DeepSeek Harness (dsh) is DeepSeek's open-source agent harness. Its Web UI can register any OpenAI-compatible endpoint as a custom provider — point one at CrossModel and the whole catalog shows up in the model picker.

Quickstart

1. Start DeepSeek Harness

Install Node.js, then run:

npx @deepseek-ai/dsh web

The command prints the Web UI address — http://127.0.0.1:3080 by default. Open it in a browser.

2. Create an API key

Sign in to the Console and create a key that starts with cm- on the API Keys page. The key is shown only once, so copy it somewhere safe.

3. Add CrossModel as a custom provider

Open Settings → Models and choose Add a custom provider.

Fill in the form:

The Provider ID is permanent — sessions, model defaults, and the stored credential all reference it, so renaming later means adding a new provider and deleting the old one. Every other field stays editable.

4. Add the models

Under Models, choose Fetch available models to read the catalog from https://api.crossmodel.ai/v1/models and pick what you want. You can also type IDs by hand — they follow the vendor/short_name format, e.g. deepseek/deepseek-v4-flash, openai/gpt-5.6-luna, anthropic/claude-sonnet-4.6. The full list is in the model catalog.

Save the provider. Model changes take effect on the next request — no need to restart dsh.

5. Pick a model and start

Select a workspace, open the model picker in the composer, and choose a model under CrossModel. The selected model also becomes the default for new sessions.

Declare images and reasoning

A model you enter by hand — including one added through Fetch available models — is described only by what the form recorded, so DeepSeek Harness treats it as text-only and non-reasoning until you say otherwise. Both are one line in $DSH_HOME/settings.yaml; the form has no field for either.

The form already wrote a llm-pi-ai block for the provider. Edit the models list it created and leave the other fields alone.

Image input

llm-pi-ai:
  providers:
    crossmodel:
      models:
        - id: deepseek/deepseek-v4-flash
        - id: anthropic/claude-sonnet-4.6
          input: [text, image]

Attaching an image to a model without input: [text, image] is refused before the request is sent. If every model on the provider takes images, set defaultInput: [text, image] on the provider instead of repeating the line. Whether a model accepts images is listed on its page in the model catalog.

Reasoning

llm-pi-ai:
  providers:
    crossmodel:
      compat:
        supportsDeveloperRole: false
      models:
        - id: deepseek/deepseek-v4-pro
          reasoningEfforts:
            off:
            high: high
            max: xhigh

Each key is a level the picker offers; each value is what travels as reasoning_effort. CrossModel accepts none, minimal, low, medium, high, and xhigh — how much of that a model actually spends depends on the model you choose. off may leave its value empty, meaning the parameter is omitted.

supportsDeveloperRole: false belongs with it. A model that declares reasoning gets its system prompt sent with role: "developer", which some models reject; system works for every model on CrossModel, so the switch is safe to set even when nothing on the provider reasons.

You do not need compat.thinkingFormat or compat.maxTokensField: CrossModel reads OpenAI's reasoning_effort, and accepts both max_tokens and max_completion_tokens.

Use an environment variable

On servers, in containers, or in CI, reference the key by environment variable instead of storing it through the form:

llm-pi-ai:
  providers:
    crossmodel:
      apiKeyEnv: CROSSMODEL_API_KEY

export CROSSMODEL_API_KEY="cm-YOUR_KEY"

Do not commit a real API key to your repository.

Troubleshooting