diff --git a/packages/app/tests/hooks/session-backup-gist.test.ts b/packages/app/tests/hooks/session-backup-gist.test.ts
new file mode 100644
index 0000000..3877836
--- /dev/null
+++ b/packages/app/tests/hooks/session-backup-gist.test.ts
@@ -0,0 +1,69 @@
+// CHANGE: add regression coverage for transient session backup paths
+// WHY: `.codex/tmp` contains ephemeral runtime files that should not break snapshot creation
+// REF: transient-session-backup-tmp
+// PURITY: SHELL (tests filesystem traversal against committed backup script)
+
+import { describe, expect, it } from "@effect/vitest"
+import { Effect } from "effect"
+import fs from "node:fs"
+import os from "node:os"
+import path from "node:path"
+
+import sessionBackupGist from "../../../../scripts/session-backup-gist.js"
+
+const { collectSessionFiles, shouldIgnoreSessionPath } = sessionBackupGist
+
+const withTempDir = Effect.acquireRelease(
+  Effect.sync(() => fs.mkdtempSync(path.join(os.tmpdir(), "session-backup-gist-"))),
+  (tmpDir) =>
+    Effect.sync(() => {
+      fs.rmSync(tmpDir, { recursive: true, force: true })
+    })
+)
+
+describe("session-backup-gist transient path filtering", () => {
+  it.effect("ignores .codex/tmp while keeping persistent session files", () =>
+    Effect.scoped(
+      Effect.gen(function*(_) {
+        const tmpDir = yield* _(withTempDir)
+        const codexDir = path.join(tmpDir, ".codex")
+        const claudeDir = path.join(tmpDir, ".claude")
+
+        yield* _(
+          Effect.sync(() => {
+            fs.mkdirSync(path.join(codexDir, "tmp", "run"), { recursive: true })
+            fs.mkdirSync(path.join(codexDir, "memory"), { recursive: true })
+            fs.mkdirSync(path.join(claudeDir, "tmp"), { recursive: true })
+
+            fs.writeFileSync(path.join(codexDir, "tmp", "run", ".lock"), "lock")
+            fs.writeFileSync(path.join(codexDir, "history.jsonl"), "{\"event\":1}\n")
+            fs.writeFileSync(path.join(codexDir, "memory", "notes.md"), "# notes\n")
+            fs.writeFileSync(path.join(claudeDir, "tmp", "should-stay.txt"), "keep")
+          })
+        )
+
+        const codexFiles = collectSessionFiles(codexDir, ".codex", false)
+        const claudeFiles = collectSessionFiles(claudeDir, ".claude", false)
+        const logicalNames = [...codexFiles, ...claudeFiles]
+          .map((file) => file.logicalName)
+          .toSorted((left, right) => left.localeCompare(right))
+
+        yield* _(
+          Effect.sync(() => {
+            expect(logicalNames).toContain(".codex/history.jsonl")
+            expect(logicalNames).toContain(".codex/memory/notes.md")
+            expect(logicalNames).toContain(".claude/tmp/should-stay.txt")
+            expect(logicalNames.some((name) => name.startsWith(".codex/tmp/"))).toBe(false)
+          })
+        )
+      })
+    ))
+
+  it.effect("marks only targeted transient .codex tmp paths for exclusion", () =>
+    Effect.sync(() => {
+      expect(shouldIgnoreSessionPath(".codex", "tmp")).toBe(true)
+      expect(shouldIgnoreSessionPath(".codex", "tmp/run/.lock")).toBe(true)
+      expect(shouldIgnoreSessionPath(".codex", "history.jsonl")).toBe(false)
+      expect(shouldIgnoreSessionPath(".claude", "tmp/run/.lock")).toBe(false)
+    }))
+})
diff --git a/scripts/session-backup-gist.d.ts b/scripts/session-backup-gist.d.ts
new file mode 100644
index 0000000..6804e5f
--- /dev/null
+++ b/scripts/session-backup-gist.d.ts
@@ -0,0 +1,23 @@
+export type SessionBackupFile = {
+  readonly logicalName: string
+  readonly sourcePath: string
+  readonly size: number
+}
+
+export declare const collectSessionFiles: (
+  dirPath: string,
+  baseName: string,
+  verbose: boolean
+) => ReadonlyArray<SessionBackupFile>
+
+export declare const shouldIgnoreSessionPath: (
+  baseName: string,
+  relativePath: string
+) => boolean
+
+declare const sessionBackupGist: {
+  readonly collectSessionFiles: typeof collectSessionFiles
+  readonly shouldIgnoreSessionPath: typeof shouldIgnoreSessionPath
+}
+
+export default sessionBackupGist
diff --git a/scripts/session-backup-gist.js b/scripts/session-backup-gist.js
index 738d9a3..492e565 100644
--- a/scripts/session-backup-gist.js
+++ b/scripts/session-backup-gist.js
@@ -41,6 +41,9 @@ const {
 } = require("./session-backup-repo.js");
 
 const SESSION_DIR_NAMES = [".codex", ".claude", ".qwen", ".gemini"];
+const TRANSIENT_SESSION_PREFIXES = {
+  ".codex": ["tmp"],
+};
 
 const isPathWithinParent = (targetPath, parentPath) => {
   const relative = path.relative(parentPath, targetPath);
@@ -71,6 +74,16 @@ const resolveAllowedSessionDir = (candidatePath, verbose) => {
   return null;
 };
 
+const toLogicalRelativePath = (relativePath) =>
+  relativePath.split(path.sep).join(path.posix.sep);
+
+const shouldIgnoreSessionPath = (baseName, relativePath) => {
+  const prefixes = TRANSIENT_SESSION_PREFIXES[baseName] ?? [];
+  const logicalPath = toLogicalRelativePath(relativePath);
+
+  return prefixes.some((prefix) => logicalPath === prefix || logicalPath.startsWith(`${prefix}/`));
+};
+
 const parseArgs = () => {
   const args = process.argv.slice(2);
   const result = {
@@ -364,6 +377,12 @@ const collectSessionFiles = (dirPath, baseName, verbose) => {
     for (const entry of entries) {
       const fullPath = path.join(currentPath, entry.name);
       const relPath = relativePath ? `${relativePath}/${entry.name}` : entry.name;
+      const logicalRelPath = toLogicalRelativePath(relPath);
+
+      if (shouldIgnoreSessionPath(baseName, logicalRelPath)) {
+        log(verbose, `Skipping transient path: ${path.posix.join(baseName, logicalRelPath)}`);
+        continue;
+      }
 
       if (entry.isDirectory()) {
         if (entry.name === "node_modules" || entry.name === ".git") {
@@ -373,7 +392,7 @@ const collectSessionFiles = (dirPath, baseName, verbose) => {
       } else if (entry.isFile()) {
         try {
           const stats = fs.statSync(fullPath);
-          const logicalName = path.posix.join(baseName, relPath.split(path.sep).join(path.posix.sep));
+          const logicalName = path.posix.join(baseName, logicalRelPath);
           files.push({
             logicalName,
             sourcePath: fullPath,
@@ -661,4 +680,11 @@ const main = () => {
   }
 };
 
-main();
+if (require.main === module) {
+  main();
+}
+
+module.exports = {
+  collectSessionFiles,
+  shouldIgnoreSessionPath,
+};