[shreyanshjain7174]

Plan

Phase 0, item 5 of the AgentGate scale-out prompt: response body limits, a single params parse, and SSE pass-through.

The gateway read upstream bodies with io.ReadAll(io.LimitReader(body, 1 MiB)) and silently truncated anything larger. It parsed params 3 times and response JSON twice. It had no streaming.

What changed

• Oversized responses. The gateway reads at most limit + 1 bytes. Over the limit, the agent gets 502 upstream_response_too_large and the receipt records it. Nothing is truncated silently. The default is 1 MiB, overridable per service with upstream.max_response_bytes (1 KiB to 64 MiB).
• Params parsed once. One decode feeds the receipt digest, URL building and the request body.
  • A dedicated canonicalizer replaces the second JCS pass.
  • Receipts hash identically: golden vectors, a corpus test, and a differential fuzz test (FuzzDigestParamsLegacy) against the old jcs.Transform path all pass.
• Response JSON scanned once. json.Valid, then the ActResponse envelope is written by hand around the validated bytes. Field order and text escaping are unchanged. Upstream JSON is no longer HTML-escaped, which is safe because the response is application/json with nosniff.
• SSE pass-through. Streaming happens only when the action sets stream: true, the agent sends Accept: text/event-stream, and the upstream answers 2xx with text/event-stream.
  • The receipt commits when upstream headers arrive, before any byte reaches the agent.
  • Bytes flush as they arrive.
  • A client disconnect closes the upstream.
  • max_stream_bytes (default 64 MiB) and an idle timeout (default 60s) end the stream.
  • No upstream headers are copied to the agent. The gateway forces text/event-stream, nosniff and no-cache.
  • A SHA-256 of the streamed bytes and the byte count are logged server-side.
  • Everything else falls back to the buffered path.
• Receipt format unchanged. docs/spec/proposals/receipt-stream-digest.md proposes a v2 field for the stream digest.

Numbers (BenchmarkHandleAct, median of 5)

	ns/op	allocs/op
Before	49,099	330
After	46,745	266

A 1 MiB JSON response envelope now costs 4 allocs/op.

Validation

• go build ./...: passed
• go vet ./...: passed
• go test -race ./...: passed
• semgrep --config auto --error --quiet internal/gateway internal/upstream internal/registry: passed. Two scoped nosemgrep lines on raw writes. Each sits on a response with a fixed Content-Type and nosniff, and is commented.

Review

Reviewed by a second model (Claude Opus 5.5). Approved with 2 minors, both fixed here: a bounded final write deadline on streams, and removing the second JSON scan.

Known limits

• An SSE receipt attests that the upstream accepted the request and started the stream. A later stream failure is logged, not receipted. Documented.
• buildURL still does not escape path params. This is unchanged from main and is a separate fix.