diff --git a/plugins/codex/scripts/lib/state.mjs b/plugins/codex/scripts/lib/state.mjs
index 2da23498f..af65c568b 100644
--- a/plugins/codex/scripts/lib/state.mjs
+++ b/plugins/codex/scripts/lib/state.mjs
@@ -6,7 +6,8 @@ import path from "node:path";
 import { resolveWorkspaceRoot } from "./workspace.mjs";
 
 const STATE_VERSION = 1;
-const PLUGIN_DATA_ENV = "CLAUDE_PLUGIN_DATA";
+export const PLUGIN_DATA_ENV = "CODEX_COMPANION_PLUGIN_DATA";
+const HOST_PLUGIN_DATA_ENV = "CLAUDE_PLUGIN_DATA";
 const FALLBACK_STATE_ROOT_DIR = path.join(os.tmpdir(), "codex-companion");
 const STATE_FILE_NAME = "state.json";
 const JOBS_DIR_NAME = "jobs";
@@ -38,7 +39,11 @@ export function resolveStateDir(cwd) {
   const slugSource = path.basename(workspaceRoot) || "workspace";
   const slug = slugSource.replace(/[^a-zA-Z0-9._-]+/g, "-").replace(/^-+|-+$/g, "") || "workspace";
   const hash = createHash("sha256").update(canonicalWorkspaceRoot).digest("hex").slice(0, 16);
-  const pluginDataDir = process.env[PLUGIN_DATA_ENV];
+  // CLAUDE_PLUGIN_DATA is scoped to this plugin while its hook runs, but the
+  // SessionStart env file is shared by every plugin. Persist that scoped value
+  // under a Codex-owned name so a sibling hook cannot redirect our later jobs.
+  // Keep the host variable as a fallback for direct and pre-upgrade callers.
+  const pluginDataDir = process.env[PLUGIN_DATA_ENV] || process.env[HOST_PLUGIN_DATA_ENV];
   const stateRoot = pluginDataDir ? path.join(pluginDataDir, "state") : FALLBACK_STATE_ROOT_DIR;
   return path.join(stateRoot, `${slug}-${hash}`);
 }
diff --git a/plugins/codex/scripts/session-lifecycle-hook.mjs b/plugins/codex/scripts/session-lifecycle-hook.mjs
index 778571e6c..8ef95b2c5 100644
--- a/plugins/codex/scripts/session-lifecycle-hook.mjs
+++ b/plugins/codex/scripts/session-lifecycle-hook.mjs
@@ -13,12 +13,12 @@ import {
   sendBrokerShutdown,
   teardownBrokerSession
 } from "./lib/broker-lifecycle.mjs";
-import { loadState, resolveStateFile, saveState } from "./lib/state.mjs";
+import { loadState, PLUGIN_DATA_ENV, resolveStateFile, saveState } from "./lib/state.mjs";
 import { TRANSCRIPT_PATH_ENV } from "./lib/claude-session-transfer.mjs";
 import { resolveWorkspaceRoot } from "./lib/workspace.mjs";
 
 export const SESSION_ID_ENV = "CODEX_COMPANION_SESSION_ID";
-const PLUGIN_DATA_ENV = "CLAUDE_PLUGIN_DATA";
+const HOST_PLUGIN_DATA_ENV = "CLAUDE_PLUGIN_DATA";
 
 function readHookInput() {
   const raw = fs.readFileSync(0, "utf8").trim();
@@ -77,7 +77,7 @@ 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]);
+  appendEnvVar(PLUGIN_DATA_ENV, process.env[HOST_PLUGIN_DATA_ENV]);
 }
 
 async function handleSessionEnd(input) {
diff --git a/tests/runtime.test.mjs b/tests/runtime.test.mjs
index 8f276835b..a366b3093 100644
--- a/tests/runtime.test.mjs
+++ b/tests/runtime.test.mjs
@@ -669,7 +669,7 @@ test("task --resume-last ignores running tasks from other Claude sessions", () =
   assert.match(resume.stderr, /No previous Codex task thread was found for this repository\./);
 });
 
