Misc. bug: Prompt cache serialization crashes on second request with SSM model on Vulkan

[task24]

Name and Version

• llama.cpp commit: 2b2cd57
• Version: master branch (2026-04-11)

Operating systems

• Linux

Modules affected

• libllama (core library)
• llama-server

Command line

llama-server -hf nvidia/NVIDIA-Nemotron-3-Nano-4B-GGUF

Problem description & steps to reproduce

1. Run llama-server with NVIDIA Nemotron-3 Nano 4B (SSM-based model)
2. Send a chat completion request — works fine
3. Send a second chat completion request
4. Server crashes during prompt cache serialization

Workaround: llama-server -hf nvidia/NVIDIA-Nemotron-3-Nano-4B-GGUF --cache-ram 0 (disabling prompt cache fixes it)

Environment

• GPU: AMD Radeon RX 7900 XTX
• Vulkan: 1.4.341
• CPU: Ryzen 16-core

Relevant log output

The crash occurs in ggml-backend.cpp:348 during llama_kv_cache::state_write_data

Error: GGML_ASSERT(tensor->data != NULL && "tensor not allocated") failed

Backtrace shows the failure during prompt cache save after first response completes successfully.