diff --git a/README.md b/README.md
index 0176dcf..89270bf 100644
--- a/README.md
+++ b/README.md
@@ -12,18 +12,18 @@ you `delete` or `clear` them.
 `requirements.txt`:
 
 ```
-aiokv @ git+ssh://git@git.rethinkstudios.io/rethink-public/aiokv.git@v0.1.1
+aiokv @ git+ssh://git@git.rethinkstudios.io/rethink-public/aiokv.git@v0.2.0
 ```
 
 Direct:
 
 ```bash
-pip install "aiokv @ git+ssh://git@git.rethinkstudios.io/rethink-public/aiokv.git@v0.1.1"
+pip install "aiokv @ git+ssh://git@git.rethinkstudios.io/rethink-public/aiokv.git@v0.2.0"
 ```
 
 Requires `aiofiles` (pulled transitively).
 
-Drop the `@v0.1.1` suffix from the line above to install the latest unpinned.
+Drop the `@v0.2.0` suffix from the line above to install the latest unpinned.
 
 ## Usage
 
@@ -62,14 +62,15 @@ Prefer `AioKV` in new code.
 ## Durability
 
 Writes are **atomic**: data is written to a temp file in the same directory and
-`os.replace()`d over the target (atomic on POSIX). A **process** crash mid-write leaves
-the previous good file intact, and a reader never observes a partial file. (This is
-process-crash safety, not power-loss durability — there's no `fsync`, so an OS/power
-failure could still lose the last write; fine for reconstructible single-process state.)
-A single
-`asyncio.Lock` guards every read and write, so concurrent operations on one instance
-are consistent and no update is lost. All blocking filesystem calls run via
-`asyncio.to_thread`, so nothing stalls the event loop.
+`os.replace()`d over the target's realpath — symlink-safe, so a symlinked store file is
+written through rather than clobbered. A **process** crash mid-write leaves the previous
+good file intact, and a reader never observes a partial file. (This is process-crash
+safety, not power-loss durability — there's no `fsync`, so an OS/power failure could
+still lose the last write; fine for reconstructible single-process state.) Orphaned
+`.<pid>.tmp` files left by a hard crash of a *different, dead* process are swept on the
+next save. A single `asyncio.Lock` guards every read and write, so concurrent operations
+on one instance are consistent and no update is lost. All blocking filesystem calls and
+JSON (de)serialization run via `asyncio.to_thread`, so nothing stalls the event loop.
 
 ## Scope — read this
 
@@ -82,10 +83,13 @@ are consistent and no update is lost. All blocking filesystem calls run via
 
 ## Error contract
 
+- Keys must be `str` — `get` / `set` raise `ValueError` on a non-str key (JSON object
+  keys are always strings, so a non-str key would silently never round-trip).
 - `get` / `set` / `get_all` raise on unexpected I/O. `_load` raises `JSONDecodeError`
   on a truncated/corrupt file, and `ValueError` when the file holds valid JSON that
   isn't an object (a bare list/number/string/null) — so corruption or a wrong-shaped
-  file is visible rather than silently masked.
+  file is visible rather than silently masked. Non-finite floats (`NaN`/`Infinity`)
+  raise `ValueError` on `set` rather than persisting invalid JSON.
 - `delete` / `clear` log the exception and return `False` on error, `True` otherwise.
 
 ## Versioning
diff --git a/pyproject.toml b/pyproject.toml
index 10d259f..d9a5d51 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -4,7 +4,7 @@ build-backend = "hatchling.build"
 
 [project]
 name = "aiokv"
-version = "0.1.1"
+version = "0.2.0"
 description = "Async file-backed key-value store for single-process local state — atomic writes, no TTL, config-free, installable."
 requires-python = ">=3.10"
 dependencies = [
diff --git a/src/aiokv/__init__.py b/src/aiokv/__init__.py
index 8c9624f..67b5f7d 100644
--- a/src/aiokv/__init__.py
+++ b/src/aiokv/__init__.py
@@ -1,3 +1,5 @@
 from .aiokv import AioKV, aiocache
 
