fix(server): initialize prometheus headers map to avoid nil map panic

CHANGELOG.md

@@ -7,6 +7,10 @@
 - `server`: Support HTTP request/response MCP with the `--mcp-enabled` and `--mcp-path` flags.
 - MCP: Add the `context`, `list_slos` and `get_slo` tools.
 
+### Fixed
+
+- `server --prometheus-header` no longer panics with "assignment to entry in nil map".
+
 ## [v0.16.0] - 2026-04-04
 
 ### Added

cmd/sloth/commands/server.go

@@ -68,6 +68,7 @@ type serverCommand struct {
 // NewServerCommand returns the UI command.
 func NewServerCommand(app *kingpin.Application) Command {
 	c := &serverCommand{}
+	c.prometheus.headers = map[string]string{}
 	cmd := app.Command("server", "Starts the Sloth web server.")
 	cmd.Flag("app-listen-address", "Application listen address.").Default(":8080").StringVar(&c.appServer.address)
 	cmd.Flag("status-listen-address", "Status (health check, metrics, pprof...) listen address.").Default(":8081").StringVar(&c.statusServer.address)

cmd/sloth/commands/server_test.go

@@ -0,0 +1,24 @@
+package commands_test
+
+import (
+	"testing"
+
+	"github.com/alecthomas/kingpin/v2"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+
+	"github.com/slok/sloth/cmd/sloth/commands"
+)
+
+// TestServerCommandPrometheusHeaderFlag checks that the `server --prometheus-header`
+// flag can be parsed without panicking. Regression test for the nil map panic
+// ("assignment to entry in nil map") reported when the backing map was not
+// initialized before being used by kingpin's StringMap value.
+func TestServerCommandPrometheusHeaderFlag(t *testing.T) {
+	app := kingpin.New("sloth", "test")
+	commands.NewServerCommand(app)
+
+	cmd, err := app.Parse([]string{"server", "--prometheus-header=Some-Header=Value"})
+	require.NoError(t, err)
+	assert.Equal(t, "server", cmd)
+}