Bitter Frontier What coding agents just made possible, where the bottleneck moved, and what deserves attention.

Evidence record / unreal-agent

A dated record of one change, kept so the writing that cites it can be checked. Compiled from the sources listed below by the research run, not written for reading. The judgment lives in the issues that cite it, below.

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

Finding metadata

Confidence
high

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

Source links

Primary links, including exact changelog lines when available.

  1. Unreal Agent
    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

Versioned source: run artifact