Prevent MCP page crash on empty tool lists

Re-apply the missing fix: Go's append(nil, empty...) encodes tools as JSON
null, and the MCP page called .length on it. Return a non-nil empty slice and
normalize tools on the client with mcpToolsOrEmpty.

Author: nvdorman

internal/mcp/client.go

@@ -205,7 +205,7 @@ func (c *Client) refreshTools(ctx context.Context) error {
 func (c *Client) Tools() []ToolDef {
 	c.mu.RLock()
 	defer c.mu.RUnlock()
-	return append([]ToolDef(nil), c.tools...)
+	return append([]ToolDef{}, c.tools...)
 }
 
 // Name returns the configured server name.

internal/mcp/client_test.go

@@ -68,6 +68,23 @@ func TestStdioReportsToolErrors(t *testing.T) {
 	}
 }
 
+func TestEmptyToolListEncodesAsArray(t *testing.T) {
+	client := &Client{}
+	got := client.Tools()
+	if got == nil {
+		t.Fatal("Tools() returned nil, want a non-nil empty slice")
+	}
+	payload, err := json.Marshal(struct {
+		Tools []ToolDef `json:"tools"`
+	}{Tools: got})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if string(payload) != `{"tools":[]}` {
+		t.Fatalf("empty tool payload = %s, want tools encoded as []", payload)
+	}
+}
+
 func TestUnknownTransport(t *testing.T) {
 	if _, err := Connect(context.Background(), "x", ServerConfig{Transport: "carrier-pigeon"}); err == nil {
 		t.Fatal("expected an unknown transport to fail")

web/src/lib/mcpPayload.test.mjs

@@ -0,0 +1,15 @@
+import { describe, expect, test } from 'bun:test'
+import { mcpToolsOrEmpty } from './mcpPayload.ts'
+
+describe('MCP payload normalization', () => {
+  test('turns null or missing tools into an empty array', () => {
+    expect(mcpToolsOrEmpty(null)).toEqual([])
+    expect(mcpToolsOrEmpty(undefined)).toEqual([])
+    expect(mcpToolsOrEmpty(null).length).toBe(0)
+  })
+
+  test('keeps valid tool arrays', () => {
+    const tools = [{ name: 'server_health', description: 'Read server health' }]
+    expect(mcpToolsOrEmpty(tools)).toBe(tools)
+  })
+})

web/src/lib/mcpPayload.ts

@@ -0,0 +1,8 @@
+export interface McpPayloadTool {
+  name: string
+  description: string
+}
+
+export function mcpToolsOrEmpty<T extends McpPayloadTool>(value: T[] | null | undefined): T[] {
+  return Array.isArray(value) ? value : []
+}

web/src/pages/McpPage.tsx

@@ -11,6 +11,7 @@ import {
 import { del, post } from '@/lib/api'
 import { useApi } from '@/lib/hooks'
 import { useI18n } from '@/lib/i18n'
+import { mcpToolsOrEmpty } from '@/lib/mcpPayload'
 import { cn } from '@/lib/utils'
 import { PageLayout } from '@/components/layout/PageLayout'
 import {
@@ -51,7 +52,7 @@ interface McpServer {
   name: string
   connected: boolean
   error?: string
-  tools: McpTool[]
+  tools: McpTool[] | null
 }
 
 export default function McpPage() {
@@ -93,7 +94,10 @@ export default function McpPage() {
 
   if (loading && !data) return <SkeletonList count={3} />
 
-  const servers = data?.servers ?? []
+  const servers = (data?.servers ?? []).map((server) => ({
+    ...server,
+    tools: mcpToolsOrEmpty(server.tools),
+  }))
 
   const header = (
     <Tabs value={tab} onValueChange={(v) => setTab(v as 'servers' | 'docs')}>