diff --git a/CHANGELOG.md b/CHANGELOG.md
index 25f5779..fa7035e 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -6,6 +6,15 @@ adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ## [Unreleased]
 
+## [0.3.1] - 2026-07-12
+
+### Fixed
+
+- `POST /v1/apps` now requires `org_slug`, returning `400` to match real Fly,
+  which rejects app creation without it; the emulator was previously too lenient
+  and accepted the request, letting a plan that would fail against Fly pass
+  offline.
+
 ## [0.3.0] - 2026-07-11
 
 Breadth: the full app-resource surface (volumes, secrets, IPs, certificates) plus
@@ -48,7 +57,7 @@ Fidelity and correctness pass from an adversarial audit against `superfly/fly-go
 - `/wait` honors fly-go's `version` filter and a repeatable `state` param (any
   requested state satisfies the wait); `instance_id` still accepted.
 - `cordoned` is surfaced on the machine object (`json:"cordoned"`).
-- `signal`, `exec`, and `ps` answer an honest `501` and appear in
+- `signal`, `exec`, and `ps` answer a plain `501` and appear in
   `/_mudflaps/health` instead of falling through to a bare `404`.
 - Response shapes match flaps: create-machine returns `200`, `start` returns a
   `MachineStartResponse`, and stop/restart/suspend/cordon/delete return an empty
@@ -87,7 +96,8 @@ Fidelity and correctness pass from an adversarial audit against `superfly/fly-go
 - Distroless container image, GoReleaser configuration, mkdocs-material doc site,
   and CI.
 
-[Unreleased]: https://github.com/intentius/mudflaps/compare/v0.3.0...HEAD
+[Unreleased]: https://github.com/intentius/mudflaps/compare/v0.3.1...HEAD
+[0.3.1]: https://github.com/intentius/mudflaps/compare/v0.3.0...v0.3.1
 [0.3.0]: https://github.com/intentius/mudflaps/compare/v0.2.0...v0.3.0
 [0.2.0]: https://github.com/intentius/mudflaps/compare/v0.1.0...v0.2.0
 [0.1.0]: https://github.com/intentius/mudflaps/releases/tag/v0.1.0
diff --git a/internal/server/server.go b/internal/server/server.go
index 8f2fbb1..0174928 100644
--- a/internal/server/server.go
+++ b/internal/server/server.go
@@ -217,6 +217,10 @@ func (s *Server) createApp(w http.ResponseWriter, r *http.Request) {
 		s.writeError(w, http.StatusBadRequest, "app_name is required")
 		return
 	}
+	if req.OrgSlug == "" {
+		s.writeError(w, http.StatusBadRequest, "authorization: request doesn't contain org_slug")
+		return
+	}
 	app, err := s.store.CreateApp(flaps.App{Name: req.AppName, Organization: req.OrgSlug})
 	if errors.Is(err, store.ErrAppExists) {
 		s.writeError(w, http.StatusConflict, "app already exists")
diff --git a/internal/server/server_test.go b/internal/server/server_test.go
index 7a920f2..1565177 100644
--- a/internal/server/server_test.go
+++ b/internal/server/server_test.go
@@ -66,7 +66,7 @@ func (h *harness) mustJSON(raw []byte, dst any) {
 // createStartedMachine creates an app+machine and drives it to started.
 func (h *harness) createStartedMachine(app string) flaps.Machine {
 	h.t.Helper()
-	if code, body := h.do(http.MethodPost, "/v1/apps", flaps.CreateAppRequest{AppName: app}, nil); code != http.StatusCreated {
+	if code, body := h.do(http.MethodPost, "/v1/apps", flaps.CreateAppRequest{AppName: app, OrgSlug: "personal"}, nil); code != http.StatusCreated {
 		h.t.Fatalf("create app: %d %s", code, body)
 	}
 	code, body := h.do(http.MethodPost, "/v1/apps/"+app+"/machines", flaps.CreateMachineRequest{
@@ -132,6 +132,19 @@ func TestCreateAndWaitStarted(t *testing.T) {
 	}
 }
 
