fix: default to first-party model alias when none specified (#1416)

[farmer-data]

Fixes #1416

Problem

When no model is specified, the action passes model: undefined all the way through to the Agent SDK, so the SDK/CLI falls back to its own bundled default model. In the currently-pinned version that resolves to the retired claude-sonnet-4-20250514, producing:

API Error: 404 {"type":"error","error":{"type":"not_found_error","message":"model: claude-sonnet-4-20250514"}}

Nothing in this repo hardcodes the bad id — it's inherited from the pinned SDK/CLI default, so it broke silently when that snapshot was retired server-side. With an OAuth token the 404 retries until the job timeout, so the run never completes. Every consumer currently has to add --model to their workflow by hand to avoid it.

How I solved it

Flow that produced the bug:

• action.yml defines no model input and never exports ANTHROPIC_MODEL.
• src/entrypoints/run.ts → runClaude(..., { model: process.env.ANTHROPIC_MODEL }) → undefined
• base-action/src/parse-sdk-options.ts → sdkOptions.model = options.model (undefined)
• base-action/src/run-claude-sdk.ts → query({ options }) → SDK supplies its stale default

The fix sets an explicit default in parseSdkOptions via a new resolveModel() helper. Key decisions:

• Use a model alias ("sonnet"), not a dated snapshot. The alias resolves server-side to the current model, so the action can't rot the same way a pinned id did.
• Preserve precedence. Explicit options.model wins → then --model in claudeArgs (kept in extraArgs and passed to the CLI, so we deliberately return undefined to avoid specifying the model twice) → then the default alias.
• Gate to the first-party provider. The default is skipped when CLAUDE_CODE_USE_BEDROCK / CLAUDE_CODE_USE_VERTEX / CLAUDE_CODE_USE_FOUNDRY is set, since "sonnet" isn't a valid Bedrock/Vertex/Foundry model id — those providers resolve their own default.

After this, users no longer need per-repo --model workarounds.

Tests

Added a model defaulting suite in base-action/test/parse-sdk-options.test.ts (env-isolated to avoid provider-var leakage):

• no model → defaults to "sonnet"
• explicit options.model preserved (and trimmed)
• --model in claudeArgs still overrides; sdkOptions.model stays undefined
• Bedrock / Vertex / Foundry → no alias injected
• explicit model preserved even for third-party providers

All 761 tests pass; tsc --noEmit and prettier --check are clean.

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