DBL Policy

Deterministic, tenant-scoped policy evaluation for DBL. This package produces DECISION events only. It does not execute tasks. Status: Stable

What it is

dbl-policy is the contract layer for policy in the DBL stack:

• It evaluates a policy from authoritative inputs only
• It returns ALLOW or DENY with stable reason codes
• It can be bridged into a dbl-core DECISION event with a strict, contract-shaped data mapping
• It is pure: no IO, no time, no randomness, no env, no network, no trace-dependence

Non-goals

• No execution
• No orchestration
• No reading or depending on observational fields (trace, runtime, exceptions, etc.)
• No mutation of event streams

Contract

The authoritative specification is in:

• docs/dbl_policy_contract.md
• docs/ARCHITECTURE.md

Architecture direction:

• dbl-policy is the contract and bridge layer
• dbl-policy-gates is the gate algebra layer
• domain policies are root-level assemblies built on top

Composition lives in dbl-policy-gates.

Key invariants enforced by this package:

• Inputs must be JSON-safe and deterministic
  • No floats (including nested)
  • Mapping keys must be exact str (no subclasses)
• Policy input is whitelisted
  • Unknown keys are rejected
• PolicyContext is snapshotted
  • Caller mutation after construction cannot affect evaluation or digest
• Output can be converted into a dbl-core DECISION event
  • DECISION.data is a Mapping with a strict shape
  • Includes policy lineage: policy_id, policy_version, tenant_id

Install

pip install dbl-policy

Requires Python 3.11+ and dbl-core>=0.3,<0.4.

Quickstart

1. Use a built-in policy

from dbl_policy import PolicyContext
from dbl_policy.model import TenantId
from dbl_policy.allow_all import POLICY as ALLOW_ALL

ctx = PolicyContext(
    tenant_id=TenantId("tenant-1"),
    inputs={"intent_type": "chat.message"},
)

decision = ALLOW_ALL.evaluate(ctx)

2. Convert a decision to a DBL DECISION event

from dbl_policy.bridge import decision_to_dbl_event

event = decision_to_dbl_event(decision, correlation_id="c1")

event.data will be shaped like:

{
  "policy_id": "...",
  "policy_version": "...",
  "tenant_id": "...",
  "gate": {
    "decision": "ALLOW" | "DENY",
    "reason_code": "...",
    # "reason_message": "..." (optional)
  }
}

Writing your own policy

Policies only implement evaluate(context) and must be deterministic.

from dataclasses import dataclass
from dbl_policy import PolicyContext, PolicyDecision
from dbl_policy.model import DecisionOutcome, PolicyId, PolicyVersion
import dbl_policy.reason_codes as reason_codes


@dataclass(frozen=True)
class ExamplePolicy:
    policy_id: PolicyId = PolicyId("example")
    policy_version: PolicyVersion = PolicyVersion("1.0.0")

    def evaluate(self, context: PolicyContext) -> PolicyDecision:
        intent_type = context.inputs.get("intent_type")
        if intent_type == "blocked":
            return PolicyDecision(
                outcome=DecisionOutcome.DENY,
                reason_code=reason_codes.TENANT_BLOCKED,
                reason_message="blocked intent_type",
                policy_id=self.policy_id,
                policy_version=self.policy_version,
                tenant_id=context.tenant_id,
            )
        return PolicyDecision(
            outcome=DecisionOutcome.ALLOW,
            reason_code=reason_codes.OK,
            policy_id=self.policy_id,
            policy_version=self.policy_version,
            tenant_id=context.tenant_id,
        )

Safe evaluation (recommended)

decide_safe wraps context validation and converts failures into a stable DENY.

• Valid inputs: evaluates policy, ensures authoritative_digest is populated
• Invalid inputs: DENY with stable reason_code
• Exceptions during evaluation: DENY with evaluation_error

from dbl_policy import decide_safe
from dbl_policy.allow_all import POLICY

d1 = decide_safe(POLICY, "tenant-1", {"intent_type": "x"})
d2 = decide_safe(POLICY, "tenant-1", {"unknown_key": "x"})  # -> DENY

Allowed context keys

The contract enforces a strict whitelist:

• principal_id
• workspace_id
• intent_type
• capability
• model_id
• provider
• max_output_tokens
• input_bytes
• input_chars
• risk_tier
• request_tags
• extensions

Legacy removal

The pre-alignment starter policy pack from dbl_policy.policies has been removed in 0.3.0.

• It is superseded by the dbl-policy-gates algebra layer.
• dbl-policy now focuses on protocol, validation, and bridge concerns only.
• See docs/ARCHITECTURE.md and docs/MIGRATION_0_3.md.

Reason codes

Reason codes are stable semantic identifiers:

• ok
• allow_all
• deny_all
• invalid_input
• unknown_context_key
• tenant_blocked
• missing_required_input
• evaluation_error
• admission.missing_required
• admission.invalid_value
• capability.denied
• model.denied
• cost.output_tokens_cap
• cost.input_bytes_cap
• cost.input_chars_cap
• risk.high_requires_override

See src/dbl_policy/reason_codes.py.

Development

python -m venv .venv
.venv\Scripts\Activate.ps1
python -m pip install -e ".[dev]"
pytest