MODELS / STEPFUN

StepFun 3.5 Flash

Step 3.5 Flash is an open-source reasoning model by StepFun with 196B total parameters (11B active) using Mixture of Experts. It features a 256K context window, deep reasoning, tool calling, and agentic capabilities, achieving 97.3 on AIME 2025 and 74.4% on SWE-bench Verified.

TEXTREASONINGREASONINGTOOL-USEIMPLICIT-CACHINGVISION
262.1K
CONTEXT WINDOW
-
MAX OUTPUT TOKENS
$0.09
INPUT PRICE / 1M TOKENS
$0.3
OUTPUT PRICE / 1M TOKENS
QUICKSTART

Call it in one request.

OpenAI-compatible: change the base URL, and the setup is complete.

curl https://enterprise.blackbox.ai/v1/chat/completions \
  -H "Authorization: Bearer $BLACKBOX_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "model": "blackboxai/stepfun/step-3.5-flash",
    "messages": [{ "role": "user", "content": "Hello!" }],
    "stream": true
  }'
SPECIFICATIONS

The full spec sheet.

Context, pricing, modalities, routing, and deployment: everything StepFun 3.5 Flash supports, in one table.

MODEL ID
blackboxai/stepfun/step-3.5-flash
DEVELOPED BY
Stepfun
MODEL TYPE
Text
RELEASED
Jan 29, 2026
CONTEXT WINDOW
262.1K
MAX OUTPUT TOKENS
-
INPUT MODALITIES
text, image
OUTPUT MODALITIES
text
INPUT PRICE
$0.09 / 1M tokens
OUTPUT PRICE
$0.3 / 1M tokens
CACHE READ
$0.02 / 1M tokens
REASONING
Supported
DATA RETENTION
Standard retention
DEDICATED DEPLOYMENT
Not available (closed-weight model)
SUPPORTED PARAMETERS
max_tokenstemperaturestoptoolstool_choicereasoninginclude_reasoning