diff --git a/apps/engine/dev.vars.example b/apps/engine/dev.vars.example
index 0eeb73f0..161698a2 100644
--- a/apps/engine/dev.vars.example
+++ b/apps/engine/dev.vars.example
@@ -24,8 +24,20 @@ STRIPE_METER_EVENT_NAME=
 # EXTERNAL: Stripe billing-meter id from your sandbox. Pairs with STRIPE_METER_EVENT_NAME.
 STRIPE_METER_ID=
 
-# LOCAL: Selects the hermetic dev KEK custodian instead of AWS KMS. Without it, creating an endpoint needs real AWS IAM credentials. Lives ONLY here — .dev.vars is never sent by `wrangler deploy`, kmsProviderFromEnv refuses it whenever AWS KMS config is bound, and scripts/kms-mode-guard.mjs keeps it out of every committed Worker config.
-KMS_MODE=local
+# EXTERNAL: REQUIRED for prod parity — the KEK ARN. An identifier, not a secret, so the vault carries it.
+KMS_KEY_ARN=
+
+# EXTERNAL: REQUIRED for prod parity — the KEK's region (us-east-2). Not a secret; the vault carries it.
+AWS_REGION=
+
+# EXTERNAL: REQUIRED for prod parity — the IAM user scoped to kms:GenerateDataKey + kms:Decrypt on that one key. NOT vaulted: it pairs with a credential that reaches production.
+AWS_ACCESS_KEY_ID=
+
+# EXTERNAL: REQUIRED for prod parity. NOT vaulted — it grants Decrypt against the production KEK, which is a production capability regardless of how narrow the policy is.
+AWS_SECRET_ACCESS_KEY=
+
+# EXTERNAL: LEAVE BLANK so the real AWS KMS custodian runs, exactly as in production. Set it to "local" ONLY if you cannot hold AWS credentials; endpoint creation then seals under a hermetic throwaway KEK and the AWS path is never exercised. Lives ONLY here — .dev.vars is never sent by `wrangler deploy`, kmsProviderFromEnv refuses it whenever AWS KMS config is bound, and scripts/kms-mode-guard.mjs keeps it out of every committed Worker config.
+KMS_MODE=
 
 # GENERATED: The dev KEK, 32 random bytes. Stable across restarts so endpoints sealed before a restart stay openable — LocalKmsProvider.generate() would mint a fresh key each process and orphan them.
 LOCAL_KEK=
diff --git a/docs/local-parity.md b/docs/local-parity.md
index 0768665c..bdc4d352 100644
--- a/docs/local-parity.md
+++ b/docs/local-parity.md
@@ -159,7 +159,7 @@ default**, and each is refused outright if it is ever seen on a deployed Worker
 | --- | --- |
 | `EMAIL_MODE=log` | mail prints to the console, link included, instead of sending |
 | `OAUTH_MODE=optional` | providers without both halves configured are not wired; magic link still works — and with Google unwired, **no Google script loads on the login page at all** |
-| `KMS_MODE=local` | a process-local KEK instead of AWS KMS, so endpoints can be created without AWS |
+| `KMS_MODE=local` | a process-local KEK instead of AWS KMS, for a contributor who cannot hold AWS credentials — **no longer the default**, see below |
 
 ### ~~`TURNSTILE_MODE`~~ — CLOSED: the playground's captcha runs locally
 
@@ -207,9 +207,34 @@ machine. Run `stripe listen --print-secret` and use that one while forwarding. T
 wrong is `400 invalid signature` with nothing naming the cause — which is exactly why the credential vault
 refuses to carry this variable in either direction.
 
