This adds a deterministic mock Anthropic-compatible /v1/messages service,
a clean-environment CLI harness, and repo docs so the first parity
milestone can be validated without live network dependencies.
Constraint: First milestone must prove Rust claw can connect from a clean environment and cover streaming, tool assembly, and permission/tool flow
Constraint: No new third-party dependencies; reuse the existing Rust workspace stack
Rejected: Record/replay live Anthropic traffic | nondeterministic and unsuitable for repeatable CI coverage
Confidence: high
Scope-risk: moderate
Reversibility: clean
Directive: Keep scenario markers and expected tool payload shapes synchronized between the mock service and the harness tests
Tested: cargo fmt --all
Tested: cargo clippy --workspace --all-targets -- -D warnings
Tested: cargo test --workspace
Tested: ./scripts/run_mock_parity_harness.sh
Not-tested: Live Anthropic responses beyond the five scripted harness scenarios
