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M3: union deserialization single-pass redesign + streaming adapter boundary (P1 P5 P3) — benchmark-gated #23
Description
Source: multi-agent review of PR #16, perf verifier verdicts. 2026-08-13. M-anchor: M3 runway — the SSE engine sits directly on this path (one union-typed event per message on a hot loop); redesign lands before/with M3, gated on baselines from the performance test project (see the perf-infrastructure issue).
- P1 · CONFIRMED · low-to-moderate now, high-plausibility M3 hot-loop concern — all seven generated union converters do
JsonDocument.ParseValue(ref reader)thenJsonSerializer.Deserialize(payload, typeInfo)(SessionMessageInfoJsonConverter.cs:35,56; emitted byUnionEmitter.cs:270-280): DOM materialization + second traversal per union level; nested assistant content/tool state multiplies. Verifier correction: theJsonElementoverload does not necessarily re-encode text — the cost is DOM + second traversal, not string round-trip. Fix direction: benchmark a reader-based discriminator dispatch that avoids the DOM for known tags while preserving unknown-payload capture. Do NOT assume the discriminator is the first property — a robust design needs a copied reader or controlled buffering. KeepJsonDocument/clone for unknown carriers. - P5 · PLAUSIBLE · negligible-to-low now — per-node
OpenCodeJsonContext.Default.GetTypeInfo(type)(SessionMessageInfoJsonConverter.cs:53-56) resolves through a cachedOptions.TryGetTypeInfo, not fresh metadata. Do not optimize speculatively; after P1, benchmark direct tag→JsonTypeInfodispatch on representative nested M3 events. Stay source-generated and AOT-safe. - P3 · CONFIRMED · low now, medium for large one-shot payloads —
Pipelineunconditionally buffers viaReadAsStringAsync(Pipeline.cs:92,146); success adapters parse and discard the UTF-16 string (RawBodyis error-only);ResponseHeadersReadalready means status is known pre-buffer. Fix direction: adapter-boundary redesign so declared success statuses can deserialize from the content stream while error statuses stay string-buffered (ADR-0007RawBodycontract survives). Not a one-liner — adapters currently takestring rawBody. The M3 SSE engine gets its own streaming path and must not reuse the one-shot buffer. - P4 rider (very low): downgrade path triple-parse (
ResponseAdapter.cs:84,90,99-102) — only if profiling justifies; correctness of the compatibility path outranks it.
Activity
docs(agents): hand off the M2 close-out — Q91 research, fix batch, me…
fix(sdk): land the small confirmed review fold-ins
Blind-Striker commentedon Aug 15, 2026
The second review of PR #26 confirmed a third mechanism for this issue's benchmark-gated redesign (R2-14c): cursor-list success pages are validated element-by-element by
NullElementRejectingListJsonConverter, then re-copied and re-scanned by the public envelope initializer's defensive copy (ListPayloadInput.CopyRejectingNullElements) — an O(n) copy + null re-scan per success page that every future list operation inherits. Candidate shape: an internal adoption path inEnvelopeEmitter(adapter-built success instances adopt the converter-validated list without the second copy) while the public init keeps its defensive copy for consumer-constructed instances. Same benchmark gate as the rest of this issue.Blind-Striker commentedon Aug 17, 2026
The
1ee8c30review adds correctness requirements to P3's adapter-boundary rewrite; they belong here rather than in a parallel buffering issue.Timeout scope
Every send uses
ResponseHeadersRead, soHttpClient.Timeoutstops governing once headers arrive.ReadBodyAsyncreceives only the caller token. A probe with a 50 ms client timeout, immediate headers, a blocked body, and a 500 ms caller token ran until the caller's 500 ms deadline.The one-shot body must remain inside one overall timeout budget from send through consumption. Caller cancellation still passes through; timeout with an untouched caller token maps to
OpenCodeTransportException. Open SSE bodies remain post-header timeout-exempt.Success media type
The adapter receives only
(status, rawBody), so a 200 health body undertext/plainor withoutContent-Typeis accepted as JSON success. P3 must preserve each operation's declared success media type and apply the chosen policy. Fail-closed recommendation: missing/different media isOpenCodeTransportException, compared case-insensitively after stripping parameters.Ownership
The current one-shot response is disposed correctly, but no test observes response/content disposal on success, API error, malformed success, or cancellation. The new handoff must pin those paths while preserving ADR-0007 error
RawBodyandNoThrowbehavior.UTF-8 link
Malformed one-shot UTF-8 is currently replacement-decoded before STJ and can return corrupted typed data. Fold strict success-body decoding into this rewrite; the SSE half is tracked separately.
Execution trigger: Arc 6 with P3, before closing
#23.Blind-Striker commentedon Aug 17, 2026
Scope correction from ADR-0014 / Q106 (7f771eb): retire the unimplemented one-shot strict-UTF-8 rider, the single-declared-materializer response-media validation rider, and the premise that generated model defensive copies must survive. Buffered JSON continues through HttpContent charset/BOM decoding; media remains a runtime dispatch key only when several materializers exist. Keep timeout budgeting, response ownership on every exit, reader-based known-union dispatch, raw error-body retention, and measured success-path representation work. #55 owns model/copy cleanup; this issue retains the remaining Arc 6 adapter/converter performance work.
Blind-Striker commentedon Aug 20, 2026
Arc 6 shipped in fa6124d and passed hosted CI run 32374393085 on Linux, Windows, and macOS. Known tagged-union arms now scan a copied reader and deserialize the original reader once; only unknown carriers retain a JsonElement DOM. Declared one-shot successes use validated UTF-8 byte adapters while charset/BOM and malformed-UTF-8 replacement behavior remain compatible; errors stay decoded strings with RawBody under throw and NoThrow. The overall HttpClient timeout now covers finite body consumption, pending reads are disposed/observed on timeout or caller cancellation, and no-content successes skip body reads while retaining response ownership. Same-environment baseline -> final: deep union 41.99 us / 13.35 KB -> 37.54 us / 12.72 KB; GetMessage 45.970 us / 21.91 KB -> 39.728 us / 19.15 KB; ListMessages 46.859 us / 22.60 KB -> 43.593 us / 19.81 KB. Health allocation moved 2.08 -> 2.05 KB; no health throughput win is claimed. PublicApi and generated-manifest membership are unchanged; 1,413 local executions and the hosted matrix are green.