diff --git a/src/server/runtime-handler/adapter-factory.test.ts b/src/server/runtime-handler/adapter-factory.test.ts
index 73b883921b..cfdffff0f4 100644
--- a/src/server/runtime-handler/adapter-factory.test.ts
+++ b/src/server/runtime-handler/adapter-factory.test.ts
@@ -522,6 +522,53 @@ describe("adapter-factory", () => {
     assertEquals("isLocalProject" in result, true);
   });
 
+  it("survives an adapter cache that drops the entry it was just given", async () => {
+    Deno.env.set("VERYFRONT_TRUST_FORWARDED_HEADERS", "1");
+    const cache = new ProjectDiscoveryCache();
+    const adapter = createMockAdapter({
+      "/trusted/project": { isDirectory: true },
+      "/trusted/project/app": { isDirectory: true },
+    });
+
+    // The adapter LRU estimates a RuntimeAdapter's size and evicts it again
+    // when it exceeds the byte budget, so set() can silently store nothing.
+    // Observed under Bun 1.3.6, where the same adapter object that caches
+    // fine on Node is dropped: set() then has()===false and size===0.
+    // resolveAdapter must not depend on reading back what it just wrote.
+    cache.adapters.set = () => {};
+
+    const result = await resolveAdapter({
+      req: await makeReq({ projectPath: "/trusted/project", trusted: true }),
+      projectDir: "/base/project",
+      adapter,
+      config: undefined,
+      projectSlug: "myproject",
+      projectId: "proj_123",
+      proxyToken: undefined,
+      releaseId: undefined,
+      proxyEnv: "preview",
+      branch: null,
+      environmentName: undefined,
+      parsedDomain: {
+        slug: null,
+        branch: null,
+        environment: null,
+        isVeryfrontDomain: false,
+        isDraft: false,
+        allowIframeEmbed: false,
+      },
+      isProxyMode: true,
+      cache,
+      prepareHostedConfigContext: preparePreviewHostedConfigContext,
+    });
+
+    assertEquals(
+      result.adapter === undefined || result.adapter === null,
+      false,
+      "resolveAdapter must hand on a real adapter even when the cache drops it",
+    );
+  });
+
   it("uses injected cache instead of default singleton", async () => {
     Deno.env.set("VERYFRONT_TRUST_FORWARDED_HEADERS", "1");
     const cache = new ProjectDiscoveryCache();
diff --git a/src/server/runtime-handler/adapter-factory.ts b/src/server/runtime-handler/adapter-factory.ts
index a082bfa924..9bb4ac91e8 100644
--- a/src/server/runtime-handler/adapter-factory.ts
+++ b/src/server/runtime-handler/adapter-factory.ts
@@ -222,17 +222,27 @@ export async function resolveAdapter(
       projectDir: effectiveProjectDir,
     });
 
-    // Get or create local adapter
-    if (!cache.adapters.has(effectiveProjectDir)) {
-      const baseAdapter = await runtime.get();
-      cache.adapters.set(effectiveProjectDir, baseAdapter);
+    // Get or create local adapter.
+    //
+    // Hold the adapter rather than reading it back: the cache is an LRU that
+    // estimates each value's size and can evict an oversized entry as part of
+    // the same set(), so a write is not guaranteed to be readable afterwards.
+    // A RuntimeAdapter crosses that budget under Bun, where set() then leaves
+    // has() === false and size === 0, and the non-null assertion this replaces
+    // turned that miss into an undefined adapter that reached getConfig and
+    // threw "undefined is not an object (evaluating 'adapter.fs')" on every
+    // request. Caching stays best effort; correctness no longer depends on it.
+    let localAdapter = cache.adapters.get(effectiveProjectDir);
+    if (!localAdapter) {
+      localAdapter = await runtime.get();
+      cache.adapters.set(effectiveProjectDir, localAdapter);
       logger.debug("Created local adapter for project", {
         projectSlug: opts.projectSlug,
         projectDir: effectiveProjectDir,
       });
     }
 
-    effectiveAdapter = cache.adapters.get(effectiveProjectDir)!;
+    effectiveAdapter = localAdapter;
 
     if (shouldDeferConfigLoad(opts)) {
       effectiveConfig = undefined;