# ADR 0017: Agent Runtime — Claude Code SDK via OpenRouter

## Status

Accepted.

## Context

The original ADR-0017 mandated a provider-agnostic agent layer: any
OpenAI-compatible model via OpenRouter, custom agent loop built on Vercel AI
SDK's `streamText`. The motivation was to avoid vendor lock-in.

In practice, the hand-rolled agent loop lacked capabilities that a production
agent needs: context compaction (long conversations), tool approval flows,
partial-message streaming, and a battle-tested multi-turn loop. Building these
ourselves meant re-implementing what the Claude Code SDK already provides.

The Feishu integration also proved costly to hand-roll — card schema quirks,
patch limitations, file support — each API quirk cost a round-trip.

## Decision

Adopt `@anthropic-ai/claude-agent-sdk` as the agent runtime, routed through
OpenRouter's **"Anthropic Skin"** (`https://openrouter.ai/api`).

OpenRouter exposes an Anthropic Messages API-compatible endpoint. Claude Code
SDK speaks its native protocol directly to OpenRouter — no proxy, no format
conversion. The SDK's `query()` owns the agent loop (tool dispatch, compaction,
streaming). Built-in tools (Read, Write, Bash, Glob, Grep) cover the entire
tool surface — no custom tools needed. `cph check` / `cph build` run via Bash.

The Hub `AgentSession` is provider/role/model-bound, and it must persist the
provider runtime cursor needed to continue a conversation. For Claude Code SDK,
that cursor is the `result.session_id`; store it in `AgentSession.metadata` as
`claudeSessionId` and pass it back to the next `query()` call as
`options.resume`. Role is part of the session binding because role prompts and
tool surfaces can differ even when the underlying model is the same; `/draft`
and `/review` must not resume the same Claude runtime cursor by accident.

Role definitions are Organization-scoped runtime data. A role bundle selects
its default model, system prompt, tool allowlist and installed Agent skill
versions. PostgreSQL is authoritative for role composition and skill metadata;
skill bytes live in a content-addressed persistent store selected only by the
recorded SHA-256 digest. Updating a role or binding skills takes effect without
a Hub release or process restart. A change to the role's execution surface
(model, prompt, tools, selected skill content) archives its active sessions so
the next run cannot resume a provider context created under stale instructions;
label and ordering-only changes preserve conversational continuity.

Environment variables:
```
ANTHROPIC_BASE_URL=https://openrouter.ai/api
ANTHROPIC_AUTH_TOKEN=<OpenRouter API key>
ANTHROPIC_API_KEY=""  # must be explicitly empty
```

Model routing: `ANTHROPIC_DEFAULT_SONNET_MODEL` etc. accept OpenRouter model
IDs (e.g. `z-ai/glm-4.7`, `anthropic/claude-sonnet-4-20250514`). This gives
limited OpenRouter-level model routing, but the runtime is still Claude Agent
SDK-shaped: non-Claude models are best-effort and may not support every
Claude Code feature.

## Consequences

- Provider-agnosticism is not fully preserved. OpenRouter can route to GLM,
  Claude, GPT, etc., but the runtime protocol and agent loop remain Claude
  Agent SDK-bound.
- The agent loop is Claude Code SDK's (compaction, tool approval, partial
  message streaming) — not hand-rolled.
- Custom tools (read_file, write_file, cph_check, cph_build) are replaced by
  the SDK's built-in Read/Write/Bash/Glob/Grep.
- The SSE event format matches Claude-to-IM's expected protocol natively.
- Per-run model selection works via role→model mapping; models are OpenRouter
  IDs, not Anthropic-only aliases.
- OpenRouter recommends setting Anthropic as the top-priority provider for
  Claude Code; non-Anthropic models may have reduced compatibility with some
  SDK features (e.g. thinking blocks).
