Buyer is charged when an agent upstream fails but returns HTTP 200

Summary

A buyer is charged when an agent upstream fails internally but still returns HTTP 200. Observed live on Base mainnet: an agent's LLM provider returned a non-retryable 404, the agent aborted its conversation loop, returned an empty reply with 200 application/json, and the payment settled on-chain. The buyer paid and received nothing.

Root cause

The settle decision is not "status < 400 after the response is known" — on the non-SSE path it happens before the body exists at all.

• settlementInterceptor.WriteHeader calls settleFunc() at internal/x402/forwardauth.go:1311, i.e. the moment ReverseProxy hands over the upstream response headers. The only gate before it is statusCode >= 400 (internal/x402/forwardauth.go:1286).
• The body-liveness guard that would catch an empty answer, bytesWritten == 0 (internal/x402/forwardauth.go:1347), is unreachable on this path: finalize returns immediately unless streamDefer is set (internal/x402/forwardauth.go:1324), and streamDefer is only set for text/event-stream (internal/x402/forwardauth.go:1297-1305).

So any upstream that reports its own failure inside a 200 is settled as a success.

Both request shapes are affected

• stream: false — settles at WriteHeader, before any body byte. This is the shape that produced the incident.
• stream: true (SSE) — reaches finalize, but a role-only delta plus data: [DONE] is roughly 60 bytes, which clears bytesWritten == 0 and settles too.

A fix that covers only one shape is not a fix.

It is recorded as a clean sale

chargedRequests increments (internal/x402/verifier.go:495-498) while upstreamFailedAfterVerify does not (internal/x402/verifier.go:499-503 requires status >= 400), so the failure is invisible in metrics.

Related defect found while investigating

settlementInterceptor.Flush (internal/x402/forwardauth.go:1362-1366) is an unconditional passthrough, and statusRecorder.Flush (internal/x402/verifier.go:1020-1024) forwards it to the real writer. httputil sets flushInterval = -1 for any response with ContentLength == -1 — chunked, not only SSE — and copyResponse arms delayedFlush before the first body byte.

A chunked application/json reply therefore commits an implicit 200 while wroteStatus is still false, which:

• silently drops the X-PAYMENT-RESPONSE / PAYMENT-RESPONSE receipt set later at internal/x402/forwardauth.go:862-865, leaving the buyer to log "auth consumed without observed settlement" (internal/x402/buyer/proxy.go:741-748) — charged on-chain with no receipt to reconcile against;
• defeats the wroteStatus guards at internal/x402/forwardauth.go:826 and :847, so a settle failure cannot flip the status to 503.

No existing test catches this: httptest handlers with small bodies get an automatic Content-Length, so flushInterval is 0. Any buffering scheme added here must gate Flush.

Proposed direction

Settle only when the upstream actually delivered a completion, behind an opt-in ForwardAuthConfig flag so existing behaviour is unchanged by default:

1. For non-SSE chat-shaped routes, hold status and body until finalize and decide there.
2. For SSE, tap the stream to detect whether any content delta was delivered — without buffering or delaying a byte, so incremental streaming timing is preserved.
3. Treat a zero-byte body as "no value" before attempting to parse it — json.Unmarshal([]byte(""), …) returns an error, so a naive parse-then-fail-open settles the exact empty-body case in this report.
4. Gate Flush under any buffering mode (see above).

Two-phase auth-capture (authorize now, capture once the answer is known good) is not available as an alternative: the facilitator rejects it with ERR_TWO_PHASE_NOT_SUPPORTED, and autoCapture: true is hardcoded at internal/x402/authcapture.go:142.

Impact

Any paid agent or inference offer whose upstream can fail while returning 2xx. With the platform fee enabled, the fee splits on-chain for the failed request too.

Start with settlementInterceptor.WriteHeader, finalize, and Flush in internal/x402/forwardauth.go, then trace statusRecorder.Flush and the chargedRequests metrics in internal/x402/verifier.go. Reproduce both non-SSE and SSE paths, including chunked application/json responses, and add coverage showing settlement and receipt handling occur only after a valid completion while preserving streaming behavior.