diff --git a/core/lambda/contract_ingestor.py b/core/lambda/contract_ingestor.py
index 8880d5d..9b8c0b1 100644
--- a/core/lambda/contract_ingestor.py
+++ b/core/lambda/contract_ingestor.py
@@ -398,6 +398,65 @@ def _validate_change_request(payload):
     }
 
 
+def _onboard_consumer(payload):
+    """P18 (REQ-182): accept a self-service onboarding request.
+
+    Validates the payload against schemas/onboarding.schema.json, then
+    writes a 'pending' row to nova-contracts (D-119). No AWS resources
+    are created by this action (D-113); the cross-account role + ABAC
+    tag grant is offline-proven Terraform (P20/REQ-184).
+    """
+    import jsonschema
+    schema_path = os.path.join(os.path.dirname(os.path.dirname(
+        os.path.dirname(os.path.abspath(__file__)))),
+        "schemas", "onboarding.schema.json")
+    try:
+        with open(schema_path) as f:
+            schema = json.load(f)
+        # Strip the Lambda dispatch envelope (action) before validating
+        # against the onboarding schema (the schema is about the request,
+        # not the Lambda wrapper).
+        onboarding_payload = {k: v for k, v in payload.items() if k != "action"}
+        jsonschema.validate(instance=onboarding_payload, schema=schema)
+    except OSError:
+        raise ValueError("onboarding schema unavailable")
+    except jsonschema.ValidationError as e:
+        raise ValueError(f"onboarding payload invalid: {e.message}")
+
+    consumer_repo = payload["consumerRepo"]
+    requested_env = payload["requestedEnvironment"]
+    owner_id = payload["ownerId"]
+    billing_tag = payload["billingTag"]
+    submitted_at = _iso8601_now()
+
+    # Write a pending CMDB row (PK consumerRepo, SK onboarding#env#timestamp).
+    table = _get_dynamodb().Table(TABLE_NAME)
+    item = {
+        "consumerRepo": consumer_repo,
+        "contractId#submittedAt": f"onboarding#{requested_env}#{submitted_at}",
+        "contractId": f"onboarding-{requested_env}",
+        "environment": requested_env,
+        "status": "pending",
+        "ownerId": owner_id,
+        "billingTag": billing_tag,
+        "notes": payload.get("notes", ""),
+        "submittedAt": submitted_at,
+    }
+    table.put_item(TableName=TABLE_NAME, Item=item)
+    return {
+        "status": "pending",
+        "consumerRepo": consumer_repo,
+        "requestedEnvironment": requested_env,
+        "action": "onboard_consumer",
+        "submittedAt": submitted_at,
+        "message": (
+            "Onboarding request received. The platform team will provision "
+            "the environment binding + cross-account role. Track the status "
+            "via the nova-contracts table (status=pending → granted)."
+        ),
+    }
+
+
 def lambda_handler(event, context):
     """AWS Lambda handler entry point.
