## Problem
We're getting 402s when an AI **agent** (chat) run hits the free
allowance / spending cap, but the frontend handles them poorly and never
pops the usage-limit modal.
The agent runs its tool calls **server-side** (loopback HTTP via
`PolicyExecutor`), so the 402 never reaches the `apiClient` interceptor
that pops the modal for direct calls. It was caught by the generic
tool-failure handler and flattened into a `CANNOT_CONTINUE` reason
string (`"The /api/v1/… tool failed: 402…"`), streamed as a `result`
event, and rendered as a scary chat bubble. This is the same gap the
policy auto-run path bridges (#6626) — one layer up.
## Fix
**Backend** (`proprietary`)
- `AiWorkflowResponse` gains `errorCode` + `errorSubscribed`.
- `AiWorkflowService` detects a downstream 401/402 entitlement sentinel
in its three tool-exec catch sites (`onToolCall`, `runPlan`,
`onConvertMarkdown`) and surfaces the structured code (+ `subscribed`)
on the terminal response instead of the raw failure text.
- Factored the 401/402 body extraction `PolicyEngine` already had into a
shared `DownstreamEntitlementError` util so the two server-side paths
can't drift.
**Frontend**
- New `usageLimitBridge` (`PAYG_LIMIT_REACHED_EVENT` +
`dispatchPaygLimitReached`) generalises the previously policy-only
bridge. Proprietary can't import the saas modal API (layering), so
server-side limit hits broadcast a window event the saas
`UsageLimitModalHost` opens the modal from. Migrated the policy path
onto it.
- `ChatContext` fires the matching modal (free → subscribe, subscribed →
raise cap) on the limit result **and** on a direct 402, replacing the
raw reason with a brief friendly line
(`chat.responses.usage_limit_reached`).
No Python engine changes — the charge/402 happens on the Java tool
endpoint that Java itself calls.
## Test plan
- [x] `:proprietary:compileJava` + `spotlessCheck` clean
- [x] `AiWorkflowServiceTest` + `PolicyEngineTest` green
- [x] eslint, proprietary + saas typechecks clean
- [ ] Manual: drive an agent run over the limit → brief line in chat +
the right modal (free vs cap)
> Note: proprietary test compilation is currently blocked on the
pre-existing `InitialSecuritySetupTest` 6-arg ctor break (unrelated,
tracked separately); verified locally by temporarily patching it.
