diff --git a/packages/prosemirror-markdown/test/fixtures/counterexamples/columns-column-width-percent.json b/packages/prosemirror-markdown/test/fixtures/counterexamples/columns-column-width-percent.json
new file mode 100644
index 00000000..c7af7dce
--- /dev/null
+++ b/packages/prosemirror-markdown/test/fixtures/counterexamples/columns-column-width-percent.json
@@ -0,0 +1,16 @@
+{
+  "_bug": "BUG #351: a `column` whose `width` is a percentage string (e.g. \"50%\") is NOT byte-stable across export->import->export (violates P2). The `column` schema's parseHTML does `parseFloat(getAttribute('data-width'))`, which silently drops the '%' unit and returns the NUMBER 50. So the first export emits data-width=\"50%\" but the re-import stores width=50, and the second export emits data-width=\"50\": md2 !== md1, a permanent GS-EDIT-REVERT churn (every git-sync pull rewrites the column width). The editor authors column widths as percentages, so this is a real data/round-trip defect. Fix belongs in src/lib/docmost-schema.ts column.width parseHTML (preserve the unit / keep the string), which is OUT OF SCOPE for this test-only PR and must be a separate, maintainer-approved change. This flat generator therefore keeps `column.width` frozen (never generates a non-default width).",
+  "doc": {
+    "type": "doc",
+    "content": [
+      {
+        "type": "columns",
+        "attrs": { "layout": "two_equal", "widthMode": "normal" },
+        "content": [
+          { "type": "column", "attrs": { "width": "50%" }, "content": [{ "type": "paragraph", "content": [{ "type": "text", "text": "L" }] }] },
+          { "type": "column", "attrs": { "width": "50%" }, "content": [{ "type": "paragraph", "content": [{ "type": "text", "text": "R" }] }] }
+        ]
+      }
+    ]
+  }
+}
diff --git a/packages/prosemirror-markdown/test/fixtures/counterexamples/ordered-list-start.json b/packages/prosemirror-markdown/test/fixtures/counterexamples/ordered-list-start.json
new file mode 100644
index 00000000..36f3de9d
--- /dev/null
+++ b/packages/prosemirror-markdown/test/fixtures/counterexamples/ordered-list-start.json
@@ -0,0 +1,19 @@
+{
+  "doc": {
+    "type": "doc",
+    "content": [
+      {
+        "type": "orderedList",
+        "attrs": { "type": null, "start": 5 },
+        "content": [
+          {
+            "type": "listItem",
+            "content": [
+              { "type": "paragraph", "content": [{ "type": "text", "text": "alpha" }] }
+            ]
+          }
+        ]
+      }
+    ]
+  }
+}
diff --git a/packages/prosemirror-markdown/test/generative/attr-arbitraries.ts b/packages/prosemirror-markdown/test/generative/attr-arbitraries.ts
new file mode 100644
index 00000000..9fa7ccf1
--- /dev/null
+++ b/packages/prosemirror-markdown/test/generative/attr-arbitraries.ts
@@ -0,0 +1,325 @@
+/**
+ * Schema-DERIVED attribute-state fast-check arbitraries (#351, PR 1).
+ *
+ * This GENERALIZES the #350 stability-matrix helper (roundtrip-stability.helper.ts)
+ * to fast-check. Where that helper sweeps a HAND-WRITTEN 2-state matrix for one
+ * node spec, this module reads the attribute list straight from
+ * `schema.nodes[type].spec.attrs` (never a hand list) and, per attribute,
+ * generates over the FOUR states the issue calls for:
+ *
+ *   - `absent`      : the attribute is OMITTED entirely (the empty-string-vs-
+ *                     absent churn class the #350 fix targets).
+ *   - `default`     : the schema default value, authored explicitly.
+ *   - `nonDefault`  : a representative legal non-default value.
+ *   - `degenerate`  : `""` for strings, `0`/negative for numbers, the flipped
+ *                     value for booleans.