+__version__ = "0.2.0"
+
 __all__ = ["AioKV", "aiocache"]
diff --git a/src/aiokv/aiokv.py b/src/aiokv/aiokv.py
index 061d3ea..71aeeca 100644
--- a/src/aiokv/aiokv.py
+++ b/src/aiokv/aiokv.py
@@ -13,10 +13,12 @@ eviction — values live until you delete or clear them.
     when = await kv.get("ran_cleanup")
     await kv.delete("last_seen")
 
-durability: writes are atomic — data is written to a temp file in the same
-directory and os.replace()d over the target, so a crash mid-write never corrupts
-the store and readers never see a partial file. a single asyncio.Lock guards every
-read and write, so concurrent operations on one instance are consistent.
+durability: atomic writes (temp file + os.replace, symlink-safe), stale
+`.<pid>.tmp` files from a hard crash are swept on save. this is process-crash
+safety, NOT power-loss durability — no fsync, so an OS/power failure can still
+lose the last write. a single asyncio.Lock guards every read and write, so
+concurrent operations on one instance are consistent. JSON (de)serialization
+runs off-loop via asyncio.to_thread so a large store never stalls other tasks.
 
 scope: SINGLE-PROCESS, single-instance local state only. the lock is per-instance —
 two AioKV instances (or two processes) pointing at the same file are NOT safe and
@@ -24,13 +26,16 @@ will clobber each other. for shared cross-process/cross-bot state, use a databas
 (e.g. mongo), not this.
 
 config-free: the file path is passed at construction; nothing is read from a global
-config. errors in delete/clear are logged and swallowed (returning False); get/set
-raise on unexpected i/o so a real failure is visible.
+config. keys must be str (ValueError otherwise — JSON object keys are always
+strings, so a non-str key would silently never round-trip). errors in delete/clear
+are logged and swallowed (returning False); get/set raise on unexpected i/o so a
+real failure is visible.
 """
 
 import os
 import json
 import time
+import glob
 import asyncio
 import logging
 from typing import Any, Dict
@@ -53,10 +58,8 @@ class AioKV:
         self.lock = asyncio.Lock()
 
     async def set(self, key: str, value: Any = None) -> None:
-        """set a value; if value is omitted or None, stores int(time.time())
-
-        the timestamp default exists for "mark that i saw/did X at time T" usage.
-        """
+        """set a value; if value is omitted or None, stores int(time.time())"""
+        self._check_key(key)
         async with self.lock:
             cache = await self._load()
             cache[key] = value if value is not None else int(time.time())
@@ -64,6 +67,7 @@ class AioKV:
 
     async def get(self, key: str, default: Any = None) -> Any:
         """return the value for key, or default if absent"""
+        self._check_key(key)
         async with self.lock:
             cache = await self._load()
             return cache.get(key, default)
@@ -71,6 +75,7 @@ class AioKV:
     async def delete(self, key: str) -> bool:
         """delete a key; returns True if removed or absent, False on error"""
         try:
+            self._check_key(key)
             async with self.lock:
                 cache = await self._load()
                 if key in cache:
@@ -82,11 +87,7 @@ class AioKV:
             return False
 
     async def clear(self) -> bool:
-        """remove the backing file entirely; returns True on success, False on error
-
-        a file already absent (or removed concurrently between the check and the remove)
-        is success — the goal state, no file, is reached.
-        """
+        """remove the backing file entirely; True on success or if already absent, False on error"""
         try:
             async with self.lock:
                 try:
@@ -103,43 +104,43 @@ class AioKV:
         async with self.lock:
             return await self._load()
 
-    async def _load(self) -> Dict[str, Any]:
-        """load the store from disk, returning {} if the file is absent or empty
+    @staticmethod
+    def _check_key(key: str) -> None:
+        """raise ValueError if key is not str (JSON object keys are always strings)"""
+        if not isinstance(key, str):
+            raise ValueError(f"aiokv keys must be str, got {type(key).__name__}")
 
