diff --git a/labs/21-interlock/integrations/pitot/SPEC.md b/labs/21-interlock/integrations/pitot/SPEC.md
index 64ae20efd..60a0295b8 100644
--- a/labs/21-interlock/integrations/pitot/SPEC.md
+++ b/labs/21-interlock/integrations/pitot/SPEC.md
@@ -221,7 +221,7 @@ JSONL over stdio (one JSON object per line). Every message carries
 `control.requested` (host → controller):
 
 ```json
-{"pitot_version":"1","type":"control.requested","kind":"interlock.decide","action_id":"<correlation>","data":{ /* the EffectRequest */ }}
+{"pitot_version":"1","type":"control.requested","kind":"interlock.effect","action_id":"<correlation>","data":{ /* the EffectRequest */ }}
 ```
 
 `control.response` (controller → host):
@@ -230,7 +230,9 @@ JSONL over stdio (one JSON object per line). Every message carries
 {"pitot_version":"1","type":"control.response","controller_id":"…","action_id":"<same correlation>","outcome":"allow","message":"…"}
 ```
 
-- `kind` selects which controller handles the request (e.g. `interlock.decide`).
+- `kind` selects which controller handles the request — the canonical value is
+  `interlock.effect` (`client.DefaultRequestKind`), and the config `kind:` map key
+  must match it or Pitot finds no controller and fails closed on every request.
 - `data` is the opaque payload — for this path, the Interlock `EffectRequest`.
 - `action_id` correlates request↔response; the runtime mints an unpredictable one.
 - `outcome` on the wire is only **`allow` | `deny`** — the controller collapses the
@@ -254,7 +256,8 @@ dependency-free. It has three parts:
   takes the request as raw bytes, runs `engine.Decide`, and returns allow only on
   `OutcomeAllow` (everything else fails closed). Unit-testable with no runtime.
 - **`cmd/interlock-pitot-controller`** — owns the Pitot wire framing (via `sdk`)
-  and calls the adapter. This is the controller `kind: interlock.decide` points at.
+  and calls the adapter. This is the controller the config `kind: interlock.effect`
+  points at.
 - **`client`** — the host-side typed decision client. Names the resource by id,
   builds the typed `EffectRequest`, sends it via Pitot, returns a typed result,
   and fails closed on build/transport/non-allow.
diff --git a/labs/21-interlock/integrations/pitot/adapter/adapter.go b/labs/21-interlock/integrations/pitot/adapter/adapter.go
index 7c593f103..1857ae312 100644
--- a/labs/21-interlock/integrations/pitot/adapter/adapter.go
+++ b/labs/21-interlock/integrations/pitot/adapter/adapter.go
@@ -38,6 +38,19 @@ func Evaluate(policy ir.Policy, data []byte) (allow bool, message string) {
 	dec := json.NewDecoder(strings.NewReader(string(data)))
 	dec.DisallowUnknownFields()
 	if err := dec.Decode(&req); err != nil {
+		// A decode failure is fail-closed either way, but the caller needs to tell
+		// two very different causes apart. An unknown-field error is schema skew —
+		// the caller is almost always a newer client sending a field this
+		// controller's protocol version does not know — not a policy denial or
+		// corrupt bytes. Naming it as such points the operator at a version
+		// mismatch (upgrade the controller / align the client) instead of hunting a
+		// phantom policy rule. Any other decode error is a genuinely malformed
+		// payload. Neither is ever confusable with an engine deny below.
