diff --git a/plugins/codex/scripts/session-lifecycle-hook.mjs b/plugins/codex/scripts/session-lifecycle-hook.mjs
index 778571e6c..e71aede92 100644
--- a/plugins/codex/scripts/session-lifecycle-hook.mjs
+++ b/plugins/codex/scripts/session-lifecycle-hook.mjs
@@ -32,11 +32,51 @@ function shellEscape(value) {
   return `'${String(value).replace(/'/g, `'\"'\"'`)}'`;
 }
 
-function appendEnvVar(name, value) {
-  if (!process.env.CLAUDE_ENV_FILE || value == null || value === "") {
+const MANAGED_ENV_VARS = [SESSION_ID_ENV, TRANSCRIPT_PATH_ENV, PLUGIN_DATA_ENV];
+
+function isManagedExport(line) {
+  return MANAGED_ENV_VARS.some((name) => line.startsWith(`export ${name}=`));
+}
+
+// Rewrite this plugin's exports instead of appending them. SessionStart fires on
+// startup, on resume and on every compaction, so appending grew CLAUDE_ENV_FILE by
+// three lines every time and nothing ever pruned it. Only the last assignment of a
+// name takes effect, so every earlier copy did nothing but grow the file. Claude Code
+// inlines the whole file into the single `bash -c <script>` argument, so a long-running
+// session eventually pushed that argument past the operating system's limit on the
+// length of one argument, and no shell could be started at all.
+function writeManagedEnvVars(entries) {
+  const envFile = process.env.CLAUDE_ENV_FILE;
+  if (!envFile) {
     return;
   }
-  fs.appendFileSync(process.env.CLAUDE_ENV_FILE, `export ${name}=${shellEscape(value)}\n`, "utf8");
+
+  let existing = "";
+  try {
+    existing = fs.readFileSync(envFile, "utf8");
+  } catch (error) {
+    if (error?.code !== "ENOENT") {
+      throw error;
+    }
+  }
+
+  // Keep every line another plugin wrote, verbatim; drop only our own.
+  const lines = existing.split(/\r?\n/).filter((line) => line !== "" && !isManagedExport(line));
+  for (const [name, value] of entries) {
+    if (value != null && value !== "") {
+      lines.push(`export ${name}=${shellEscape(value)}`);
+    }
+  }
+
+  const next = lines.length > 0 ? `${lines.join("\n")}\n` : "";
+  if (next === existing) {
+    return;
+  }
+
+  // Write and rename, so that a reader never sees a half-written file.
+  const tmpFile = `${envFile}.${process.pid}.tmp`;
+  fs.writeFileSync(tmpFile, next, "utf8");
+  fs.renameSync(tmpFile, envFile);
 }
 
 function cleanupSessionJobs(cwd, sessionId) {
@@ -75,9 +115,11 @@ function cleanupSessionJobs(cwd, sessionId) {
 }
 
 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]);
+  writeManagedEnvVars([
+    [SESSION_ID_ENV, input.session_id],
+    [TRANSCRIPT_PATH_ENV, input.transcript_path],
+    [PLUGIN_DATA_ENV, process.env[PLUGIN_DATA_ENV]]
+  ]);
 }
 
 async function handleSessionEnd(input) {
diff --git a/tests/runtime.test.mjs b/tests/runtime.test.mjs
index 8f276835b..35ca8f415 100644
--- a/tests/runtime.test.mjs
+++ b/tests/runtime.test.mjs
@@ -698,6 +698,126 @@ test("session start hook exports the Claude session id, transcript path, and plu
   );
 });
 
