A new agent, and nothing else moved

A developer added DeepSeek as the tenth agent in the OpenSpec-UI project by writing a single new file, deepseek-acp.ts, that implements the shared AgentAdapter interface over the Agent Client Protocol, with no changes needed in either the standalone app or the VS Code extension. Testing the integration surfaced two undocumented behaviors: dsh exits with code 0 and no output on Node 22.11 (working on Node 24.18), and the agent reports no usage data, so only time-based spending ceilings apply. The author notes DeepSeek responds best to literal, step-by-step instructions and now prepends a preamble to every prompt.

First published on openspec-ui.dev https://openspec-ui.dev/articles/a-new-agent-and-nothing-else-moved/ . The last article argued that a shared core is what keeps a VS Code extension and a web app from drifting apart. Two days after it went out, a new agent arrived, and it is as good a test of that argument as any: DeepSeek, over the Agent Client Protocol, requested by the owner and added the same day. One new file, deepseek-acp.ts , implementing the same AgentAdapter interface every other agent implements. It runs dsh --profile acp , the DeepSeek CLI's own Agent Client Protocol mode, through the driver the other ACP-flavored adapters already share. It is added to the registry, allowed by the default allowlist for exactly those arguments, and detected on PATH like every other agent. Neither host's code changed. Both the standalone application and the VS Code extension picked it up because both read the same registry. DeepSeek is the tenth agent, and the fifth that speaks ACP, beside Claude, Copilot, Codex and Gemini. initialize answers as deepseek-harness-acp , with no auth method to run: the key lives in the person's own dsh profile, never in this project. session/new offers DeepSeek-V4-Flash as the default model and DeepSeek-V4-Pro as an option, though nothing here sends that option yet, so V4-Flash is what runs. Two things worth a person's attention, both found by running it rather than by reading its documentation. On Node 22.11, dsh exits with code 0 before answering, saying nothing at all. This repository pins Node 22.11 with Volta for its own tooling, and Volta puts it first on the PATH of anything it starts, including an agent CLI. The same run on Node 24.18 completed. "ACP connection closed" is not a sentence a person can act on, so a run that ends this way is now told which Node it met and that a newer one is needed on the PATH a host starts agents with. It reports no usage at all. The live run sent no usage update and no usage figure on its answer, so a spending ceiling over deepseek-cli-acp counts nothing and cannot fire, the same as over five of the other nine agents. Only a time ceiling has any force over it. The owner has used DeepSeek enough to know it does best with instructions taken literally, one at a time, rather than left to infer how strictly to read what follows. So every prompt to it now opens with a short preamble: follow the steps in order, do exactly what each one asks, name a step as it finishes, and stop and say why rather than improvise if a step cannot be done as written. What it does not do yet: choose a model. DeepSeek's model is an Agent Client Protocol session option session/set config option , and nothing in this project's shared driver sends session options. Whichever model dsh defaults to is the one that runs. Fixing the Node a host starts agents with is also out of reach from here: the product cannot choose what is on someone's PATH ; it can only say which one it met. Adding a tenth agent is not, by itself, news. What is worth a reader's time is that adding it stayed inside the one package the architecture says it should: core , and nowhere else. An implement run against this same change wrote its own file and ticked its own task in 22 seconds, over the very mechanism the article describes. The code is at github.com/VeryComplexAndLongName/OpenSpec-UI https://github.com/VeryComplexAndLongName/OpenSpec-UI , where the repository and packages keep the name OpenSpec-UI. The architecture this rests on is in One core, two hosts https://openspec-ui.dev/articles/one-core-two-hosts/ . initialize / session/new answers, the Node 22.11 exit and its explanation, and the preamble: packages/core/src/agents/deepseek-acp.ts deepseek-joins-as-an-acp-agent HARNESS.md 's own row for deepseek-cli-acp .