diff --git a/src/user_engine/domain/models.py b/src/user_engine/domain/models.py
index b0b41f0..2e69d4d 100644
--- a/src/user_engine/domain/models.py
+++ b/src/user_engine/domain/models.py
@@ -579,6 +579,8 @@ class RegistrationSession:
     started_by_subject: str | None = None
     applicant_username: str | None = None
     client_id: str | None = None
+    start_idempotency_hash: str | None = None
+    start_request_hash: str | None = None
     provisioning_resume_hash: str | None = None
     correlation_id: str | None = None
     created_at: datetime = field(default_factory=utc_now)
diff --git a/src/user_engine/web.py b/src/user_engine/web.py
index 1b6009b..938ba3b 100644
--- a/src/user_engine/web.py
+++ b/src/user_engine/web.py
@@ -198,8 +198,10 @@ class PortalApplication:
             if not self.public_registration or self.registration_verification is None:
                 raise NotFoundError("public registration is unavailable")
             token = secrets.token_urlsafe(32)
+            idempotency_key = secrets.token_urlsafe(24)
             return self._html(
-                start_response, self._registration_form(token), correlation_id,
+                start_response,
+                self._registration_form(token, idempotency_key), correlation_id,
                 extra_headers=[("Set-Cookie", self._registration_csrf_cookie(token))],
             )
         if path == "/register" and method == "POST":
@@ -919,6 +921,34 @@ class PortalApplication:
         if tenant not in self.registration_tenants:
             raise ValidationError("tenant is not eligible for registration")
 
+        raw_idempotency_key = str(
+            body.get("idempotency_key") if browser
+            else environ.get("HTTP_IDEMPOTENCY_KEY", "")
+        )
+        if len(raw_idempotency_key) < 16 or len(raw_idempotency_key) > 256:
+            raise ValidationError("registration idempotency key is invalid")
+        idempotency_hash = hmac.new(
+            self.trusted_proxy_secret.encode(), raw_idempotency_key.encode(),
+            hashlib.sha256,
+        ).hexdigest()
+        request_hash = hashlib.sha256(json.dumps({
+            "username": username, "email": email, "display_name": display_name,
+            "client_id": client_id, "tenant": tenant,
+        }, sort_keys=True, separators=(",", ":")).encode()).hexdigest()
+        previous = next((
+            item for item in self.service.store.all_registration_sessions()
+            if item.start_idempotency_hash
+            and hmac.compare_digest(item.start_idempotency_hash, idempotency_hash)
+        ), None)
+        if previous is not None:
+            if not previous.start_request_hash or not hmac.compare_digest(
+                previous.start_request_hash, request_hash
+            ):
+                raise ConflictError("registration idempotency key was reused")
+            return self._registration_requested_response(
+                start_response, correlation_id, browser
+            )
+
         applicant_subject = f"applicant_{secrets.token_hex(16)}"
         actor = Actor(
             issuer="urn:netkingdom:public-registration",
@@ -937,6 +967,11 @@ class PortalApplication:
             applicant_username=username,
             client_id=client_id,
         )
+        session = replace(
+            session, start_idempotency_hash=idempotency_hash,
+            start_request_hash=request_hash,
+        )
+        self.service.store.save_registration_session(session)
         self.registration_verification.request(
             RegistrationVerificationRequest(
                 registration_id=session.registration_id,
@@ -948,6 +983,13 @@ class PortalApplication:
                 display_name=display_name,
             )
         )
+        return self._registration_requested_response(
+            start_response, correlation_id, browser
+        )
+
+    def _registration_requested_response(
+        self, start_response: StartResponse, correlation_id: str, browser: bool
+    ) -> Iterable[bytes]:
         if browser:
             return self._html(
                 start_response,
@@ -960,10 +1002,8 @@ class PortalApplication:
                 correlation_id,
             )
         return self._json(
-            start_response,
-            "202 Accepted",
-            {"status": "verification_requested"},
-            correlation_id,
+            start_response, "202 Accepted",
+            {"status": "verification_requested"}, correlation_id,
         )
 
     def _verify_public_registration(
@@ -1325,7 +1365,7 @@ class PortalApplication:
             + identity,
         )
 
-    def _registration_form(self, csrf_token: str) -> str:
+    def _registration_form(self, csrf_token: str, idempotency_key: str) -> str:
         client_options = "".join(
             f'<option value="{escape(item)}">{escape(item)}</option>'
             for item in sorted(self.registration_clients)
@@ -1340,6 +1380,7 @@ class PortalApplication:
 <p>We will verify your email before creating an identity.</p>
 <form method="post" action="/register">
 <input type="hidden" name="csrf_token" value="{escape(csrf_token)}">
+<input type="hidden" name="idempotency_key" value="{escape(idempotency_key)}">
 <label>Username <input name="username" required minlength="3" maxlength="32" pattern="[A-Za-z][A-Za-z0-9._-]+" autocomplete="username"></label>
 <label>Email <input name="email" type="email" required autocomplete="email"></label>
 <label>Display name <input name="display_name" maxlength="200" autocomplete="name"></label>
diff --git a/tests/test_web.py b/tests/test_web.py
index 2a0167f..91aea5b 100644
--- a/tests/test_web.py
+++ b/tests/test_web.py
@@ -1,4 +1,5 @@
 import io
+import hashlib
 import json
 import re
 import unittest
@@ -53,6 +54,8 @@ def invoke(
     if claims is not None:
         environ["HTTP_X_VERIFIED_OIDC_CLAIMS"] = json.dumps(claims)
         environ["HTTP_X_USER_ENGINE_PROXY_SECRET"] = marker
+    if path == "/api/v1/public/registrations" and method == "POST":
+        environ["HTTP_IDEMPOTENCY_KEY"] = hashlib.sha256(payload).hexdigest()
     environ.update(headers or {})
     captured = {}
 
