\newcommand{\WPabstract}{%
Agents built on language models hand information to one another as text, summaries, structured evidence, shared-memory references or---when
they share a checkpoint---attention state (KV cache). We ask under which task, model, cache, network and reuse conditions each representation
preserves task-relevant information at the lowest end-to-end cost, and answer with a reproducible study executed by an autonomous research
operator (Claude Code) on Apple-Silicon hardware with the MLX runtime, whose caches are inspectable tensors. Twenty-two conditions (truncation, extractive selection, summaries,
structured evidence, metered retrieval, a lossless codec, LLMLingua-2, native/serialized caches, packed 8/4-bit quantization, four eviction
rules) are measured on Qwen2.5-3B and Llama-3.2-3B across private-fact ledgers, LongBench QA and executable tasks, in question-blind and question-aware regimes, with
measured bytes, synchronized latency spans and peak memory (13\,280 trials; frozen protocol, cluster bootstrap, Holm-corrected tests). Further
experiments measure two-node cache transfer over a 1~GbE LAN, relays of 1--8 handoffs with an evaluator-only fact ledger, fan-in with
conflicting evidence, a same-family learned KV mapper, hosted receivers and a development-fitted router. Lossless
representations are interchangeable in quality and differ only in cost, whose ranking inverts between cold and reused regimes; on the measured
link, moving an 8-bit cache beats re-reading the text at every length. Lossy choices are model- and regime-dependent: 4-bit quantization is free for Llama and
fatal for Qwen; attention-based eviction keeps 78--98\,\% of private-fact accuracy with the question and 12--19\,\% without it; no eviction
survives a code specification. At a 512-token budget prose summaries beat structured evidence (38--44 vs 12--15\,\%); relays are
nearly lossless after the first hop and invent almost nothing. A ridge KV mapper transfers no
content while producing coherent abstention---an argument for compatibility manifests and negative controls. The router halves amortized latency at equal quality when questions are unknown. Cross-family cache transfer was not executed. Code, records, protocol, checkpoint and a minimal reproduction are released.%
}