diff --git a/apps/api/src/modules/deployments/prepare.service.ts b/apps/api/src/modules/deployments/prepare.service.ts
index 623c3bb4..a256bc00 100644
--- a/apps/api/src/modules/deployments/prepare.service.ts
+++ b/apps/api/src/modules/deployments/prepare.service.ts
@@ -24,6 +24,7 @@ import {
   type RepoTreeEntry,
 } from "../../lib/project-root-detector";
 import {
+  ALL_PACKAGE_MANAGERS,
   parseDeploymentMetadata,
   parseOpenshipConfigJson,
   METADATA_FILES,
@@ -184,7 +185,18 @@ export interface ProjectInfo {
   stack: StackResult["stack"];
   projectType: ProjectType;
   category: string;
-  packageManager: string;
+  /**
+   * Absent when the source carries no package manager at all — a stock Compose
+   * project has neither a manifest nor a lockfile, and `detectPackageManager`
+   * reports the `"unknown"` sentinel for it. Only values in
+   * `ALL_PACKAGE_MANAGERS` are surfaced, which is the same list every write
+   * body's `packageManager` is generated from: a scan echoed back into
+   * `POST /projects/ensure` (exactly what the wizard does) is then valid by
+   * construction rather than because the one sentinel we know about was
+   * filtered out (#389). Omitted is the shape the field being optional on
+   * every write body already allows.
+   */
+  packageManager?: string;
   buildCommand: string;
   installCommand: string;
   startCommand: string;
@@ -838,7 +850,9 @@ function toProjectInfo(
     stack: stack.stack,
     projectType,
     category: stack.category,
-    packageManager: stack.packageManager,
+    ...(ALL_PACKAGE_MANAGERS.includes(stack.packageManager) && {
+      packageManager: stack.packageManager,
+    }),
     buildCommand: stack.buildCommand,
     installCommand: stack.installCommand,
     startCommand: stack.startCommand,
diff --git a/apps/api/test/modules/deployments/prepare.service.test.ts b/apps/api/test/modules/deployments/prepare.service.test.ts
index f2f8ddc8..9547b0aa 100644
--- a/apps/api/test/modules/deployments/prepare.service.test.ts
+++ b/apps/api/test/modules/deployments/prepare.service.test.ts
@@ -2,8 +2,10 @@ import { afterEach, describe, expect, it } from "vitest";
 import { mkdtemp, mkdir, rm, writeFile } from "node:fs/promises";
 import { tmpdir } from "node:os";
 import { join } from "node:path";
+import { Value } from "@sinclair/typebox/value";
 
 import { resolveProjectInfo } from "../../../src/modules/deployments/prepare.service";
+import { EnsureProjectBody } from "../../../src/modules/projects/project.schema";
 
 describe("resolveProjectInfo", () => {
   const tempDirs: string[] = [];
@@ -109,6 +111,30 @@ describe("resolveProjectInfo", () => {
     expect(result.rootEnv).toEqual({ PORT: "9090" });
   });
 
