Milestone 4.C.1 — Anthropic Adapter Hardening

Status: Planned
Goal: Track C — Multi-Provider Adapters & Resilience
Phase: 4 — Operator Platform & Multi-Provider
Estimated effort: 3 days
Track: Track C — Multi-Provider Adapters & Resilience

Why This Milestone Exists

Phase 2.5 stood up a first-cut Anthropic adapter to prove the interface generalizes. This milestone hardens it to production parity with the OpenAI adapter: full SSE dual-write, anthropic-version header handling, and the message-role translation Anthropic requires (no system role inside messages; it's a top-level system field).

This is not new design — it's closing gaps against the same bar OpenAI already cleared. The packages/provider.Provider interface (ADR-0025) should be satisfied without any handler/middleware changes.

Non-Goals

• Per-org model routing (4.C.2)
• Circuit breaker (4.C.3)
• Azure OpenAI or Bedrock adapters (future)

Orientation (indicative)

Named paths, package layouts, libraries, schemas, env vars, and commands anywhere on this page are rough sketches for orientation — inspiration and a baseline, not a required change list.

During implementation, expect to:

• open the live tree and follow existing patterns before inventing new ones
• research current constraints (latency, tenancy, deploy shape, libraries) more deeply than this page can
• advance the design beyond the sketch where measurement or code reality says so
• land work in different filenames, merged packages, deferred docs, or new surfaces when the situation calls for it

Prefer outcomes over matching any particular file tree or command sequence.

Areas that may be involved (situational — not a checklist):

• Provider abstraction / adapters
• Proxy service (HTTP, bootstrap, config)

Suggested naming (provisional)

Rename freely to match the change that actually lands.

• Branch: feature/m4-c-1-anthropic-adapter-hardening
• PR title: feat(provider): production-hardened Anthropic adapter with SSE dual-write (m4.C.1)

Message-Role Translation

Anthropic's API does not accept a system role inside the messages array. It requires a top-level system field:

// Illustrative — exact path may differ
 
// toAnthropicRequest translates an IBEX provider.Request to Anthropic's native wire format.
// Key translation: system messages extracted from Messages array → top-level system field.
func toAnthropicRequest(req provider.Request) anthropicRequest {
 var systemParts []string
 var messages []anthropicMessage
 
 // Inject directive as first system content
 if req.SystemDirective != "" {
 systemParts = append(systemParts, req.SystemDirective)
 }
 
 for _, m := range req.Messages {
 if m.Role == "system" {
 systemParts = append(systemParts, m.Content)
 } else {
 messages = append(messages, anthropicMessage{
 Role: m.Role,
 Content: m.Content,
 })
 }
 }
 
 return anthropicRequest{
 Model: req.Model,
 System: strings.Join(systemParts, "\n\n"),
 Messages: messages,
 MaxTokens: maxTokensOrDefault(req.MaxTokens),
 Stream: req.Stream,
 }
}

Anthropic SSE Event Mapping

Anthropic's SSE stream uses different event types than OpenAI's:

// Illustrative — exact path may differ
 
// Anthropic event types that matter for IBEX:
// - content_block_delta: contains text delta (maps to OpenAI's delta.content)
// - message_stop: signals stream completion (maps to OpenAI's [DONE])
// - error: mid-stream error event — should be forwarded verbatim per ADR-0027
 
type anthropicStreamEvent struct {
 Type string `json:"type"`
 Delta *anthropicDelta `json:"delta,omitempty"`
 Error *anthropicError `json:"error,omitempty"`
}
 
type anthropicDelta struct {
 Type string `json:"type"` // "text_delta"
 Text string `json:"text"`
}

Success signals

Outcome-oriented signals that the milestone is in good shape. Exact filenames, package layouts, and commands may differ from any sketches above.

• Registry.For("claude-*") resolves to the Anthropic adapter; NewRegistry panics on duplicate model prefixes across providers
• Streaming dual-write behavior matches ADR-0027 exactly (no JSON-swap mid-stream) — same test harness reused from OpenAI, parameterized by provider
• writeProviderFailure/error mapping paths reused unmodified — Anthropic errors map through the same provider.MapError (do not special-case in the handler)
• Golden-fixture tests pass for both non-streaming and streaming request/response pairs
• Zero changes to any proxy handler or middleware code (pure adapter addition)

Prerequisites

• Phase 2.5 Anthropic prototype merged
• packages/provider.Provider interface stable (M2.1.1)