Two cleanups stacked on PR #2418:
1. Refactor `send_a2a_message(target_url, msg)` →
`send_a2a_message(peer_id, msg)`. After #2418 every caller passes
`${PLATFORM_URL}/workspaces/{peer_id}/a2a` — the function's
parameter pretended to accept arbitrary URLs but in practice only
one shape is meaningful. Owning URL construction inside the
function makes the contract honest and centralises the peer-id
validation introduced below.
2. Add `_validate_peer_id` UUID-shape check at the trust boundary.
`discover_peer` and `send_a2a_message` are the entry points where
agent-controlled strings flow into URL paths; rejecting non-UUID
input at this layer eliminates the URL-interpolation class of
bug (`workspace_id="../admin"` etc.) regardless of how the rest
of the codebase interpolates ids elsewhere. Auth was already
gating malicious access — this is consistency + clear failure
over silent platform 4xx.
In-container tests cover positive UUIDs, malformed input
(``"ws-abc"``, ``"../admin"``, empty), and the contract that
``tool_delegate_task`` hands the peer_id to ``send_a2a_message``
without building URLs itself.
Live-verified: external delegation 8dad3e29 → 97ac32e9 returned
"refactor verified" from Claude Code Agent through the refactored
code; ``_validate_peer_id`` rejects ``"ws-abc"`` and ``"../admin"``
and accepts canonical UUIDs.
Stacked on PR #2418 (proxy-routing fix). Will rebase onto staging
once #2418 merges.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
993f8c494e