+ *
+ * ── Why a per-attribute override table ──────────────────────────────────────
+ * Everything that CAN be derived generically from the default's runtime type is
+ * (booleans flip; the degenerate value follows the runtime type). But two facts
+ * force a small, DOCUMENTED override table:
+ *
+ *  1. CONSTRAINED domains the schema does not encode. `image.align ∈
+ *     {left,center,right}`, `heading.level ∈ 1..6`, `callout.type ∈
+ *     {info,success,warning,danger}`, `columns.layout`, table-cell `align`,
+ *     `status.color`, `orderedList.start ≥ 1`, etc. A generic "default + 1"
+ *     would emit an ILLEGAL value, so these get an explicit legal domain.
+ *  2. ROUND-TRIP-safety, established EMPIRICALLY by probing the live converter
+ *     (the classification captured in flat-roundtrip.property.test.ts). A frozen
+ *     attribute falls into ONE of TWO explicitly-distinguished classes — never a
+ *     silent "it just doesn't round-trip":
+ *
+ *       (a) ACCEPTED LIMITATION — the attribute has NO markdown representation,
+ *           so the loss is inherent to targeting markdown, not a converter
+ *           defect. These: `paragraph`/`heading` `indent`, `callout.icon`,
+ *           `orderedList.type` (a/A/i markers), table `colwidth` /
+ *           `backgroundColor(Name)` (dropped by the raw-<table> fallback). Each is
+ *           tagged `// ACCEPTED:` inline. Freezing them is correct — there is
+ *           nothing to preserve in the target format.
+ *
+ *       (b) PINNED BUG — the attribute IS representable in markdown but the
+ *           converter drops it anyway (a real defect). These are NOT silently
+ *           frozen: each is captured as a LOUD `it.fails` counterexample in
+ *           test/fixtures/counterexamples/ + counterexamples.test.ts, and the
+ *           freeze here only keeps the P1/P2 union green until a MAINTAINER rules
+ *           on accept-vs-fix (the epic guardrail reserves that call). These:
+ *           `column.width` (parseFloat drops `%`), `orderedList.start` (non-1
+ *           start renders as `1.`). Tagged `// PINNED-BUG:` inline.
+ *
+ *       (c) DEFERRED-BUG — representable AND round-trips, frozen only because the
+ *           flat generator can't yet build a valid instance. Table
+ *           `colspan`/`rowspan` round-trip via the raw-<table> fallback, but a
+ *           geometrically-valid spanned table is PR-2 structural work; the flat
+ *           generator hardcodes span = 1. Tagged `// DEFERRED-BUG:` inline so a
+ *           maintainer does not read them as an inherent limitation.
+ *       - Several non-null-default attrs are MATERIALIZED on import but are not
+ *         in canonicalize's KNOWN_DEFAULTS (`callout.type`, `status.color`,
+ *         table `colspan`/`rowspan`, `columns.layout`/`widthMode`,
+ *         `embed.width`/`height`, `heading.level`, `taskItem.checked`,
+ *         `details.open`, `subpages.recursive`, `orderedList.start`). If left
+ *         `absent` they re-materialize as a non-canonical default and diverge
+ *         under P1. We mark them `always` so they are authored explicitly.
+ *       - The documented numeric→string coercion set (`width height size
+ *         aspectRatio`) is generated as STRINGS for the media family (a stored
+ *         number re-parses as a string), EXCEPT `embed.width/height` which the
+ *         embed schema keeps numeric — handled per-attr.
+ *
+ * Both PINNED-BUG attrs (`column.width` P2 churn, `orderedList.start` P1 loss)
+ * are captured as committed `it.fails` counterexamples — NOT hidden here.