-test("session start hook exports the Claude session id, transcript path, and plugin data dir", () => {
+test("session start hook exports the Claude session id, transcript path, and a Codex-owned plugin data variable", () => {
   const repo = makeTempDir();
   const envFile = path.join(makeTempDir(), "claude-env.sh");
   fs.writeFileSync(envFile, "", "utf8");
@@ -694,7 +694,7 @@ test("session start hook exports the Claude session id, transcript path, and plu
   assert.equal(result.status, 0, result.stderr);
   assert.equal(
     fs.readFileSync(envFile, "utf8"),
-    `export CODEX_COMPANION_SESSION_ID='sess-current'\nexport CODEX_COMPANION_TRANSCRIPT_PATH='${transcriptPath}'\nexport CLAUDE_PLUGIN_DATA='${pluginDataDir}'\n`
+    `export CODEX_COMPANION_SESSION_ID='sess-current'\nexport CODEX_COMPANION_TRANSCRIPT_PATH='${transcriptPath}'\nexport CODEX_COMPANION_PLUGIN_DATA='${pluginDataDir}'\n`
   );
 });
 
diff --git a/tests/state.test.mjs b/tests/state.test.mjs
index 0f8f57cea..600c75e3f 100644
--- a/tests/state.test.mjs
+++ b/tests/state.test.mjs
@@ -5,21 +5,47 @@ import test from "node:test";
 import assert from "node:assert/strict";
 
 import { makeTempDir } from "./helpers.mjs";
-import { resolveJobFile, resolveJobLogFile, resolveStateDir, resolveStateFile, saveState } from "../plugins/codex/scripts/lib/state.mjs";
+import {
+  resolveJobFile,
+  resolveJobLogFile,
+  resolveStateDir,
+  resolveStateFile,
+  saveState
+} from "../plugins/codex/scripts/lib/state.mjs";
 
 test("resolveStateDir uses a temp-backed per-workspace directory", () => {
   const workspace = makeTempDir();
-  const stateDir = resolveStateDir(workspace);
+  const previousCodexPluginDataDir = process.env.CODEX_COMPANION_PLUGIN_DATA;
+  const previousPluginDataDir = process.env.CLAUDE_PLUGIN_DATA;
+  delete process.env.CODEX_COMPANION_PLUGIN_DATA;
+  delete process.env.CLAUDE_PLUGIN_DATA;
+
+  try {
+    const stateDir = resolveStateDir(workspace);
 
-  assert.equal(stateDir.startsWith(os.tmpdir()), true);
-  assert.match(path.basename(stateDir), /.+-[a-f0-9]{16}$/);
-  assert.match(stateDir, new RegExp(`^${os.tmpdir().replace(/[.*+?^${}()|[\]\\]/g, "\\$&")}`));
+    assert.equal(stateDir.startsWith(os.tmpdir()), true);
+    assert.match(path.basename(stateDir), /.+-[a-f0-9]{16}$/);
+    assert.match(stateDir, new RegExp(`^${os.tmpdir().replace(/[.*+?^${}()|[\]\\]/g, "\\$&")}`));
+  } finally {
+    if (previousCodexPluginDataDir == null) {
+      delete process.env.CODEX_COMPANION_PLUGIN_DATA;
+    } else {
+      process.env.CODEX_COMPANION_PLUGIN_DATA = previousCodexPluginDataDir;
+    }
+    if (previousPluginDataDir == null) {
+      delete process.env.CLAUDE_PLUGIN_DATA;
+    } else {
+      process.env.CLAUDE_PLUGIN_DATA = previousPluginDataDir;
+    }
+  }
 });
 
-test("resolveStateDir uses CLAUDE_PLUGIN_DATA when it is provided", () => {
+test("resolveStateDir falls back to CLAUDE_PLUGIN_DATA when it is provided", () => {
   const workspace = makeTempDir();
   const pluginDataDir = makeTempDir();
+  const previousCodexPluginDataDir = process.env.CODEX_COMPANION_PLUGIN_DATA;
   const previousPluginDataDir = process.env.CLAUDE_PLUGIN_DATA;
+  delete process.env.CODEX_COMPANION_PLUGIN_DATA;
   process.env.CLAUDE_PLUGIN_DATA = pluginDataDir;
 
