SessionStart hook appends duplicate exports to CLAUDE_ENV_FILE until every Bash call fails

[Vadevious]

SessionStart hook appends duplicate exports to CLAUDE_ENV_FILE until every Bash call fails

Plugin: codex 1.0.5 (marketplace openai-codex, commit 80c31f9)
Platform: Windows 10 Pro 19045, Claude Code, Git Bash (/usr/bin/bash)

Summary

handleSessionStart appends three export lines to $CLAUDE_ENV_FILE unconditionally. SessionStart fires again on every resume, clear, and compact, so a long-lived session accumulates the same three lines over and over. Claude Code inlines that file into the bash -c prelude for every Bash tool call, and the prelude is truncated past roughly 8 KB. The cut lands mid-string inside a quoted export, so from that point on every Bash invocation in that session fails:

/usr/bin/bash: -c: line 84: unexpected EOF while looking for matching `''

echo ok included. The failure is total and permanent for the affected session — the truncation offset is fixed, so the reported line number never changes and no command is small enough to avoid it.

Cause

scripts/session-lifecycle-hook.mjs:

function appendEnvVar(name, value) {
  if (!process.env.CLAUDE_ENV_FILE || value == null || value === "") {
    return;
  }
  fs.appendFileSync(process.env.CLAUDE_ENV_FILE, `export ${name}=${shellEscape(value)}\n`, "utf8");
}

function handleSessionStart(input) {
  appendEnvVar(SESSION_ID_ENV, input.session_id);
  appendEnvVar(TRANSCRIPT_PATH_ENV, input.transcript_path);
  appendEnvVar(PLUGIN_DATA_ENV, process.env[PLUGIN_DATA_ENV]);
}

hooks/hooks.json registers SessionStart with no matcher, so it runs for startup, resume, clear and compact alike. session_id, transcript_path and CLAUDE_PLUGIN_DATA are all constant for the life of a session, so every fire after the first writes bytes that were already there.

Reproduction

Resume or compact a session ~28 times (≈286 bytes per fire on this setup; the threshold is bytes, not fires, so longer paths get there sooner). Then any Bash tool call fails as above.

Observed on this machine — ~/.claude/session-env/<session-id>/sessionstart-hook-3.sh:

201 lines, 3 unique, 19162 bytes   # 67 SessionStart fires, broken
 90 lines, 3 unique,  8490 bytes   # broken
 87 lines, 3 unique,  8555 bytes   # broken
 45 lines, 3 unique,  4440 bytes   # still under the limit, works
  3 lines, 3 unique,   286 bytes   # fresh session, works

Eleven of the session-env files on disk had duplicates; the three over ~8 KB were the ones with dead Bash.

Why it is hard to diagnose

The error names bash and a line number in a string the user never wrote, so it reads as a broken shell profile or a broken PreToolUse hook. It is also session-scoped and correlates with session age, so it looks project-specific or intermittent. Nothing in the plugin appears in the error.

Suggested fix

Make appendEnvVar idempotent:

function appendEnvVar(name, value) {
  if (!process.env.CLAUDE_ENV_FILE || value == null || value === "") {
    return;
  }
  const line = `export ${name}=${shellEscape(value)}\n`;
  try {
    if (fs.readFileSync(process.env.CLAUDE_ENV_FILE, "utf8").includes(line)) {
      return;
    }
  } catch {
    // No env file yet (or unreadable) — fall through and write it.
  }
  fs.appendFileSync(process.env.CLAUDE_ENV_FILE, line, "utf8");
}

Verified locally against the real hook with a real CLAUDE_ENV_FILE: five identical SessionStart fires produce 3 lines instead of 15; a subsequent fire with a different session_id correctly appends the two changed exports and skips the unchanged CLAUDE_PLUGIN_DATA; the resulting file sources cleanly under set -e.

Alternatives, if you prefer: rewrite the file rather than append (the values are per-session constants, so there is nothing to accumulate), or gate the hook on matcher: "startup" — though that would drop the exports for resumed sessions.

Workaround for anyone hitting this now

Deduplicate the file; content is preserved, since every line is a duplicate of one of three:

f=~/.claude/session-env/<session-id>/sessionstart-hook-3.sh
awk '!seen[$0]++' "$f" > "$f.tmp" && mv "$f.tmp" "$f"

Takes effect on the next Bash call — no restart needed.