+		if strings.Contains(err.Error(), "unknown field") {
+			return false, fmt.Sprintf("interlock: schema skew — effect request has a field this controller does not recognize; "+
+				"the caller is likely a newer client than this controller's protocol %s (fail closed): %v",
+				protocol.EffectRequestProtocol, err)
+		}
 		return false, fmt.Sprintf("interlock: undecodable effect request (fail closed): %v", err)
 	}
 
diff --git a/labs/21-interlock/integrations/pitot/adapter/adapter_test.go b/labs/21-interlock/integrations/pitot/adapter/adapter_test.go
index e12b76314..f2df074c8 100644
--- a/labs/21-interlock/integrations/pitot/adapter/adapter_test.go
+++ b/labs/21-interlock/integrations/pitot/adapter/adapter_test.go
@@ -105,6 +105,53 @@ func TestEvaluateRequireBecomesDenyWithMissingEvidence(t *testing.T) {
 	}
 }
 
+func TestEvaluateSchemaSkewIsDiagnosableAndDistinct(t *testing.T) {
+	pol, hash := testPolicy(t)
+	// An otherwise well-formed request carrying one field this controller does not
+	// know — the shape a newer client sends against an older controller. It must
+	// fail closed (like every decode error), but its message must point at a
+	// version skew, name the controller's protocol, and NOT read as a policy deny.
+	data := []byte(`{
+		"protocol":"interlock.effect.v1","request_id":"r4","run_id":"run1",
+		"actor":"publisher","operation":"artifact.publish",
+		"resource":{"kind":"file","uri":"repo://out/result.json"},
+		"claimed_policy_hash":"` + hash + `",
+		"evidence":[{"kind":"staged_hash_match"}],
+		"future_field":"from a newer client"
+	}`)
+	allow, msg := Evaluate(pol, data)
+	if allow {
+		t.Fatalf("schema skew must fail closed, got allow: %s", msg)
+	}
+	if !strings.Contains(msg, "schema skew") {
+		t.Errorf("skew message should name the schema skew explicitly: %q", msg)
+	}
+	if !strings.Contains(msg, "interlock.effect.v1") {
+		t.Errorf("skew message should name the controller protocol version: %q", msg)
+	}
+	// It must be distinguishable from a policy denial: an operator reading it must
+	// not go hunting for a phantom rule.
+	if strings.Contains(msg, "rule") {
+		t.Errorf("skew message must not read as a policy deny (no rule mention): %q", msg)
+	}
+	// And distinct from a generic malformed-payload message.
+	if strings.Contains(msg, "undecodable") {
+		t.Errorf("skew message should be labeled schema skew, not generic undecodable: %q", msg)
+	}
+
+	// A genuinely malformed payload keeps the generic diagnostic — not the skew one.
+	allow, msg = Evaluate(pol, []byte("{not json"))
+	if allow {
+		t.Fatalf("malformed data must fail closed")
+	}
+	if strings.Contains(msg, "schema skew") {
+		t.Errorf("corrupt bytes should not be reported as schema skew: %q", msg)
+	}
+	if !strings.Contains(msg, "undecodable") {
+		t.Errorf("corrupt bytes should keep the generic undecodable diagnostic: %q", msg)
+	}
+}
+
 func TestEvaluateFailsClosedOnMalformedData(t *testing.T) {
 	pol, _ := testPolicy(t)
 	for _, tc := range []struct {
diff --git a/labs/21-interlock/integrations/pitot/client/client.go b/labs/21-interlock/integrations/pitot/client/client.go
index bde53dda7..87a5fd9de 100644
--- a/labs/21-interlock/integrations/pitot/client/client.go
+++ b/labs/21-interlock/integrations/pitot/client/client.go
@@ -23,9 +23,19 @@ import (
 	"github.com/operatorstack/pitot/schema"
 )
 
-// DefaultRequestKind is the conventional Pitot request kind an Interlock decision
-// controller registers under. A deployment may choose another; pass it to New.
-const DefaultRequestKind = "interlock.decide"
+// DefaultRequestKind is the canonical Pitot request kind an Interlock decision
+// controller is registered under. It is the single source of truth for that
+// string: the Pitot config's controller map key (`kind:`), the documentation, and
+// scripts/validate.sh all use this same value, and kind_consistency_test.go fails
+// the build if any of them drift.
+//
+// Why this matters: Pitot routes a request to a controller by matching the
+// request kind against the config `kind:`. If the two disagree, Pitot finds no
+// controller and fails closed — every decision is denied, silently, with no
+// engine ever consulted. Keeping one canonical value avoids that footgun; a
+// deployment that deliberately chooses another kind must pass the same string to
+// both New and the controller's config `kind:`.
