Finding / unreal-agent
2026-10-13-unreal-agent-v0-3-1-adds-an-anthropic-provider-that-requests-server-side-model-fallback-unless-anthropi
v0.3.1 adds an Anthropic provider that requests server-side model fallback unless ANTHROPIC_FALLBACK=false. UNREAL_HARNESS_LLM_PROVIDER=anthropic (or -provider anthropic in the TUI) uses the Messages API with ANTHROPIC_API_KEY; the default model is claude-opus-5-5. When ANTHROPIC_FALLBACK is unset, the adapter adds the header Anthropic-Beta: server-side-fallback-2026-07-01 and the body field "fallbacks": "default" to every request (“Nil enables server-side fallback”, adapter.go L54). It accepts a fallback output block, which must name a from model and a to model. The setting moved from opt-in (2026-09-30) to on by default (2026-10-02) within the v0.3.0 Go-tag range, first available as a binary in v0.3.1, and usage is now recorded per model (Usage.ByModel). Requests default to 128,000 max output tokens, and ANTHROPIC_CACHE_TTL accepts 5m or 1h. The adapter answers each tool call once and sends later progress for that call as text (request.go L239-L240), which on this provider avoids the duplicate-result rejection behind #11. I did not read Anthropic’s documentation for the beta. What a fallback actually serves is Anthropic’s behavior; this record covers only what the harness sends and parses.
Channel: tagged-release. Half: both. Date: 2026-10-06.
Operator consequence: If a run has to stay on one Claude model (for evals, a cost ceiling or a data-handling rule), set ANTHROPIC_FALLBACK=false. The default requests fallback and the adapter recognizes returned fallback blocks. The beta’s service availability, substitute-model selection and billing are unverified; do not turn request construction into a guarantee about the service. Check per-model usage in the session output before you compare costs across runs. The runner README does not list this provider, so read providers.go for its switches.
Receipt
- https://github.com/unreallabsai/unreal-agent/blob/v0.3.1/cmd/internal/providers/providers.go#L95-L118
- https://github.com/unreallabsai/unreal-agent/blob/v0.3.1/harness/llm/messagesapi/adapter.go#L92-L105
- https://github.com/unreallabsai/unreal-agent/blob/v0.3.1/harness/llm/messagesapi/request.go#L17-L40
- https://github.com/unreallabsai/unreal-agent/commit/20b14c090bb72e2ee461179e9bb5c0fc39a92596
Finding metadata
Run: 2026-10-13-profile-refresh-2026-09-21_2026-10-13-frontier-v0
Finding ID: 2026-10-13-unreal-agent-v0-3-1-adds-an-anthropic-provider-that-requests-server-side-model-fallback-unless-anthropi
Profile citations
- unreal-agent / claim / anthropic-fallback-default
Source links
Primary links, including exact changelog lines when available.
- tagged commit file2026-10-13-unreal-agent-v0-3-1-adds-an-anthropic-provider-that-requests-server-side-model-fallback-unless-anthropiunreallabsai/unreal-agent / cmd/internal/providers/providers.go#L95-L118tagged commit file2026-10-13-unreal-agent-v0-3-1-adds-an-anthropic-provider-that-requests-server-side-model-fallback-unless-anthropiunreallabsai/unreal-agent / harness/llm/messagesapi/adapter.go#L92-L105tagged commit file2026-10-13-unreal-agent-v0-3-1-adds-an-anthropic-provider-that-requests-server-side-model-fallback-unless-anthropiunreallabsai/unreal-agent / harness/llm/messagesapi/request.go#L17-L40commit2026-10-13-unreal-agent-v0-3-1-adds-an-anthropic-provider-that-requests-server-side-model-fallback-unless-anthropigithub.com/unreallabsai/unreal-agent/commit/20b14c090bb72e2ee461179e9bb5c0fc39a92596