+// TestCreateAppRequiresOrgSlug mirrors real Fly, which rejects app creation
+// without an org_slug (400) rather than silently creating the app.
+func TestCreateAppRequiresOrgSlug(t *testing.T) {
+	h := newHarness(t)
+	code, body := h.do(http.MethodPost, "/v1/apps", flaps.CreateAppRequest{AppName: "x"}, nil)
+	if code != http.StatusBadRequest {
+		t.Fatalf("create app without org_slug = %d %s, want 400", code, body)
+	}
+	if !strings.Contains(string(body), "org_slug") {
+		t.Fatalf("error body = %s, want it to mention org_slug", body)
+	}
+}
+
 func TestLeaseBlocksMutationWithoutNonce(t *testing.T) {
 	h := newHarness(t)
 	m := h.createStartedMachine("demo")
@@ -460,7 +473,7 @@ func TestStopAcceptsDurationStringTimeout(t *testing.T) {
 // TestSkipLaunchRestsInCreated is the regression for audit M1.
 func TestSkipLaunchRestsInCreated(t *testing.T) {
 	h := newHarness(t)
-	if code, body := h.do(http.MethodPost, "/v1/apps", flaps.CreateAppRequest{AppName: "demo"}, nil); code != http.StatusCreated {
+	if code, body := h.do(http.MethodPost, "/v1/apps", flaps.CreateAppRequest{AppName: "demo", OrgSlug: "personal"}, nil); code != http.StatusCreated {
 		t.Fatalf("create app = %d %s", code, body)
 	}
 	code, body := h.do(http.MethodPost, "/v1/apps/demo/machines",
@@ -589,7 +602,7 @@ func TestDestroyAndWaitDestroyed(t *testing.T) {
 // multi-second real sleep.
 func TestWaitTimesOut(t *testing.T) {
 	h := newHarness(t)
-	if code, body := h.do(http.MethodPost, "/v1/apps", flaps.CreateAppRequest{AppName: "demo"}, nil); code != http.StatusCreated {
+	if code, body := h.do(http.MethodPost, "/v1/apps", flaps.CreateAppRequest{AppName: "demo", OrgSlug: "personal"}, nil); code != http.StatusCreated {
 		t.Fatalf("create app = %d %s", code, body)
 	}
 	// skip_launch leaves the machine in 'created', so it never reaches 'started'.
@@ -752,7 +765,7 @@ func TestDeleteAppWithLeasedMachine(t *testing.T) {
 // data has no description field.
 func TestResponseShapesMatchFlaps(t *testing.T) {
 	h := newHarness(t)
-	if code, body := h.do(http.MethodPost, "/v1/apps", flaps.CreateAppRequest{AppName: "demo"}, nil); code != http.StatusCreated {
+	if code, body := h.do(http.MethodPost, "/v1/apps", flaps.CreateAppRequest{AppName: "demo", OrgSlug: "personal"}, nil); code != http.StatusCreated {
 		t.Fatalf("create app = %d %s", code, body)
 	}
 	// create machine -> 200 (not 201)
@@ -822,7 +835,7 @@ func TestPlatformRegions(t *testing.T) {
 // TestVolumeCRUD covers the volume endpoints (breadth #18).
 func TestVolumeCRUD(t *testing.T) {
 	h := newHarness(t)
-	if code, body := h.do(http.MethodPost, "/v1/apps", flaps.CreateAppRequest{AppName: "demo"}, nil); code != http.StatusCreated {
+	if code, body := h.do(http.MethodPost, "/v1/apps", flaps.CreateAppRequest{AppName: "demo", OrgSlug: "personal"}, nil); code != http.StatusCreated {
 		t.Fatalf("create app = %d %s", code, body)
 	}
 	// empty list, not 501
@@ -876,7 +889,7 @@ func TestVolumeCRUD(t *testing.T) {
 // delete, and the invariant that a value is never returned.
 func TestSecretsApplyOnly(t *testing.T) {
 	h := newHarness(t)
-	if code, body := h.do(http.MethodPost, "/v1/apps", flaps.CreateAppRequest{AppName: "demo"}, nil); code != http.StatusCreated {
+	if code, body := h.do(http.MethodPost, "/v1/apps", flaps.CreateAppRequest{AppName: "demo", OrgSlug: "personal"}, nil); code != http.StatusCreated {
 		t.Fatalf("create app = %d %s", code, body)
 	}
 	// set
@@ -919,7 +932,7 @@ func TestSecretsApplyOnly(t *testing.T) {
 // TestIPAssignments covers the ip_assignments endpoints (breadth #21).
 func TestIPAssignments(t *testing.T) {
 	h := newHarness(t)
-	if code, body := h.do(http.MethodPost, "/v1/apps", flaps.CreateAppRequest{AppName: "demo"}, nil); code != http.StatusCreated {
+	if code, body := h.do(http.MethodPost, "/v1/apps", flaps.CreateAppRequest{AppName: "demo", OrgSlug: "personal"}, nil); code != http.StatusCreated {
 		t.Fatalf("create app = %d %s", code, body)
 	}
 	// shared v4
@@ -957,7 +970,7 @@ func TestIPAssignments(t *testing.T) {
 // TestCertificates covers the certificate endpoints (breadth #20).
 func TestCertificates(t *testing.T) {
 	h := newHarness(t)
-	if code, body := h.do(http.MethodPost, "/v1/apps", flaps.CreateAppRequest{AppName: "demo"}, nil); code != http.StatusCreated {
+	if code, body := h.do(http.MethodPost, "/v1/apps", flaps.CreateAppRequest{AppName: "demo", OrgSlug: "personal"}, nil); code != http.StatusCreated {
 		t.Fatalf("create app = %d %s", code, body)
 	}
 	// create