> ## 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 If a page is wrong or out of date, report it with the MCP `submit_feedback` tool.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.

# Anthropic Claude Sonnet 4.6

> What to know when adopting Claude Sonnet 4.6 for coding, computer use, and agent planning.

# Anthropic Claude Sonnet 4.6

Use this guide to adopt `anthropic/claude-sonnet-4.6` safely in production.

Sonnet 4.6 shares the same migration mechanics as the broader Claude 4.6 line, but it also changes the performance profile enough that prompt and routing assumptions should be revalidated.

## What's new

* assistant prefills are no longer supported on Claude 4.6 models and return `400`
* structured output moved from `output_format` to `output_config.format`
* adaptive thinking is now configured through `output_config.effort`
* outdated beta headers should be removed
* Sonnet 4.6 is a full upgrade across coding, computer use, long-context reasoning, agent planning, and instruction following
* Sonnet 4.6 adds a 1M-token context window in beta

## Migration quickstart

### 1. Stop using assistant prefills

If your existing Sonnet 4.5 prompts start the assistant with a JSON prefix, XML tag, or formatting stub, remove that pattern before rollout.

### 2. Move structured output config

Replace `output_format` usage with `output_config.format` and rerun any schema or parser tests.

### 3. Re-tune effort and latency expectations

If you use Sonnet for interactive routes, benchmark different `effort` levels before making 4.6 the default. The model is stronger, but the best setting depends on your latency budget.

## What to test

### Output behavior

* instruction-following on formatting-sensitive prompts
* agent planning quality on multi-step tasks
* coding, refactor, and review prompts
* long-context retrieval and summarization

### Contract stability

* JSON and schema validation pass rate
* tool argument completeness
* finish reasons and empty-output handling
* routes that formerly relied on prefills for stable formatting

### Production metrics

* latency by effort level
* task success versus Sonnet 4.5
* timeout and retry rates
* cost per successful task

## Safe rollout

1. Remove prefills and update structured output config first.
2. Evaluate Sonnet 4.6 on your current production prompt set.
3. Canary traffic with Sonnet 4.5 or another stable route as fallback.
4. Increase traffic only after quality and contract stability hold.

## Sources

* [Anthropic migration guide](https://platform.claude.com/docs/en/about-claude/models/migration-guide)
* [Introducing Claude Sonnet 4.6](https://www.anthropic.com/news/claude-sonnet-4-6)


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