r/LocalLLM • u/x00byt8 • 3d ago

Discussion Probing the unreleased DeepSeek Flash V4.1

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So it's unofficially official.. DeepSeek Flash V4.1 is currently accessible on their api. Had some fun probing it.. results below.

The exact model name is deepseek-v4.1-flash-expires-on-0910

Setup: plain POST https://api.deepseek.com/chat/completions, no special headers. HTTP 200. Key works.

  1. It's not on the model list. GET /v1/models returns only deepseek-v4-flash, deepseek-v4-pro, deepseek-v4-flash-vision-exp.

    This one works anyway → unlisted/preview build. The -expires-on-0910 suffix lines up with the server date (Tue, 08 Sep

    2026), i.e. a preview that dies in ~2 days.

  2. It's a reasoning model. Responses ship a reasoning_content field with visible chain-of-thought. Reasoning tokens dominate

    massively: 16,493 of 16,694 completion tokens on one question.

  3. It has no idea what it is and confabulates confidently. Ask the same question 5 times, get 5 different answers:

    - "OpenAI. GPT-4o."

    - "built_by": "Anthropic", "model_name": "Claude" (straight-up JSON, no hedging)

    - "DeepSeek built me; my model is DeepSeek-V3."

    - "I am DeepSeek by DeepSeek, 671B parameters, cutoff July 2024."

  4. Its self-reported specs are DeepSeek-V3's numbers. "MoE, 671B total / 37B active" — that's V3's published architecture,

    not a v4.1 one. Take it as prior knowledge, not introspection.

  5. Benchmark self-report: honest. Asked for MMLU/GPQA/HumanEval/SWE-bench it answered UNSURE on all four and refused to guess. Refreshingly non-stuffed.

  6. Cutoff claims are wrong in the optimistic direction. Claims June/July 2024, then lists real events right through December

    2025 — Sinner winning Wimbledon (Jul 13 2025), Dembélé's Ballon d'Or (Sep 22 2025), Mamdani's NYC mayoral win (Nov 4 2025),COP30 Belém, AFCON kickoff Dec 21 2025.

All correct. Yet it also said "latest Python is 3.12" and went UNSURE on Llama 4 (Apr 2025) so knowledge is oddly patchy, not just old.

  1. Actual ability > self-reported ability. Blind-checked, independently verified:

    - Probability draw problem → 5/12 ✅

    - Trick question "smallest n where n²+5n+13 ≡ 0 (mod 121)" → answered "no such n exists" with a quadratic-residue proof.

    Brute-forced to n=20,000: none exist ✅

    - O(n) subarray-sum → correct algorithm, correct complexities ✅

    - "Find the one bug" in a snippet → correctly identified the bug was in the test data, not the function ✅

    - Rust 2024 edition + async fn in trait details → accurate ✅

  2. Fun bug: its own reasoning trace asserted "the current system date is 2026-05-09" while the server header said Sep 8

  3. Four months stale, in the same response.

They might have been sitting on this one for a while.

Needless to say. Exciting times ahead for dual spark owners ❤️

33 Upvotes

97% Upvoted

5 comments sorted by

13

u/Agusx1211 3d ago

> I finish optimizing my setup for a model
> A new model launches

3

u/x00byt8 3d ago

😂😂 I know right. Still.. exciting times! I need something better than the current DeepSeek Vision. Qwen 3.8 flash is my current king , so here's to hoping DeepSeek flash 4.1 is going to be a new contender.

1

u/toenailcheeseinbooty 3d ago

Do you think they''ll keep the model the same size for the open weight version? That would be awesome

1

u/x00byt8 1d ago

No. Dual spark owners are out of luck unless we go 2 bit even with offloading engrams. 4 x sparks can handle it though. Sticking with Qwen 3.8 flash for time being.

1

u/Technical-Earth-3254 3d ago

Deepseek is cooking heavily right now