fix(pi): make skill loading token-lean by default

packages/mosaic/src/commands/launch.spec.ts

@@ -1,6 +1,6 @@
 import { describe, it, expect, vi, beforeEach, afterEach, type MockInstance } from 'vitest';
 import { Command } from 'commander';
-import { registerRuntimeLaunchers, type RuntimeLaunchHandler } from './launch.js';
+import { buildPiSkillArgs, registerRuntimeLaunchers, type RuntimeLaunchHandler } from './launch.js';
 
 /**
  * Tests for the commander wiring between `mosaic <runtime>` / `mosaic yolo <runtime>`
@@ -22,6 +22,8 @@ function buildProgram(handler: RuntimeLaunchHandler): Command {
   return program;
 }
 
+const fakeSkills = ['--skill', '/skills/test-driven-development', '--skill', '/skills/pdf'];
+
 // `process.exit` returns `never`, so vi.spyOn demands a replacement with the
 // same signature. We throw from the mock to short-circuit into test-land.
 const exitThrows = (): never => {
@@ -63,6 +65,30 @@ describe('registerRuntimeLaunchers — non-yolo subcommands', () => {
   });
 });
 
+describe('buildPiSkillArgs', () => {
+  it('defaults to disabling Pi skill discovery to keep startup context small', () => {
+    expect(buildPiSkillArgs([], {}, fakeSkills)).toEqual(['--no-skills']);
+  });
+
+  it('keeps explicit user skills while disabling automatic discovery', () => {
+    expect(buildPiSkillArgs(['--skill', '/tmp/custom'], {}, fakeSkills)).toEqual(['--no-skills']);
+  });
+
+  it('supports legacy all-skills mode without double-loading settings skills', () => {
+    expect(buildPiSkillArgs([], { MOSAIC_PI_SKILL_MODE: 'all' }, fakeSkills)).toEqual([
+      '--no-skills',
+      '--skill',
+      '/skills/test-driven-development',
+      '--skill',
+      '/skills/pdf',
+    ]);
+  });
+
+  it('supports native Pi discovery when explicitly requested', () => {
+    expect(buildPiSkillArgs([], { MOSAIC_PI_SKILL_MODE: 'discover' }, fakeSkills)).toEqual([]);
+  });
+});
+
 describe('registerRuntimeLaunchers — yolo <runtime>', () => {
   let mockExit: MockInstance<typeof process.exit>;
   let mockError: MockInstance<typeof console.error>;

packages/mosaic/src/commands/launch.ts

@@ -447,6 +447,32 @@ function discoverPiSkills(): string[] {
   return args;
 }
 
+type PiSkillMode = 'none' | 'all' | 'discover';
+
+function normalizePiSkillMode(env: NodeJS.ProcessEnv): PiSkillMode {
+  const value = env['MOSAIC_PI_SKILL_MODE']?.trim().toLowerCase();
+  if (value === 'all' || value === 'discover') return value;
+  return 'none';
+}
+
+export function buildPiSkillArgs(
+  _runtimeArgs: string[],
+  env: NodeJS.ProcessEnv = process.env,
+  discoveredSkillArgs: string[] = discoverPiSkills(),
+): string[] {
+  const mode = normalizePiSkillMode(env);
+
+  if (mode === 'discover') {
+    return [];
+  }
+
+  if (mode === 'all') {
+    return ['--no-skills', ...discoveredSkillArgs];
+  }
+
+  return ['--no-skills'];
+}
+
 function discoverPiExtension(): string[] {
   const ext = join(MOSAIC_HOME, 'runtime', 'pi', 'mosaic-extension.ts');
   return existsSync(ext) ? ['--extension', ext] : [];
@@ -523,7 +549,7 @@ function launchRuntime(runtime: RuntimeName, args: string[], yolo: boolean): nev
     case 'pi': {
       const prompt = buildRuntimePrompt('pi');
       const cliArgs = ['--append-system-prompt', prompt];
-      cliArgs.push(...discoverPiSkills());
+      cliArgs.push(...buildPiSkillArgs(args));
       cliArgs.push(...discoverPiExtension());
       if (hasMissionNoArgs) {
         cliArgs.push(missionPrompt);