diff --git a/src/main/steam-activity-watch.ts b/src/main/steam-activity-watch.ts
index 98ffff5..5c9bce9 100644
--- a/src/main/steam-activity-watch.ts
+++ b/src/main/steam-activity-watch.ts
@@ -151,7 +151,8 @@ export class SteamActivityWatch {
       const stillListed = new Set(this.deps.listSteamGames().map((game) => game.id));
       for (const game of games) {
         if (!stillListed.has(game.id)) continue;
-        this.apply(game, statuses.get(game.appid) ?? { state: 'absent' });
+        const status = statuses.get(game.appid);
+        if (status !== undefined) this.apply(game, status);
       }
     } catch (cause) {
       log.warn('[steam-watch] tick failed:', describe(cause));
@@ -189,14 +190,20 @@ export class SteamActivityWatch {
     if (game.id === this.deps.sessionGameId()) return;
     if (this.applyUninstallRequest(game, status)) return;
     const before = this.tracked.get(game.id);
-    const updating = status.state === 'downloading' && (before?.state === 'installed' || before?.updating === true);
+    const updating =
+      status.state === 'downloading' &&
+      (before?.state === 'installed' || before?.updating === true);
     this.tracked.set(game.id, { state: status.state, updating });
     if (status.state === 'downloading') {
-      this.deps.registry.set(game.id, downloadActivity(updating ? 'steam-updating' : 'steam-installing', status));
+      this.deps.registry.set(
+        game.id,
+        downloadActivity(updating ? 'steam-updating' : 'steam-installing', status),
+      );
     } else {
       this.clearSteamActivity(game.id);
     }
-    const changed = before === undefined ? status.state === 'downloading' : before.state !== status.state;
+    const changed =
+      before === undefined ? status.state === 'downloading' : before.state !== status.state;
     if (!changed) return;
     log.info(`[steam-watch] appid=${game.appid} ${before?.state ?? 'unknown'} → ${status.state}`);
     this.deps.onGameChanged(game.id);
@@ -219,7 +226,9 @@ export class SteamActivityWatch {
         this.deps.registry.set(game.id, { kind: 'steam-uninstalling' });
         return true;
       }
-      log.info(`[steam-uninstall] appid=${game.appid} still installed after timeout — assuming cancel`);
+      log.info(
+        `[steam-uninstall] appid=${game.appid} still installed after timeout - assuming cancel`,
+      );
       this.uninstallRequests.delete(game.appid);
       this.clearSteamActivity(game.id);
       return false;
diff --git a/src/main/steam.ts b/src/main/steam.ts
index 589db36..55c5fa7 100644
--- a/src/main/steam.ts
+++ b/src/main/steam.ts
@@ -105,15 +105,15 @@ function fraction(num: number | null, den: number | null): number | null {
   return Math.max(0, Math.min(1, num / den));
 }
 
-async function readAcfState(acfPath: string): Promise<AcfState | null> {
+async function readAcfState(acfPath: string): Promise<AcfState | 'missing' | 'unreadable'> {
   let content: string;
   try {
     content = await fse.readFile(acfPath, 'utf8');
   } catch {
-    return null; // no .acf in this library
+    return 'missing'; // no .acf in this library
   }
   const flags = readAcfNumber(content, 'StateFlags');
-  if (flags === null) return null;
+  if (flags === null) return 'unreadable';
   // Staged progress matches Steam's displayed percent best; fall back to download progress.
   const progress =
     fraction(readAcfNumber(content, 'BytesStaged'), readAcfNumber(content, 'BytesToStage')) ??
@@ -131,14 +131,20 @@ async function readAcfState(acfPath: string): Promise<AcfState | null> {
 
 /**
  * Where `appid` stands across the given Steam library roots: the `appmanifest_<appid>.acf` walk shared by
- * the single lookup and the batch one below.
+ * the single lookup and the batch one below. Null when the only `.acf` found could not be parsed - most
+ * likely caught halfway through Steam rewriting it - so the state is unknown rather than "absent".
  */
-async function statusInLibraries(appid: number, libs: readonly string[]): Promise<SteamInstallStatus> {
+async function statusInLibraries(appid: number, libs: readonly string[]): Promise<SteamInstallStatus | null> {
   let downloading: SteamInstallStatus | null = null;
+  let unreadable = false;
   for (const lib of libs) {
     const acfPath = path.join(lib, 'steamapps', `appmanifest_${appid}.acf`);
     const acf = await readAcfState(acfPath);
-    if (acf === null) continue;
+    if (acf === 'missing') continue;
+    if (acf === 'unreadable') {
+      unreadable = true;
+      continue;
+    }
     if (acf.fullyInstalled) return { state: 'installed' };
     // .acf exists but not fully installed → downloading/updating in this library. The percent is only
     // trustworthy when paused (byte counters are stale while actively downloading).
@@ -149,7 +155,8 @@ async function statusInLibraries(appid: number, libs: readonly string[]): Promis
       preloaded: acf.preloaded,
     };
   }
-  return downloading ?? { state: 'absent' };
+  if (downloading !== null) return downloading;
+  return unreadable ? null : { state: 'absent' };
 }
 