+ */
+import fc from 'fast-check';
+import { getSchema } from '@tiptap/core';
+import { docmostExtensions } from '../../src/lib/index.js';
+import { phraseArb, letterPhraseArb, urlArb } from './text-arbitraries.js';
+
+/** The exact ProseMirror schema the converter targets. */
+export const schema = getSchema(docmostExtensions as any);
+
+/** Sentinel: this attribute is OMITTED (the `absent` state). */
+export const ABSENT = Symbol('ABSENT');
+
+/** The documented numeric→string coercion set (issue + roundtrip-stability.helper). */
+export const NUMERIC_STRING_ATTRS = ['width', 'height', 'size', 'aspectRatio'];
+
+/** Read the schema default for every attribute of a node type. */
+export function schemaAttrDefaults(type: string): Record<string, unknown> {
+  const specAttrs = (schema.nodes[type]?.spec?.attrs ?? {}) as Record<
+    string,
+    { default: unknown }
+  >;
+  const out: Record<string, unknown> = {};
+  for (const [k, v] of Object.entries(specAttrs)) out[k] = v.default;
+  return out;
+}
+
+/** Attribute names for a node type, straight from the schema (never hand-listed). */
+export function schemaAttrNames(type: string): string[] {
+  return Object.keys((schema.nodes[type]?.spec?.attrs ?? {}) as object);
+}
+
+/**
+ * Per-attribute policy. Everything unlisted falls back to a generic policy:
+ *   - a BOOLEAN default is fuzzable (its non-default is the flipped value);
+ *   - any other default is `frozen` (only `absent`/`default` are generated) so
+ *     we never invent an unverified non-default that might not round-trip.
+ * Listed attrs override this with a legal `arb` domain and/or flags.
+ */
+interface AttrPolicy {
+  /** Arbitrary for the `nonDefault` state's value. */
+  arb?: fc.Arbitrary<unknown>;
+  /** Value for the `degenerate` state (fuzz mode only). Omit to skip degenerate. */
+  degen?: unknown;
+  /** Never emit `absent` — the attr must be authored (materialized default class). */
+  always?: boolean;
+  /** Never emit the schema default value (required-ish attrs like `src`). Implies always. */
+  noDefault?: boolean;
+  /** Never emit non-default/degenerate — attr has no md representation or churns. */
+  frozen?: boolean;
+}
+
+const num = (...xs: number[]) => fc.constantFrom(...xs);
+const str = (...xs: string[]) => fc.constantFrom(...xs);
+const widthStr = str('120', '320', '640');
+
+// The documented override table, keyed `type.attr`. Every entry is grounded in
+// the empirical converter probe (see flat-roundtrip.property.test.ts header).
+const OVERRIDES: Record<string, AttrPolicy> = {
+  // ── block text containers ────────────────────────────────────────────────
+  // 'left' is the IMPLICIT default alignment: the converter drops it on export
+  // (empirically confirmed), so it never round-trips. Only center/right/justify
+  // carry through the `<!--attrs {textAlign}-->` comment.
+  'paragraph.textAlign': { arb: str('center', 'right', 'justify') },
+  'paragraph.indent': { frozen: true }, // ACCEPTED: no md representation
+  'heading.level': { always: true, arb: num(2, 3, 4, 5, 6) },
+  'heading.textAlign': { arb: str('center', 'right', 'justify') },
+  'heading.indent': { frozen: true }, // ACCEPTED: no md representation
+  // ── lists ────────────────────────────────────────────────────────────────
+  // PINNED-BUG: markdown CAN express a non-1 start ("5."), but the converter
+  // renders "1." and drops it -> P1 loss. See counterexamples.test.ts
+  // (ordered-list-start.json). Frozen only until the maintainer rules accept-vs-fix.
