Report unsupported Claude peer tool-selection capabilities clearly #55

Problem

The v1.7.5 peer path explicitly selects builtins with --tools and denies discovered unselected MCP tools with --disallowedTools. The final Fable review noted that an older CLI which rejects a required flag may fail with opaque CLAUDE_TURN_FAILED, rather than explaining the unsupported capability.

This is a compatibility/diagnostic observation from source review, not a reproduced failure on the installed CLI. Claude Code 2.1.266 passed actual Fable 5.1 / Opus 5 memo and critique qualification, including ToolSearch and native StructuredOutput. No minimum supported flag version was established by that review.

Baseline: PR #53, released commit ab53e31e57022b6e5f88e2ac3706d135938c48c8; see scripts/lib/claude-cli.mjs argument construction and scripts/claude-companion.mjs readiness / shared peer execution.

Follow-up after v1.7.5 usage

• First confirm behavior with a controlled CLI lacking the required capability; do not invent a minimum version.
• Reuse existing readiness/capability/error classification where possible.
• Return a bounded actionable unsupported-capability diagnostic and upgrade guidance.
• Keep the peer path fail-closed: never retry by dropping --tools, exact denies, sandbox requirements or hook isolation.
• Test unsupported and supported cases without changing ordinary task/review behavior or storing raw stderr/secrets.

Deferred until after experience with the current release. No implementation or broad compatibility layer is requested now.