Rosenblatt
Goldilocks Zone
AI Engineering Playbook · August 2026

Finding the Goldilocks Zone with Open Source Models

How we get frontier-grade results from Chinese open-weight models — orchestrated across families, hosted on trusted US infrastructure, and priced to run in loops.
Rosenblatt · Open-Closed Hybrid Inference
The Strategy

The Goldilocks Zone: 1.5B tokens for $400/m


Model strategy has a habitable band. Most teams camp at one of the extremes.

Too Hot

All-frontier, all the time

Flagship intelligence on every call — and unit economics that collapse the moment you build loops or scale up delegation.

Just Right

Multi-family, OSS models

Ensembles of OSS models hit frontier-level quality at loop-friendly prices — with true frontier models held back for surgical moments.

Too Cold

Self-hosting for everything

Hits an intelligence ceiling. The model's blind spots become your blind spots — and a single point of failure.

Part 1
The Infrastructure
The Trust Model

Chinese Models, American Providers


Open weight models are simple, verifiable math formulas — not software.

Anyone can download the weights and run them anywhere. Nothing about inference has to touch the lab that trained the model.

The provider is the risk surface — not the model.

We don't trust Chinese-hosted endpoints, so we don't use them. Distrusting a host doesn't mean discarding the model.

US providers with zero data retention — no exceptions.

Every token we run goes through American providers with ZDR policies: prompts and outputs aren't stored, logged, or used for training.

The Providers

Two Providers, Full Model Redundancy


Primary
Fireworks AI

From the creators of PyTorch, NVIDIA-backed, and publicly praised by Jensen Huang. Raised a $1.5B Series D in July 2026 at a $17.5B valuation on $1B+ annualized revenue.

Our default endpoint for every family we run.

Fallback
DeepInfra

Newer, and usually beats Fireworks on price. $107M Series B raised in May 2026, new Toronto data center opened in August 2026. Latency is less consistent and we've hit the occasional small bug.

Crucially: it hosts the same models we already run.

Same weights on both sides means failover is a routing change — not a prompt-engineering project.

Failover in Action

IRL: the GLM-5.2 incident


01

Fireworks wobbles

Fireworks starts having trouble serving GLM-5.2, our most popular endpoint, and a 90 minute outage ensues.

02

Flip the switch

Traffic reroutes to DeepInfra, which hosts the same GLM-5.2. Same weights, same prompts, same harness.

03

Nothing else changes

Work carries on without incident — near-100% behavioral consistency, because we never changed the model.

Redundancy across providers. Consistency within the model. That's the whole design.

Subscribe, Don't Meter

Keep the frontier — Subscribe, don't meter


The Key

Run Anthropic's flagship models exclusively through flat $25–$125/m subscription tiers on a business team agreement, never API rates. Use Opus & Fable as surgeons' tools; break glass in case of emergency. Extremely useful — deliberately rare.

DO
  • Reserve it for absolute-flagship reasoning.
  • Leverage it for complex debugging and long-form writing.
  • Make it your default for cheap deep research.
DON'T
  • Build loops on frontier models.
  • Assume planning must start at the absolute frontier.
  • Pay API rates for capacity a subscription already covers.
Part 2
The Models
The Shift

The ground shifted — We rebuilt around it.


100%

of our day-to-day inference now runs on open-weight models.

14x

cheaper than Anthropic models running on Claude Code.

6+

production model families (e.g. DeepSeek, GLM, Qwen, Kimi, Nvidia, MiniMax).

The frontier labs still own the very top end — the game is deciding where those tokens are actually worth it.

Auxilary Models

Auxiliary Models: delegate case by case


flowchart LR T["Task arrives
prompt + screenshot"] Q["Qwen 3.8 Max (Vision)
describes the image in detail"] G["GLM-5.2 (Default)
text-only default"] A["DeepSeek Flash (Worker)
executes the task"] T --> G T --> Q Q --> G G <--> A classDef box fill:#141414,stroke:#2a2a2a,stroke-width:1.4px,color:#ece8e3; classDef cold fill:#141414,stroke:#7fa5c4,stroke-width:1.6px,color:#7fa5c4; classDef gold fill:#141414,stroke:#c676e4,stroke-width:1.6px,color:#c676e4; classDef out fill:transparent,stroke:#c2825c,stroke-width:1.6px,color:#ece8e3; classDef green fill:#141414,stroke:#5fc770,stroke-width:1.6px,color:#5fc770; class T box class Q cold class G gold class A green

Pick the default on text strength alone.

Vision inference only as neeeded.

Swap per gap without touching the harness.

Mixture of Agents

MoA: Don't bet on a single model family


flowchart LR P["Prompt"] D["DeepSeek V4 Pro
reference model"] G["GLM 5.2
reference model"] K["Kimi K3
reference model"] A["Qwen 3.8 Max
aggregator model"] O["Frontier-level
answer"] P --> D P --> G P --> K D --> A G --> A K --> A A ==> O classDef box fill:#141414,stroke:#2a2a2a,stroke-width:1.4px,color:#ece8e3; classDef cold fill:#141414,stroke:#7fa5c4,stroke-width:1.6px,color:#7fa5c4; classDef gold fill:#141414,stroke:#c676e4,stroke-width:1.6px,color:#c676e4; classDef green fill:#141414,stroke:#5fc770,stroke-width:1.6px,color:#5fc770; class P box class D,G,K cold class O green class A gold

Share the prompt across multiple models.

Aggregate each model response.

Return a final answer with frontier quality.

Split The Work

Planner → Workers → Reviewers


Stop asking one model to do everything. Route each phase to the cheapest model that can own it.

Planner 30%

Use a top OSS model or MoA to turn the task into an explicit plan before any execution starts. This is where quality gets decided.

Workers 60%

The plan is delegated to one or more DeepSeek Flash-class models that grind through execution. This is where loop volume lives.

Reviewers 10%

High-intelligence, but lower context window models grade the output — ideally several independent reviewers across several families.

The Playbook

The Goldilocks Playbook — and what's next


  • 01

    Open weights on trusted hosts — Fireworks AI primary + DeepInfra fallback.

  • 02

    Primary + fallback, same models — Fireworks first, DeepInfra right behind it.

  • 03

    Keep the frontier subscription — use for hard calls and edge cases, not loops.

  • 04

    Diversify across model families — balance your quality and spend.

Not too hot, not too cold — frontier results at open-source prices.