feat(models): add Fable 5 as a selectable Anthropic model

[doomspork]

Summary

Adds Fable 5 as a new Opus-class Anthropic model, selectable in the picker without changing the default (Opus 4.8 stays the default). This mirrors the prior Opus 4.8 transition, but for a brand-new, non-aliased name that also ships a 1M-context variant.

• Registry — two MODELS entries: claude-fable-5 (200K) and claude-fable-5[1m] (1M), plus a NON_1M_FALLBACKS mapping for the 1M → 200K fallback.
• Capabilities — Fable 5 joins the effort / xhigh / max sets (it is not added to fast mode, which is Opus-4.6-only).
• Pricing — claude-fable-5 at $30 / $150 per Mtok (2× Opus), for local cost aggregation (e.g. via Pi).
• Docs + help text — model-selection table, default-model row, --model flag examples, and the xhigh requires Opus 4.7+ strings (CLI help + command palette) now include Fable 5.

The key design choice is the concrete id claude-fable-5 (no bare fable alias). Because it starts with claude-, it passes the one hard allow-list gate (is_claude_code_model_alias in gateway_translate.rs) for free — so no Rust routing/validation or default-model changes were needed. The id flows verbatim to the Claude CLI --model arg, and every picker / /model / command-palette surface renders it automatically from the registry.

flowchart LR
    A["claude-fable-5 in MODELS"] --> B["buildModelRegistry()"]
    B --> C["Picker / /model / palette<br/>(auto-render)"]
    A --> D["--model claude-fable-5"]
    D --> E{"is_claude_code_model_alias?"}
    E -->|"starts_with('claude-') ✓"| F["Claude CLI resolves it"]
    A --> G["modelCapabilities Sets<br/>effort / xhigh / max"]
    A --> H["pricing.rs prefix lookup<br/>covers [1m] too"]

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Complexity Notes

• The capability Sets are the silent-failure risk. A model omitted from modelCapabilities.ts still works, but its effort/xhigh UI silently disappears. Fable 5 is added to EFFORT, XHIGH, and MAX (both the bare and [1m] ids), with matching tests pinning the exact profile — including an explicit isFastSupported('claude-fable-5') === false to lock in the "no fast mode" decision.
• One pricing entry covers both ids. pricing::lookup matches by starts_with, so claude-fable-5[1m] resolves to the same claude-fable-5 row. A test asserts both.
• The 1M variant uses the [1m] suffix, not a bare alias. Unlike the opus alias (whose id lacks [1m] and needs a special case in is1mContextModel), the suffixed id is detected automatically.

Test Steps

1. Frontend unit: cd src/ui && bun run test — registry + capability suites pass (73 in those two files; 2809 total).
2. Type-check: cd src/ui && bunx tsc -b — exits 0.
3. Rust pricing: cargo test -p claudette pricing — includes lookup_fable_5_priced_at_2x_opus (asserts $30/$150 and that claude-fable-5[1m] resolves).
4. Lint/format: cargo fmt --all --check, RUSTFLAGS="-Dwarnings" cargo clippy -p claudette -p claudette-cli --all-targets --all-features, cd src/ui && bun run lint:css — all clean.
5. Manual (running app): open the chat model picker → Fable 5 and Fable 5 1M appear as primary (non-legacy) entries → select Fable 5 → the effort selector offers up to XHigh/Max, and fast mode is hidden → /model claude-fable-5 resolves the same way.

Checklist

• Tests added/updated
• Documentation updated (if applicable)

[Copilot]

Pull request overview

Adds Anthropic Claude Fable 5 (200K + [1m] variant) as a selectable Claude Code model across the UI/CLI surfaces, including capability gating (effort/xhigh/max), 1M→non‑1M fallback behavior, pricing for local aggregation, and docs/help-text updates—without changing the default model.

Changes:

• Register claude-fable-5 and claude-fable-5[1m] in the UI model registry, including a 1M fallback mapping and coverage in registry tests.
• Extend model capability allow-lists so Fable 5 supports effort + xhigh + max (but not fast), with targeted unit tests.
• Add pricing lookup entry + test, and update CLI/help/docs strings to include Fable 5 in model/effort guidance.

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/usage/pricing.rs	Adds pricing table entry for claude-fable-5 (covers [1m] via prefix match) and a test.
src/ui/src/components/command-palette/commands.ts	Updates effort command description text to include Fable 5 for XHigh.
src/ui/src/components/chat/modelRegistry.ts	Adds Fable 5 (200K + 1M) to MODELS and adds [1m] fallback mapping.
src/ui/src/components/chat/modelRegistry.test.ts	Pins fallback behavior for claude-fable-5[1m] in tests.
src/ui/src/components/chat/modelCapabilities.ts	Adds Fable 5 ids to effort/xhigh/max supported sets.
src/ui/src/components/chat/modelCapabilities.test.ts	Adds tests ensuring Fable 5 supports effort/xhigh/max and does not support fast mode.
src-cli/src/commands/chat.rs	Updates --effort help text to include Fable 5 for xhigh eligibility.
site/src/content/docs/features/settings.mdx	Updates the Settings reference to list Fable 5 in the “Default model” description.
site/src/content/docs/features/cli-client.mdx	Updates CLI docs: --model examples and --effort xhigh note include Fable 5.
site/src/content/docs/features/agent-configuration.mdx	Documents Fable 5 model IDs and updates XHigh availability text.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.00%. Comparing base (558b686) to head (6a40faf).

Additional details and impacted files

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  Coverage   81.00%   81.00%           
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  Files         123      123           
  Lines       46958    46965    +7     
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+ Hits        38039    38046    +7     
  Misses       8919     8919           

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