# The decision point for tool-call parameters.
#
# Cluster-scoped and platform-owned, like everything else in this repo: every
# domain gateway asks the same server, so a rule is written once rather than
# five times, and changing it does not mean redeploying five gateways.
apiVersion: v1
kind: Namespace
metadata:
  name: opa
---
apiVersion: v1
kind: ConfigMap
metadata:
  name: opa-policy
  namespace: opa
data:
  policy.rego: |
    # Authorisation on tool-call *parameters*, not just the tool name.
    #
    # The estate already answers "may this identity call this tool?" — the MCP
    # server filters tool discovery by the caller's roles. What nothing answered is
    # "may it call it with *these* arguments". `write` authorises writing; it does
    # not authorise this particular record. A read cap counts calls; it does not
    # stop one call asking for ten thousand rows.
    #
    # This runs at the gateway, so it holds whether the caller is an agent behaving
    # itself, an agent that has been talked into something, or a person with curl.

    package kong

    import rego.v1

    default allow := false

    # Everything that is not an MCP tool call passes: the session handshake, the
    # tool listing, the REST surface. This policy has an opinion about arguments
    # and no opinion about anything else.
    allow if not is_tool_call

    allow if {
    	is_tool_call
    	count(violations) == 0
    }

    is_tool_call if {
    	input.request.http.parsed_body.method == "tools/call"
    }

    tool := input.request.http.parsed_body.params.name

    args := object.get(input.request.http.parsed_body.params, "arguments", {})

    # --- how much one call may read -------------------------------------------
    #
    # The rate limit counts requests. Without this, one request asking for a
    # hundred thousand rows costs the same as one asking for ten — which is how a
    # volume cap is defeated without breaking it.
    max_page_size := 200

    violations contains msg if {
    	size := to_number(object.get(args, "query_page_size", 0))
    	size > max_page_size
    	msg := sprintf("query_page_size %d exceeds the maximum of %d", [size, max_page_size])
    }

    # --- what one call may commit ---------------------------------------------
    #
    # A price is the parameter that turns a permitted write into an expensive
    # mistake. The bound is deliberately wide: this is not pricing policy, it is a
    # guard against a value that can only be an error or an attack.
    max_unit_price := 100000

    violations contains msg if {
    	body := object.get(args, "body", {})
    	some field in ["buyprice", "msrp"]
    	value := to_number(object.get(body, field, 0))
    	value > max_unit_price
    	msg := sprintf("%s %v exceeds the maximum of %d", [field, value, max_unit_price])
    }

    violations contains msg if {
    	body := object.get(args, "body", {})
    	some field in ["buyprice", "msrp"]
    	to_number(object.get(body, field, 1)) <= 0
    	msg := sprintf("%s must be greater than zero", [field])
    }

    # What the caller is told. A refusal that does not say which argument was
    # refused sends someone looking in the wrong place.
    reason := concat("; ", violations)
---
apiVersion: apps/v1
kind: Deployment
metadata:
  name: opa
  namespace: opa
spec:
  # Stateless: every decision is a function of the request. Two replicas because
  # it sits on the request path of every write in the estate — an OPA that is
  # down is a gateway that refuses.
  replicas: 2
  selector:
    matchLabels:
      app: opa
  template:
    metadata:
      labels:
        app: opa
    spec:
      securityContext:
        runAsNonRoot: true
        runAsUser: 1000
        seccompProfile:
          type: RuntimeDefault
      containers:
        - name: opa
          image: openpolicyagent/opa:1.11.0
          args:
            - run
            - --server
            - --addr=0.0.0.0:8181
            - --log-level=info
            - /policy/policy.rego
          ports:
            - name: http
              containerPort: 8181
          volumeMounts:
            - name: policy
              mountPath: /policy
              readOnly: true
          readinessProbe:
            httpGet:
              path: /health
              port: http
            initialDelaySeconds: 3
            periodSeconds: 10
          livenessProbe:
            httpGet:
              path: /health
              port: http
            initialDelaySeconds: 10
            periodSeconds: 20
          resources:
            requests:
              cpu: 20m
              memory: 64Mi
            limits:
              cpu: 500m
              memory: 256Mi
          securityContext:
            allowPrivilegeEscalation: false
            readOnlyRootFilesystem: true
            capabilities:
              drop: ["ALL"]
      volumes:
        - name: policy
          configMap:
            name: opa-policy
---
apiVersion: v1
kind: Service
metadata:
  name: opa
  namespace: opa
spec:
  selector:
    app: opa
  ports:
    - name: http
      port: 8181
      targetPort: http