Reasoning summary events can contain and render literal `<!-- -->` placeholders

What happened?

Reasoning summaries can contain a literal empty HTML comment placeholder, <!-- -->. This placeholder then shows up in user-facing surfaces such as the TUI progress/history view and codex exec --json output.

Example observed in the TUI:

• <!-- -->

  Clarifying Codex comment leaks and prevention
  <!-- -->

  Proposing global instruction update
  <!-- -->

A later clean-isolation run reproduced the same issue in exec --json and persisted session JSONL, so this is broader than a TUI-only rendering issue.

Environment

codex-cli: 0.143.0
release tag checked: rust-v0.143.0
model: gpt-5.5
model_reasoning_effort: xhigh
model_reasoning_summary: auto / concise / detailed / none tested
OS: macOS 26.5.1, arm64
terminal: Ghostty 1.3.2-HEAD-+05c3e29
install: Homebrew cask
subscription/auth: ChatGPT Pro auth

The regular user config originally had:

model_reasoning_summary = "auto"
model_reasoning_effort = "xhigh"

codex doctor --json reported overallStatus = warning, but the warning was about rollout/session inventory mismatch with an empty historical session file; it appears unrelated. Config parsing and auth were OK, and the responses websocket handshake succeeded.

Clean-isolation reproduction

I reproduced with a fresh temporary HOME and CODEX_HOME, copying only auth.json so the CLI could authenticate. The invocation used --ignore-user-config --ignore-rules, a temp working directory, and no user hooks, personal skills, AGENTS.md, project instructions, or user config.

Command shape:

ISO_ROOT=$(mktemp -d /tmp/codex-clean-iso2.XXXXXX)
ISO_HOME="$ISO_ROOT/home"
ISO_CODEX="$ISO_ROOT/codex"
mkdir -p "$ISO_HOME" "$ISO_CODEX"
cp "$HOME/.codex/auth.json" "$ISO_CODEX/auth.json"

HOME="$ISO_HOME" CODEX_HOME="$ISO_CODEX" \
  codex --ask-for-approval never --sandbox workspace-write \
  exec --json --ignore-user-config --ignore-rules --skip-git-repo-check \
  -C "$ISO_ROOT" -m gpt-5.5 \
  -c 'model_reasoning_effort="xhigh"' \
  -c 'model_reasoning_summary="auto"' \
  < "$ISO_ROOT/prompt.txt" > "$ISO_ROOT/out.jsonl"

Prompt:

Without using the network, inspect this empty temp workspace. Run `pwd` and `ls -la`, then answer with two concise bullet points: what files are present and the current working directory basename.

Result: reproduced in exec --json:

{"type":"item.completed","item":{"id":"item_0","type":"reasoning","text":"**Planning parallel command execution**\n\n<!-- -->"}}

The persisted isolated session JSONL also contained the placeholder in both agent_reasoning and completed reasoning.summary_text:

{"type":"event_msg","payload":{"type":"agent_reasoning","text":"**Planning parallel command execution**\n\n<!-- -->"}}
{"type":"response_item","payload":{"type":"reasoning","summary":[{"type":"summary_text","text":"**Planning parallel command execution**\n\n<!-- -->"}]}}

A minimal no-tool prompt in the same isolation style did not reproduce; its session had "summary": []. So the issue appears task-shape dependent and intermittent, but it reproduces without my local config/hooks/skills/project instructions.