 /**
@@ -164,7 +171,7 @@ export async function steamInstallStatus(
   try {
     const steamPath = await locator.locateSteam();
     if (steamPath === null) return { state: 'absent' };
-    return await statusInLibraries(appid, await steamLibraryDirs(steamPath));
+    return (await statusInLibraries(appid, await steamLibraryDirs(steamPath))) ?? { state: 'absent' };
   } catch (cause) {
     log.warn(
       '[steam] install check failed:',
@@ -177,7 +184,8 @@ export async function steamInstallStatus(
 /**
  * Where each of `appids` stands locally, from ONE Steam lookup and ONE library listing for the whole batch.
  * Null when Steam is not found or the lookup fails, so a caller polling many games keeps what it last knew
- * instead of reading every one of them as removed.
+ * instead of reading every one of them as removed; an appid whose `.acf` could not be parsed is left out of
+ * the map for the same reason.
  */
 export async function steamInstallStatuses(
   appids: readonly number[],
@@ -190,7 +198,9 @@ export async function steamInstallStatuses(
     const statuses = await Promise.all(
       appids.map(async (appid) => [appid, await statusInLibraries(appid, libs)] as const),
     );
-    return new Map(statuses);
+    return new Map(
+      statuses.flatMap(([appid, status]) => (status === null ? [] : [[appid, status] as const])),
+    );
   } catch (cause) {
     log.warn(
       '[steam] batch install check failed:',
diff --git a/test/steam-activity-watch.test.ts b/test/steam-activity-watch.test.ts
index d912842..17c2c60 100644
--- a/test/steam-activity-watch.test.ts
+++ b/test/steam-activity-watch.test.ts
@@ -136,7 +136,11 @@ describe('SteamActivityWatch transitions', () => {
     expect(fixture.registry.get('a')).toEqual({ kind: 'steam-updating', paused: false });
     await fixture.acf(A.appid, 'paused', 0.25);
     await fixture.watch.scanNow();
-    expect(fixture.registry.get('a')).toEqual({ kind: 'steam-updating', paused: true, pausedProgress: 0.25 });
+    expect(fixture.registry.get('a')).toEqual({
+      kind: 'steam-updating',
+      paused: true,
+      pausedProgress: 0.25,
+    });
     await fixture.acf(A.appid, 'installed');
     await fixture.watch.scanNow();
     expect(fixture.registry.has('a')).toBe(false);
@@ -148,7 +152,11 @@ describe('SteamActivityWatch transitions', () => {
     await fixture.watch.scanNow();
     await fixture.acf(A.appid, 'paused', 0.4);
     await fixture.watch.scanNow();
-    expect(fixture.registry.get('a')).toEqual({ kind: 'steam-installing', paused: true, pausedProgress: 0.4 });
+    expect(fixture.registry.get('a')).toEqual({
+      kind: 'steam-installing',
+      paused: true,
+      pausedProgress: 0.4,
+    });
     expect(fixture.events).toEqual(['changed:a']);
   });
 
@@ -176,10 +184,15 @@ describe('SteamActivityWatch transitions', () => {
     await fixture.acf(B.appid, 'installed');
     await fixture.watch.scanNow();
     expect(fixture.registry.snapshot()).toEqual({ a: { kind: 'steam-installing', paused: false } });
-    expect(fixture.events.filter((event) => event.startsWith('installed:'))).toEqual(['installed:b']);
+    expect(fixture.events.filter((event) => event.startsWith('installed:'))).toEqual([
+      'installed:b',
+    ]);
     await fixture.acf(A.appid, 'installed');
     await fixture.watch.scanNow();
-    expect(fixture.events.filter((event) => event.startsWith('installed:'))).toEqual(['installed:b', 'installed:a']);
+    expect(fixture.events.filter((event) => event.startsWith('installed:'))).toEqual([
+      'installed:b',
+      'installed:a',
+    ]);
   });
 });
 
@@ -216,7 +229,11 @@ describe('SteamActivityWatch pre-loads', () => {
   it('the same result before every byte is in stays an ordinary paused download', async () => {
     await writeAcf(A.appid, preloadAcf(A.appid, 1000));
     await fixture.watch.scanNow();
-    expect(fixture.registry.get('a')).toEqual({ kind: 'steam-installing', paused: true, pausedProgress: 1 });
+    expect(fixture.registry.get('a')).toEqual({
+      kind: 'steam-installing',
+      paused: true,
+      pausedProgress: 1,
+    });
   });
 
   it('the release turns the pre-load into an install, with its notification', async () => {
@@ -229,6 +246,23 @@ describe('SteamActivityWatch pre-loads', () => {
   });
 });
 
+describe('SteamActivityWatch and a half-written .acf', () => {
+  it('an .acf caught mid-rewrite changes nothing, so the finished download still notifies', async () => {
+    await fixture.acf(A.appid, 'downloading');
+    await fixture.watch.scanNow();
+    await fs.writeFile(
+      path.join(root, 'steamapps', `appmanifest_${A.appid}.acf`),
+      '"AppState"\n{\n',
+    );
+    await fixture.watch.scanNow();
+    expect(fixture.registry.get('a')).toEqual({ kind: 'steam-installing', paused: false });
+    expect(fixture.events).toEqual(['changed:a']);
+    await fixture.acf(A.appid, 'installed');
+    await fixture.watch.scanNow();
+    expect(fixture.events).toEqual(['changed:a', 'changed:a', 'installed:a']);
+  });
+});
+
 describe('SteamActivityWatch uninstall requests', () => {
   it('shows uninstalling while the game is still installed, then notifies once it is gone', async () => {
     await fixture.acf(A.appid, 'installed');