Fixes#5148: with aggregator upstreams (e.g. newapi) that end SSE
streams without a proper terminal event, every such request was
silently missing from usage logs and billing.
Root cause (tracked via the nested audit issue): the low-level
Anthropic SSE readers already return the partially collected usage
together with the stream error (missing terminal event, read error,
interval timeout), but Forward converted every such result to
(nil, err) and the handler returned before submitting RecordUsage.
Changes:
- Add partialStreamUsageResult: on stream errors, wrap observed usage
into a ForwardResult and return it alongside the error, for both the
regular Anthropic path and the API-key passthrough path. Invariants:
UpstreamFailoverError always keeps result=nil (failover retries are
billed as the successful attempt, never twice), and zero observed
usage returns no partial result (no phantom zero-usage records).
- Messages handler: hoist the usage submission block into a closure
shared by the success path and the new partial-result error path.
- Usage record worker pool: distinguish pool-stopped drops
(dropped_stopped) from operator-configured drop/sample overflow
drops; billing tasks now fall back to inline synchronous execution
only during the shutdown window, while explicit drop/sample overflow
semantics are preserved. Image usage keeps its mandatory fallback
for both drop kinds via the new mode.Dropped() helper.
Tests: Forward-level regressions for missing-terminal / read-error /
no-usage / failover-invariant on both paths, plus handler-level
stopped-pool sync fallback and drop-policy preservation tests.
When an upstream API gateway (e.g. new-api) relays real Claude Code
requests, the User-Agent becomes Go-http-client while the body retains
the full Claude Code fingerprint (billing attribution block +
metadata.user_id + cache_control breakpoints).
Previously, the OAuth mimicry path relied solely on UA matching to
detect Claude Code clients. Without a matching UA, the gateway would
rewrite the system prompt — replacing the client's carefully structured
system blocks and cache_control breakpoints with its own injection.
This breaks Anthropic's prefix-based prompt cache: since the cache key
evaluates tools → system → messages in order, a changed system
invalidates all downstream message caching.
Symptoms observed:
- cache_read permanently locked at ~25K (only system prompt cached)
- cache_creation growing monotonically every turn (full messages rewrite)
- Single-request costs $17-27 instead of normal $1-2
Fix: when UA does not match but the body contains a valid billing
attribution block (x-anthropic-billing-header with cc_entrypoint=),
treat the request as proxied Claude Code traffic and skip mimicry.
This preserves the client's original system structure and cache_control
breakpoints, allowing Anthropic's prompt cache to function correctly.
IanShaw