[ako]

Fixes mendixlabs#1593

Symptom (as reported)

alter page over the MCP backend (Studio Pro 11.12.5) fails for every page inside a folder:

failed to open page for mutation: module not found: <uuid>

The same statement works on a page at the module root. check --references passes, and the trace shows no PED tool call before the error.

Cause

resolvePageUnit (mdl/backend/mcp/page_mutator.go) looked up the module with GetModule(page.ContainerID). A page's ContainerID is its immediate parent, which for a foldered page is the folder, so the <uuid> in the error is the folder's ID. The backend already has a folder-aware resolver, moduleNameForContainer, which CreatePage, DeleteConstant, the microflow mutator and others use. The page mutator didn't.

Fix

• resolvePageUnit now resolves through moduleNameForContainer, for both saved pages and pages created this session.
• The same direct lookup was in four sibling paths, now fixed the same way:
  • UpdateConstant: create or modify on a foldered constant failed the same way.
  • UpdateWorkflow: create or modify on a foldered workflow failed the same way.
  • forgetSessionConstant / forgetSessionEnum: DROP of a foldered document created this session left it registered.
• The two remaining GetModule(x.ContainerID) calls in the package are correct: a domain model's container is always its module, and the other is the folder walk itself.

Evidence

mdl/backend/mcp/page_mutator_folder_test.go covers:

• pages at the module root (control), one folder deep, and nested (A/B/Pages), plus a page created this session in a folder;
• an end-to-end run over the fake Studio Pro server, asserting pg_read_page is called with MyFirstModule / Customer_Overview;
• the constant, workflow and DROP-cleanup paths.

The fake reader returns module not found: <id> for any non-module ID, as the codec engine does.

Control. With the production change stashed, the module-root case passes and the rest fail with the reported symptom:

page_mutator_folder_test.go:75: resolvePageUnit(pFolder): module not found: fA
page_mutator_folder_test.go:75: resolvePageUnit(pNested): module not found: fPages
page_mutator_folder_test.go:75: resolvePageUnit(pSession): module not found: fA
page_mutator_folder_test.go:117: OpenPageForMutation: module not found: fPages
page_mutator_folder_test.go:169: UpdateConstant: resolve module for constant "ApiUrl": module f2 not found
page_mutator_folder_test.go:196: UpdateWorkflow: resolve module for workflow "WF": module f1 not found
page_mutator_folder_test.go:215: dropped foldered documents still registered: 1 constant(s), 1 enumeration(s)
FAIL

All pass with the fix.

Not run against a live Studio Pro. The MCP server is a Studio Pro feature and none is available in this environment. The fix doesn't depend on what a Mendix tool accepts; it changes which ID is used for a lookup.

Regression script. mdl-examples/bug-tests/1593-mcp-alter-page-in-folder.mdl passes mxcli check and make check-mdl. It also runs cleanly against testdata/pedapp on the default engine, and describe page shows the nested-folder page altered. The default engine never had this bug, as the script's header notes; run it with --mcp <url> against Studio Pro to reproduce live.

Bookkeeping

• Finding: .claude/skills/fix-issue/findings/mdl-backend/2026-10-20-mcp-alter-page-in-folder-module-not-found.json. make check-findings passes.
• docs-wiki/bug-patterns/duplicate-resolver-drift.md: one paragraph on the "which module owns this document?" variant (Fix script set checking to resolve files against accumulated state #1071 in the executor, alter page over the Studio Pro MCP backend fails with "module not found: <id>" for every page inside a folder; check passes mendixlabs/mxcli#1593 in the MCP backend).

Checks

• make build, make test, make check-mdl and make check-findings all pass locally on the branch after merging main.
• CI is green on the current head. The earlier build-and-test failure came from the v1 BSON driver import in container_rendermode_test.go, which was already broken on main. fix(test): point container_rendermode_test at the v2 BSON driver #1500 fixed it there, and main has been merged into this branch.

🤖 Generated with Claude Code

https://claude.ai/code/session_016ZCjbSMETkeDrr8bHi6LCD

[ako]

build-and-test is red for a reason that predates this PR.

The job fails in two places, both from one file this PR doesn't touch:

FAIL	github.com/mendixlabs/mxcli/mdl/backend/pagemutator [setup failed]
--- FAIL: TestNoV1DriverImports
    the deprecated v1 BSON driver is imported again by 1 file(s):
      mdl/backend/pagemutator/container_rendermode_test.go

container_rendermode_test.go (added in 4a3571d) imports go.mongodb.org/mongo-driver/bson. That's the v1 driver: go.mod doesn't require it, and the guard test bans it. Both failures reproduce on main at fa57680, which is this PR's base. The package this PR changes, mdl/backend/mcp, passes in the same run.

No open PR fixes it yet. The fix is a one-line patch, verified locally (go test ./mdl/backend/pagemutator/ ./modelsdk/bsoncompat/ → both ok):

--- a/mdl/backend/pagemutator/container_rendermode_test.go
+++ b/mdl/backend/pagemutator/container_rendermode_test.go
-	"go.mongodb.org/mongo-driver/bson"
+	"go.mongodb.org/mongo-driver/v2/bson"

I've left it out of this PR to keep it scoped to mendixlabs#1593. Once that fix lands on main, this PR's CI should go green.

---

Generated by Claude Code