[thedancingdeveloper]

Fixes a compaction loop seen on 2026-11-01 (Vogt WI-2832). Operator-approved one-off PR into fork/main: the fork has no graft line.

Root cause

• Two swack-haiku sessions compacted 235 and 143 times in one turn, using about 60M tokens.
• Each cycle: a ~132k-token request (over model_auto_compact_token_limit = 128000), a compaction, one tool call, then ~132k again.
• History was not what survived compaction. A session in the same checkout 23 s earlier had byte-identical history, world state and turn context, and started at 42.9k tokens. The thrashing session started at 130.5k.
• The extra ~87k tokens were MCP tool definitions. These sessions were launched without Vogt's worker MCP set, so Codex loaded every server in ~/.codex/config.toml. Compaction cannot shrink tool definitions, so the fixed prefix alone was over the limit.
• In core/src/session/turn.rs, the turn loop compacted whenever token_limit_reached was true and continued, without checking whether compaction helped. The old comment read: "as long as compaction works well … we shouldn't worry about being in an infinite loop".

The launch-side cause is tracked separately in Vogt (WI-2833: start Codex workers with the worker MCP set).

Fix

• After a context-limit compaction (pre-turn or mid-turn), the turn checks the prompt size of the first request that follows (last_token_usage.input_tokens).
• If that prompt alone is at or over the auto-compact limit, or the usable context window, the turn:
  • emits a Warning naming both sizes and suggesting fewer MCP servers or a higher model_auto_compact_token_limit;
  • emits Error ContextWindowExceeded (non-retryable) and ends without compacting again.
• With the body_after_prefix scope, only the body after the prefix counts against the limit.
• run_pre_sampling_compact now returns whether it compacted, so a pre-turn compaction is checked too.

Tests

• Unit tests session::turn::tests::prompt_over_limit_detects_a_prefix_that_compaction_cannot_shrink and prompt_over_limit_ignores_the_prefix_for_body_after_prefix_scope.
• Integration test suite::compact::auto_compact_stops_when_compaction_cannot_get_below_limit: a 400-token step against a limit of 300, one compaction, then a 400-token step. It asserts exactly three requests, a ContextWindowExceeded error and the warning. With the guard disabled, the test fails.
• Run locally (RUST_MIN_STACK=8388608):
  • cargo check -p codex-core --tests: clean
  • cargo clippy -p codex-core --tests -- -D warnings: clean
  • rustfmt --check on the changed files: clean
  • cargo test -p codex-core --lib prompt_over_limit: 2/2
  • cargo test -p codex-core --test all -- suite::compact::auto_compact: 13/13

FORK.md

Adds a carried-patch row describing the auto-compaction loop guard: the files touched, the tests, and the branch.

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