-        a truncated/corrupt file raises JSONDecodeError, and a file holding valid
-        JSON that is not an object (e.g. a bare list, number, or null) raises
-        ValueError — both surfaced to the caller rather than silently masking a
-        real corruption or returning a non-dict that breaks every other method.
-        """
-        if not await asyncio.to_thread(os.path.exists, self.file):
+    async def _load(self) -> Dict[str, Any]:
+        """load the store, returning {} if absent or blank; raises JSONDecodeError on
+        corrupt content and ValueError on valid-but-non-object JSON rather than masking it"""
+        try:
+            async with aiofiles.open(self.file, mode="r", encoding="utf-8") as f:
+                data = await f.read()
+        except FileNotFoundError:
             return {}
-        async with aiofiles.open(self.file, mode="r", encoding="utf-8") as f:
-            data = await f.read()
-        if not data:
+        if not data.strip():
             return {}
-        loaded = json.loads(data)
+        loaded = await asyncio.to_thread(json.loads, data)
         if not isinstance(loaded, dict):
             raise ValueError(f"store file {self.file} does not hold a JSON object")
         return loaded
 
     async def _save(self, cache: Dict[str, Any]) -> None:
-        """write the store atomically: temp file in the same dir, then os.replace
-
-        os.replace is atomic on POSIX, so a reader never sees a partial file and a
-        process crash mid-write leaves the previous good file intact. note this is
-        process-crash safety, NOT power-loss durability — there is no fsync of the temp
-        file or the directory, so an OS/power failure could still lose the most recent
-        write (acceptable here: this is reconstructible single-process state, not a db).
-        """
-        directory = os.path.dirname(self.file) or "."
+        """write atomically: temp file in the same dir, then os.replace over the realpath'd
+        target (symlink-safe). process-crash safe (a crash mid-write leaves the prior good
+        file intact), NOT power-loss safe — no fsync, so an OS/power failure can still lose
+        the last write (acceptable: reconstructible single-process state, not a db)"""
+        target = await asyncio.to_thread(os.path.realpath, self.file)
+        directory = os.path.dirname(target) or "."
         await asyncio.to_thread(os.makedirs, directory, exist_ok=True)
+        await self._sweep_stale_tmp(directory)
 
-        payload = json.dumps(cache)
-        tmp = f"{self.file}.{os.getpid()}.tmp"
+        payload = await asyncio.to_thread(json.dumps, cache, allow_nan=False)
+        tmp = f"{target}.{os.getpid()}.tmp"
         try:
             async with aiofiles.open(tmp, mode="w", encoding="utf-8") as f:
                 await f.write(payload)
-            await asyncio.to_thread(os.replace, tmp, self.file)
+            await asyncio.to_thread(os.replace, tmp, target)
         except Exception:
             if await asyncio.to_thread(os.path.exists, tmp):
                 try:
@@ -148,6 +149,37 @@ class AioKV:
                     log.exception("aiokv: failed to clean up temp file %s", tmp)
             raise
 
+    async def _sweep_stale_tmp(self, directory: str) -> None:
+        """remove orphaned .<pid>.tmp files left by a hard crash of a different, dead process"""
+        base = os.path.basename(await asyncio.to_thread(os.path.realpath, self.file))
+        pattern = os.path.join(directory, f"{base}.*.tmp")
+        for path in await asyncio.to_thread(glob.glob, pattern):
+            try:
+                pid = int(path.rsplit(".", 2)[-2])
+            except (ValueError, IndexError):
+                continue
+            if pid == os.getpid():
+                continue
+            if await asyncio.to_thread(self._pid_alive, pid):
+                continue
+            try:
+                await asyncio.to_thread(os.remove, path)
+            except FileNotFoundError:
+                pass
+            except Exception:
+                log.exception("aiokv: failed to sweep stale temp file %s", path)
+
+    @staticmethod
+    def _pid_alive(pid: int) -> bool:
+        """check whether pid refers to a live process, without permission to signal counting as alive"""
+        try:
+            os.kill(pid, 0)
+        except ProcessLookupError:
+            return False
+        except PermissionError:
+            return True
+        return True
+
 
 # back-compat: this lib was originally named aiocache; legacy call sites using
 # `aiocache(...)` keep working via this alias. prefer AioKV in new code.