diff --git a/internal/modules/arduinocli/arduinocli.go b/internal/modules/arduinocli/arduinocli.go
index d098be8..6e115ef 100644
--- a/internal/modules/arduinocli/arduinocli.go
+++ b/internal/modules/arduinocli/arduinocli.go
@@ -4,7 +4,6 @@ package arduinocli
 import (
 	"bytes"
 	"context"
-	"errors"
 	"fmt"
 	"os"
 	"path/filepath"
@@ -61,6 +60,30 @@ func (m Module) Detect(ctx context.Context) (module.State, error) {
 	return s, nil
 }
 
+// downloadBinary fetches, extracts, and installs arduino-cli to bp.
+func (m Module) downloadBinary(ctx context.Context, bp string) error {
+	tmpDir, err := os.MkdirTemp("", "arduinocli-")
+	if err != nil {
+		return err
+	}
+	defer os.RemoveAll(tmpDir)
+	archive := filepath.Join(tmpDir, "arduino-cli.tar.gz")
+	if err := platform.Download(ctx, m.cfg.URL, archive, 0o644); err != nil {
+		return err
+	}
+	if err := platform.ExtractTarGz(ctx, archive, tmpDir); err != nil {
+		return err
+	}
+	if err := os.MkdirAll(filepath.Dir(bp), 0o755); err != nil {
+		return err
+	}
+	data, err := os.ReadFile(filepath.Join(tmpDir, "arduino-cli"))
+	if err != nil {
+		return err
+	}
+	return os.WriteFile(bp, data, 0o755)
+}
+
 func (m Module) Plan(ctx context.Context, cur module.State) (module.Plan, error) {
 	s, _ := cur.(state)
 	var actions []module.Action
@@ -69,28 +92,7 @@ func (m Module) Plan(ctx context.Context, cur module.State) (module.Plan, error)
 		actions = append(actions, module.Action{
 			Description: "download + install arduino-cli " + m.cfg.Version,
 			Kind:        module.ActionFileWrite,
-			Exec: func(ctx context.Context) error {
-				tmpDir, err := os.MkdirTemp("", "arduinocli-")
-				if err != nil {
-					return err
-				}
-				defer os.RemoveAll(tmpDir)
-				archive := filepath.Join(tmpDir, "arduino-cli.tar.gz")
-				if err := platform.Download(ctx, m.cfg.URL, archive, 0o644); err != nil {
-					return err
-				}
-				if err := platform.ExtractTarGz(ctx, archive, tmpDir); err != nil {
-					return err
-				}
-				if err := os.MkdirAll(filepath.Dir(bp), 0o755); err != nil {
-					return err
-				}
-				data, err := os.ReadFile(filepath.Join(tmpDir, "arduino-cli"))
-				if err != nil {
-					return err
-				}
-				return os.WriteFile(bp, data, 0o755)
-			},
+			Exec:        func(ctx context.Context) error { return m.downloadBinary(ctx, bp) },
 		})
 	}
 	for _, core := range s.MissingCores {
@@ -138,6 +140,33 @@ func (m Module) Verify(ctx context.Context) error {
 	return nil
 }
 
+// PlanForce redownloads the arduino-cli binary, refreshes the core index,
+// and reinstalls every configured core unconditionally. arduino-cli's
+// `core install` is idempotent, so reissuing it is safe.
 func (m Module) PlanForce(ctx context.Context) (module.Plan, error) {
-	return module.Plan{}, errors.New("PlanForce not implemented")
+	bp, err := binaryPath()
+	if err != nil {
+		return module.Plan{}, err
+	}
+	actions := []module.Action{
+		{
+			Description: "redownload + reinstall arduino-cli " + m.cfg.Version + " (force)",
+			Kind:        module.ActionFileWrite,
+			Exec:        func(ctx context.Context) error { return m.downloadBinary(ctx, bp) },
+		},
+		{
+			Description: "arduino-cli core update-index (force)",
+			Kind:        module.ActionCommand,
+			Exec:        func(ctx context.Context) error { return m.runner.Run(ctx, bp, "core", "update-index") },
+		},
+	}
+	for _, core := range m.cfg.Cores {
+		core := core
+		actions = append(actions, module.Action{
+			Description: "arduino-cli core install " + core + " (force)",
+			Kind:        module.ActionCommand,
+			Exec:        func(ctx context.Context) error { return m.runner.Run(ctx, bp, "core", "install", core) },
+		})
+	}
+	return module.Plan{Actions: actions}, nil
 }
diff --git a/internal/modules/arduinocli/arduinocli_test.go b/internal/modules/arduinocli/arduinocli_test.go
index 90dbec3..c293f6e 100644
--- a/internal/modules/arduinocli/arduinocli_test.go
+++ b/internal/modules/arduinocli/arduinocli_test.go
@@ -91,4 +91,64 @@ func TestArduinoCLIDownloadsAndInstallsCore(t *testing.T) {
 	}
 }
 
+func TestPlanForceRedownloadsAndReinstallsCores(t *testing.T) {
+	home := t.TempDir()
+	t.Setenv("HOME", home)
+	// Pre-populate a stale binary to prove force overwrites.
+	os.MkdirAll(filepath.Join(home, ".local", "bin"), 0o755)
+	bp := filepath.Join(home, ".local", "bin", "arduino-cli")
+	os.WriteFile(bp, []byte("stale"), 0o755)
+
+	archive := makeTarGz(t, map[string][]byte{"arduino-cli": []byte("#!/bin/sh\nfresh\n")})
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		data, _ := os.ReadFile(archive)
+		w.Write(data)
+	}))
+	defer srv.Close()
+
+	r := &fakeRunner{coreListOut: []byte("arduino:avr\nesp32:esp32\n")}
+	m := New(config.ArduinoCLIConfig{
+		Version: "1.0.0",
+		URL:     srv.URL + "/arduino-cli.tgz",
+		Cores:   []string{"arduino:avr", "esp32:esp32"},
+	}, r)
+
+	plan, err := m.PlanForce(context.Background())
+	if err != nil {
+		t.Fatalf("PlanForce: %v", err)
+	}
+	if plan.IsEmpty() {
+		t.Fatal("force plan must not be empty")
+	}
+	for _, a := range plan.Actions {
+		if err := a.Exec(context.Background()); err != nil {
+			t.Fatalf("Exec %q: %v", a.Description, err)
+		}
+	}
+
+	// Binary got overwritten with fresh payload.
+	got, _ := os.ReadFile(bp)
+	if string(got) == "stale" {
+		t.Fatalf("binary not overwritten by force")
+	}
+
+	// update-index and both `core install` calls fired.
+	sawUpdate := false
+	installed := map[string]bool{}
+	for _, c := range r.runCalls {
+		if filepath.Base(c[0]) == "arduino-cli" && len(c) >= 3 && c[1] == "core" && c[2] == "update-index" {
+			sawUpdate = true
+		}
+		if filepath.Base(c[0]) == "arduino-cli" && len(c) >= 4 && c[1] == "core" && c[2] == "install" {
+			installed[c[3]] = true
+		}
+	}
+	if !sawUpdate {
+		t.Errorf("no core update-index call; calls=%v", r.runCalls)
+	}
+	if !installed["arduino:avr"] || !installed["esp32:esp32"] {
+		t.Errorf("expected both cores force-installed; got=%v calls=%v", installed, r.runCalls)
+	}
+}
+
 var _ module.Module = Module{}