Vulkan: vkCreateDevice fails with ErrorExtensionNotPresent on a device reporting bf16: 0

Every model fails to load on the Vulkan backend on an AMD Radeon 880M (RDNA 3.5 integrated).
The backend enumerates the device, reports bf16: 0, and then requests a bfloat16 device extension
the driver does not expose, so vkCreateDevice fails and no model can be loaded at all.

Setting GGML_VK_DISABLE_BFLOAT16=1 fixes it completely, with cooperative matrices still enabled.

Environment

parakeet.cpp	v0.5.0, prebuilt parakeet-v0.5.0-bin-win-vulkan-x64.zip
	sha256 717c416fab299755e8140137e3a0115121ce1acb6379d13c60f2f0613f6c13a3
OS	Windows 11 (10.0.26200), x64
GPU	AMD Radeon 880M (integrated, RDNA 3.5), driver 32.0.13022.3006
Vulkan	API 1.3.302, driverVersion 2.0.331
Model	tdt-0.6b-v3-f16.gguf (also reproduced with tdt-0.6b-v3-q4_k.gguf)

Reproduction

Using only the prebuilt CLI from the release:

parakeet-cli transcribe --model tdt-0.6b-v3-f16.gguf --input short.wav

Output:

ggml_vulkan: Found 1 Vulkan devices:
ggml_vulkan: 0 = AMD Radeon(TM) 880M Graphics (AMD proprietary driver) | uma: 1 | fp16: 1 | bf16: 0 |
             warp size: 64 | shared memory: 32768 | int dot: 1 | matrix cores: KHR_coopmat
transcribe failed: vk::PhysicalDevice::createDevice: ErrorExtensionNotPresent
ERROR:             vkDestroyFence: Invalid device [VUID-vkDestroyFence-device-parameter]

The process then exits with 0xC0000409 (STATUS_STACK_BUFFER_OVERRUN) rather than a clean error
exit — the vkDestroyFence line is teardown running against the device that was never created.

Any 10-second 16 kHz mono PCM WAV reproduces it; the failure is at device creation, before the audio
is used.

Note that parakeet-cli info <model.gguf> succeeds on this machine. It reads GGUF metadata and
never creates a backend, so it is not a useful check for this.

The cause

VK_ERROR_EXTENSION_NOT_PRESENT from vkCreateDevice means a requested device extension is not
available. vulkaninfo on this device shows:

• VK_KHR_cooperative_matrix — present, extension revision 2
• no bfloat16 extension of any kind — absent

The backend has already worked this out: it prints bf16: 0. It then requests a bfloat16 device
extension anyway, which cannot succeed. The capability check and the extension request appear to
disagree.

Workarounds, both confirmed

Setting	Result	Device line
(none)	ErrorExtensionNotPresent	matrix cores: KHR_coopmat
GGML_VK_DISABLE_BFLOAT16=1	works	matrix cores: KHR_coopmat
GGML_VK_DISABLE_COOPMAT=1	works	matrix cores: none

GGML_VK_DISABLE_BFLOAT16=1 is the better of the two — it keeps cooperative matrices enabled and is
about 1.36x faster than disabling coopmat wholesale (measured over a 600 s file, 3 runs each,
mean real-time factor 0.0349 against 0.0473, run-to-run range 3.2% and 5.3%).

That GGML_VK_DISABLE_COOPMAT=1 also avoids it suggests the bfloat16 extension is requested from
the cooperative-matrix path rather than independently.

Ruled out

• Not the model or its size. f16 (1.34 GiB) and q4_k (644 MiB) fail identically. Both load
  and decode correctly on the cpu backend on the same machine, from the same files.
• Not memory. maxMemoryAllocationSize and maxBufferSize are both 2 GiB, against a 7.75 GiB
  device-local heap with 7.36 GiB budget.
• Not these, all of which still fail: GGML_VK_DISABLE_F16,
  GGML_VK_DISABLE_INTEGER_DOT_PRODUCT, GGML_VK_PREFER_HOST_MEMORY,
  GGML_VK_ALLOW_SYSMEM_FALLBACK, GGML_VK_DISABLE_HOST_VISIBLE_VIDMEM.

Once it loads, it works correctly

With the workaround, Vulkan output matches the CPU backend closely on a 600 s file: all 106 segment
boundaries identical, 1,605 words against 1,606 on CPU, first divergence a punctuation difference.
So this is purely a device-creation problem, not a correctness one.

Suggested fix

Do not request the bfloat16 device extension when the device does not advertise it — the same
condition that already produces bf16: 0 in the device line. A device that lacks it currently cannot
create a Vulkan device at all, which makes the backend unusable rather than degraded.

Two smaller points that would have saved a lot of time here:

1. The C API (parakeet_capi_load) returns NULL with no message on this path, so anything using
   the shared library instead of the CLI sees a bare load failure with no cause. Surfacing the
   underlying error through parakeet_capi_last_error would make it diagnosable.
2. Exiting via STATUS_STACK_BUFFER_OVERRUN in teardown after a failed device creation looks like
   memory corruption and sends you down the wrong path; guarding teardown on a device that was never
   created would fail cleanly.

Start at the Vulkan backend's device-creation path and compare the bf16 capability check with the device-extension request. Reproduce with the parakeet-cli command and verify that the AMD device loads without GGML_VK_DISABLE_BFLOAT16=1 while cooperative matrices remain enabled. The C API entry point parakeet_capi_load and parakeet_capi_last_error are additional context for failure reporting.