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Feature Request: --expert-override flag for MoE models #19528

Description

@muscleriot

Summary

Add a command-line flag --expert-override or --moe-topk to allow users to change the number of active experts at runtime for Mixture of Experts (MoE) models.

Motivation

Currently, the expert_used_count (top-k) is hardcoded in the GGUF file metadata (LLM_KV_EXPERT_USED_COUNT). Users cannot experiment with different expert counts without re-conver
ting models.

Real-world use case: The Nemotron-3-Nano-30B-A3B model defaults to 6 experts, but testing shows that 8-10 experts significantly improves summarization quality for long-context ta
sks while 4-6 is sufficient for chat.

Proposed API

# Override to use 8 experts instead of GGUF default
./llama-cli -m model.gguf --expert-override 8 -p "Hello"

# Or --moe-topk alias
./llama-cli -m model.gguf --moe-topk 8 -p "Hello"

Implementation

The change would be minimal:

• Add expert_override to common_params
• Modify llm_graph_params initialization in llama-graph.cpp
• Add CLI argument in common/arg.cpp

Benefits

1. Experimentation: Users can tune expert counts for specific tasks
2. Flexibility: Same model file works for different scenarios
3. No re-quantization: Saves disk space and time

Affected Models

• Nemotron-H series
• Mixtral 8x7B/8x22B
• DeepSeek-V2/V3
• Qwen2.5-MoE

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