server: resolve Responses response ids to KV prefixes

[perfloop-agent]

Summary

• Generate one resp_ id before response serialization and retain a bounded record for its exact model frontier, prompt configuration, and generated tool-call bindings.
• Resolve valid append-only previous_response_id requests from that record, render and tokenize only the supplied continuation tail, and synchronize it onto the saved prefix.
• Preserve the existing full-replay path for complete histories, edits, configuration mismatches, expired state, and unavailable checkpoints.

Persistence and limits

Disk-backed response state now follows the normal continued-cache boundary instead of copying a checkpoint after every completed turn. A failed or over-budget write still advances the recorded boundary, preventing short tails from repeatedly staging the same payload. Durable records restore their response metadata and tool-call bindings after restart; ids between durable boundaries remain live-process continuations and require replay after restart or slot displacement.

The response-state index is FIFO-bounded. The endpoint benchmark covers HTTP dispatch through prefix synchronization with a byte-tokenizer/checkpoint fixture; it does not measure model or GPU inference throughput.

Testing

• make -B cpu -j2
• make -B ds4_test -j2 && ./ds4_test --server
• make -B tests/responses_replay_bench -j2 && ./tests/responses_replay_bench --turns 172 --iterations 20
• Disk-cache fixture runs covering a shaped retained checkpoint, short steady-state tails, restart restoration, and an over-budget checkpoint rejection.

No model-backed Metal, SSD-streaming, CUDA, or distributed run was performed.

Workload: in-memory steady-state 8k-token Responses response-id host ingress

Metric	Before	After	Change
host_ingress_ns/op	3181149	483897	84.8% lower
All 5 declared correctness checks passed.

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Authored and verified by Perfloop: every claim above was co-measured on both trees and independently re-verified before submission — the full record is public: case_avefhzk2jy. Replies from this account are human-approved, and a human operator is accountable for this contribution.