+  it("omits the package manager when the source has none, so the scan stays a valid project body", async () => {
+    const tempDir = await mkdtemp(join(tmpdir(), "openship-prepare-"));
+    tempDirs.push(tempDir);
+
+    // #389: a stock Compose project has no package.json and no lockfile, so
+    // detection reports the "unknown" sentinel. The wizard echoes the scan
+    // straight back into POST /projects/ensure, whose packageManager is drawn
+    // from ALL_PACKAGE_MANAGERS, and the deploy 400s. The scan must only ever
+    // surface a value from that same list, whatever detection reports.
+    await writeFile(
+      join(tempDir, "docker-compose.yml"),
+      ["services:", "  immich-server:", "    image: ghcr.io/immich-app/immich-server:release"].join(
+        "\n",
+      ),
+    );
+
+    const result = await resolveProjectInfo({ source: "local", path: tempDir });
+
+    expect(result.packageManager).toBeUndefined();
+    expect(
+      Value.Check(EnsureProjectBody, { name: "immich", packageManager: result.packageManager }),
+    ).toBe(true);
+  });
+
   // ── Declared compose path (issue #330) ────────────────────────────────────
   //
   // The detector only PROMOTES a nested compose root when the repo root is
diff --git a/apps/dashboard/src/context/deployment/mode-config.test.ts b/apps/dashboard/src/context/deployment/mode-config.test.ts
new file mode 100644
index 00000000..5acee387
--- /dev/null
+++ b/apps/dashboard/src/context/deployment/mode-config.test.ts
@@ -0,0 +1,54 @@
+import { describe, it, expect } from "vitest";
+import { buildSingleModeSnapshot } from "./mode-config";
+import type { DeploymentConfig } from "./types";
+
+/**
+ * #389: the scan omits `packageManager` when the source has none — a stock
+ * Compose project carries neither a manifest nor a lockfile. The snapshot is
+ * persisted through `POST /projects/ensure`, whose `packageManager` is drawn
+ * from the real package-manager list, so an absent value has to resolve to a
+ * real one here rather than travel as undefined.
+ */
+describe("buildSingleModeSnapshot — compose primary without a package manager", () => {
+  const config = {
+    projectType: "services",
+    projectName: "immich",
+    repo: "immich",
+    services: [],
+    publicEndpoints: [],
+    buildStrategy: "auto",
+    options: {
+      productionPort: "",
+      rootDirectory: "./",
+      installCommand: "",
+      buildCommand: "",
+      startCommand: "",
+      outputDirectory: "",
+      productionPaths: "",
+      hasServer: false,
+      hasBuild: false,
+    },
+    singleAppCandidate: {
+      stack: "docker-compose",
+      projectType: "services",
+      category: "docker",
+      // No packageManager: this is exactly what the scan now returns.
+      buildCommand: "",
+      installCommand: "",
+      startCommand: "",
+      buildImage: "node:22",
+      outputDirectory: "dist",
+      rootDirectory: "./",
+      productionPaths: [],
+      port: 2283,
+      hasServer: false,
+      hasBuild: false,
+    },
+  } as unknown as DeploymentConfig;
+
+  it("falls back to npm instead of carrying an absent package manager", () => {
+    const snapshot = buildSingleModeSnapshot(config);
+
+    expect(snapshot?.packageManager).toBe("npm");
+  });
+});
diff --git a/apps/dashboard/src/context/deployment/mode-config.ts b/apps/dashboard/src/context/deployment/mode-config.ts
index b2293e02..5862da76 100644
--- a/apps/dashboard/src/context/deployment/mode-config.ts
+++ b/apps/dashboard/src/context/deployment/mode-config.ts
@@ -305,7 +305,7 @@ function pickComposePrimary(
     const primary: SingleAppPrimary = {
       framework: candidate.stack as FrameworkId,
       detectedFramework: candidate.stack as FrameworkId,
-      packageManager: candidate.packageManager,
+      packageManager: candidate.packageManager || "npm",
       buildImage: candidate.buildImage,
       rootDirectory: candidate.rootDirectory,
       installCommand: candidate.installCommand,
diff --git a/apps/dashboard/src/lib/api/deploy.ts b/apps/dashboard/src/lib/api/deploy.ts
index 4a24253b..1ab06bce 100644
--- a/apps/dashboard/src/lib/api/deploy.ts
+++ b/apps/dashboard/src/lib/api/deploy.ts
@@ -74,7 +74,9 @@ export interface PrepareAppConfig {
   stack: StackId;
   projectType: "app" | "docker" | "services" | "monorepo";
   category: string;
-  packageManager: string;
+  /** Absent when the source has none — a stock Compose project carries neither
+   *  a manifest nor a lockfile. Every read falls back to npm. */
+  packageManager?: string;
   buildCommand: string;
   installCommand: string;
   startCommand: string;