+  'orderedList.start': { always: true, frozen: true },
+  'orderedList.type': { frozen: true }, // ACCEPTED: a/A/i markers not expressible in GFM
+  'taskItem.checked': { always: true, arb: fc.constant(true) }, // boolean, default false
+  // ── codeBlock ────────────────────────────────────────────────────────────
+  'codeBlock.language': { arb: str('js', 'ts', 'python', 'go', 'rust', 'bash') },
+  // ── image / media (numeric→string width family) ──────────────────────────
+  'image.src': { noDefault: true, arb: urlArb, degen: '' },
+  'image.align': { arb: str('left', 'right') },
+  'image.alt': { arb: letterPhraseArb, degen: '' },
+  'image.title': { arb: letterPhraseArb },
+  'image.width': { arb: widthStr, degen: '' },
+  'image.height': { arb: widthStr, degen: '' },
+  'video.src': { noDefault: true, arb: urlArb, degen: '' },
+  'video.alt': { arb: letterPhraseArb },
+  'video.width': { arb: widthStr },
+  'video.height': { arb: widthStr },
+  'audio.src': { noDefault: true, arb: urlArb, degen: '' },
+  'youtube.src': { noDefault: true, arb: urlArb },
+  'pdf.src': { noDefault: true, arb: urlArb },
+  'pdf.name': { arb: phraseArb },
+  'drawio.src': { noDefault: true, arb: urlArb },
+  'excalidraw.src': { noDefault: true, arb: urlArb },
+  'attachment.url': { noDefault: true, arb: urlArb },
+  'attachment.name': { arb: phraseArb },
+  // ── callout / status ─────────────────────────────────────────────────────
+  'callout.type': { always: true, arb: str('success', 'warning', 'danger') },
+  'callout.icon': { frozen: true }, // ACCEPTED: no md representation (dropped on export)
+  'status.text': { noDefault: true, arb: phraseArb, degen: '' },
+  'status.color': { always: true, arb: str('green', 'orange', 'red', 'blue', 'yellow', 'purple') },
+  // ── table cells ────────────────────────────────────────────────────────────
+  // DEFERRED-BUG (not ACCEPTED): colspan/rowspan ARE representable and round-trip
+  // — a spanned cell makes the converter emit the whole table as a raw <table>
+  // with colspan/rowspan attrs (markdown-converter.ts tableToHtml), which the
+  // tiptap parser reads back. They are frozen only because generating a
+  // geometrically-valid spanned table is deferred STRUCTURAL work (the flat
+  // generator hardcodes colspan/rowspan = 1), NOT a markdown limitation.
+  'tableCell.colspan': { always: true, frozen: true },
+  'tableCell.rowspan': { always: true, frozen: true },
+  // ACCEPTED: colwidth / backgroundColor(Name) have no representation — the
+  // raw-<table> fallback (tableToHtml) drops them, so there is nothing to preserve.
+  'tableCell.colwidth': { frozen: true },
+  'tableCell.backgroundColor': { frozen: true },
+  'tableCell.backgroundColorName': { frozen: true },
+  'tableCell.align': { arb: str('left', 'center', 'right') },
+  'tableHeader.colspan': { always: true, frozen: true }, // DEFERRED-BUG (see tableCell.colspan)
+  'tableHeader.rowspan': { always: true, frozen: true }, // DEFERRED-BUG (see tableCell.rowspan)
+  'tableHeader.colwidth': { frozen: true }, // ACCEPTED: no representation
+  'tableHeader.backgroundColor': { frozen: true }, // ACCEPTED: no representation
+  'tableHeader.backgroundColorName': { frozen: true }, // ACCEPTED: no representation
+  'tableHeader.align': { arb: str('left', 'center', 'right') },
+  // ── details ──────────────────────────────────────────────────────────────
+  'details.open': { always: true, arb: fc.constant(true) }, // boolean, default false
+  // ── columns ──────────────────────────────────────────────────────────────
+  'columns.layout': { always: true, arb: str('three_equal', 'left_sidebar', 'right_sidebar') },
+  // widthMode round-trips via the `data-width-mode` attribute (verified P1+P2),
+  // so it is fuzzed, not frozen.
+  'columns.widthMode': { always: true, arb: str('custom') },
+  // PINNED-BUG: parseFloat import drops the `%` unit -> P2 churn. See
+  // counterexamples.test.ts (columns-column-width-percent.json).
