[rushilbhat]

Summary

• make synthetic replay text fail closed at the exact requested token count, including shared byte-offset tokenizers
• keep future trajectory text lazy and memory-bounded with deterministic per-agent/per-turn streams
• prepare rolling replacements concurrently while preserving FIFO admission and routing slots
• serialize requests from borrowed live history into an immutable retryable byte body

Validation

• cargo fmt --manifest-path rust/Cargo.toml -- --check
• cargo check --manifest-path rust/Cargo.toml --all-targets
• cargo check --manifest-path rust/Cargo.toml --features python
• cargo test --manifest-path rust/Cargo.toml (51 passed)
• real GLM-5.2 tokenizer regression at 2,857 tokens
• 65,536-token initial-prompt stress case

This keeps the v1 JSONL schema and CLI unchanged.

[Copilot]

Pull request overview

This PR tightens and restructures the batchbench-agent synthetic text and admission pipeline so that replay/synthetic prompts are generated exactly at the requested token counts (failing closed if not achievable), while keeping trajectory text generation lazy, deterministic, and memory-bounded during agent execution.

Changes:

• Introduces a SyntheticTextGenerator that deterministically generates per-agent/per-turn text streams, bounds concurrent materialization via a semaphore, and enforces exact re-encode token counts.
• Moves initial prompt materialization out of the benchmark clock and adds concurrent replacement preparation while preserving FIFO admission semantics.
• Serializes request bodies into an immutable Bytes payload (instead of building a mutable serde_json::Value) to avoid cloning large message trees during retries/in-flight work.

Reviewed changes

Copilot reviewed 3 out of 4 changed files in this pull request and generated no comments.

File	Description
rust/src/agent.rs	Adds deterministic, bounded synthetic text generation; materializes initial prompts pre-benchmark; concurrent FIFO replacement preparation; serializes request bodies to Bytes.
rust/Cargo.toml	Adds bytes dependency and enables Tokio sync for semaphore-based bounded work.
rust/Cargo.lock	Locks the new bytes dependency.
README.md	Updates documentation to reflect exact-token fail-closed generation and concurrent replacement preparation semantics.

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