-`KMS_MODE=local` is the one that has no alternative today: the engine is the sole KEK custodian and nobody
-should hold production AWS credentials to develop. The other two are conveniences, and using them means
-accepting that you are not testing what production does.
+### ~~`KMS_MODE=local`~~ — CLOSED: local seals against the real KEK
+
+**Local dev now uses the same KEK custodian production does**, through the same `AwsKmsProvider` seam. It
+used to default to the hermetic `LocalKmsProvider`, which meant the envelope-encryption path — the one that
+seals provider secrets and ingest tokens — was never exercised before a deploy. That was the
+compliance-critical path running only in production.
+
+The page used to say this had "no alternative today: nobody should hold production AWS credentials to
+develop". The premise was wrong in the way this page warns about: **the credential already existed**. IAM
+user `webhook-co-claude-code` is already least-privilege — `kms:GenerateDataKey` + `kms:Decrypt`, on one
+key, and nothing else. AGENTS.md says to look for the real credential before inventing a substitute, and it
+was there the whole time.
+
+Verified with a real round-trip: a DEK wrapped by the KEK, unwrapped, and the two proven identical by
+encrypting with one and decrypting with the other.
+
+⚠️ **What a machine holds is the ability to wrap and unwrap, not the key** — the KEK never leaves AWS. It
+is still a production capability, so `AWS_ACCESS_KEY_ID` / `AWS_SECRET_ACCESS_KEY` are **excluded from the
+credential vault** for the same reason `RESEND_API_KEY` is: a passphrase sitting beside the ciphertext is
+not where a production capability belongs. `KMS_KEY_ARN` and `AWS_REGION` do travel — an ARN and a region
+name are identifiers, and withholding them would be friction without security.
+
+`KMS_MODE=local` survives as the opt-out for a contributor who cannot hold AWS credentials. The engine
+**refuses** it outright when any AWS field is bound, so the two configurations cannot be half-applied —
+four tests cover that fence, including the partial-config case.
+
+The remaining two flags are conveniences, and using them means accepting that you are not testing what
+production does.
 
