A passthrough stream could end with no usage even though the client asked for it via stream_options: {include_usage: true}, so providers that do not meter always showed 0 tokens. The fix estimates usage at the finish marker when the upstream stays silent (flagged estimated: true) and drops any duplicate trailing usage chunk so the client never sees two.
open-sse/utils/stream.ts:1982,1749 · open-sse/utils/usageTracking.ts:651,664
Six cases: the predicate (finish without usage but with content, trailing valid, empty response, tool-only) plus two SSE harness cases through createSSEStream passthrough.
Note on base: this branch forked 442 commits back and carried a base-red marker for #12109, which is now closed — the release tip has no open base-red issue. It merged cleanly against the current tip regardless.
Verified in a combined batch worktree: 174/174 focused tests across all 11 PRs of this batch (this PR's stream-passthrough-usage-estimation suite included), typecheck:core clean, check:cycles and check:docs-counts green.
Thanks @maxmad64bis.
5e6c9a92dc