diff --git a/scripts/build.ts b/scripts/build.ts
index 02dab0ae..1fca2ca8 100644
--- a/scripts/build.ts
+++ b/scripts/build.ts
@@ -34,6 +34,7 @@ const featureFlags: Record<string, boolean> = {
   WEB_BROWSER_TOOL: false,        // Built-in browser automation (source not mirrored)
   CHICAGO_MCP: false,             // Computer-use MCP (native Swift modules stubbed)
   COWORKER_TYPE_TELEMETRY: false, // Telemetry for agent/coworker type classification
+  MCP_SKILLS: false,              // Dynamic MCP skill discovery (src/skills/mcpSkills.ts not mirrored; enabling this causes "fetchMcpSkillsForClient is not a function" when MCP servers with resources connect — see #856)
 
   // ── Enabled: upstream defaults ──────────────────────────────────────
   COORDINATOR_MODE: true,             // Multi-agent coordinator with worker delegation
@@ -56,7 +57,6 @@ const featureFlags: Record<string, boolean> = {
   EXTRACT_MEMORIES: true,             // Auto-extract durable memories from conversations
   FORK_SUBAGENT: true,                // Implicit context-forking when omitting subagent_type
   VERIFICATION_AGENT: true,           // Built-in read-only agent for test/verification
-  MCP_SKILLS: true,                   // Discover skills dynamically from MCP server resources
   PROMPT_CACHE_BREAK_DETECTION: true, // Detect & log unexpected prompt cache invalidations
   HOOK_PROMPTS: true,                 // Allow tools to request interactive user prompts
 }
diff --git a/scripts/feature-flags-source-guard.test.ts b/scripts/feature-flags-source-guard.test.ts
new file mode 100644
index 00000000..fc8bb9ff
--- /dev/null
+++ b/scripts/feature-flags-source-guard.test.ts
@@ -0,0 +1,47 @@
+import { existsSync, readFileSync } from 'fs'
+import { join } from 'path'
+import { expect, test } from 'bun:test'
+
+// Regression guard for #856. Several build feature flags require source files
+// that are not mirrored into the open build. When such a flag is set to `true`
+// without the source present, the bundler falls back to a missing-module stub
+// that only exports `default`, which causes runtime errors like
+// `fetchMcpSkillsForClient is not a function` when downstream code reaches
+// through the `require()` to a named export.
+//
+// This test fails fast at test-time if someone re-enables one of these flags
+// without first mirroring the corresponding source file.
+
+const BUILD_SCRIPT = join(import.meta.dir, 'build.ts')
+const REPO_ROOT = join(import.meta.dir, '..')
+
+type FlagGuard = {
+  flag: string
+  source: string // path relative to repo root
+}
+
+const FLAG_REQUIRES_SOURCE: FlagGuard[] = [
+  { flag: 'MCP_SKILLS', source: 'src/skills/mcpSkills.ts' },
+]
+
+test('build feature flags are not enabled without their source files', () => {
+  const buildScript = readFileSync(BUILD_SCRIPT, 'utf-8')
+
+  for (const { flag, source } of FLAG_REQUIRES_SOURCE) {
+    const enabledRe = new RegExp(`^\\s*${flag}\\s*:\\s*true\\b`, 'm')
+    const isEnabled = enabledRe.test(buildScript)
+    const sourceExists = existsSync(join(REPO_ROOT, source))
+
+    if (isEnabled && !sourceExists) {
+      throw new Error(
+        `Feature flag ${flag} is enabled in scripts/build.ts, but its required source file "${source}" does not exist. ` +
+          `Enabling this flag without the source will cause runtime errors (missing named exports from the missing-module stub). ` +
+          `Either mirror the source file or set ${flag}: false.`,
+      )
+    }
+
+    // When the source IS present, the flag can be either true or false; either
+    // is fine. We only care about the "enabled but missing" combination.
+    expect(true).toBe(true)
+  }
+})