[ako]

Symptom

Found while verifying #1534. Over --mcp, this fails:

create microflow MyFirstModule.X ($T: System.UserType) begin … end;

ped_create_document … {"/parameters/0/entity":"Reference with qualified name System.UserType of type DomainModels$Entity not found."}

A module enumeration (Rules.RuleResult) works.

It was never MCP-only

The local engines write the same wrong document. They just don't check references at write time:

• mxcli bson dump shows DataTypes$ObjectType with Entity: "System.UserType".
• mx check (11.14.0) reports [CE1613] "The selected entity 'System.UserType' no longer exists." at Parameter 'T'. A returns System.DeviceType gets the same error.
• DESCRIBE hid it. It prints an enumeration type as enum X and an object type as a bare X, so the plausible-looking $T: System.UserType in describe output was the symptom.

The explicit $T: enum System.UserType form is what DESCRIBE emits for an enumeration parameter. It was refused with enumeration 'System.UserType' not found.

Cause

The visitor types every bare qualified name in a flow parameter or return position as TypeEntity. The executor only falls back from entity to enumeration after the entity lookup misses. For the System module it skips that lookup entirely (isBuiltinModuleEntity, because System entities are not stored in the project), so a System enumeration kept the entity guess.

The explicit form failed for a separate reason: findEnumeration reads ListEnumerations, which never includes System enumerations.

Fix

• settleSystemEnumeration changes a TypeEntity to TypeEnumeration before validation when its name is in modelsdk/meta.SystemEnumerations. It runs at all six sites: microflow, nanoflow and rule, each for parameters and the return type.
• isKnownEnumeration treats a System enumeration as known even though findEnumeration cannot see it.
• System.User still becomes an object type. A test controls for this.

findEnumeration is deliberately left unchanged. Returning a synthetic System enumeration from it would make ALTER/DROP ENUMERATION System.X reachable, which is the "making a synthesized element readable makes it writable" trap in docs-wiki/bug-patterns/platform-semantics-gaps.md.

Verification

• Unit (mdl/executor/system_enum_flow_type_test.go). The tests parse real MDL, so the visitor's entity guess is exercised. They cover a bare parameter, a bare return type, an explicit enum parameter, a nanoflow parameter, and a System-entity control. With settleSystemEnumeration stubbed to identity, the three bare-name tests fail with got *microflows.ObjectType. The explicit-enum test failed before the fix with the "not found" refusal.
• Live over --mcp, Studio Pro 11.15.0-rc.4. Created MyFirstModule.MCPE_SysEnumParam and MCPE_SysEnumReturn. ped_read_document reads back:

  "/objectCollection/objects/0/variableType": {"$Type": "DataTypes$EnumerationType", "enumeration": "System.UserType"}
  "/microflowReturnType":                     {"$Type": "DataTypes$EnumerationType", "enumeration": "System.DeviceType"}

  Control: the same build with the fix stubbed reproduces the reported PED error verbatim.
• Local (default engine) + mx check 11.14.0 on mdl-examples/bug-tests/mcp-system-enumeration-parameter.mdl against a copy of TestApp: 2× CE1613 before, 0 errors after. DESCRIBE now round-trips as $T: enum System.UserType.
• go test ./mdl/... ./cmd/mxcli/... ./modelsdk/...: all 84 packages pass. mdl/executor and mdl/backend/modelsdk hit the 10-minute default timeout on a heavily loaded devcontainer (load average ~11), then passed in a rerun with -timeout 40m (609s and 589s). go vet, gofmt and make check-findings are clean.

Checklist

• Fix-issue findings consulted (closest: the System-module paragraph of platform-semantics-gaps.md; nothing on enum-vs-entity for System references)
• Finding recorded: .claude/skills/fix-issue/findings/mdl-executor/2026-10-21-system-enumeration-flow-parameter-written-object-type.json
• Test written first and shown to fail with the reported symptom
• Bug-test script: mdl-examples/bug-tests/mcp-system-enumeration-parameter.mdl
• No backend / BSON changes; the executor still goes through ctx.Backend

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