Model error diagnosis

xAI: Grok 4 Fast | model capability unsupported

xAI: Grok 4 Fast returning 400 / model capability unsupported means you should first confirm whether model ID x-ai/grok-4-fast is visible for this xAI key, then separate permission, context, capability, rate limit, or route issues.

Model

x-ai/grok-4-fast

xAI: Grok 4 Fast

Provider

xAI

10 models in catalog

Error type

model capability unsupported

capability-unsupported

Status code

400

global model route

Model error summary

Model

x-ai/grok-4-fast

Error type

capability-unsupported

Status code

400

Read-only check. Detection data burns after 5 minutes.

What this model error usually means

xAI: Grok 4 Fast returning 400 / model capability unsupported means you should first confirm whether model ID x-ai/grok-4-fast is visible for this xAI key, then separate permission, context, capability, rate limit, or route issues.

Model: x-ai/grok-4-fast
Provider: xAI
Status code: 400

How to prove it during checking

Read-only check. Detection data burns after 5 minutes.

List models: confirm whether x-ai/grok-4-fast is actually visible instead of guessing from the display name.
Light probe: use minimal input to verify whether xAI returns the same 400, then record the error body.
Compare model facts: context 2,000,000, input $0.200, output $0.500.
supports include_reasoning
supports logprobs
supports max_tokens
supports reasoning

Next action

xAI: Grok 4 Fast model capability unsupported should not end at failed. Return the next move: change model ID, add permission, reduce context, disable unsupported capability, change route, monitor, or hold listing.

Context: 2,000,000
Price: $0.200 / $0.500
Read-only check. Detection data burns after 5 minutes.

Open key checker

Read-only check. Detection data burns after 5 minutes.