+const DefaultRequestKind = "interlock.effect"
 
 // Client issues Interlock decision requests over a Pitot runtime.
 type Client struct {
diff --git a/labs/21-interlock/integrations/pitot/client/kind_consistency_test.go b/labs/21-interlock/integrations/pitot/client/kind_consistency_test.go
new file mode 100644
index 000000000..a12af26f2
--- /dev/null
+++ b/labs/21-interlock/integrations/pitot/client/kind_consistency_test.go
@@ -0,0 +1,52 @@
+package client
+
+import (
+	"os"
+	"strings"
+	"testing"
+)
+
+// The Pitot request kind lives in three places that must never disagree: the
+// DefaultRequestKind constant (what the client sends), the config fragment in the
+// integration README (the controller map key `kind:`), and scripts/validate.sh
+// (which registers the controller and drives `pitot request <kind>`). Pitot routes
+// by matching the request kind against the config `kind:`; if they drift, Pitot
+// finds no controller and fails closed on every request — a silent deny-all. These
+// tests fail the build the moment any of the three moves without the others.
+
+// canonicalKind is the value the whole integration is expected to converge on.
+// It is stated here independently of DefaultRequestKind so a change to the
+// constant is a deliberate, reviewed edit to this test too, not a silent rename.
+const canonicalKind = "interlock.effect"
+
+// staleKind is the previous value; no shipped surface may reference it as a kind.
+const staleKind = "interlock.decide"
+
+func TestDefaultRequestKindIsCanonical(t *testing.T) {
+	if DefaultRequestKind != canonicalKind {
+		t.Fatalf("DefaultRequestKind = %q, want %q", DefaultRequestKind, canonicalKind)
+	}
+}
+
+func TestDocsAndScriptAgreeOnKind(t *testing.T) {
+	// Paths are relative to this package dir (integrations/pitot/client).
+	surfaces := map[string]string{
+		"integration README": "../README.md",
+		"validate.sh":        "../scripts/validate.sh",
+	}
+	for name, path := range surfaces {
+		b, err := os.ReadFile(path)
+		if err != nil {
+			t.Fatalf("%s: %v", name, err)
+		}
+		text := string(b)
+		if !strings.Contains(text, DefaultRequestKind) {
+			t.Errorf("%s (%s) does not mention the canonical request kind %q; it must register the controller under this kind or Pitot fails closed on every request",
+				name, path, DefaultRequestKind)
+		}
+		if strings.Contains(text, staleKind) {
+			t.Errorf("%s (%s) still references the stale request kind %q; a config/client kind mismatch silently denies every request",
+				name, path, staleKind)
+		}
+	}
+}
diff --git a/labs/21-interlock/interlock-distribution/release-notes/2026-07-31-interface-and-client-readiness.md b/labs/21-interlock/interlock-distribution/release-notes/2026-07-31-interface-and-client-readiness.md
new file mode 100644
index 000000000..c87f13821
--- /dev/null
+++ b/labs/21-interlock/interlock-distribution/release-notes/2026-07-31-interface-and-client-readiness.md
@@ -0,0 +1,42 @@
+### Start building over Pitot without the silent deny-all footgun
+
+This release hardens the Interlock↔Pitot decision-transport interface and the
+language clients so external builders can start testing against them — with no
+change to the enforcement guarantee. The engine still decides, the broker still
+performs, and every path still fails closed. What changed is that the two ways a
+newcomer gets *silently* stuck are now *loudly* diagnosed.
+
+**One canonical request kind, guarded against drift.** Pitot routes a request to a
+controller by matching the request kind against the config key; if the client's
+kind and the config key disagree, Pitot finds no controller and denies **every**
+request silently, with no engine ever consulted. The client, the integration
+README, and `scripts/validate.sh` now all converge on one canonical value
+(`client.DefaultRequestKind = "interlock.effect"`), and a build-time drift guard
+fails CI the moment any of them moves apart. A mismatch is now a failed build, not
+a mysterious deny-all in production.
