Handle explicit --config flag in determineBaseAppConfig

When --config is explicitly specified (e.g., by the deployer passing
--config fly.api-server.toml), treat it as copy-config to prevent
prompting and source scanning fallback. Add comprehensive tests for
determineBaseAppConfig behavior and ensure LoadAppConfigIfPresent is
called in runGenerate so custom config paths are available in context.

Author: lubien

internal/command/launch/plan_builder.go

@@ -465,9 +465,13 @@ func determineBaseAppConfig(ctx context.Context) (*appconfig.Config, bool, error
 
 		// if --attach is specified, we should return the config as the base config
 		attach := flag.GetBool(ctx, "attach")
-		copyConfig := flag.GetBool(ctx, "copy-config") || attach
+		// An explicit --config flag means the caller deliberately chose the file
+		// (e.g. the deployer passing --config fly.api-server.toml). Treat it as
+		// copy-config so we never prompt and never fall back to source scanning.
+		explicitConfig := flag.IsSpecified(ctx, "config")
+		copyConfig := flag.GetBool(ctx, "copy-config") || attach || explicitConfig
 
-		if !flag.IsSpecified(ctx, "copy-config") && !attach && !flag.GetYes(ctx) {
+		if !flag.IsSpecified(ctx, "copy-config") && !attach && !explicitConfig && !flag.GetYes(ctx) {
 			var err error
 			copyConfig, err = prompt.Confirm(ctx, colorize.Yellow("Would you like to use this fly.toml configuration for this app?"))
 			fmt.Fprintln(io.Out)

internal/command/launch/plan_builder_test.go

@@ -8,9 +8,11 @@ import (
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 	fly "github.com/superfly/fly-go"
+	"github.com/superfly/flyctl/internal/appconfig"
 	"github.com/superfly/flyctl/internal/flag/flagctx"
 	"github.com/superfly/flyctl/internal/flyutil"
 	"github.com/superfly/flyctl/internal/mock"
+	"github.com/superfly/flyctl/iostreams"
 )
 
 func newDetermineOrgCtx(t *testing.T, orgFlag string) context.Context {
@@ -112,3 +114,80 @@ func TestDetermineOrg(t *testing.T) {
 		assert.Equal(t, "personal", org.Slug)
 	})
 }
+
+// newDetermineBaseAppConfigCtx builds a context wired with the flags that
+// determineBaseAppConfig reads. Pass configPath="" to leave --config unset.
+func newDetermineBaseAppConfigCtx(t *testing.T, copyConfigFlag, explicitConfigPath bool) context.Context {
+	t.Helper()
+
+	ctx := context.Background()
+	ctx = iostreams.NewContext(ctx, iostreams.System())
+
+	flagSet := pflag.NewFlagSet("test", pflag.ContinueOnError)
+	flagSet.String("config", "", "")
+	flagSet.Bool("copy-config", false, "")
+	flagSet.Bool("attach", false, "")
+	flagSet.Bool("yes", false, "")
+
+	if copyConfigFlag {
+		require.NoError(t, flagSet.Set("copy-config", "true"))
+	}
+	if explicitConfigPath {
+		require.NoError(t, flagSet.Set("config", "fly.custom.toml"))
+	}
+
+	return flagctx.NewContext(ctx, flagSet)
+}
+
+func TestDetermineBaseAppConfig(t *testing.T) {
+	// existingCfg simulates what LoadAppConfigIfPresent puts in context when
+	// the customer has a custom fly.toml with a non-default dockerfile.
+	existingCfg := appconfig.NewConfig()
+	existingCfg.Build = &appconfig.Build{
+		Dockerfile: "docker.ui-server.dockerfile",
+	}
+
+	t.Run("no flags and no existing config returns blank config", func(t *testing.T) {
+		ctx := newDetermineBaseAppConfigCtx(t, false, false)
+		// No config in context — simulates no fly.toml present.
+
+		cfg, copied, err := determineBaseAppConfig(ctx)
+		require.NoError(t, err)
+		assert.False(t, copied)
+		assert.Nil(t, cfg.Build)
+	})
+
+	t.Run("--copy-config adopts existing config without prompting", func(t *testing.T) {
+		ctx := newDetermineBaseAppConfigCtx(t, true, false)
+		ctx = appconfig.WithConfig(ctx, existingCfg)
+
+		cfg, copied, err := determineBaseAppConfig(ctx)
+		require.NoError(t, err)
+		assert.True(t, copied)
+		assert.Equal(t, "docker.ui-server.dockerfile", cfg.Build.Dockerfile)
+	})
+
+	t.Run("explicit --config adopts existing config without prompting", func(t *testing.T) {
+		// This is the deployer scenario: --config fly.custom.toml is passed but
+		// --copy-config is not. The explicit path signals intent, so we must
+		// not fall through to source scanning with an empty config.
+		ctx := newDetermineBaseAppConfigCtx(t, false, true)
+		ctx = appconfig.WithConfig(ctx, existingCfg)
+
+		cfg, copied, err := determineBaseAppConfig(ctx)
+		require.NoError(t, err)
+		assert.True(t, copied)
+		assert.Equal(t, "docker.ui-server.dockerfile", cfg.Build.Dockerfile)
+	})
+
+	t.Run("no flags in non-interactive mode returns error", func(t *testing.T) {
+		ctx := newDetermineBaseAppConfigCtx(t, false, false)
+		ctx = appconfig.WithConfig(ctx, existingCfg)
+		// Non-interactive iostreams → prompt.Confirm returns ErrNonInteractive.
+		ios, _, _, _ := iostreams.Test()
+		ctx = iostreams.NewContext(ctx, ios)
+
+		_, _, err := determineBaseAppConfig(ctx)
+		assert.Error(t, err)
+	})
+}

internal/command/launch/plan_commands.go

@@ -184,6 +184,12 @@ func newGenerate() *cobra.Command {
 		flag.Region(),
 		flag.Org(),
 		flag.AppConfig(),
+		flag.Yes(),
+		flag.Bool{
+			Name:        "copy-config",
+			Description: "Use the configuration file if present without prompting",
+			Default:     false,
+		},
 		flag.Bool{
 			Name:        "no-deploy",
 			Description: "Don't deploy the app",
@@ -269,5 +275,16 @@ func runTigris(ctx context.Context) error {
 func runGenerate(ctx context.Context) error {
 	flag.SetString(ctx, "from-manifest", flag.FirstArg(ctx))
 
+	// LoadAppConfigIfPresent is registered on the parent "plan" command, but
+	// because that command has no Run func cobra never executes its RunE and the
+	// preparer is never called. Load the config here explicitly so that a custom
+	// path supplied via --config (e.g. fly.api-server.toml) is in context before
+	// buildManifest → determineBaseAppConfig reads it.
+	var err error
+	ctx, err = command.LoadAppConfigIfPresent(ctx)
+	if err != nil {
+		return err
+	}
+
 	return RunPlan(ctx, "generate")
 }