diff --git a/server/services/arduino-compiler.ts b/server/services/arduino-compiler.ts
index c6e8ac7e8..b1fe3edfe 100644
--- a/server/services/arduino-compiler.ts
+++ b/server/services/arduino-compiler.ts
@@ -362,6 +362,7 @@ export class ArduinoCompiler {
         await writeOutputToCache(this.defaultBinaryStorageDir, sketchHash, cliOutput).catch(() => undefined);
       }
       await runHexCacheCleanup(hexCacheDir);
+      await runHexCacheCleanup(this.defaultBinaryStorageDir);
     }
 
     return { cliOutput, cliErrors, parsedErrors };
diff --git a/server/services/compiler/cache-manager.ts b/server/services/compiler/cache-manager.ts
index 3a7cfbfa6..08db80389 100644
--- a/server/services/compiler/cache-manager.ts
+++ b/server/services/compiler/cache-manager.ts
@@ -124,7 +124,7 @@ export async function runHexCacheCleanup(
 ): Promise<void> {
   try {
     const entries = await readdir(hexCacheDir);
-    const files: Array<{ path: string; size: number; atimeMs: number }> = [];
+    const files: Array<{ path: string; sidecarPath: string; size: number; atimeMs: number }> = [];
     let totalSize = 0;
 
     for (const entry of entries) {
@@ -136,6 +136,7 @@ export async function runHexCacheCleanup(
         totalSize += fileStat.size;
         files.push({
           path: fullPath,
+          sidecarPath: join(hexCacheDir, `${entry.slice(0, -".hex".length)}.output.txt`),
           size: fileStat.size,
           atimeMs: fileStat.atimeMs || fileStat.mtimeMs,
         });
@@ -150,6 +151,8 @@ export async function runHexCacheCleanup(
     for (const file of files) {
       if (totalSize <= maxBytes) break;
       await rm(file.path, { force: true });
+      // The output sidecar is useless without its binary and must not outlive it.
+      await rm(file.sidecarPath, { force: true });
       totalSize -= file.size;
     }
   } catch (error) {
diff --git a/server/services/workers/compile-worker-utils.ts b/server/services/workers/compile-worker-utils.ts
index dedce9c88..9d4af219b 100644
--- a/server/services/workers/compile-worker-utils.ts
+++ b/server/services/workers/compile-worker-utils.ts
@@ -153,6 +153,18 @@ export async function evictLruEntries(
   }
 }
 
+/**
+ * Directories under the build cache that grow per sketch hash and need LRU eviction.
+ * The instant-binary store (written by ArduinoCompiler) is included so it cannot grow unbounded.
+ */
+export function buildCacheEvictionTargets(
+  buildCacheDir: string,
+  hexCacheDir: string,
+  coreCacheBuildPath: string,
+): string[] {
+  return [hexCacheDir, join(buildCacheDir, "binaries"), coreCacheBuildPath];
+}
+
 /**
  * LRU cleanup of build cache directories, debounced via marker file.
  */
diff --git a/server/services/workers/compile-worker.ts b/server/services/workers/compile-worker.ts
index e90837112..427ced5d4 100644
--- a/server/services/workers/compile-worker.ts
+++ b/server/services/workers/compile-worker.ts
@@ -32,6 +32,7 @@ import {
   buildSketchHash,
   checkBinaryExists,
   checkFileExists,
+  buildCacheEvictionTargets,
   cleanupCacheLru,
   ensureDirectories,
   execArduinoCliJson,
@@ -170,7 +171,7 @@ async function buildCoreFingerprint(task: CompileRequestPayload, fqbn: string):
 }
 
 async function cleanupCacheLruLocal(): Promise<void> {
-  await cleanupCacheLru(BUILD_CACHE_DIR, [HEX_CACHE_DIR, CORE_CACHE_BUILD_PATH], buildCacheMaxBytes);
+  await cleanupCacheLru(BUILD_CACHE_DIR, buildCacheEvictionTargets(BUILD_CACHE_DIR, HEX_CACHE_DIR, CORE_CACHE_BUILD_PATH), buildCacheMaxBytes);
 }
 
 async function acquireCoreCache(coreReadyMarker: string, coreLockPath: string, coreFingerprint: string): Promise<{ coreCacheWarm: boolean; acquiredCoreLock: boolean; activeBuildCachePath: string }> {
diff --git a/tests/server/services/compiler/cache-manager.test.ts b/tests/server/services/compiler/cache-manager.test.ts
index 799ab6140..eca866946 100644
--- a/tests/server/services/compiler/cache-manager.test.ts
+++ b/tests/server/services/compiler/cache-manager.test.ts
@@ -242,6 +242,20 @@ describe("cache-manager cache cleanup", () => {
     await expect(readFile(paths[2])).resolves.toEqual(Buffer.alloc(3, 3));
   });
 
+  it("removes the output sidecar together with an evicted HEX entry", async () => {
+    const hexCacheDir = await createHexCacheDirectory();
+    const oldHex = join(hexCacheDir, "old.hex");
+    await writeFile(oldHex, Buffer.alloc(4, 1));
+    await writeFile(join(hexCacheDir, "old.output.txt"), "old output", "utf8");
+    await writeFile(join(hexCacheDir, "new.hex"), Buffer.alloc(4, 2));
+    await writeFile(join(hexCacheDir, "new.output.txt"), "new output", "utf8");
+    await utimes(oldHex, new Date("2020-01-01T00:00:00Z"), new Date("2020-01-01T00:00:00Z"));
+
+    await runHexCacheCleanup(hexCacheDir, 4);
+
+    await expect(readdir(hexCacheDir)).resolves.toEqual(["new.hex", "new.output.txt"]);
+  });
+
   it("does not remove HEX entries or sidecars when the cache is within the limit", async () => {
     const hexCacheDir = await createHexCacheDirectory();
     await writeFile(join(hexCacheDir, "sketch.hex"), Buffer.alloc(4, 1));
diff --git a/tests/server/workers/compile-worker-utils.test.ts b/tests/server/workers/compile-worker-utils.test.ts
index bd507ab63..7e0eaec1c 100644
--- a/tests/server/workers/compile-worker-utils.test.ts
+++ b/tests/server/workers/compile-worker-utils.test.ts
@@ -10,6 +10,7 @@ import {
   acquireCoreCacheLock,
   collectDirectoryRecords,
   evictLruEntries,
+  buildCacheEvictionTargets,
   cleanupCacheLru,
   ensureDirectories,
   execArduinoCliJson,
@@ -390,3 +391,13 @@ describe("execArduinoCliJson", () => {
     expect(result).toBeNull();
   });
 });
+
+describe("buildCacheEvictionTargets", () => {
+  it("covers the instant-binary store written by ArduinoCompiler", () => {
+    expect(buildCacheEvictionTargets("/cache", "/cache/hex-cache", "/core/build-cache")).toEqual([
+      "/cache/hex-cache",
+      "/cache/binaries",
+      "/core/build-cache",
+    ]);
+  });
+});