Flux 2 Klein — Pro Pack on RunComfy

runcomfy.com

·

9B model

·

4B model

·

GitHub

Black Forest Labs'

Flux 2 Klein

(the distilled, low-latency variant of Flux 2) hosted on the

RunComfy Model API

— no API key, async REST.

npx skills

add

agentspace-so/runcomfy-skills

--skill

flux-2-klein

-g

When to pick this model (vs siblings)

Flux 2 Klein's distinct strength is

latency-first creative iteration

sub-second feedback enables live art-direction sessions and rapid product visualization that batch-style models can't sustain. Pick it when

iteration speed matters more than ceiling resolution

.

You want

Use

Real-time / live art-direction sessions

Flux 2 Klein 4B

Fast iteration with strong detail at the end

Flux 2 Klein 9B

Multi-reference brand styling with consistent looks

Flux 2 Klein

2K–4K hero images, max resolution

Seedream 5

Maximum prompt adherence + extreme detail

Flux 2 Pro

Embedded text, logos, multilingual signage

GPT Image 2

Hyperrealistic portrait

Nano Banana Pro

If the user said "Flux 2 Klein" / "BFL Klein" / "flux klein" explicitly, route here regardless. If they said "Flux 2" generically, ask whether they want

Klein

(fast) or

Pro

(max quality) before defaulting.

Prerequisites

RunComfy CLI

—

npm i -g @runcomfy/cli

RunComfy account

—

runcomfy login

opens a browser device-code flow.

CI / containers

— set

RUNCOMFY_TOKEN=

instead of

runcomfy login

.

Endpoints + input schema

Two variants, same endpoint shape, same prompt grammar.

blackforestlabs/flux-2-klein/9b/text-to-image

The fidelity-first variant. Use for polish / final output.

Field

Type

Required

Default

Notes

prompt

string

yes

—

Up to ~512 tokens. Longer degrades.

steps

int

no

25

4–50.

Step-distilled architecture

— 4–8 enough for concepting; ~25 for polish; >25 buys little.

width

int

no

1024

512–1536 typical.

Aspect ratio capped at 16:9

, max ~2K total.

height

int

no

1024

Match

width

's aspect intent.

blackforestlabs/flux-2-klein/4b/text-to-image

The latency-first variant. Sub-second 4-step inference. Use for live iteration / concepting.

Same field set as 9B. Default

steps

is effectively 4 — the variant is built for that step count.

Reference images (both variants)

Up to

4 simultaneous reference images

are supported on the same endpoint for style transfer / guided composition. The exact field name in the JSON body is documented on the

model's API tab

— pass it through the CLI verbatim. Reference-image use enables editing-style workflows without a separate

/edit

endpoint.

How to invoke

Fast concepting (4B, sub-second):

runcomfy run blackforestlabs/flux-2-klein/4b/text-to-image

\

--input

'{"prompt": ""}'

\

--output-dir

<

absolute/path

>

Polish / final (9B, ~25 steps):

runcomfy run blackforestlabs/flux-2-klein/9b/text-to-image

\

--input

'{

"prompt": "",

"steps": 25,

"width": 1024,

"height": 1024

}'

\

--output-dir

<

absolute/path

>

Wide-format poster:

runcomfy run blackforestlabs/flux-2-klein/9b/text-to-image

\

--input

'{"prompt": "", "width": 1536, "height": 864}'

\

--output-dir

<

absolute/path

>

The CLI submits, polls every 2s until terminal, then downloads any

*.runcomfy.net

/

*.runcomfy.com

URL from the result into

--output-dir

. Stdout is the result JSON. Stderr is progress.

For pipe-friendly usage:

runcomfy

--output

json run blackforestlabs/flux-2-klein/4b/text-to-image

\

--input

'{"prompt":"..."}'

--no-wait

|

jq

-r

.request_id

Prompting — what actually works

These are model-specific patterns that empirically improve output quality.

Subject-first declarative grammar.

The structure Flux 2 Klein was trained on is

"Subject + action + scene + style + lighting + camera + quality"

. Front-load the subject; trail with directives. Example:

"A vibrant hummingbird mid-flight sipping nectar from a bright pink hibiscus, iridescent feathers in morning sun, soft bokeh tropical garden, macro photography, razor-sharp detail, cinematic lighting"

.

Specificity wins over flowery language.

"4k product photo, softbox lighting, reflective table, 35mm, f/2.8" guides predictably. "A really pretty product image" doesn't.

Step-count by phase.

Concepting

4–8 steps on the 4B variant — sub-second feedback for live exploration.

Refinement

8–15 steps still on 4B, locking in subject + framing.

Polish

~25 steps on the 9B variant — texture, microdetail, fine typography.

Multi-reference alignment.

