StepFun Step 3.7 Flash + OpenCode: Cheapest Reasoning Setup

StepFun’s Step 3.7 Flash is one of the cheapest reasoning-capable models you can put behind a coding agent — a multimodal mixture-of-experts design that activates only a small slice of its parameters per token, keeping it fast and inexpensive. Paired with free, open OpenCode, it makes a low-cost agent that can also reason over images if you need that.

This is the OpenCode + StepFun setup. For OpenCode’s general provider model, see OpenCode custom providers.

Step 1: Install OpenCode and get a StepFun key#

npm install -g opencode-ai

Sign up on the StepFun platform and create an API key (its quickstart covers key creation). On Windows, WSL gives the cleaner shell — see install OpenCode on Windows with WSL.

Step 2: Add the StepFun provider#

{
  "$schema": "https://opencode.ai/config.json",
  "provider": {
    "stepfun": {
      "npm": "@ai-sdk/openai-compatible",
      "name": "StepFun",
      "options": {
        "baseURL": "https://api.stepfun.com/v1",
        "apiKey": "{env:STEPFUN_API_KEY}"
      },
      "models": {
        "step-3.7-flash": { "name": "Step 3.7 Flash" },
        "step-3.5-flash": { "name": "Step 3.5 Flash" }
      }
    }
  },
  "model": "stepfun/step-3.7-flash"
}

Export the key:

export STEPFUN_API_KEY="your-stepfun-key"

Confirm the exact base URL and model IDs on StepFun’s docs.

Step 3: Run it#

opencode

The default is stepfun/step-3.7-flash. For the lowest cost on simpler tasks, switch to step-3.5-flash.

Pricing#

StepFun’s pricing is at or below the floor most mid-tier providers held — Step 3.7 Flash in the region of $0.20 input / $1.15 output per million, and Step 3.5 Flash even lower (around $0.10 / $0.30), illustrative. Multimodal images bill at a fixed token count each. Confirm current rates on StepFun’s pricing page.

Troubleshooting#

• Provider not found — the provider key (stepfun) must match the model prefix.
• Auth errors — STEPFUN_API_KEY not set in the launching shell.
• Model rejected — confirm the Step model ID on StepFun.
• Format errors — confirm you’re using StepFun’s OpenAI-compatible endpoint.

Wrapping up#

StepFun Step 3.7 Flash with OpenCode is about the cheapest reasoning-capable coding agent you can assemble: a free, open tool on a fast, multimodal MoE model at floor-level pricing. Add the provider block, set step-3.7-flash as default (drop to 3.5 Flash for simple work), and launch. Verify the endpoint and rates on StepFun’s docs.

For the broader budget picture, see cheapest AI coding API in 2026.