+
+**Schema skew reads as schema skew.** When a newer client sends a field an older
+controller does not recognize, the controller still fails closed — but the message
+now names it as a version skew against the controller's protocol
+(`interlock.effect.v1`), distinct from both a corrupt payload and a policy denial.
+An operator reading the response is pointed at "upgrade the controller / align the
+client," not sent hunting for a phantom policy rule.
+
+**The language clients ship as one version-locked release.** A static CI check
+(no toolchain required) fails the build if the TypeScript and Python package
+versions drift, so an external builder can never pin mismatched clients against one
+controller and silently reason about two different wire contracts. This rides the
+existing client bar: generated-from-Go types (diff-gated), frozen-corpus canonical
+parity, and the no-foreign-enforcement guardrail.
+
+**A clearer onramp for building on Interlock.** The public README gains a *Build on
+Interlock* section — installing and running the TypeScript/Python clients, and
+running a decision over Pitot with the canonical request kind — plus a
+`CONTRIBUTING.md` covering the six-point client support bar and the one guardrail
+that cannot move: enforcement stays the trusted Go executable. Clients transport,
+shape, hash, and observe decisions; they never decide or enforce.
+
+None of this touches the enforcement boundary. The Pitot leg remains
+decision-transport, not the guarantee — the broker is still the sole authority that
+lands the protected artifact, only on a policy-allowed request with truthful,
+hash-bound evidence.
diff --git a/labs/21-interlock/public-readme-preview/CONTRIBUTING.md b/labs/21-interlock/public-readme-preview/CONTRIBUTING.md
new file mode 100644
index 000000000..e1ed14f3f
--- /dev/null
+++ b/labs/21-interlock/public-readme-preview/CONTRIBUTING.md
@@ -0,0 +1,77 @@
+# Contributing to Interlock
+
+Interlock's core guarantee is **authority, not interception**: one protected effect
+is landed only by the broker, only on a policy-allowed request with truthful
+evidence. Contributions must preserve that separation — and the honesty about what
+is and is not guaranteed. Read [The enforcement boundary](README.md#the-enforcement-boundary-what-is-and-isnt-guaranteed)
+before proposing a change.
+
+## The one guardrail that cannot move
+
+**Enforcement stays the trusted Go executable.** The engine decides and the broker
+performs; nothing else does. A language client, an integration, or an example may
+**transport, shape, hash, or observe** a decision — it may never **decide** or
+**enforce** one.
+
+Concretely, forbidden anywhere outside the Go engine/broker:
+
+- a `decide` / `publish` / `broker` callable, or a decider/publisher/broker class;
+- inferring a file effect from a command string (Interlock rejects this dishonesty —
+  a command string does not describe an opaque process's writes);
+- an arbitrary decision-time callback (it would reintroduce hidden I/O,
+  nondeterminism, and replay failure into the one component that must stay pure).
+
+[`scripts/check-clients.sh`](https://github.com/operatorstack/intelligence-flow/blob/main/labs/21-interlock/scripts/check-clients.sh)
+fails the build if any client grows an executable enforcement surface.
+
+## Adding or changing a language client
+
+The clients carry protocol **data types** and a **canonical encoder** only. Every
+shipped language must clear the same six-point support bar (all CI-enforced):
+
+1. the generator is not stale — types are generated from the Go wire structs
+   (`go run ./clients/gen/main.go`) and diff-gated, so a DTO can never drift from
+   the protocol it mirrors;
+2. the types compile / import;
+3. the package installs;
+4. the canonical hash matches the **frozen Go corpus** (`conformance/compat/v0.1.0/`)
+   — this is the real bar, not "the structs compile";
+5. the decision fixtures round-trip through the DTOs within the closed vocabulary;
+6. the example runs and exits 0.
+
+Two more standing rules for clients:
+
+- **Version lockstep.** All language clients ship as one release; bump every
+  package to the same version together. A static CI check fails the build on drift,
+  even where a toolchain is absent.
+- **No foreign enforcement.** See the guardrail above — a client that grows a
+  `decide`/`publish`/`broker` callable is rejected, type declarations aside.
+
+Do not hand-edit a generated file; change the Go source struct and regenerate.
