> ## Documentation Index
> Fetch the complete documentation index at: https://phaseo.app/docs/llms.txt
> Use this file to discover all available pages before exploring further.

> ## Agent Instructions
> Phaseo Gateway base URL: https://api.phaseo.app/v1. Send `Authorization: Bearer $PHASEO_API_KEY` with every request.
> Model IDs look like `provider/model`, for example `openai/gpt-6-astra`. Do not guess IDs: list them with `GET /v1/models` or check https://phaseo.app/models. `phaseo/free` routes to an available free model and needs no wallet balance.
> For new text integrations prefer `POST /v1/responses`. `POST /v1/chat/completions` (OpenAI format) and `POST /v1/messages` (Anthropic format) suit existing code.
> The full API contract is https://phaseo.app/docs/openapi/v1/openapi.yaml. Official SDKs: `@phaseo/sdk` (TypeScript) and `phaseo` (Python), plus Go, C#, PHP, Ruby and Rust.
> Search these docs over MCP at https://phaseo.app/docs/mcp, and install Phaseo skills with `npx skills add https://phaseo.app/docs`. If a page is wrong or out of date, report it with the MCP `submit_feedback` tool.
> Manage Phaseo accounts with the `phaseo` CLI (`npm install -g @phaseo/cli`, then `phaseo login` or `phaseo login --device-code`), the `/v1` Management API or the MCP server at `https://mcp.phaseo.app/mcp?access=manage`. Do not send users to the dashboard for account tasks. A person is still needed for sign-up, approving a sign-in, paying the first top-up on the checkout link that `phaseo credits add` returns when no card is saved, confirming each saved-card top-up from the emailed link, and account security (password, MFA, passkeys). Create API keys with `phaseo keys create --name <name> --json`; never guess one. Credit top-ups need `phaseo login --scopes billing:write` first. Task map: https://phaseo.app/docs/v1/developers/manage-from-an-agent.md.

# Fusion Server Tool

> Run several analysis models and synthesize their findings.

Use `phaseo:fusion` when one answer benefits from several independent model perspectives. Phaseo runs the analysis models in parallel, then asks the selected analyst model to synthesize their findings for the calling model.

## Playground setup

In the Playground server-tools menu, Fusion lets you choose:

* analysis models that independently inspect the request
* an optional judge model that compares the analysis outputs
* a max tool-call limit for the Fusion advisor tools

The main model sees those named advisors during the same tool loop and can use their outputs before writing the final response.

## API shape

```json theme={null}
{
  "tools": [
    {
      "type": "phaseo:fusion",
      "parameters": {
        "analysis_models": ["anthropic/claude-opus-5", "openai/gpt-5"],
        "model": "google/gemini-pro-latest",
        "instructions": "Reconcile disagreements and return concise synthesis guidance.",
        "max_uses": 1,
        "max_completion_tokens": 1400
      }
    }
  ]
}
```

## When to use Fusion

Use Fusion for:

* planning decisions where a second or third model may catch missing assumptions
* research synthesis where models may weigh evidence differently
* code or architecture reviews where you want independent critique
* high-value responses where a little extra latency is acceptable

Use a single Advisor instead when you only need one review pass.

## Usage and pricing

Each Fusion invocation increments:

```json theme={null}
{
  "usage": {
    "server_tool_use": {
      "fusion_requests": 1
    }
  }
}
```

The advisor models' tokens are included in total usage and can be priced at each selected advisor model's rates. Server-tool pricing can also use `server_tool_advisor_requests`.

## Related

* [Advisor](https://phaseo.app/docs/v1/guides/server-tools/advisor.md)
* [Subagent](https://phaseo.app/docs/v1/guides/server-tools/subagent.md)
* [Server Tools](https://phaseo.app/docs/v1/guides/server-tools/index.md)


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