Milestone 4.C.1 — Anthropic Adapter HardeningCopy Anchor Link

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

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Why This Milestone ExistsCopy Anchor Link

The Anthropic adapter already shipped in Phase 2.5 (2.5.G1.M1) and is documented in ADR-0040 (Accepted 2026-08-21): OpenAI-compatible SSE translation, anthropic-version on every request, system-role extraction (leading + mid-conversation fold), and mid-stream event: error → pipe CloseWithError (no JSON-swap). Phase 2.5’s exit gate already checked Anthropic test-depth parity with OpenAI.

This milestone is therefore not a greenfield hardening pass. It closes roadmap drift against live code and adds two small, additive stream-translate test assertions (incomplete mid-stream body; explicit ignore of non-text delta types). No adapter behavior change; no proxy handler/middleware changes.

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Non-GoalsCopy Anchor Link

• Tool-use / image / vision / thinking passthrough
• ANTHROPIC_API_VERSION env wiring
• Shared parameterized OpenAI/Anthropic stream harness
• Per-org model routing (4.C.2)
• Circuit breaker (4.C.3)
• Azure OpenAI or Bedrock adapters (future)
• Changes to packages/provider/openai/** or services/proxy/** handlers/middleware

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Shipped surface (already in tree)Copy Anchor Link

Request constructionCopy Anchor Link

• packages/provider/anthropic/request.go — peel leading system messages + fold mid-conversation systems into top-level system; reject tool role; deny-by-default passthrough allowlist (text-only).
• packages/provider/anthropic/client.go — newMessagesRequest sets x-api-key, anthropic-version, Content-Type on every Messages call.

Streaming translationCopy Anchor Link

• packages/provider/anthropic/stream_translate.go — Anthropic named events → OpenAI-compatible SSE; non-text_delta content deltas ignored; mid-stream event: error → CloseWithError (no [DONE], no fabricated OpenAI error chunk in the pipe body).
• Retry does not re-issue after a live stream body has started (retry.go).

Registry / capabilitiesCopy Anchor Link

• Built-in IDs: claude-sonnet-4-5, claude-haiku-4-5, claude-opus-4-5 (+ ExtraModels).
• Capability rows in capability_catalog.go (context window / streaming flags); tools/vision flags are vendor-capability metadata, not a claim that the adapter implements tool/image passthrough.

Success signalsCopy Anchor Link

• Exact-ID Registry.For("claude-sonnet-4-5") (and sibling Claude IDs) resolves when Anthropic is registered; NewRegistry returns ErrDuplicateModel on colliding model IDs
• Streaming translation emits OpenAI-shaped chunks suitable for ADR-0027 dual-write; mid-stream Anthropic event: error terminates the translate pipe as incomplete (no data: [DONE], no fabricated OpenAI-shaped error chunk) — asserted in stream_translate_test.go
• Anthropic HTTP / mapped provider errors continue through provider.MapError / existing failure paths — no proxy handler special-case for Anthropic
• Non-streaming and streaming fixture coverage lives in packages/provider/anthropic (inline httptest / SSE fixtures; no separate golden-file harness required)
• Zero proxy handler or middleware changes for this milestone (docs + Anthropic package tests only)
• Non-text stream deltas (input_json_delta, thinking_delta, signature_delta) are ignored without leaking into OpenAI-shaped output

Local verificationCopy Anchor Link

1. go test ./packages/provider/anthropic/...
2. go test ./packages/provider/...
3. go test ./services/proxy/... (regression only; no proxy edits)

PrerequisitesCopy Anchor Link

• Phase 2.5 Anthropic adapter merged (2.5.G1.M1 / ADR-0040)
• packages/provider.Provider interface stable (M2.1.1)