+
+## Changing the protocol or the Pitot transport
+
+- Any change to the wire types is a change to the **single source of truth** (the Go
+  structs). Regenerate the clients and the JSON schema, and update the frozen corpus
+  deliberately — a changed *old* hash or *old* decision is a **breaking change** and
+  is meant to fail CI.
+- The Pitot integration ([`integrations/pitot`](integrations/pitot)) lives in its
+  own module; the interlock core never imports Pitot. Keep the request kind
+  canonical (`client.DefaultRequestKind`) — the drift guard exists because a
+  client/config kind mismatch silently denies every request.
+
+## Development
+
+The reference implementation is a Go module. From the lab directory:
+
+```bash
+bash scripts/validate.sh          # gofmt, vet, tests (+ -race), purity boundary,
+                                  # IR determinism, CGO_ENABLED=0 build, CLI lifecycle,
+                                  # and check-clients.sh
+bash scripts/check-determinism.sh # equivalent Go source → identical IR + hash
+```
+
+Fixtures live under `conformance/` as embedded positive/negative vectors: add a
+positive vector for every new behavior and a negative control for every boundary
+fault. The engine stays pure — no I/O, deterministic — so it can be replayed; real
+hashing and receipt correlation live in the broker.
diff --git a/labs/21-interlock/public-readme-preview/README.md b/labs/21-interlock/public-readme-preview/README.md
index 571ca7eb4..3a0dc1edb 100644
--- a/labs/21-interlock/public-readme-preview/README.md
+++ b/labs/21-interlock/public-readme-preview/README.md
@@ -277,6 +277,91 @@ by the broker tests, not just asserted here.
 See [Enforcement model — transport is not authority](docs/concepts/enforcement-model.md)
 for how this guarantee holds across local, agent, and cloud-sandbox environments.
 
+## Build on Interlock
+
+Two supported ways to build against Interlock, both honest about the boundary
+above: they **transport and shape** decisions; they never decide or enforce.
+
+### Language clients (TypeScript, Python)
+
+The `clients/` directory ships generated protocol types plus a hand-written,
+parity-gated canonical encoder for **TypeScript** and **Python**. They carry
+exactly three things — the wire **data types**, the **canonical encoder** that
+produces the same bytes (and therefore the same SHA-256) as the Go source, and the
+ergonomics to **shape** a request. They carry **no `decide`, no `publish`, no
+broker**: enforcement stays the trusted Go executable, and a build-time guardrail
+([`scripts/check-clients.sh`](https://github.com/operatorstack/intelligence-flow/blob/main/labs/21-interlock/scripts/check-clients.sh))
+fails the build if a client ever grows an executable decision surface.
+
+The two packages ship as **one version-locked release** — the same CI gate fails
+the build if the TypeScript and Python package versions drift, so an external
+builder can never pin mismatched clients against one controller.
+
+```bash
+# TypeScript (Node >= 22 runs .ts natively; zero runtime deps)
+cd clients/typescript && npm install
+node examples/decision-request.ts     # resolve a resource by id → typed, canonically-encoded EffectRequest
+
+# Python (>= 3.11; zero runtime deps — stdlib hashlib/json only)
+cd clients/python && pip install -e .
+python examples/decision-request.py
+```
+
+The types are **generated from the Go wire structs** (`go run ./clients/gen/main.go`)
+and diff-gated in CI, so a DTO can never silently drift from the protocol. Each
+client's canonical encoder is proven byte-for-byte against a **frozen Go corpus**
+(`conformance/compat/v0.1.0/`): the parity example re-canonicalizes every frozen
+policy and asserts the hash matches. That frozen-corpus parity is the real bar —
+"a client compiles" is not enough to ship one.
+
+What a client is *for*: hashing a policy identically across languages, and shaping
+an `EffectRequest` to send to a decision controller. What it is **not** for:
+deciding the request (that is the Go engine) or performing the effect (that is the
+Go broker). Porting either into another language is an explicit non-goal.
