Make dynamic skill-menu failures deterministic across Anthropic and pi

Problem

Dynamic-skill programs depend on load_skill_menu, but a failed menu fetch produces different and model-dependent behavior across the two harnesses.

On the Anthropic path, createWizardToolsMcpServer prefetches skill-menu.json to build a Zod enum. When the fetch fails, the enum falls back to only integration. A valid call such as load_skill_menu { category: "feature-flags" } is then schema-rejected before the handler can return an actionable menu error.

On the pi path, the category is an unrestricted string, but a failed fetch returns the plain tool result Error: could not load the skill menu. Neither path is converted by the host into a terminal Wizard error. The agent must interpret the tool failure and emit the program's configured [ABORT] signal. That usually works when the prompt names this case, but it is not deterministic and is not covered by a failure-path test.

Why this matters

Programs without a fixed skillId, including ai-observability and feature-flags, cannot proceed without the menu. A transient registry outage should fail once with a useful explanation. It should not depend on the selected model to translate a schema error, retry, or eventually stop for another reason.

Relevant code

• src/lib/wizard-tools/mcp.ts:157-169 - failed prefetch leaves categoryNames as ['integration'].
• src/lib/wizard-tools/mcp.ts:340-369 - Anthropic schema and handler.
• src/lib/agent/runner/harness/pi/tools.ts:86-112 - pi fetch and error result.
• src/lib/agent/runner/sequence/linear.ts:142-263 - terminal error routing has no skill-menu failure case.

Suggested direction

Keep the category input callable even when startup discovery fails, then represent registry unavailability explicitly. The exact implementation can stay shared-tool-level, but Anthropic and pi should expose the same failure semantics and the runner should not rely on agent wording to finish the run.

Acceptance

• A failed skill-menu.json fetch does not narrow Anthropic's category schema to an unrelated fallback value.
• Anthropic and pi return equivalent structured failures when the menu is unavailable.
• A dynamic-skill program terminates with one actionable error rather than relying on model retries or prose.
• Focused tests cover menu-fetch failure for both harnesses.

Start with the failure paths in src/lib/wizard-tools/mcp.ts and src/lib/agent/runner/harness/pi/tools.ts, then trace terminal routing in src/lib/agent/runner/sequence/linear.ts. Run the focused harness tests if available and add coverage for failed skill-menu fetches in both Anthropic and pi. Done means both harnesses expose equivalent structured failures and a dynamic-skill run terminates with one actionable error.