feat(capabilities): allow free verbs with no billing unit

surfsense_backend/app/capabilities/billing.py

@@ -17,9 +17,11 @@ if TYPE_CHECKING:
 
 
 async def charge_capability(
-    output: BillableOutput, unit: BillingUnit, ctx: CapabilityContext
+    output: BillableOutput, unit: BillingUnit | None, ctx: CapabilityContext
 ) -> None:
-    """Bill the workspace owner for this result's billable successes (03c)."""
+    """Bill the workspace owner for this result's billable successes (03c). ``None`` = free."""
+    if unit is None:
+        return
     units = output.billable_units
     if units <= 0:
         return

surfsense_backend/app/capabilities/types.py

@@ -13,7 +13,7 @@ if TYPE_CHECKING:
 
 
 class BillingUnit(StrEnum):
-    """The meter a verb charges on (priced by the billing service, 03c)."""
+    """The meter a verb charges on (priced by the billing service, 03c). ``None`` = free."""
 
     WEB_CRAWL = "web_crawl"
 
@@ -44,4 +44,4 @@ class Capability:
     input_schema: type[BaseModel]
     output_schema: type[BaseModel]
     executor: Executor
-    billing_unit: BillingUnit
+    billing_unit: BillingUnit | None

surfsense_backend/tests/unit/capabilities/test_billing.py

@@ -114,3 +114,14 @@ async def test_disabled_is_noop(monkeypatch, record_usage):
     record_usage.assert_not_awaited()
     session.execute.assert_not_called()
     assert user.credit_micros_balance == 100_000
+
+
+async def test_free_verb_without_a_unit_is_noop(monkeypatch, record_usage):
+    monkeypatch.setattr(config, "WEB_CRAWL_CREDIT_BILLING_ENABLED", True)
+    session, user = _make_session(_OWNER, balance_micros=100_000)
+
+    await charge_capability(_output("success", "success"), None, _ctx(session))
+
+    record_usage.assert_not_awaited()
+    session.execute.assert_not_called()
+    assert user.credit_micros_balance == 100_000