Three things the write surface claimed but did not hold.
Scope: the run bodies never re-checked ctx.canWrite. Enforcement was
real but lived entirely in the registration decision, and the test
asserted registration only — so a refactor that registered the write
tools unconditionally and meant to refuse at call time would open the
surface with the suite still green. Wrap every run body in a scope
guard at the registry level, so a new write tool inherits it, and drive
the run bodies directly from a read-only context in the test.
Free text: medication names, doses, treatment classes and lab analyte
names were interpolated into tool prose with no delimiter. A lab
analyte name is transcribed out of an uploaded document, so a hostile
document chooses it, and the write tools ride the same wire. Fence the
assembled prose with the shared data-fence markers and scrub short
result titles, which would leak markers into a host's result list.
Confirm: the header called the confirm flag a PROTOCOL-LEVEL CONFIRM
GATE. It is a client-asserted boolean with nothing binding it to a
prior preview. Binding it to a server-issued preview token would prove
only that a preview call happened, which any client can do, so it buys
no human-in-the-loop for a round trip and server state the stateless
transport does not want. Describe it as the convention it is and name
what actually bounds the blast radius.
The confirm:false preview echoed the would-be record without running the
type / range / instant validation, so a preview could show a value the
commit then refused. Run the same shared validation in the preview and
surface wouldFail with the error + reason the commit would return, so a
preview and its commit never disagree.
Also note at the REST read-method allowlist that the MCP-audience guard
assumes GET/HEAD/OPTIONS are side-effect-free — a future side-effecting
GET must revisit that audience binding.
Blood pressure is two values, so it cannot ride the single-value
log_measurement tool. Add log_blood_pressure: it writes the systolic and
diastolic rows atomically under one shared idempotency namespace and one
measured-at instant, range-validates each value, rejects an implausible
pair (systolic at or below diastolic), and keeps the same confirm-flag
preview gate, per-credential write budget, MCP provenance, and audit
trail the existing writes use. Declare a structured output schema on
every write tool so a preview, a commit, an already-logged no-op, and a
refusal all validate as structured content.
Register two write tools — log_measurement and log_mood — only for a
health:write-scoped session; a read-only session never sees them advertised.
Each write mirrors the proven Telegram capture path (same range guard,
canonical unit, cache invalidation and rollup recompute) pinned to the MCP
source, and is idempotent on a hashed externalId derived from a client key, so
a retried call is a no-op rather than a duplicate.
Because the remote wire is stateless (no elicitation), the tools carry a
protocol-level confirm gate: without confirm:true they write nothing and
return the exact record they would write plus an instruction to confirm the
value with the user and re-call with the same idempotency key. Commits run
under a tighter per-credential write budget on top of the per-binding request
limiter.
Harden the resource-server audience binding: a token whose scopes are all in
{health:read, health:write} is bound to the MCP surface and refused on every
REST write/delete. A health:write token therefore admits the in-process MCP
write tools only and can never become a general REST write credential; admin
stays unreachable (cookie-only). Refuse, by construction, any medication, lab,
delete/update, or data-export tool on this surface.
Marc-André Bombeck