Context: yoyo #1184 (owner-filed 2026-11-12) replaces yoyo's hand line-by-line TOML config reader with the toml crate (serde typed structs) and toml_edit for writes. Versions on crates.io 2026-11-12: toml 1.2.1+spec-1.1.0, toml_edit 0.25.19+spec-1.1.0.

Precedent: OpenAI Codex CLI (Rust) parses ~/.codex/config.toml with toml + serde and writes with toml_edit::DocumentMut so comments/order survive (codex-rs/core/src/config/mod.rs set_default_oss_provider). Its error UX (PR openai/codex#9746, module codex-rs/config/src/diagnostics.rs, config_error_from_toml / format_config_error): uses toml::de::Error::span() to print path:line:col: message, then the offending line with a gutter and ^^^ carets under the span; non-zero exit in CLI. Issue openai/codex#11946: silent config failures were the user complaint; fix = file + text range + non-zero exit. Also openai/codex#13464: duplicate [mcp_servers.*] blocks made the app unresponsive -> a real TOML parser rejects duplicate tables/keys, which a hand parser silently last-wins.

Separate fact (vendor page platform.claude.com/docs/en/about-claude/pricing, read 2026-11-12): Claude Fable 5.2 (claude-fable-5-2, released 2026-11-09 per Claude Code 2.1.321 changelog, now Claude Code default Fable, 1M context) =  in / 0 out per MTok, 5m cache write 0, 1h write 6, cache hits bash.20 (0.025x). Fast mode 6/0. Batch /0. Claude Mythos 5.2 shares the 0.025x cache multiplier. yoagent 0.27.7 prices.json has claude-fable-5 (0/0, hits .0) and claude-fable-5-1 but no 5-2, so yoyo  shows 0/0 (25% over, cache reads 5x over).