diff --git a/CHANGELOG.md b/CHANGELOG.md
index 83c66c5..1dd06cf 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,3 +1,15 @@
+## v0.266.0 — a failed install removes what it started (2026-09-23, R-649)
+
+**MinAgent: 0.131.0** (unchanged). No new strings. No hub change.
+
+- **Operator ruling 2026-09-23 (R-649):** when the deploy's `compose up -d` fails for its own reasons (e.g. a
+  dependency whose healthcheck never passes after its container started), `runComposeDeploy` now runs
+  `compose down` before recording „not deployed" — so „not installed" never stands over running
+  containers (R-634's promise, completed). `down` WITHOUT `-v`: named volumes are kept. A failed `down` is
+  logged; the household's Remove clears what is left (v0.262.0 `halfStateEvidence`).
+- Tests: `TestR649_AFailedInstallRemovesWhatItStarted` (stub compose, PATH = the stub only),
+  `TestR649_ASuccessfulInstallIsNotTakenDown`. Red-proof: the `down` removed → `calls=["up -d"]`.
+
 ## v0.265.0 — the cause of "runs but not installed", a held app that says so, a louder OOM storm (2026-09-23, R-634, R-625, R-636, R-647)
 
 **MinAgent: 0.131.0** (unchanged). **Needs hub v0.121.0** (deployed first). New strings: yes (hu + en).
diff --git a/controller/README.md b/controller/README.md
index 7a3b43c..53bdb95 100644
--- a/controller/README.md
+++ b/controller/README.md
@@ -656,6 +656,11 @@ sentence, in the reader's language) and no Update button is rendered; the API st
 held update's error is stored as the key `update.error.held` and rendered per reader, so a reader in the
 other language than the box reads the hold in theirs (R-647).
 
+**A failed install removes what it started (v0.266.0, R-649, operator ruling 2026-09-23).** When the
+deploy's `compose up -d` fails, the controller runs `compose down` (WITHOUT `-v`: named volumes stay, so a
+reinstall after „keep my data" finds its data) before the record reads „not deployed". „Failed" therefore
+means nothing runs; the household presses Install again. A failed `down` is logged; Remove clears the rest.
+
 **Deploys are not interrupted (v0.265.0, R-634).** A deploy still running is not in any backup leg's app
 list, the volume leg asks again right before it stops an app, and `StopStack` / `StartStack` refuse a
 deploying stack (`ErrStackDeploying`) for every caller. Measured cause: the whole-box backup's `compose down`
diff --git a/controller/internal/stacks/deploy.go b/controller/internal/stacks/deploy.go
index 05cfdb4..7fa42f6 100644
--- a/controller/internal/stacks/deploy.go
+++ b/controller/internal/stacks/deploy.go
@@ -422,6 +422,17 @@ func (m *Manager) runComposeDeploy(name, stackDir string, env map[string]string,
 
