SessionStart hook appends duplicate exports to CLAUDE_ENV_FILE on every start, resume and compaction, so every Bash call fails with E2BIG in long sessions #783

Summary

scripts/session-lifecycle-hook.mjs appends three export lines to $CLAUDE_ENV_FILE each time SessionStart fires, with no de-duplication. Claude Code fires SessionStart again, under the same session id, on resume and after every compaction. In a long session the file therefore fills with hundreds of identical copies.

Claude Code inlines that env file into the single bash -c argument of every Bash command. Once the argument passes Linux's 128 KiB per-argument limit (MAX_ARG_STRLEN), every Bash and Monitor call fails before it starts, and so does every sub-agent's shell. The session loses its shell completely, and restarting does not recover it, because resuming re-reads the bloated file and adds three more lines.

Environment

• codex plugin 1.0.6 (codex@openai-codex, from openai/codex-plugin-cc)
• Claude Code 2.1.281
• Linux 7.0.0-29-generic, GNU bash 5.3.9
• getconf ARG_MAX = 2097152. The limit hit is the 128 KiB per-argument cap, not ARG_MAX.

Evidence

1. The error. Every Bash call failed with this, including echo ok:

Could not start /bin/bash: the command line plus environment exceed the OS exec argument limit (E2BIG).
At spawn: command line 133KB across 3 args (largest single arg 133KB); environment 3.3KB across 47 vars (largest: LS_COLORS at 1.7KB).

The environment is only 3.3 KB. Almost all of the ~133 KB is the one bash -c argument.

2. The argument grew steadily through the session. Sizes from successive errors: 128.1 → 128.2 → 129.1 → 131.8 → 131.9 → 132.1 → 132.8 → 133.0 → 133.3 → 133.5 KB. At first only long commands failed; once the base passed 128 KiB, every command did. The jump over a restart and resume (133.0 → 133.3 KB, about 0.3 KB) matches one appended block of three lines (~311 bytes).

3. The env file held the same three lines, repeated. ~/.claude/session-env/<session-id>/sessionstart-hook-0.sh:

export CODEX_COMPANION_SESSION_ID='<session-id>'
export CODEX_COMPANION_TRANSCRIPT_PATH='~/.claude/projects/<project>/<session-id>.jsonl'
export CLAUDE_PLUGIN_DATA='~/.claude/plugins/data/codex-openai-codex'
export CODEX_COMPANION_SESSION_ID='<session-id>'
export CODEX_COMPANION_TRANSCRIPT_PATH='~/.claude/projects/<project>/<session-id>.jsonl'
export CLAUDE_PLUGIN_DATA='~/.claude/plugins/data/codex-openai-codex'
... (the same triplet, over and over)

The session transcript has 136 compaction boundaries, and each is followed by a SessionStart.

4. Trimming the file fixed it. I reduced the file to one triplet (3 lines) and restarted with claude --resume. echo ok then succeeded.

5. It keeps growing after the fix. Shortly after the trim, the file already held 9 lines, 3 copies of each variable (sort | uniq -c):

      3 export CLAUDE_PLUGIN_DATA='~/.claude/plugins/data/codex-openai-codex'
      3 export CODEX_COMPANION_SESSION_ID='<session-id>'
      3 export CODEX_COMPANION_TRANSCRIPT_PATH='~/.claude/projects/<project>/<session-id>.jsonl'

6. Causes ruled out. Disabling the sandbox for the call, deleting large scratch directories, and a plain restart each left the argument size unchanged. No settings file configures a sandbox or deny list, and the user's .bashrc/.profile are stock.

Root cause

scripts/session-lifecycle-hook.mjs:

• lines 35-40: appendEnvVar() calls fs.appendFileSync without de-duplicating;
• lines 77-81: handleSessionStart() calls it three times on every SessionStart, whatever its source.

Steps to reproduce

1. Enable the codex plugin.
2. Start a session, then resume it (claude --resume) or let it compact, several times.
3. Run sort ~/.claude/session-env/<session-id>/sessionstart-hook-0.sh | uniq -c. The count goes up by one per SessionStart.
4. At about 420 copies (~128 KiB), every Bash call fails with E2BIG.

Expected

Each variable is exported once per session, whatever number of times SessionStart fires.

Suggested fix

function appendEnvVar(name, value) {
  const file = process.env.CLAUDE_ENV_FILE;
  if (!file || value == null || value === "") {
    return;
  }
  const line = `export ${name}=${shellEscape(value)}`;
  const existing = fs.existsSync(file) ? fs.readFileSync(file, "utf8").split("\n") : [];
  if (existing.includes(line)) {
    return;
  }
  fs.appendFileSync(file, `${line}\n`, "utf8");
}

An alternative is to export only when the hook input's source is startup. De-duplicating is more robust, because it also covers a changed value on resume.

Workaround

Overwrite ~/.claude/session-env/<session-id>/sessionstart-hook-0.sh with one copy of the three lines, then restart Claude Code and resume the session. The file is read at session start.