@@ -255,6 +258,9 @@ class PortalApplicationTests(unittest.TestCase):
         self.assertIn(b"Create your account", html)
         cookie = page["headers"]["Set-Cookie"]
         token = re.search(rb'name="csrf_token" value="([^"]+)"', html).group(1).decode()
+        idempotency_key = re.search(
+            rb'name="idempotency_key" value="([^"]+)"', html
+        ).group(1).decode()
         denied, _ = invoke(self.app, "/register", method="POST", form={
             "csrf_token": "wrong", "username": "new.person",
             "email": "new@example.test", "client_id": "coulomb-social",
@@ -262,7 +268,8 @@ class PortalApplicationTests(unittest.TestCase):
         }, cookie=cookie)
         self.assertEqual("403 Forbidden", denied["status"])
         started, html = invoke(self.app, "/register", method="POST", form={
-            "csrf_token": token, "username": "new.person",
+            "csrf_token": token, "idempotency_key": idempotency_key,
+            "username": "new.person",
             "email": "new@example.test", "display_name": "New Person",
             "client_id": "coulomb-social", "tenant": "tenant:coulomb",
         }, cookie=cookie)
@@ -436,6 +443,38 @@ class PortalApplicationTests(unittest.TestCase):
         )
         self.assertEqual("400 Bad Request", result["status"])
 
+    def test_public_registration_start_is_idempotent_and_key_is_payload_bound(self):
+        verifier = FakeRegistrationVerification()
+        self.app.registration_verification = verifier
+        self.app.registration_clients = frozenset({"coulomb-social"})
+        self.app.registration_tenants = frozenset({"tenant:coulomb"})
+        body = {
+            "username": "idem.person", "email": "idem@example.test",
+            "client_id": "coulomb-social", "tenant": "tenant:coulomb",
+        }
+        headers = {"HTTP_IDEMPOTENCY_KEY": "registration-key-123456789"}
+        first, _ = invoke(
+            self.app, "/api/v1/public/registrations", method="POST",
+            body=body, headers=headers,
+        )
+        registration_id = verifier.requested.registration_id
+        replay, _ = invoke(
+            self.app, "/api/v1/public/registrations", method="POST",
+            body=body, headers=headers,
+        )
+        conflicting, _ = invoke(
+            self.app, "/api/v1/public/registrations", method="POST",
+            body={**body, "email": "different@example.test"}, headers=headers,
+        )
+        sessions = self.app.service.store.all_registration_sessions()
+        self.assertEqual("202 Accepted", first["status"])
+        self.assertEqual("202 Accepted", replay["status"])
+        self.assertEqual("409 Conflict", conflicting["status"])
+        self.assertEqual(1, len(sessions))
+        self.assertEqual(1, verifier.request_count)
+        self.assertEqual(registration_id, sessions[0].registration_id)
+        self.assertNotIn("registration-key-123456789", repr(sessions[0]))
+
     def test_public_registration_rate_limit_uses_peer_not_forwarded_header(self):
         self.app.registration_verification = FakeRegistrationVerification()
         self.app.registration_rate_limit = 2
@@ -896,8 +935,10 @@ class FakeRegistrationVerification:
     def __init__(self):
         self.requested = None
         self.registration_id = None
+        self.request_count = 0
 
     def request(self, request):
+        self.request_count += 1
         self.requested = request
         return RegistrationVerificationReceipt(request_id="vrq_test")
 
diff --git a/workplans/USER-WP-0022-public-registration-and-jit-application-profiles.md b/workplans/USER-WP-0022-public-registration-and-jit-application-profiles.md
index b047e7b..85834d3 100644
--- a/workplans/USER-WP-0022-public-registration-and-jit-application-profiles.md
+++ b/workplans/USER-WP-0022-public-registration-and-jit-application-profiles.md
@@ -102,6 +102,14 @@ atomically invalidate one another, replay fails closed, and no identity is
 created from cancellation. The full user-engine suite passes 133 tests with
 three environment-dependent skips; email-connect passes all 24 tests.
 
+2026-08-10 idempotency completion: public API starts now require an
+`Idempotency-Key`, while the browser form receives a cryptographically random
+hidden key. user-engine persists only a keyed HMAC of that value plus a hash of
+the normalized request. Exact retries return the same generic response without
+creating another session or requesting another email; reuse with different
+inputs returns 409. Raw keys and applicant fields are absent from idempotency
+evidence. The full suite passes 134 tests with three external skips.
+
 ## T02 - Orchestrate provider identity creation
 
 ```task