diff --git a/apps/server/src/auth/ServerSecretStore.test.ts b/apps/server/src/auth/ServerSecretStore.test.ts
index 539b35dc8f..af7bc0774d 100644
--- a/apps/server/src/auth/ServerSecretStore.test.ts
+++ b/apps/server/src/auth/ServerSecretStore.test.ts
@@ -3,6 +3,7 @@ import { assert, it } from "@effect/vitest";
 import * as Cause from "effect/Cause";
 import * as Deferred from "effect/Deferred";
 import * as Effect from "effect/Effect";
+import * as Fiber from "effect/Fiber";
 import * as FileSystem from "effect/FileSystem";
 import * as Layer from "effect/Layer";
 import * as Option from "effect/Option";
@@ -185,6 +186,57 @@ it.layer(NodeServices.layer)("ServerSecretStore.layer", (it) => {
     }).pipe(Effect.provide(layerConcurrentCreateSecretStore())),
   );
 
+  it.effect("never exposes a secret before its bytes are written", () =>
+    Effect.gen(function* () {
+      const writing = yield* Deferred.make<void>();
+      const readDone = yield* Deferred.make<void>();
+      const layerPausedWriteFileSystem = Layer.effect(
+        FileSystem.FileSystem,
+        Effect.gen(function* () {
+          const fileSystem = yield* FileSystem.FileSystem;
+
+          return {
+            ...fileSystem,
+            open: (path, options) =>
+              fileSystem.open(path, options).pipe(
+                Effect.map((file) =>
+                  Object.assign(Object.create(file) as FileSystem.File, {
+                    writeAll: (buffer: Uint8Array) =>
+                      Deferred.succeed(writing, void 0).pipe(
+                        Effect.andThen(Deferred.await(readDone)),
+                        Effect.andThen(file.writeAll(buffer)),
+                      ),
+                  }),
+                ),
+              ),
+          } satisfies FileSystem.FileSystem;
+        }),
+      ).pipe(Layer.provide(NodeServices.layer));
+
+      yield* Effect.gen(function* () {
+        const secretStore = yield* ServerSecretStore.ServerSecretStore;
+        const value = Uint8Array.from([1, 2, 3]);
+
+        const creating = yield* Effect.forkChild(secretStore.create("session-signing-key", value));
+        yield* Deferred.await(writing);
+        const midWrite = yield* secretStore.get("session-signing-key");
+        yield* Deferred.succeed(readDone, void 0);
+        yield* Fiber.join(creating);
+
+        assert.isTrue(Option.isNone(midWrite));
+        const persisted = Option.getOrThrow(yield* secretStore.get("session-signing-key"));
+        assert.deepEqual(Array.from(persisted), Array.from(value));
+      }).pipe(
+        Effect.provide(
+          ServerSecretStore.layer.pipe(
+            Layer.provide(layerServerConfig()),
+            Layer.provideMerge(layerPausedWriteFileSystem),
+          ),
+        ),
+      );
+    }).pipe(Effect.provide(NodeServices.layer)),
+  );
+
   it.effect("uses restrictive permissions for the secret directory and files", () =>
     Effect.gen(function* () {
       const chmodCalls: Array<{ readonly path: string; readonly mode: number }> = [];
diff --git a/apps/server/src/auth/ServerSecretStore.ts b/apps/server/src/auth/ServerSecretStore.ts
index 1ee11d3e19..8f20f22b7d 100644
--- a/apps/server/src/auth/ServerSecretStore.ts
+++ b/apps/server/src/auth/ServerSecretStore.ts
@@ -224,26 +224,42 @@ export const make = Effect.gen(function* () {
     );
   };
 
+  // Writes the complete secret to a temporary file, then hard-links it into place. The link
+  // fails with AlreadyExists when another creator won, and a reader never sees partial bytes.
   const create: ServerSecretStore["Service"]["create"] = (name, value) => {
     const secretPath = resolveSecretPath(name);
-    return Effect.scoped(
-      Effect.gen(function* () {
-        const file = yield* fileSystem.open(secretPath, {
-          flag: "wx",
-          mode: 0o600,
-        });
-        yield* file.writeAll(value);
-        yield* file.sync;
-        yield* fileSystem.chmod(secretPath, 0o600);
-      }),
-    ).pipe(
+    return crypto.randomUUIDv4.pipe(
       Effect.mapError(
         (cause) =>
-          new SecretStorePersistError({
+          new SecretStoreTemporaryPathError({
             resource: `secret ${name}`,
             cause,
           }),
       ),
+      Effect.flatMap((uuid) => {
+        const tempPath = `${secretPath}.${uuid}.tmp`;
+        return Effect.scoped(
+          Effect.gen(function* () {
+            const file = yield* fileSystem.open(tempPath, {
+              flag: "wx",
+              mode: 0o600,
+            });
+            yield* file.writeAll(value);
+            yield* file.sync;
+          }),
+        ).pipe(
+          Effect.andThen(fileSystem.chmod(tempPath, 0o600)),
+          Effect.andThen(fileSystem.link(tempPath, secretPath)),
+          Effect.ensuring(fileSystem.remove(tempPath).pipe(Effect.ignore)),
+          Effect.mapError(
+            (cause) =>
+              new SecretStorePersistError({
+                resource: `secret ${name}`,
+                cause,
+              }),
+          ),
+        );
+      }),
     );
   };
 
diff --git a/docs/fork-features.md b/docs/fork-features.md
index 57cd68ae6c..ae8767f463 100644
--- a/docs/fork-features.md
+++ b/docs/fork-features.md
@@ -1313,3 +1313,21 @@ Child creation paths are owned by child-threads and shared with thread-people.
   "keywords": ["realPath", "/private/var", "npm-global"]
 }
 ```
+
+## secret-store-atomic-create
+
+```json
+{
+  "id": "secret-store-atomic-create",
+  "purpose": "ServerSecretStore.create writes the complete secret to a temporary file and hard-links it into place, so a creator that loses the race never reads a half-written secret. Drop when upstream makes create atomic.",
+  "issues": ["https://github.com/toolboxmd/chromeria/issues/249"],
+  "prs": ["https://github.com/toolboxmd/chromeria/pull/261"],
+  "newFiles": [],
+  "upstreamFiles": [
+    "apps/server/src/auth/ServerSecretStore.test.ts",
+    "apps/server/src/auth/ServerSecretStore.ts"
+  ],
+  "sharedFiles": [],
+  "keywords": ["getOrCreateRandom", "fileSystem.link", "half-written secret"]
+}
+```
diff --git a/scripts/fork-upstream-edits.txt b/scripts/fork-upstream-edits.txt
index ef88bf33e9..a122467f0d 100644
--- a/scripts/fork-upstream-edits.txt
+++ b/scripts/fork-upstream-edits.txt
@@ -198,3 +198,5 @@ apps/server/src/orchestration-v2/Adapters/GrokAdapterV2.ts
 apps/web/src/components/chat/MermaidDiagram.tsx
 apps/server/src/cli/invocation.test.ts
 apps/server/src/environment/ServerEnvironment.test.ts
+apps/server/src/auth/ServerSecretStore.test.ts
+apps/server/src/auth/ServerSecretStore.ts