Issue #503 reported a hardBreak sent via updatePageJson being «silently
cut» — no error, no line break — forcing the agent to split lines into
separate paragraphs.
A faithful repro shows the bug does NOT reproduce against develop:
hardBreak is fully representable in the markdown canon in BOTH directions
and survives the real Yjs write path.
- pm -> md: the serializer (now in @docmost/prosemirror-markdown after the
#293 STEP 5 consolidation) emits the CommonMark hard break ` \n`, not a
bare `\n`.
- md -> pm: marked tokenizes ` \n` to `<br>`, which generateJSON parses
back to a hardBreak node in ONE paragraph (not split, not dropped).
- updatePageJson (PM JSON -> applyDocToFragment = PMNode.fromJSON +
updateYFragment, the real collab-session write encoder -> read back)
keeps text + hardBreak + text in one paragraph.
The issue's diagnosis points at packages/mcp/.../markdown-converter.ts as
the serializer, but that file is a 15-line re-export shim since #293 STEP 5
— the diagnosis predates the converter consolidation that already closed
both sides. P1 (semantic round-trip) + P2 (byte-fixpoint) hold, and the
node already has broad property/corpus/golden coverage in the package.
No converter change is warranted (switching the break form to `\`+newline
would churn the whole #351 byte-fixpoint corpus against the established
` \n` repo convention for zero functional gain). This adds one
integration guard at the MCP canon seam covering all three seams of the
reported updatePageJson path. Mutation-verified: neutering the serializer
arm or the importer `<br>` handling reddens seams 1-2; neutering the real
applyDocToFragment write path reddens seam 3.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
agent_coder