Matrix

I then ran a small clean-isolation matrix: model_reasoning_summary x task shape.

summary_mode  task_shape  stdout <!-- -->  session <!-- -->  agent_reasoning events  reasoning summary items
auto          no_tool     0               0                 0                       0
auto          tool        0               0                 0                       0
concise       no_tool     0               0                 1                       1
concise       tool        0               0                 1                       1
detailed      no_tool     0               0                 0                       0
detailed      tool        1               2                 1                       1
none          no_tool     0               0                 0                       0
none          tool        0               0                 0                       0

The detailed + tool run produced:

{"kind":"agent_reasoning","text":"**Running parallel directory commands**\n\n<!-- -->"}
{"kind":"reasoning","summary":["**Running parallel directory commands**\n\n<!-- -->"]}

I also repeated the auto + tool case five times with the same isolated prompt. Runs 1-4 did not emit summaries; run 5 emitted two reasoning summary items, both with the placeholder:

run  stdout <!-- -->  session <!-- -->  agent_reasoning events  reasoning summary items
1    0               0                 0                       0
2    0               0                 0                       0
3    0               0                 0                       0
4    0               0                 0                       0
5    2               4                 2                       2

Run 5 summary texts:

**Planning parallel execution of pwd and ls**\n\n<!-- -->
**Preparing to execute pwd and ls commands**\n\n<!-- -->

Local session frequency

In my normal ~/.codex/sessions/2026/07/08 data, before the clean-isolation checks, I found:

files with agent_reasoning containing <!-- -->: 9
agent_reasoning entries containing <!-- -->: 1680

Representative normal-session lines:

{"type":"event_msg","payload":{"type":"agent_reasoning","text":"**Clarifying Codex comment leaks and prevention**\n\n<!-- -->"}}
{"type":"event_msg","payload":{"type":"agent_reasoning","text":"**Proposing global instruction update**\n\n<!-- -->"}}
{"type":"response_item","payload":{"type":"reasoning","summary":[{"type":"summary_text","text":"**Clarifying Codex comment leaks and prevention**\n\n<!-- -->"},{"type":"summary_text","text":"**Proposing global instruction update**\n\n<!-- -->"}]}}

Those entries predated the user later copying the visible leak back into the conversation, so they were not merely echoes of user-provided text.

Source-path notes from rust-v0.143.0

I checked the release tag corresponding to the installed version. The user-facing path appears to pass summary text through without filtering:

• codex-rs/codex-api/src/sse/responses.rs:354-369 maps response.reasoning_summary_text.delta/done into ResponseEvent::ReasoningSummaryDelta/Done using the server-provided delta/text as-is.
• codex-rs/core/src/session/turn.rs:2337-2368 turns ReasoningSummaryDone.text into ReasoningContentDeltaEvent.delta as-is for streaming to the client.
• codex-rs/core/src/event_mapping.rs:177-195 maps completed ResponseItem::Reasoning.summary[].text into ReasoningItem.summary_text with text.clone().
• codex-rs/protocol/src/legacy_events.rs:104-110 turns every ReasoningItem.summary_text into AgentReasoningEvent { text: summary.clone() }.
• codex-rs/exec/src/event_processor_with_jsonl_output.rs:150-158 joins ThreadItem::Reasoning.summary and emits it as an exec --json reasoning item if text.trim() is non-empty.
• codex-rs/exec/src/event_processor_with_human_output.rs:468-482 similarly joins summary entries for human output.
• codex-rs/tui/src/chatwidget/streaming.rs:200-235 appends reasoning deltas directly into reasoning_buffer and later calls new_reasoning_summary_block.
• codex-rs/tui/src/history_cell/messages.rs:486-499 splits **header** from the remaining content. For **Header**\n\n<!-- -->, the remaining content is non-empty, so the final reasoning cell displays the literal placeholder.

Expected behavior

User-facing reasoning summaries should not expose literal empty HTML comment placeholders.

Either:

1. Do not generate <!-- --> in reasoning summary text, or
2. Normalize/filter placeholder-only HTML comment lines before emitting user-facing reasoning summary events and rendering them.

If raw ResponseItem persistence should remain untouched for debugging, the filter could live in the user-facing conversion/rendering layers, such as the AgentReasoningEvent / ThreadItem::Reasoning / exec --json / TUI history conversion path.

Workaround

model_reasoning_summary = "none" parses successfully and avoided the issue in the matrix, but it disables reasoning summaries entirely.