[MetalRT][macOS 15.7.4][M3] Crash in metalrt_cache_prompt during pipeline init (Failed to initialize pipeline)

Summary

RCLI fails to initialize pipeline on MetalRT and exits with:
Failed to initialize pipeline
Error: Failed to initialize (check models dir: ~/Library/RCLI/models)

This is NOT a models-missing issue. Model files exist and are readable.

Environment

• RCLI: 0.3.6 (Homebrew)
• MetalRT: v0.2.1 (from rcli metalrt status)
• macOS: 15.7.4 (24G517)
• Hardware: Apple Silicon (arm64), Mac15,12
• Engine config: engine=metalrt

Repro

1. Install/setup RCLI and MetalRT models.
2. Run:

   rcli ask "hello" --no-speak -v
   
   

3. Observe failure:
   • [RCLI] Initializing pipeline...
   • [RCLI] Failed to initialize pipeline
   • [LLM] Shutdown complete

Expected

Pipeline initializes and returns a response.

Actual

Pipeline init fails every time; in some runs process crashes (segfault).

Verbose log excerpt

[RCLI] Personality: default
[RCLI] MetalRT fallback to largest: /Users/black_knife_air/Library/RCLI/models/metalrt/LFM2.5-1.2B-MLX-
4bit
[RCLI] MetalRT STT: Whisper Tiny (MLX 4-bit) (...)
[RCLI] MetalRT TTS: Kokoro 82M (bf16) (...)
[Tools] Set external tool definitions (1103 bytes, 9 tools parsed)
[RCLI] Initializing pipeline...
[RCLI] Failed to initialize pipeline
[LLM] Shutdown complete

Crash report

File:
~/Library/Logs/DiagnosticReports/rcli-2026-03-13-032414.ips

Key stack frames:

• metalrt_cache_prompt
• rastack::MetalRTEngine::cache_system_prompt(...)
• AGX::ComputeContext...::setPipelineCommon(...)
• Exception: EXC_BAD_ACCESS (SIGSEGV), KERN_INVALID_ADDRESS at 0x50

Additional notes

• ~/Library/RCLI/models/metalrt/* exists with model/config/tokenizer files.
• Downloading Qwen3-0.6B does not resolve issue.
• Failure reproduces with --no-speak (not microphone related).

Reproduce with rcli ask "hello" --no-speak -v using the reported MetalRT setup, then inspect the crash report at ~/Library/Logs/DiagnosticReports/rcli-2026-03-13-032414.ips. Trace the metalrt_cache_prompt and rastack::MetalRTEngine::cache_system_prompt frames; done means pipeline initialization returns a response without failing or crashing.