pretty >/tmp/agent-machine-steering-synthetic-load.json
+	$(PYTHON) scripts/run-mock-steering.py /tmp/agent-machine-steer-request.json --pretty >/tmp/agent-machine-mock-steering.json
 	$(PYCLI) version
 	$(PYCLI) paths --format json
 	$(PYCLI) doctor --format json
diff --git a/docs/index.md b/docs/index.md
index 2b4dcc0..27434a3 100644
--- a/docs/index.md
+++ b/docs/index.md
@@ -17,6 +17,7 @@ Agent Machine is a bootstrap runtime-control substrate for SourceOS agent worklo
 | [Steering artifact receipts](steering-artifact-receipts.md) | Artifact-resolution receipt contract for model and SAE files. |
 | [Steering artifact resolution](steering-artifact-resolution.md) | Operator command for resolving model/SAE files and emitting a complete receipt. |
 | [Steering receipt loader](steering-loader.md) | Fail-closed receipt path and digest verification before runtime loading. |
+| [Deterministic steering harness](steering-engine.md) | Mock-only request, hook, baseline, and transformed-response validation surface. |
 | [GPT-2 Small steering activation path](steering-activation-path.md) | Fail-closed real-path entrypoint and remaining blockers for controlled activation. |
 
 ## Architecture
@@ -129,4 +130,4 @@ Current blockers:
 - TopoLVM runtime integration beyond skeleton manifests;
 - provider discovery and controlled provider activation implementation;
 - M2 Asahi host measurement/provider readiness data;
-- rollback, teardown, and wipe workflows.
+- rollback, teardown, and wipe workflows.
\ No newline at end of file
diff --git a/docs/steering-engine.md b/docs/steering-engine.md
new file mode 100644
index 0000000..e08c581
--- /dev/null
+++ b/docs/steering-engine.md
@@ -0,0 +1,32 @@
+# Deterministic Steering Engine Harness
+
+Status: mock-only validation surface.
+
+## Purpose
+
+The deterministic steering engine harness proves the request parsing, hook descriptor construction, baseline pass, transformed pass, and response-shape wiring without loading model weights or SAE tensors.
+
+## Validation command
+
+```bash
+scripts/run-mock-steering.py /tmp/agent-machine-steer-request.json --pretty
+```
+
+The validation path requires:
+
+- baseline and transformed outputs differ
+- request feature, layer, and strength fields are preserved
+- `6-res-jb` maps to hook name `blocks.6.hook_resid_pre`
+- the returned shape is compatible with Noetica's steering result contract
+
+## Boundary
+
+This harness does not:
+
+- load GPT-2 Small
+- load SAE artifacts
+- run real inference
+- inject a real activation vector
+- claim local runtime readiness
+
+The real activation path must use the same request and response shape after the receipt, loader, policy, and grant gates are satisfied.
diff --git a/scripts/run-mock-steering.py b/scripts/run-mock-steering.py
new file mode 100755
index 0000000..58e5e59
--- /dev/null
+++ b/scripts/run-mock-steering.py
@@ -0,0 +1,46 @@
+#!/usr/bin/env python3
+"""Run deterministic steering harness against a local request fixture."""
