Which AI APIs Support Negative Prompts? A 2026 Endpoint Map

Key Takeaways

• Of the 48 image endpoints callable on Siray today, five accept a negative_prompt field, and all five are Midjourney. Z-Image, Seedream and Flux do not.
• On video the field tracks model generation, not vendor: kuaishou/kling-3.0-t2v accepts one, while Kling 2.6, 2.1 and 1.6 do not.
• A missing field is not a missing capability. Every constraint you would put in a negative box can be written as a positive, and on most endpoints that is the only way to write it.

Check the Spec, Not the Habit

Negative prompting is muscle memory from Stable Diffusion, where a negative box sat beside the positive one and "blurry, extra fingers, watermark" was standard boilerplate. Most hosted APIs never adopted the pattern. Porting the habit costs real calls: you send negative_prompt, the endpoint drops an unknown field or rejects the body, and you walk away thinking the model follows instructions badly.

Everything below was read on August 11, 2026 from the request schemas at docs.siray.ai/api-reference/openapi-spec/, endpoint by endpoint rather than from vendor marketing. Endpoints without a product page, and any in retirement, are out of scope.

Image: One Vendor Family Has It

Midjourney is the exception, not the rule. Z-Image Turbo takes exactly four fields — model, prompt, seed, size — and nothing else. Seedream's request bodies differ from each other in size options and reference handling, never in negative support. The 2026 image model roundup compares these models on output rather than on this field, because for 43 of 48 endpoints the field is simply absent.

Video: It Tracks Generation, Not Vendor

Video is where the assumption breaks hardest.

Same vendor, four generations, one flip: Kling gained the field at 3.0 and did not carry it at 2.6, 2.1 or 1.6. PixVerse is the only vendor whose whole line has it, six endpoints across C1, V5.6 and V6. LTX 2.3 22B has it on both t2v and i2v, which makes it the one endpoint in the NSFW video lineup that offers a negative prompt at all — per-second rates are a separate question.

So "does this API support negative prompts" has no vendor-level answer. It has a per-endpoint answer, and it can change between releases of the same model line.

Writing Constraints Without the Field

Everywhere else, the constraint travels in the positive prompt. The rewrite is mechanical:

The rule underneath: name what should be there. "No blur" hands the model the token blur; "sharp focus" hands it the target. A full worked example on a model with no negative field is in the Z-Image prompt guide.

FAQ

Which AI image APIs support negative prompts?

On Siray, only Midjourney's five text-to-image endpoints: midjourney-v7-t2i, -v6.1-t2i, -v6-t2i, -niji-7-t2i and -niji-6-t2i. The other 43 callable image endpoints accept no negative field.

Does Kling support negative prompts?

Kling 3.0 does, on both kuaishou/kling-3.0-t2v and kling-3.0-i2v. Kling 2.6, 2.1 and 1.6 do not. The field arrived with the 3.0 generation, so check the endpoint you are calling rather than the model line.

What happens if I send negative_prompt to an endpoint that does not have it?

Nothing useful. It is not in the request schema, so it is either dropped or the request fails validation, and either way the constraint never reaches the model. Move it into the positive prompt instead.

Summary

Negative prompt support is a property of the endpoint, not of the vendor or the modality. Five Midjourney endpoints have it on the image side; on video it belongs to Kling 3.0, PixVerse and LTX 2.3 and to nothing else callable. Read the request schema for the exact endpoint before designing a prompt around a field that may not exist.

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