sourcegraph
diff --git a/cmd/src/version_check.go b/cmd/src/version_check.go
@@ -0,0 +1,119 @@
+package main
+
+import (
+	"context"
+	"fmt"
+	"io"
+	"os"
+	"strings"
+	"time"
+
+	"github.com/Masterminds/semver"
+
+	"github.com/sourcegraph/src-cli/internal/api"
+	"github.com/sourcegraph/src-cli/internal/version"
+)
+
+// envSkipVersionCheck disables the automatic out-of-date warning when set to a
+// non-empty value.
+const envSkipVersionCheck = "SRC_SKIP_VERSION_CHECK"
+
+// versionCheckTimeout bounds how long the background version check may take so it
+// never noticeably delays the command the user actually asked to run.
+const versionCheckTimeout = 3 * time.Second
+
+// maybeWarnOutdatedVersion checks, on a best-effort basis, whether the running
+// src-cli is older than the version recommended by the configured Sourcegraph
+// instance and prints a warning to stderr if so.
+//
+// It is intentionally fail-open: any problem (missing config, network failure,
+// unparseable version, ...) results in no warning, and it never blocks or delays
+// the actual command by more than versionCheckTimeout.
+func maybeWarnOutdatedVersion(subcommand string) {
+	if shouldSkipVersionCheck(subcommand) {
+		return
+	}
+
+	// Resolve config independently and ignore any error: a malformed config is the
+	// dispatched command's problem to report, not ours.
+	c, err := readConfig()
+	if err != nil || c == nil {
+		return
+	}
+
+	ctx, cancel := context.WithTimeout(context.Background(), versionCheckTimeout)
+	defer cancel()
+
+	// Build a quiet client: discard incidental output and skip user-agent telemetry
+	// for this background request.
+	client := c.apiClient(api.NewFlagsFromValues(false, false, false, false, false), io.Discard)
+
+	recommended, err := getRecommendedVersion(ctx, client)
+	if err != nil || recommended == "" {
+		return
+	}
+
+	if outdated, ok := isOutdated(version.BuildTag, recommended); ok && outdated {
+		fmt.Fprintf(os.Stderr,
+			"Warning: src-cli is out of date: you are running %s, but your Sourcegraph "+
+				"instance recommends %s or later. Some commands may not work as expected; see "+
+				"https://github.com/sourcegraph/src-cli#installation to upgrade. "+
+				"Set %s=1 to silence this warning.\n",
+			version.BuildTag, recommended, envSkipVersionCheck)
+	}
+}
+
+// shouldSkipVersionCheck reports whether the version check should be skipped for
+// this invocation.
+func shouldSkipVersionCheck(subcommand string) bool {
+	// Respect an explicit opt-out (also the simplest way to silence the warning in
+	// CI or scripts).
+	if os.Getenv(envSkipVersionCheck) != "" {
+		return true
+	}
+
+	// Dev builds have no meaningful version to compare against.
+	if version.BuildTag == version.DefaultBuildTag {
+		return true
+	}
+
+	// Nothing to do for help/version/no-subcommand invocations. `version` already
+	// prints the recommended version itself.
+	switch subcommand {
+	case "", "help", "version":
+		return true
+	}
+
+	// A leading dash means no subcommand was given (e.g. `src -h`).
+	if strings.HasPrefix(subcommand, "-") {
+		return true
+	}
+
+	return false
+}
+
+// isOutdated reports whether current is an older release than recommended,
+// comparing only the major/minor/patch components so that prerelease or build
+// metadata never produces a spurious warning.
+//
+// ok is false when either version cannot be parsed, in which case callers should
+// not warn.
+func isOutdated(current, recommended string) (outdated bool, ok bool) {
+	cur, err := semver.NewVersion(current)
+	if err != nil {
+		return false, false
+	}
+	rec, err := semver.NewVersion(recommended)
+	if err != nil {
+		return false, false
+	}
+
+	curParts := [3]int64{cur.Major(), cur.Minor(), cur.Patch()}
+	recParts := [3]int64{rec.Major(), rec.Minor(), rec.Patch()}
+	for i := range curParts {
+		if curParts[i] != recParts[i] {
+			return curParts[i] < recParts[i], true
+		}
+	}
+	return false, true
+}