The arithmetic
V4-Pro is $0.66 / $1.98. Sonnet 5 is $2 / $10. On output-heavy agent traces, DeepSeek can be five to eight times cheaper. V4-Flash ($0.22 / $0.66) is cheaper still and should stay on short tasks.
The ranking
On our mirrored LMArena Agent board, Sonnet 5 is #7 and DeepSeek-V4-Pro is #8 — close enough that cost often decides. Opus 5 still sits at #1 if quality is the only knob.
Ecosystem
Claude Code assumes Anthropic. DeepSeek shines in Cline, Aider, and self-hosted OpenHands. Cursor can route either. Chinese teams often run DeepSeek as default and keep a Claude key for the nasty PR.