 
@@ -426,6 +485,8 @@ def lambda_handler(event, context):
             result = _report_error(payload)
         elif action == "validate_change_request":
             result = _validate_change_request(payload)
+        elif action == "onboard_consumer":
+            result = _onboard_consumer(payload)
         else:
             return {
                 "statusCode": 400,
diff --git a/schemas/onboarding.schema.json b/schemas/onboarding.schema.json
new file mode 100644
index 0000000..f982052
--- /dev/null
+++ b/schemas/onboarding.schema.json
@@ -0,0 +1,39 @@
+{
+  "$schema": "https://json-schema.org/draft/2020-12/schema",
+  "$id": "https://nova.cloudinit.dev/schemas/onboarding.schema.json",
+  "title": "Nova Consumer Onboarding Request",
+  "description": "A self-service onboarding request from a consumer repo. Submitted to the contract_ingestor Lambda 'onboard_consumer' action (D-113, P18/REQ-182). The Lambda validates the payload against this schema, then writes a 'pending' CMDB row to nova-contracts. No AWS resources are created by this action (D-119); the cross-account role + ABAC tag grant is offline-proven Terraform (P20/REQ-184).",
+  "type": "object",
+  "required": ["consumerRepo", "requestedEnvironment", "ownerId", "billingTag"],
+  "additionalProperties": false,
+  "properties": {
+    "consumerRepo": {
+      "type": "string",
+      "description": "The consumer repository in org/repo format.",
+      "pattern": "^[a-zA-Z0-9_.-]+/[a-zA-Z0-9_.-]+$",
+      "maxLength": 128
+    },
+    "requestedEnvironment": {
+      "type": "string",
+      "description": "The environment the consumer requests (must exist as a core/environments/<name>.json).",
+      "enum": ["dev", "qa", "prod", "dr"]
+    },
+    "ownerId": {
+      "type": "string",
+      "description": "The owning team or individual (for ABAC nova:owner tag + CMDB).",
+      "minLength": 1,
+      "maxLength": 64
+    },
+    "billingTag": {
+      "type": "string",
+      "description": "The cost-center / billing tag for the consumer's resources.",
+      "minLength": 1,
+      "maxLength": 64
+    },
+    "notes": {
+      "type": "string",
+      "description": "Optional free-form notes for the platform team.",
+      "maxLength": 500
+    }
+  }
+}
\ No newline at end of file
diff --git a/tests/test_contract_ingestor.py b/tests/test_contract_ingestor.py
index 89a30fd..58d3242 100644
--- a/tests/test_contract_ingestor.py
+++ b/tests/test_contract_ingestor.py
@@ -593,4 +593,52 @@ class TestV14IdentityValidation:
         """The _validate_caller_identity docstring documents the ABAC reliance."""
         docstring = ingestor._validate_caller_identity.__doc__
         assert "ABAC" in docstring
-        assert "PrincipalTag" in docstring
\ No newline at end of file
+        assert "PrincipalTag" in docstring
+
+class TestOnboardConsumer:
+    """P18 (REQ-182): the onboard_consumer action writes a pending CMDB row."""
+
+    _ARN = "arn:aws:sts::000:assumed-role/nova-deploy/test"
+
+    def test_valid_onboarding_writes_pending_row(self, moto_contracts_table):
+        payload = {
+            "action": "onboard_consumer",
+            "consumerRepo": "acdl/consumer-b",
+            "requestedEnvironment": "dev",
+            "ownerId": "team-b",
+            "billingTag": "cost-center-b",
+        }
+        event = {"body": json.dumps(payload), "requestContext": {"identity": {"userArn": self._ARN}}}
+        resp = ingestor.lambda_handler(event, None)
+        assert resp["statusCode"] == 200
+        body = json.loads(resp["body"])
+        assert body["status"] == "pending"
+        assert body["action"] == "onboard_consumer"
+        assert body["requestedEnvironment"] == "dev"
+
+    def test_invalid_onboarding_rejected(self, moto_contracts_table):
+        # An invalid consumerRepo (no /) fails the identity format check
+        # (which runs for all actions) before the onboarding schema.
+        payload = {
+            "action": "onboard_consumer",
+            "consumerRepo": "not-a-repo-format",
+            "requestedEnvironment": "dev",
+            "ownerId": "team-b",
+            "billingTag": "cost-center-b",
+        }
+        event = {"body": json.dumps(payload), "requestContext": {"identity": {"userArn": self._ARN}}}
+        resp = ingestor.lambda_handler(event, None)
+        assert resp["statusCode"] == 400
+        assert "invalid consumerRepo" in json.loads(resp["body"])["error"]
+
+    def test_missing_onboarding_field_rejected(self, moto_contracts_table):
+        payload = {
+            "action": "onboard_consumer",
+            "consumerRepo": "acdl/consumer-b",
+            "requestedEnvironment": "dev",
+            # ownerId + billingTag missing
+        }
+        event = {"body": json.dumps(payload), "requestContext": {"identity": {"userArn": self._ARN}}}
+        resp = ingestor.lambda_handler(event, None)
+        assert resp["statusCode"] == 400
+        assert "onboarding payload invalid" in json.loads(resp["body"])["error"]