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Michael Suchacz
95a2c2ba02
## Stack Context
This stack removes native chat cost tracking and native chat usage
limits, making the AI Gateway the single source of AI spend data and
budget enforcement.
1. **This PR:** re-back the per-chat cost endpoint with AI Gateway data.
2. Remove native chat usage limits end to end, rewiring the sidebar
indicator to gateway spend.
3. Remove native chat cost tracking end to end, deleting the
Analytics/Spend cost UI.
## What?
`GET /api/experimental/chats/{chat}/cost` summed
`chat_messages.total_cost_micros`, which native chat cost tracking
maintained. It now aggregates AI Gateway interception data instead, and
has no native fallback.
- New `GetAIBridgeChatCost` query, authorized through the root chat so
members can read their own chat's cost without gaining access to raw
interception rows.
- Response fields renamed: `priced_message_count` -> `request_count`,
`unpriced_messages_having_usage_count` -> `unpriced_request_count`.
- The chat summary sidebar keys its cost cache by root chat, and hides
the cost row where the AI Gateway is off or unlicensed. The root cost is
invalidated when a chat leaves an active status and when a generated
title lands, since title generation bills its own gateway request.
`GetChatModelUsageCostByChatID` and the rest of native cost tracking are
untouched here; PR 3 removes them.
## Why?
Native cost tracking duplicates what the AI Gateway already records, and
the two disagree. Repointing the endpoint first means the cost UI keeps
working while the native implementation is deleted later in the stack.
Two behaviour changes follow from gateway semantics and are intentional:
- **Requests, not messages.** The gateway records interceptions, so
counts are requests. Title-generation traffic now counts.
- **Whole-tree totals.** The gateway records the *spawning* chat's ID as
the interception session ID, so a subagent's requests are attributed to
its immediate parent, not always the root. Only a whole chat tree can be
summed, so the query resolves the root and aggregates the tree, and
every chat in a tree reports the same total. Native returned per-subtree
totals.
## Attribution and counting semantics
The aggregate groups token usage per interception before counting, so
the reported numbers are per request even though a request records one
usage row per provider response:
- `RequestCount` counts finished `Coder Agents` interceptions in the
tree, including unpriced ones.
- `UnpricedRequestCount` counts requests with at least one usage row the
gateway could not price. It is a subset of `RequestCount`.
- `TotalCostMicros` omits only unpriced usage, so a partially priced
request still contributes its priced portion. The sidebar therefore says
`Excludes unpriced usage from N request(s)` rather than claiming whole
requests were dropped.
A recorded cost of zero is a free request, not an unpriced one. Usage
without an effective group is excluded, matching what never reached
`ai_user_daily_spend`.
## Authorization
Reads go through `ExtractChatParam` plus `ResourceChat`, with no
cost-specific RBAC widening. `TestGetChatCost/MemberCanReadOwnChat`
covers a scoped `agents-access` member reading their own chat's cost,
and `MemberCannotReadOtherUsersChat` still asserts 404 for a non-owner.
Plain members without `agents-access` cannot create or read chats at
all, so they never reach this endpoint.
## Known limitation
AI Gateway data has its own retention period, 60 days by default and
configured independently of chat retention, so spend for requests older
than that is no longer reported. A chat whose gateway records have all
been purged reports zero cost, which is indistinguishable from genuinely
free usage under this contract. The endpoint documents the caveat;
#27330 documents it on the Spend Management page.
In-flight interceptions are excluded, since cost is only known once the
response is recorded. A chat's cost therefore lags the active turn by
one request.
## Rebase note
Rebased onto `main` after #27579 removed the `ai-gateway-cost-control`
experiment. The per-chat cost row is now gated on the `aibridge` feature
alone, matching how #27579 degated the other cost-control surfaces.
> Mux prepared this PR on Mike's behalf.