When passing reference images,

keep their aesthetics aligned

. Mixing a watercolor + a photoreal + a 3D render in the same call confuses the editor. Pick one consistent visual register across all refs.

Conditional edits

state what stays, then what changes.

"Same composition and lighting as reference, but change the background from beach to mountain studio."

This pattern holds composition stable.

For text rendering

(Klein has the 8B Qwen3 embedder, decent but not GPT Image 2 territory): add

"crisp typography, high-contrast label"

and bump steps to ~25 if the text comes out soft. For heavy in-image text or multilingual rendering, route to GPT Image 2 instead.

Anti-patterns

:

Don't conflict adjectives. "minimalist + ornate" cancels.

Don't exceed ~512 tokens. The model degrades, doesn't truncate gracefully.

Don't ask for 4K — the model's resolution ceiling is ~2K.

Don't ask for ultra-wide (>16:9) — the model crops.

Where it shines

Use case

Why Flux 2 Klein

Live art-direction sessions

Sub-second feedback (4B) enables real-time iteration

Interactive product visualization

Fast UI previews and product comps without batch waits

Multi-reference brand styling

Strong style consistency across references for unified asset packs

Rapid concepting → polish workflow

4B for exploration, 9B for the final pass — same prompt grammar throughout

Consumer-GPU-friendly inference

4B variant runs on modest hardware; relevant for self-host comparisons but RunComfy-hosted is fine

Sample prompts (verified to produce strong results)

From the model page (BFL example):

A vibrant hummingbird mid-flight sipping nectar from a bright pink hibiscus

flower, iridescent emerald and sapphire feathers catching the morning sun,

soft bokeh tropical garden background, macro photography, razor-sharp

detail, cinematic lighting

Product-photo pattern:

A matte ceramic mug on a reclaimed-wood table, soft northern window light

from the left, shallow depth of field, 50mm prime, f/2.0, neutral

background, e-commerce ready, 4K product photography

Brand-consistent pair (multi-ref):

Same composition and lighting as the reference image, but the bottle

label is now blue with white sans-serif typography reading "AURA";

keep the bottle silhouette, table, and shadow exactly as in the reference

Limitations

Resolution ceiling ~2K

— for higher native res, route to Seedream 5.

Aspect ratio cap 16:9

— extreme wide/tall ratios get cropped.

Prompt cap ~512 tokens

— longer degrades quality; doesn't truncate gracefully.

Reference image cap 4

— more than 4 increases latency and dilutes guidance.

Text rendering

— the 8B Qwen3 embedder helps but GPT Image 2 still wins for embedded text precision.

Exit codes

The

runcomfy

CLI uses sysexits-style codes:

code

meaning

0

success

64

bad CLI args

65

bad input JSON / schema mismatch (e.g.

width: 4096

would 422)

69

upstream 5xx

75

retryable: timeout / 429

77

not signed in or token rejected

Full reference:

docs.runcomfy.com/cli/troubleshooting

.

How it works

The skill invokes

runcomfy run blackforestlabs/flux-2-klein//text-to-image

with a JSON body matching the schema.

The CLI POSTs to

https://model-api.runcomfy.net/v1/models/blackforestlabs/flux-2-klein//text-to-image

with the user's bearer token.

The Model API returns a

request_id

; the CLI polls

GET .../requests//status

every 2 seconds.

On terminal status, the CLI fetches

GET .../requests//result

and downloads any URL whose host ends with

.runcomfy.net

or

.runcomfy.com

into

--output-dir

. Other URLs are listed but not fetched.

Ctrl-C

while polling sends

POST .../requests//cancel

so you don't get billed for GPU you stopped.

What this skill is not

Not a self-hosted Flux runner. Not a capability grant — depends on a working RunComfy account. Not multi-tenant.

Security & Privacy

Token storage

:

runcomfy login

writes the API token to

~/.config/runcomfy/token.json

with mode 0600 (owner-only read/write). Set

RUNCOMFY_TOKEN

env var to bypass the file entirely in CI / containers.

Input boundary

the user prompt is passed as a JSON string to the CLI via

--input

. The CLI does NOT shell-expand the prompt; it transmits the JSON body directly to the Model API over HTTPS. No shell injection surface from prompt content.

Third-party content

image / mask / video URLs you pass are fetched by the RunComfy model server, not by the CLI on your machine. Treat external URLs as untrusted; image-based prompt injection is a known risk for any image-edit / video-edit model.

Outbound endpoints

only

model-api.runcomfy.net

(request submission) and

*.runcomfy.net

/

*.runcomfy.com

(download whitelist for generated outputs). No telemetry, no callbacks.

Generated-file size cap

the CLI aborts any single download > 2 GiB to prevent disk-fill from a malicious or runaway model output.