Allow compaction-specific reasoning effort and service tier

Proposal

Allow compaction requests to use a separate reasoning effort and service tier from normal turns.

Suggested config shape:

model_reasoning_effort = "high"
service_tier = "default"

compact_model_reasoning_effort = "low"
compact_service_tier = "fast"

This lets normal interactive turns keep their usual quality/cost profile, while auto/manual compaction can opt into a faster and lower-reasoning request path.

Motivation

Long-running sessions can become unusable when compaction itself is slow or times out. In local testing, using fast mode for compact substantially reduced compact stalls, but setting global service_tier = "fast" changes the behavior of every normal turn too.

The current config surface appears to make compact inherit the session-level model_reasoning_effort and service_tier. That is convenient as a default, but it prevents users from making compaction cheaper/faster without also changing day-to-day model behavior.

Prepared implementation

I prepared a branch here:

https://github.com/Cccceb/codex/tree/compact-specific-reasoning-service-tier

Commit:

https://github.com/Cccceb/codex/commit/64b6b1e00ad40558c9e4d78c4a6d17b4d01f3b7a

Summary of the change:

• add optional compact_service_tier and compact_model_reasoning_effort config fields
• use those compact-specific settings for local compact, legacy remote compact, and remote compaction v2
• keep existing behavior unchanged when the compact-specific fields are unset
• normalize compact_service_tier = "fast" through the same fast-mode service-tier normalization path as regular service_tier

Touched areas:

• codex-rs/config/src/config_toml.rs
• codex-rs/core/config.schema.json
• codex-rs/core/src/compact.rs
• codex-rs/core/src/compact_remote.rs
• codex-rs/core/src/compact_remote_v2.rs
• codex-rs/core/src/config/mod.rs
• config and remote compact tests

Testing

Ran locally:

cargo fmt --check
env CARGO_HOME=/private/tmp/codex-cargo-home cargo test -p codex-core config_toml_compact_
env CARGO_HOME=/private/tmp/codex-cargo-home cargo test -p codex-core remote_manual_compact_uses_compact_specific_service_tier_and_reasoning_effort

All passed.

I attempted to open this as a PR, but this repository currently restricts pull request creation to collaborators, so I am filing the proposal and implementation details here instead.