+
+### Run a decision over Pitot
+
+To route a decision through a running host, Interlock ships an ordinary
+[Pitot](https://github.com/operatorstack/pitot) subprocess Controller in a separate
+module ([`integrations/pitot`](integrations/pitot)) — **no Pitot source change**,
+and the interlock core never imports Pitot. Pitot launches the controller and
+streams `control.requested` events; the controller runs the pure `engine.Decide`
+and answers allow/deny, with Pitot's deadline and fail-closed
+`on_timeout`/`on_unavailable` defaults layered on top.
+
+```bash
+# the integration is its own Go module (keeps the interlock core Pitot-free)
+cd integrations/pitot && go build -o interlock-pitot-controller ./cmd/interlock-pitot-controller
+interlock compile ./examples/exclusive-publish -o policy.json
+```
+
+Register the controller under the canonical request kind `interlock.effect`, then
+issue a normal Pitot request whose `data` is an Interlock `EffectRequest`:
+
+```yaml
+controllers:
+  interlock.effect:                      # the request kind Pitot routes on
+    id: interlock-effect
+    command: [./interlock-pitot-controller, --id, interlock-effect, --policy, policy.json]
+    deadline_ms: 5000
+    on_timeout: deny
+    on_unavailable: deny
+```
+
+```bash
+pitot run --config config.yaml --runtime rt.json &
+pitot request interlock.effect --runtime rt.json --data '{"protocol":"interlock.effect.v1", ...}'
+```
+
+**One request kind, or fail closed.** Pitot routes a request to a controller by
+matching the request kind against the config key. If the client's kind and the
+config key disagree, Pitot finds no controller and denies **every** request
+silently — an engine is never consulted. Interlock keeps a single canonical value
+(`client.DefaultRequestKind = "interlock.effect"`) and a build-time drift guard
+that fails CI the moment the constant, the README, or `scripts/validate.sh` move
+apart, so a mismatch is diagnosed, never silent. This is transport plumbing, not
+the enforcement guarantee — the broker remains that.
+
 ## Install
 
 **Prebuilt binary (no Go toolchain).** The installer detects your platform,
diff --git a/labs/21-interlock/scripts/check-clients.sh b/labs/21-interlock/scripts/check-clients.sh
index 871ed6dd2..ce04d67ab 100755
--- a/labs/21-interlock/scripts/check-clients.sh
+++ b/labs/21-interlock/scripts/check-clients.sh
@@ -79,6 +79,31 @@ if ! git diff --exit-code -- \
 fi
 echo "ok (generated types match the Go source structs)"
 
+# ---------------------------------------------------------------------------
+# 2a'. Version lockstep — the language clients ship as ONE release. An external
+#      builder installs the TypeScript and Python clients expecting the same
+#      protocol surface; if the two package versions drift, a consumer can pin
+#      mismatched clients against one controller and silently reason about two
+#      different wire contracts. This is a static file check (no toolchain), so it
+#      runs even where node/python are absent — a skipped bar must never let a
+#      version skew through. Bump both packages together or this fails the build.
+# ---------------------------------------------------------------------------
+step "clients are version-locked (TypeScript and Python share one release version)"
+ts_pkg="clients/typescript/package.json"
+py_pkg="clients/python/pyproject.toml"
+ts_ver="$(sed -n 's/.*"version"[[:space:]]*:[[:space:]]*"\([^"]*\)".*/\1/p' "$ts_pkg" | head -1)"
+py_ver="$(sed -n 's/^version[[:space:]]*=[[:space:]]*"\([^"]*\)".*/\1/p' "$py_pkg" | head -1)"
+if [ -z "$ts_ver" ] || [ -z "$py_ver" ]; then
+  echo "could not read a client version (typescript='$ts_ver' python='$py_ver')"
+  exit 1
+fi
+if [ "$ts_ver" != "$py_ver" ]; then
+  echo "client version drift: typescript=$ts_ver python=$py_ver"
+  echo "the language clients ship as one release; bump both packages to the same version"
+  exit 1
+fi
+echo "ok (both clients at v$ts_ver)"
+
 # ---------------------------------------------------------------------------
 # 2b. TypeScript support bar (gated on Node >= 22, which runs .ts natively).
 # ---------------------------------------------------------------------------