 **Setting them:** put the value in the app's `.dev.vars`. `pnpm dev:secrets` writes a commented template
 listing exactly what each app wants and which values are required for parity.
diff --git a/scripts/dev-preflight.test.mjs b/scripts/dev-preflight.test.mjs
index c4ccde49..d4d4db9a 100644
--- a/scripts/dev-preflight.test.mjs
+++ b/scripts/dev-preflight.test.mjs
@@ -197,3 +197,48 @@ test("a blank play secret with the challenge ON is reported by findings()", () =
     "the playground would mint with an unverifiable challenge and nothing would say so",
   );
 });
+
+// The engine's KEK custodian, mirroring the dmarc Resend and play Turnstile pins. This PR's load-bearing
+// change is that real AWS KMS is now the local DEFAULT — drop `parityRequired` or break `relaxedBy` and
+// local dev silently returns to sealing under a throwaway KEK, with the vault, example and kms-provider
+// suites all still green. The runtime fence does not help: it only fires once a provider is constructed.
+const AWS_KMS_SPECS = ["KMS_KEY_ARN", "AWS_REGION", "AWS_ACCESS_KEY_ID", "AWS_SECRET_ACCESS_KEY"];
+
+test("every AWS KMS field is ENFORCED for the engine, with the same opt-out", () => {
+  const required = requiredSpecs("engine");
+  for (const name of AWS_KMS_SPECS) {
+    const spec = required.find((s) => s.name === name);
+    assert.ok(spec, `${name} blank would still start \`pnpm dev\` — local would not use real KMS`);
+    assert.deepEqual(
+      spec.relaxedBy,
+      { name: "KMS_MODE", value: "local" },
+      `${name} must be relaxed by the documented opt-out and nothing else`,
+    );
+  }
+});
+
+test("a blank KMS_MODE means REAL AWS, so the four fields are demanded", () => {
+  // Blank is the parity path here, which inverts the usual convention — the substitute is the flag, not
+  // its absence. If blank started relaxing them, the default would quietly become the throwaway KEK again.
+  const problems = findings([{ app: "engine", exists: true, source: "KMS_MODE=\n" }]);
+  const missing = problems.flatMap((p) => p.missing ?? []);
+  for (const name of AWS_KMS_SPECS) {
+    assert.ok(missing.includes(name), `${name} was not demanded when KMS_MODE is blank`);
+  }
+});
+
+test("KMS_MODE=local is the ONLY thing that relaxes them", () => {
+  const spec = requiredSpecs("engine").find((s) => s.name === "KMS_KEY_ARN");
+  assert.ok(
+    isRelaxed(spec, new Map([["KMS_MODE", "local"]])),
+    "the documented opt-out does not work",
+  );
+  assert.ok(
+    !isRelaxed(spec, new Map([["KMS_MODE", ""]])),
+    "blank must NOT relax — blank is the parity path",
+  );
+  assert.ok(
+    !isRelaxed(spec, new Map([["KMS_MODE", "aws"]])),
+    "an unrecognised value must not relax either",
+  );
+});
diff --git a/scripts/dev-secrets-manifest.mjs b/scripts/dev-secrets-manifest.mjs
index 43b968de..9419976c 100644
--- a/scripts/dev-secrets-manifest.mjs
+++ b/scripts/dev-secrets-manifest.mjs
@@ -143,11 +143,55 @@ export const APPS = {
         scope: "external",
         note: "Stripe billing-meter id from your sandbox. Pairs with STRIPE_METER_EVENT_NAME.",
       },
+      // --- The KEK custodian ------------------------------------------------------------------
+      // Local dev now uses the SAME custodian production does: real AWS KMS, through the same
+      // `AwsKmsProvider` seam, with the team's existing least-privilege IAM user (kms:GenerateDataKey +
+      // kms:Decrypt on one key, and nothing else). The KEK itself never leaves AWS — only wrapped data
+      // keys cross the wire — so what a machine holds is the ability to wrap and unwrap, not the key.
+      //
+      // This used to default to the hermetic LocalKmsProvider, which meant the envelope-encryption path
+      // that seals provider secrets and ingest tokens — the compliance-critical one — was never exercised
+      // before a deploy. AGENTS.md: look for the real credential before inventing a substitute. It
+      // existed the whole time.
+      //
+      // ⚠️ apps/engine REFUSES `KMS_MODE=local` when any AWS field is bound, so these two configurations
+      // cannot be half-applied: bind AWS and the flag is rejected outright rather than quietly sealing
+      // secrets under a throwaway key nobody custodies.
+      {
+        name: "KMS_KEY_ARN",
+        scope: "external",
+        parityRequired: true,
+        relaxedBy: { name: "KMS_MODE", value: "local" },
+        note: "REQUIRED for prod parity — the KEK ARN. An identifier, not a secret, so the vault carries it.",
+      },
+      {
+        name: "AWS_REGION",
+        scope: "external",
+        parityRequired: true,
+        relaxedBy: { name: "KMS_MODE", value: "local" },
+        note: "REQUIRED for prod parity — the KEK's region (us-east-2). Not a secret; the vault carries it.",
+      },
+      {
+        name: "AWS_ACCESS_KEY_ID",
+        scope: "external",
+        parityRequired: true,
+        relaxedBy: { name: "KMS_MODE", value: "local" },
+        note: "REQUIRED for prod parity — the IAM user scoped to kms:GenerateDataKey + kms:Decrypt on that one key. NOT vaulted: it pairs with a credential that reaches production.",
+      },
+      {
+        name: "AWS_SECRET_ACCESS_KEY",
+        scope: "external",
+        parityRequired: true,
+        relaxedBy: { name: "KMS_MODE", value: "local" },
+        note: "REQUIRED for prod parity. NOT vaulted — it grants Decrypt against the production KEK, which is a production capability regardless of how narrow the policy is.",
+      },
       {
         name: "KMS_MODE",
-        scope: "local",
-        value: "local",
-        note: "Selects the hermetic dev KEK custodian instead of AWS KMS. Without it, creating an endpoint needs real AWS IAM credentials. Lives ONLY here — .dev.vars is never sent by `wrangler deploy`, kmsProviderFromEnv refuses it whenever AWS KMS config is bound, and scripts/kms-mode-guard.mjs keeps it out of every committed Worker config.",
+        // `external`, and correct value BLANK: blank means real AWS KMS, matching production. It was
+        // `local`/"local", which made the substitute the default for everyone. Set it to "local" only if
+        // you cannot hold AWS credentials — the endpoint-creation path then seals under a throwaway KEK.
+        scope: "external",
+        note: 'LEAVE BLANK so the real AWS KMS custodian runs, exactly as in production. Set it to "local" ONLY if you cannot hold AWS credentials; endpoint creation then seals under a hermetic throwaway KEK and the AWS path is never exercised. Lives ONLY here — .dev.vars is never sent by `wrangler deploy`, kmsProviderFromEnv refuses it whenever AWS KMS config is bound, and scripts/kms-mode-guard.mjs keeps it out of every committed Worker config.',
       },
       {
         name: "LOCAL_KEK",
diff --git a/scripts/dev-secrets-vault.mjs b/scripts/dev-secrets-vault.mjs
index 7131179c..ad8c4916 100644
--- a/scripts/dev-secrets-vault.mjs
+++ b/scripts/dev-secrets-vault.mjs
@@ -149,6 +149,21 @@ export const NOT_SHAREABLE = new Map([
     // secrets live in a KMS, and an offline-attackable git blob is not one.
     "it is the same key production sends with — a prod capability does not belong in a passphrase-protected git blob",
   ],
+  [
+    "AWS_SECRET_ACCESS_KEY",
+    // Blast radius, same test as RESEND_API_KEY: this grants kms:Decrypt against the PRODUCTION KEK. The
+    // policy is narrow — one key, two actions — but "narrow" is not "not production". The vault's
+    // protection is a passphrase sitting beside the ciphertext, and a production capability does not
+    // belong behind one. KMS_KEY_ARN and AWS_REGION DO travel: an ARN and a region name are identifiers,
+    // and withholding them would cost setup friction for no security.
+    "it grants Decrypt against the production KEK — narrow is not the same as non-production",
+  ],
+  [
+    "AWS_ACCESS_KEY_ID",
+    // Useless alone, but it names the principal the secret half belongs to. Splitting a credential pair
+    // across "vaulted" and "fetch by hand" invites someone to wonder which half is the secret one.
+    "it is one half of a credential whose other half reaches production; they travel together or not at all",
+  ],
   [
     "STRIPE_WEBHOOK_SIGNING_SECRET",
     // Machine scope. A registered Stripe endpoint POSTs to a public URL and can never reach localhost, so
diff --git a/scripts/dev-secrets-vault.test.mjs b/scripts/dev-secrets-vault.test.mjs
index 33dd8f91..a7c265d9 100644
--- a/scripts/dev-secrets-vault.test.mjs
+++ b/scripts/dev-secrets-vault.test.mjs
@@ -66,12 +66,14 @@ test("never shares a name that is a LOCAL literal in any app", () => {
 
