diff --git a/internal/agents/claudecode_plugin.go b/internal/agents/claudecode_plugin.go
index ab10a26..c6169c5 100644
--- a/internal/agents/claudecode_plugin.go
+++ b/internal/agents/claudecode_plugin.go
@@ -100,11 +100,11 @@ func EnsureClaudeCodePlugin() (ClaudePluginResult, error) {
 		return res, nil
 	}
 
-	if _, err := claudeExec("plugin", "marketplace", "add", claudePluginMarketplaceRef); err != nil {
-		return res, fmt.Errorf("claude plugin marketplace add %s: %w", claudePluginMarketplaceRef, err)
+	if out, err := claudeExec("plugin", "marketplace", "add", claudePluginMarketplaceRef); err != nil {
+		return res, fmt.Errorf("claude plugin marketplace add %s: %w%s", claudePluginMarketplaceRef, err, outputTail(out))
 	}
-	if _, err := claudeExec("plugin", "install", claudePluginSpec()); err != nil {
-		return res, fmt.Errorf("claude plugin install %s: %w", claudePluginSpec(), err)
+	if out, err := claudeExec("plugin", "install", claudePluginSpec()); err != nil {
+		return res, fmt.Errorf("claude plugin install %s: %w%s", claudePluginSpec(), err, outputTail(out))
 	}
 	res.Installed = true
 	return res, nil
@@ -127,13 +127,31 @@ func RemoveClaudeCodePlugin() (ClaudePluginResult, error) {
 		return res, nil
 	}
 
-	if _, err := claudeExec("plugin", "uninstall", claudePluginSpec()); err != nil {
-		return res, fmt.Errorf("claude plugin uninstall %s: %w", claudePluginSpec(), err)
+	if out, err := claudeExec("plugin", "uninstall", claudePluginSpec()); err != nil {
+		return res, fmt.Errorf("claude plugin uninstall %s: %w%s", claudePluginSpec(), err, outputTail(out))
 	}
 	res.Removed = true
 	return res, nil
 }
 
+// outputTail formats a command's combined output for inclusion in an
+// error — a ": " separator + the trimmed tail (capped), or "" when
+// the command said nothing. Without this the install verb only
+// surfaced "exit status 1"; claude's actual reason (auth, network,
+// unknown plugin) lives in the output and is what makes the failure
+// actionable.
+func outputTail(out string) string {
+	out = strings.TrimSpace(out)
+	if out == "" {
+		return ""
+	}
+	const max = 600
+	if len(out) > max {
+		out = "…" + out[len(out)-max:]
+	}
+	return ": " + out
+}
+
 // ClaudeCodePluginStatus reports whether the plugin is currently
 // installed. ClaudeFound=false (with installed=false) when the CLI
 // isn't on PATH.
diff --git a/internal/agents/claudecode_plugin_test.go b/internal/agents/claudecode_plugin_test.go
index 9b55a6d..588ba9e 100644
--- a/internal/agents/claudecode_plugin_test.go
+++ b/internal/agents/claudecode_plugin_test.go
@@ -2,6 +2,7 @@ package agents
 
 import (
 	"errors"
+	"strings"
 	"testing"
 )
 
@@ -126,6 +127,39 @@ func TestEnsureClaudeCodePlugin_InstallFails(t *testing.T) {
 	}
 }
 
+// TestEnsureClaudeCodePlugin_InstallError_IncludesOutput guards the
+// diagnostics fix: when claude exits non-zero, its combined output
+// (the real reason — auth, unknown plugin, etc.) must appear in the
+// error rather than being swallowed into a bare "exit status 1".
+func TestEnsureClaudeCodePlugin_InstallError_IncludesOutput(t *testing.T) {
+	stubClaude(t, map[string]stubResp{
+		"plugin list":        {out: "none\n"},
+		"plugin marketplace": {out: "ok\n"},
+		"plugin install":     {out: "Error: marketplace clawtool-marketplace not found", err: errors.New("exit status 1")},
+	})
+	_, err := EnsureClaudeCodePlugin()
+	if err == nil {
+		t.Fatalf("expected error")
+	}
+	if !strings.Contains(err.Error(), "marketplace clawtool-marketplace not found") {
+		t.Errorf("error did not surface claude's output: %v", err)
+	}
+}
+
+func TestOutputTail(t *testing.T) {
+	if got := outputTail("   "); got != "" {
+		t.Errorf("blank output should yield empty, got %q", got)
+	}
+	if got := outputTail("boom"); got != ": boom" {
+		t.Errorf("got %q, want ': boom'", got)
+	}
+	long := strings.Repeat("x", 800)
+	got := outputTail(long)
+	if len(got) > 610 || !strings.HasPrefix(got, ": …") {
+		t.Errorf("long output not capped/elided: len=%d", len(got))
+	}
+}
+
 func TestRemoveClaudeCodePlugin_NotInstalled(t *testing.T) {
 	calls := stubClaude(t, map[string]stubResp{
 		"plugin list": {out: "no plugins\n"},