Compacted replacement_history retains active startup/context packets, duplicating AGENTS/developer/memory/skills context

What version of the Codex App are you using (From “About Codex” dialog)?

Version 26.429.61741 (2429)

What subscription do you have?

Pro

What platform is your computer?

Darwin 24.6.0 arm64 arm

What issue are you seeing?

Codex compaction appears to preserve active startup/context instruction packets inside payload.replacement_history as if they were ordinary conversation history. On the next turn, those same instruction surfaces are also supplied again through the normal active turn-context path.

This creates a large fixed post-compaction prompt floor before any meaningful new work is added. In one representative Codex Desktop session, the first post-compaction request started at 33,093 input tokens with only 3,456 cached, leaving about 29,637 uncached input.

This is not just “my local startup packet is large.” Larger local memory/skills payloads make the symptom easier to measure, but the underlying bug is independent of that size: active startup/context packets should not be retained in compacted history and then supplied again as active context. Any user with AGENTS/project instructions, developer context, plugins, skills, or memories can pay the same duplicated-context cost at smaller scale.

Impact is cost/headroom and possible steering risk: retained startup/context packets appear as history while current versions are also supplied through the active instruction path.

Representative session:

/Users/<user>/.codex/sessions/2026/05/05/rollout-2026-05-05T12-43-22-019df905-d546-7100-b619-4571226c3b53.jsonl

Representative cwd:

/Users/<user>/work/repo-with-agents

Representative compaction timestamp:

2026-05-05T17:22:37.986Z

In the type == "compacted" record:

• payload.message == ""
• replacement_history entries: 9
• serialized replacement_history size: 98,519 bytes
• retained developer text: about 58.9 KB
• retained AGENTS wrapper text: about 16.1 KB
• largest retained developer sections: memory about 26.6 KB, skills about 24.0 KB, app context about 3.5 KB

Token counts around compaction, from last_token_usage:

• before compaction: 221,525 input, 220,032 cached
• first nonzero post-compaction request: 33,093 input, 3,456 cached
• first post-compaction uncached input: about 29,637

Redacted proof shape:

• payload.replacement_history[N].role == "user" contains # AGENTS.md instructions...
• the normalized/hash of that AGENTS body matches the adjacent turn_context.user_instructions
• payload.replacement_history[M].role == "developer" contains <app-context>, ## Memory, and <skills_instructions>
• the adjacent post-compaction turn_context also contains current active context surfaces

I can provide exact redacted hashes privately if useful.

What steps can reproduce the bug?

1. Start a Codex Desktop thread in a repo with an AGENTS.md.
2. Use normal startup/context surfaces, especially skills and/or memories if available.
3. Drive the thread to compaction.
4. Inspect the session JSONL type == "compacted" event.
5. Check whether payload.replacement_history contains:

• a user item with # AGENTS.md instructions...
• a developer item containing <app-context>, ## Memory, <skills_instructions>, or plugin/app instructions

1. Inspect the adjacent turn_context.
2. Compare whether the AGENTS body is also present in turn_context.user_instructions.
3. Inspect the first post-compaction event_msg / token_count.
4. Confirm whether post-compaction input remains high before new work is added.

Focused scan:

• window: 2026-04-28T00:00:00Z through 2026-05-05T23:59:59Z
• 70 compactions across 34 session files
• 63/70 compactions retained developer/app-context packets
• 63/70 retained # AGENTS.md instructions... wrappers
• 63/63 normal compactions with an AGENTS wrapper contained the same body already present in turn_context.user_instructions
• 63/70 retained skills instruction markers
• 36/70 retained memory sections
• replacement history size: median 107,290 bytes, p95 1,130,774, max 1,470,356
• first post-compaction input tokens: median 32,514, max 90,411

Broader scan across all available local sessions:

• 1,015 compaction events across 479 session files
• retained AGENTS/developer packet pattern observed across every week with compactions in the local corpus
• Desktop: 938/986 compactions included retained AGENTS wrappers and developer packets
• TUI: 22/29 compactions included retained AGENTS wrappers and developer packets
• earliest retained AGENTS sample observed:

2026-02-27T21:18:49.602Z, Codex Desktop 0.105.0, cwd /Users/<user>/work/another-repo-with-agents

The retention pattern predates my recent local startup/config setup. Later memory/skills configuration amplified the duplicated payload size but did not create the underlying compaction behavior.

What is the expected behavior?

After compaction, replacement_history should contain compacted conversational history and/or the compaction summary.

It should not retain raw current startup/context packets when those same surfaces are already supplied through active turn context on the next request.

After compaction:

• replacement_history does not contain raw active AGENTS/developer/memory/skills/app/plugin startup packets
• active startup/context surfaces are still present exactly once in the next model request
• first post-compaction prompt size reflects compacted conversation state plus one current context injection, not duplicated startup/context payload
• disabling memories/skills may reduce baseline size, but should not be required to avoid duplication

Additional information

Related prior work:

• #10574: https://github.com/openai/codex/pull/10574

Closely related. It handled stale developer-role messages in compacted replacement_history and reinjected fresh developer instructions. This report shows active startup/context packets still being retained through another path or broader surface area, including contextual user wrappers such as AGENTS.

• #12252: https://github.com/openai/codex/pull/12252

Possibly related. It addressed compaction context reinjection and duplicate context concerns. This report is specifically about post-compaction replacement_history retaining active startup/context surfaces.

• #14313: https://github.com/openai/codex/pull/14313

Possibly related. It deferred fresh-session initial context insertion until the first real turn.

• #14365: https://github.com/openai/codex/issues/14365

Related open issue about remote compaction preserving too much replacement history and failing to create meaningful headroom.

Local mitigation tests:

Measured with:

codex debug prompt-input 'probe'

From /Users/<user>/work/repo-with-agents:

• default: 76,270 serialized prompt bytes
• -c features.memories=false: 49,302 bytes
• -c memories.use_memories=false: 49,302 bytes
• -c skills.include_instructions=false: 52,097 bytes

These mitigations reduce the size of my local duplicated payload. They do not address the general bug: active context should not be copied into replacement_history and re-sent as history for any user.

I can provide the representative JSONL privately if useful. I am not attaching the full session publicly because it contains local paths and model-visible context, but the event paths, timestamps, counts, and reproduction checks above are exact.