 	if composeErr != nil {
 		m.logger.Printf("[ERROR] [stacks] Stack %s deploy failed after %.1fs: %v", name, time.Since(start).Seconds(), composeErr)
+		// R-649 (v0.266.0, operator ruling 2026-09-23): a failed install REMOVES what it started, so
+		// „not installed" never stands over running containers (R-634's promise, for the deploy's OWN
+		// failures — e.g. a dependency whose healthcheck never passes after its container started).
+		// `down` WITHOUT -v: containers and the network go, named volumes stay — a reinstall of an app
+		// removed with „keep my data" must find its data again. A failed `down` is logged and the record
+		// still reads not-deployed; the household's Remove clears what is left (halfStateEvidence).
+		if _, downErr := m.composeExecWithEnv(stackDir, env, "down"); downErr != nil {
+			m.logger.Printf("[ERROR] [stacks] Stack %s: removing what the failed deploy started ALSO failed: %v — containers may remain; Remove clears them", name, downErr)
+		} else {
+			m.logger.Printf("[INFO] [stacks] Stack %s: the failed deploy's containers were removed (volumes kept) — R-649", name)
+		}
 		// Revert in-memory and disk state
 		m.mu.Lock()
 		if s, ok := m.stacks[name]; ok {
diff --git a/controller/internal/stacks/r649_failed_deploy_cleanup_test.go b/controller/internal/stacks/r649_failed_deploy_cleanup_test.go
new file mode 100644
index 0000000..6d78f23
--- /dev/null
+++ b/controller/internal/stacks/r649_failed_deploy_cleanup_test.go
@@ -0,0 +1,69 @@
+package stacks
+
+import (
+	"os"
+	"path/filepath"
+	"runtime"
+	"strings"
+	"testing"
+)
+
+// R-649 (v0.266.0, operator ruling 2026-09-23) — a failed install removes the containers it started.
+// The stub compose binary FAILS `up` and records every call, so the test sees what the deploy ran
+// across the real process boundary.
+//
+// COMPANION RED-PROOF (REPORT.md): delete the `down` call in runComposeDeploy's failure branch — the
+// recorded calls are only [up -d] and this fails.
+func TestR649_AFailedInstallRemovesWhatItStarted(t *testing.T) {
+	if runtime.GOOS != "linux" {
+		t.Skip("the stub compose binary is a shell script")
+	}
+	m, dir := newInstalledManager(t, "services:\n  web:\n    image: nginx:1.27\n", "deployed: true\nenv: {}\n")
+	bin := t.TempDir()
+	calls := filepath.Join(t.TempDir(), "calls")
+	script := "#!/bin/sh\necho \"$*\" >> " + calls + "\ncase \"$*\" in *up*) exit 1;; esac\nexit 0\n"
+	if err := os.WriteFile(filepath.Join(bin, "docker-compose"), []byte(script), 0o755); err != nil {
+		t.Fatal(err)
+	}
+	t.Setenv("PATH", bin) // ONLY the stub: no real docker can ever be reached from here (R-650)
+	m.composeCmd = "docker-compose"
+	m.execFn = func(string, ...string) (string, error) { return "", nil }
+
+	m.runComposeDeploy("bookstack", dir, map[string]string{}, &AppConfig{Deployed: true})
+
+	b, _ := os.ReadFile(calls)
+	got := strings.Split(strings.TrimSpace(string(b)), "\n")
+	if len(got) != 2 || !strings.Contains(got[0], "up -d") || !strings.HasSuffix(got[1], "down") {
+		t.Fatalf("a failed install must run `down` after its failed `up`; calls=%q", got)
+	}
+	if strings.Contains(got[1], "-v") || strings.Contains(got[1], "--volumes") {
+		t.Fatalf("the cleanup must KEEP named volumes (a reinstall finds its data); got %q", got[1])
+	}
+	if cfg := LoadAppConfig(dir); cfg != nil && cfg.Deployed {
+		t.Fatal("the record must read not deployed")
+	}
+}
+
+// A successful install never runs `down` (control: the cleanup is on the failure branch only).
+func TestR649_ASuccessfulInstallIsNotTakenDown(t *testing.T) {
+	if runtime.GOOS != "linux" {
+		t.Skip("the stub compose binary is a shell script")
+	}
+	m, dir := newInstalledManager(t, "services:\n  web:\n    image: nginx:1.27\n", "deployed: true\nenv: {}\n")
+	bin := t.TempDir()
+	calls := filepath.Join(t.TempDir(), "calls")
+	script := "#!/bin/sh\necho \"$*\" >> " + calls + "\nexit 0\n"
+	if err := os.WriteFile(filepath.Join(bin, "docker-compose"), []byte(script), 0o755); err != nil {
+		t.Fatal(err)
+	}
+	t.Setenv("PATH", bin)
+	m.composeCmd = "docker-compose"
+	m.execFn = func(string, ...string) (string, error) { return "", nil }
+
+	m.runComposeDeploy("bookstack", dir, map[string]string{}, &AppConfig{Deployed: true})
+
+	b, _ := os.ReadFile(calls)
+	if strings.Contains(string(b), "down") {
+		t.Fatalf("a successful install must not be taken down; calls=%q", string(b))
+	}
+}