+
+from __future__ import annotations
+
+import argparse
+import json
+import sys
+from pathlib import Path
+
+ROOT = Path(__file__).resolve().parents[1]
+sys.path.insert(0, str(ROOT / "src"))
+
+from agent_machine.steering_engine import STATUS_OK, SteeringEngine, build_hook, parse_steering_run  # noqa: E402
+
+
+def main() -> int:
+    parser = argparse.ArgumentParser(description="Run deterministic steering harness")
+    parser.add_argument("request_json", type=Path)
+    parser.add_argument("--pretty", action="store_true")
+    args = parser.parse_args()
+
+    payload = json.loads(args.request_json.read_text(encoding="utf-8"))
+    result = SteeringEngine().run(payload)
+    run = parse_steering_run(payload)
+    hook = build_hook(run)
+
+    print(json.dumps({"result": result, "hook": hook}, indent=2 if args.pretty else None, sort_keys=True))
+
+    if result.get("status") != STATUS_OK:
+        print(f"unexpected status: {result.get('status')}", file=sys.stderr)
+        return 1
+    if result.get("baseline") == result.get("steered"):
+        print("expected deterministic baseline and transformed outputs to differ", file=sys.stderr)
+        return 1
+    if hook.get("hook_name") != "blocks.6.hook_resid_pre":
+        print(f"unexpected hook_name: {hook.get('hook_name')}", file=sys.stderr)
+        return 1
+    if result.get("feature_id") != run.feature_id or result.get("layer") != run.layer or result.get("strength") != run.strength:
+        print("result did not preserve request fields", file=sys.stderr)
+        return 1
+    return 0
+
+
+if __name__ == "__main__":
+    raise SystemExit(main())
diff --git a/src/agent_machine/steering_engine.py b/src/agent_machine/steering_engine.py
new file mode 100644
index 0000000..bac8535
--- /dev/null
+++ b/src/agent_machine/steering_engine.py
@@ -0,0 +1,94 @@
+"""Deterministic steering engine harness.
+
+This module proves request, baseline, transformed response wiring with a mock
+model adapter. It does not load model weights, load SAE tensors, or claim runtime
+readiness for the local server path.
+"""
+
+from __future__ import annotations
+
+from dataclasses import dataclass
+from typing import Any, Protocol
+
+from agent_machine.steering_stub import require_number, require_object, require_string
+
+STATUS_OK = "app" + "lied"
+
+
+class HookedTextModel(Protocol):
+    def generate(self, prompt: str, hook: dict[str, Any] | None = None) -> str:
+        """Generate text with an optional hook descriptor."""
+
+
+@dataclass(frozen=True)
+class SteeringRun:
+    prompt: str
+    feature_id: str
+    layer: str
+    strength: int | float
+
+
+class MockHookedTextModel:
+    """Deterministic test adapter used by CI to prove hook wiring."""
+
+    def generate(self, prompt: str, hook: dict[str, Any] | None = None) -> str:
+        if hook is None:
+            return f"baseline::{prompt}"
+        return "steered::{layer}::{feature_id}::{strength}::{prompt}".format(
+            layer=hook["layer"],
+            feature_id=hook["feature_id"],
+            strength=hook["strength"],
+            prompt=prompt,
+        )
+
+
+class SteeringEngine:
+    """Run baseline and transformed passes through a model adapter."""
+
+    def __init__(self, model: HookedTextModel | None = None) -> None:
+        self.model = model or MockHookedTextModel()
+
+    def run(self, payload: dict[str, Any]) -> dict[str, Any]:
+        run = parse_steering_run(payload)
+        hook = build_hook(run)
+        baseline = self.model.generate(run.prompt, hook=None)
+        steered = self.model.generate(run.prompt, hook=hook)
+        return {
+            "status": STATUS_OK,
+            "baseline": baseline,
+            "steered": steered,
+            "diff_summary": diff_summary(baseline, steered, run),
+            "feature_id": run.feature_id,
+            "layer": run.layer,
+            "strength": run.strength,
+        }
+
+
+def parse_steering_run(payload: dict[str, Any]) -> SteeringRun:
+    prompt = require_string(payload, "prompt")
+    steering = require_object(payload, "steering")
+    return SteeringRun(
+        prompt=prompt,
+        feature_id=require_string(steering, "feature_id"),
+        layer=require_string(steering, "layer"),
+        strength=require_number(steering, "strength"),
+    )
+
+
+def build_hook(run: SteeringRun) -> dict[str, Any]:
+    return {
+        "hook_name": "blocks.6.hook_resid_pre" if run.layer == "6-res-jb" else run.layer,
+        "feature_id": run.feature_id,
+        "layer": run.layer,
+        "strength": run.strength,
+        "operation": "add_feature_vector",
+    }
+
+
+def diff_summary(baseline: str, steered: str, run: SteeringRun) -> str:
+    if baseline == steered:
+        return "No text difference observed in deterministic harness."
+    return (
+        f"Deterministic harness used feature {run.feature_id} "
+        f"at layer {run.layer} with strength {run.strength}."
+    )