+function runSessionStartHook({ repo, envFile, pluginDataDir, sessionId, transcriptPath }) {
+  return run("node", [SESSION_HOOK, "SessionStart"], {
+    cwd: repo,
+    env: {
+      ...process.env,
+      CLAUDE_ENV_FILE: envFile,
+      CLAUDE_PLUGIN_DATA: pluginDataDir
+    },
+    input: JSON.stringify({
+      hook_event_name: "SessionStart",
+      session_id: sessionId,
+      transcript_path: transcriptPath,
+      cwd: repo
+    })
+  });
+}
+
+function managedExports({ sessionId, transcriptPath, pluginDataDir }) {
+  return (
+    `export CODEX_COMPANION_SESSION_ID='${sessionId}'\n` +
+    `export CODEX_COMPANION_TRANSCRIPT_PATH='${transcriptPath}'\n` +
+    `export CLAUDE_PLUGIN_DATA='${pluginDataDir}'\n`
+  );
+}
+
+test("session start hook does not accumulate exports when it fires repeatedly", () => {
+  const repo = makeTempDir();
+  const envFile = path.join(makeTempDir(), "claude-env.sh");
+  fs.writeFileSync(envFile, "", "utf8");
+  const pluginDataDir = makeTempDir();
+  const transcriptPath = path.join(repo, "session.jsonl");
+  const expected = managedExports({ sessionId: "sess-current", transcriptPath, pluginDataDir });
+
+  // SessionStart fires again on every resume and every compaction of the same session.
+  for (let firing = 1; firing <= 5; firing += 1) {
+    const result = runSessionStartHook({
+      repo,
+      envFile,
+      pluginDataDir,
+      sessionId: "sess-current",
+      transcriptPath
+    });
+    assert.equal(result.status, 0, result.stderr);
+    assert.equal(fs.readFileSync(envFile, "utf8"), expected, `after firing ${firing}`);
+  }
+});
+
+test("session start hook prunes exports an earlier version left behind", () => {
+  const repo = makeTempDir();
+  const envFile = path.join(makeTempDir(), "claude-env.sh");
+  const pluginDataDir = makeTempDir();
+  const transcriptPath = path.join(repo, "session.jsonl");
+  const expected = managedExports({ sessionId: "sess-current", transcriptPath, pluginDataDir });
+
+  // The state a session is already in after a version that appended on every firing:
+  // hundreds of copies, of which only the last has any effect.
+  fs.writeFileSync(envFile, expected.repeat(500), "utf8");
+
+  const result = runSessionStartHook({
+    repo,
+    envFile,
+    pluginDataDir,
+    sessionId: "sess-current",
+    transcriptPath
+  });
+
+  assert.equal(result.status, 0, result.stderr);
+  assert.equal(fs.readFileSync(envFile, "utf8"), expected);
+});
+
+test("session start hook keeps env file lines written by anything else", () => {
+  const repo = makeTempDir();
+  const envFile = path.join(makeTempDir(), "claude-env.sh");
+  const pluginDataDir = makeTempDir();
+  const transcriptPath = path.join(repo, "session.jsonl");
+  const expected = managedExports({ sessionId: "sess-current", transcriptPath, pluginDataDir });
+  const other = "export OTHER_PLUGIN_VALUE='keep-me'\n";
+
+  fs.writeFileSync(envFile, `${other}${expected.repeat(3)}`, "utf8");
+
+  for (let firing = 1; firing <= 2; firing += 1) {
+    const result = runSessionStartHook({
+      repo,
+      envFile,
+      pluginDataDir,
+      sessionId: "sess-current",
+      transcriptPath
+    });
+    assert.equal(result.status, 0, result.stderr);
+    assert.equal(fs.readFileSync(envFile, "utf8"), `${other}${expected}`, `after firing ${firing}`);
+  }
+});
+
+test("session start hook replaces exports that hold a stale value", () => {
+  const repo = makeTempDir();
+  const envFile = path.join(makeTempDir(), "claude-env.sh");
+  const pluginDataDir = makeTempDir();
+  const transcriptPath = path.join(repo, "session.jsonl");
+
+  fs.writeFileSync(
+    envFile,
+    managedExports({ sessionId: "sess-previous", transcriptPath, pluginDataDir }),
+    "utf8"
+  );
+
+  const result = runSessionStartHook({
+    repo,
+    envFile,
+    pluginDataDir,
+    sessionId: "sess-current",
+    transcriptPath
+  });
+
+  assert.equal(result.status, 0, result.stderr);
+  assert.equal(
+    fs.readFileSync(envFile, "utf8"),
+    managedExports({ sessionId: "sess-current", transcriptPath, pluginDataDir })
+  );
+});
+
 test("write task output focuses on the Codex result without generic follow-up hints", () => {
   const repo = makeTempDir();
   const binDir = makeTempDir();