Both sit in the harness layer with similar commercial terms, so the practical split is language: DeepSeek Harness targets TypeScript and Letta targets Python.
Everything is a plugin — the model adapter, the tool registry, the sandbox and the agent loop itself are all swappable.
Web UI first — `dsh web` serves on 127.0.0.1:3080 rather than taking over the terminal. Built on Cordis. Still a developer preview: DeepSeek warns of compatibility-breaking changes, so pin the version.
Stateful agent server where memory is the primary abstraction.
| DeepSeek Harness | Letta | |
|---|---|---|
| Licence | Open source | Open source |
| Pricing | open source | freemium |
| Self-hostable | Yes | Yes |
| Languages | TypeScript | Python, TypeScript |
| Install | npx @deepseek-ai/dsh web | pip install letta |
| Keys required | None | None |
Both sit in the harness layer with similar commercial terms, so the practical split is language: DeepSeek Harness targets TypeScript and Letta targets Python. DeepSeek Harness: Everything is a plugin — the model adapter, the tool registry, the sandbox and the agent loop itself are all swappable. Letta: Stateful agent server where memory is the primary abstraction.
DeepSeek Harness can run on your own infrastructure. Letta can run on your own infrastructure.
DeepSeek Harness is open source (open source). Letta is open source (freemium).
Every page here answers to Accept: text/markdown and returns the same content at roughly a tenth the tokens. No separate site, no toggle — same URL.
curl -s -H "Accept: text/markdown" https://newagent.build/compare/deepseek-harness-vs-letta