   try {
@@ -32,6 +58,94 @@ test("resolveStateDir uses CLAUDE_PLUGIN_DATA when it is provided", () => {
       new RegExp(`^${path.join(pluginDataDir, "state").replace(/[.*+?^${}()|[\]\\]/g, "\\$&")}`)
     );
   } finally {
+    if (previousCodexPluginDataDir == null) {
+      delete process.env.CODEX_COMPANION_PLUGIN_DATA;
+    } else {
+      process.env.CODEX_COMPANION_PLUGIN_DATA = previousCodexPluginDataDir;
+    }
+    if (previousPluginDataDir == null) {
+      delete process.env.CLAUDE_PLUGIN_DATA;
+    } else {
+      process.env.CLAUDE_PLUGIN_DATA = previousPluginDataDir;
+    }
+  }
+});
+
+test("resolveStateDir prefers the Codex plugin data dir over another plugin's host-scoped value", () => {
+  const workspace = makeTempDir();
+  const codexPluginDataDir = makeTempDir();
+  const siblingPluginDataDir = makeTempDir();
+  const previousCodexPluginDataDir = process.env.CODEX_COMPANION_PLUGIN_DATA;
+  const previousPluginDataDir = process.env.CLAUDE_PLUGIN_DATA;
+  process.env.CODEX_COMPANION_PLUGIN_DATA = codexPluginDataDir;
+  process.env.CLAUDE_PLUGIN_DATA = siblingPluginDataDir;
+
+  try {
+    const stateDir = resolveStateDir(workspace);
+
+    assert.equal(stateDir.startsWith(path.join(codexPluginDataDir, "state")), true);
+    assert.equal(stateDir.startsWith(path.join(siblingPluginDataDir, "state")), false);
+  } finally {
+    if (previousCodexPluginDataDir == null) {
+      delete process.env.CODEX_COMPANION_PLUGIN_DATA;
+    } else {
+      process.env.CODEX_COMPANION_PLUGIN_DATA = previousCodexPluginDataDir;
+    }
+    if (previousPluginDataDir == null) {
+      delete process.env.CLAUDE_PLUGIN_DATA;
+    } else {
+      process.env.CLAUDE_PLUGIN_DATA = previousPluginDataDir;
+    }
+  }
+});
+
+test("saveState does not prune another plugin's job when the host-scoped data dir was overwritten", () => {
+  const workspace = makeTempDir();
+  const codexPluginDataDir = makeTempDir();
+  const siblingPluginDataDir = makeTempDir();
+  const previousCodexPluginDataDir = process.env.CODEX_COMPANION_PLUGIN_DATA;
+  const previousPluginDataDir = process.env.CLAUDE_PLUGIN_DATA;
+  process.env.CODEX_COMPANION_PLUGIN_DATA = siblingPluginDataDir;
+  process.env.CLAUDE_PLUGIN_DATA = siblingPluginDataDir;
+
+  try {
+    const siblingStateFile = resolveStateFile(workspace);
+    const siblingJobFile = resolveJobFile(workspace, "sibling-job");
+    const siblingLogFile = resolveJobLogFile(workspace, "sibling-job");
+    const siblingState = {
+      version: 1,
+      config: { stopReviewGate: false },
+      jobs: [
+        {
+          id: "sibling-job",
+          status: "completed",
+          logFile: siblingLogFile,
+          createdAt: "2026-08-25T00:00:00.000Z",
+          updatedAt: "2026-08-25T00:00:00.000Z"
+        }
+      ]
+    };
+    fs.writeFileSync(siblingJobFile, '{"owner":"sibling"}\n', "utf8");
+    fs.writeFileSync(siblingLogFile, "sibling output\n", "utf8");
+    fs.writeFileSync(siblingStateFile, `${JSON.stringify(siblingState, null, 2)}\n`, "utf8");
+
+    process.env.CODEX_COMPANION_PLUGIN_DATA = codexPluginDataDir;
+    saveState(workspace, {
+      version: 1,
+      config: { stopReviewGate: false },
+      jobs: []
+    });
+
+    assert.deepEqual(JSON.parse(fs.readFileSync(siblingStateFile, "utf8")), siblingState);
+    assert.equal(fs.readFileSync(siblingJobFile, "utf8"), '{"owner":"sibling"}\n');
+    assert.equal(fs.readFileSync(siblingLogFile, "utf8"), "sibling output\n");
+    assert.equal(fs.existsSync(resolveStateFile(workspace)), true);
+  } finally {
+    if (previousCodexPluginDataDir == null) {
+      delete process.env.CODEX_COMPANION_PLUGIN_DATA;
+    } else {
+      process.env.CODEX_COMPANION_PLUGIN_DATA = previousCodexPluginDataDir;
+    }
     if (previousPluginDataDir == null) {
       delete process.env.CLAUDE_PLUGIN_DATA;
     } else {