 // The exact set, not a floor. A floor of 6 passed while every STRIPE_* name silently dropped out of
 // discovery — the tool would still report success, just quietly stop sharing half of what it claims to.
-test("the shared set is exactly the documented 9 names", () => {
+test("the shared set is exactly the documented 11 names", () => {
   assert.deepEqual(sharedSecretNames(), [
+    "AWS_REGION",
     "GITHUB_CLIENT_ID",
     "GITHUB_CLIENT_SECRET",
     "GOOGLE_CLIENT_ID",
     "GOOGLE_CLIENT_SECRET",
+    "KMS_KEY_ARN",
     "STRIPE_METER_ID",
     "STRIPE_PLANS",
     "STRIPE_PORTAL_CONFIGURATION_ID",
@@ -361,3 +363,24 @@ test("collecting skips apps with no .dev.vars, and blank values", () => {
   );
   assert.deepEqual(conflicts, []);
 });
+
+// The KEK custodian's credential reaches production, so it must never enter a passphrase-protected vault
+// — the same test applied to RESEND_API_KEY. Its non-secret companions must still travel, or closing the
+// KMS parity gap would cost four manual values instead of two.
+test("the AWS credential pair is excluded, but its identifiers still travel", () => {
+  const shared = sharedSecretNames();
+  for (const secret of ["AWS_SECRET_ACCESS_KEY", "AWS_ACCESS_KEY_ID"]) {
+    assert.ok(!shared.includes(secret), `${secret} reaches production and must not be vaulted`);
+    assert.deepEqual(
+      [...pullSet(new Map([[secret, "leaked"]]), [secret])],
+      [],
+      `${secret} pulled anyway`,
+    );
+  }
+  for (const id of ["KMS_KEY_ARN", "AWS_REGION"]) {
+    assert.ok(
+      shared.includes(id),
+      `${id} is an identifier, not a secret — withholding it is pure friction`,
+    );
+  }
+});