+  'column.width': { frozen: true },
+  // ── embed (schema keeps width/height NUMERIC, not string-coerced) ─────────
+  'embed.src': { noDefault: true, arb: urlArb, degen: '' },
+  'embed.provider': { noDefault: true, arb: str('iframe', 'youtube', 'vimeo') },
+  'embed.width': { always: true, frozen: true },
+  'embed.height': { always: true, frozen: true },
+  // ── subpages / math / htmlEmbed ──────────────────────────────────────────
+  'subpages.recursive': { always: true, arb: fc.constant(true) }, // boolean, default false
+  'mathBlock.text': { noDefault: true, arb: str('x^2', 'a < b', '\\frac{1}{2}'), degen: '' },
+  'mathInline.text': { noDefault: true, arb: str('x^2', 'a < b', '\\frac{1}{2}'), degen: '' },
+  'htmlEmbed.source': { noDefault: true, arb: str('<b>hi</b>', '<i>x</i>', '<span>y</span>'), degen: '' },
+  'htmlEmbed.height': { arb: num(200, 300, 400) },
+  // ── footnotes / transclusion / pageEmbed / mention ───────────────────────
+  'footnoteDefinition.id': { noDefault: true, arb: str('fn1', 'fn2', 'note') },
+  'footnoteReference.id': { noDefault: true, arb: str('fn1', 'fn2', 'note') },
+  'pageEmbed.sourcePageId': { noDefault: true, arb: fc.uuid() },
+  'transclusionSource.id': { noDefault: true, arb: str('src1', 'src2') },
+  'transclusionReference.sourcePageId': { noDefault: true, arb: fc.uuid() },
+  'transclusionReference.transclusionId': { noDefault: true, arb: str('tr1', 'tr2') },
+  'mention.id': { noDefault: true, arb: fc.uuid() },
+  'mention.label': { noDefault: true, arb: phraseArb },
+  'mention.entityType': { noDefault: true, arb: str('user') },
+  'mention.entityId': { noDefault: true, arb: fc.uuid() },
+};
+
+/** Resolve the effective policy for one attribute (override merged over generic). */
+function policyFor(type: string, attr: string, def: unknown): AttrPolicy {
+  const override = OVERRIDES[`${type}.${attr}`];
+  if (override) return override;
+  // Generic: booleans are fuzzable via their flipped value; everything else is
+  // frozen (only absent/default) so no unverified non-default is invented.
+  if (typeof def === 'boolean') return { arb: fc.constant(!def) };
+  return { frozen: true };
+}
+
+/**
+ * Whether an attribute is actually exercised at a NON-DEFAULT value (i.e. its
+ * policy has an `arb`, which the generic fallback does not). Used by the
+ * attribute-coverage snapshot test to make the generic-frozen space VISIBLE: any
+ * string/number attr not in OVERRIDES is silently only tested at absent/default,
+ * so the snapshot pins exactly which attrs are NOT value-fuzzed and forces a
+ * reviewer to look when a new attr lands in that invisible bucket.
+ */
+export function attrIsValueFuzzed(type: string, attr: string): boolean {
+  const def = schemaAttrDefaults(type)[attr];
+  return !!policyFor(type, attr, def).arb;
+}
+
+/** Every node `type.attr` in the schema (excluding the auto `id`), sorted. */
+export function allSchemaAttrKeys(): string[] {
+  const keys: string[] = [];
+  for (const type of Object.keys(schema.nodes)) {
+    for (const attr of schemaAttrNames(type)) {
+      if (attr === 'id') continue;
+      keys.push(`${type}.${attr}`);
+    }
+  }
+  return keys.sort();
+}
+
+/**
+ * Every MARK attribute in the schema, keyed `mark:<name>.<attr>`, sorted. Marks
+ * are not driven by the node OVERRIDES table (they are fuzzed by the text
+ * generator, text-arbitraries.ts), so their value-fuzz coverage is tracked with a
+ * separate snapshot (see flat-roundtrip.property.test.ts) — without this the
+ * "no invisible coverage hole" guarantee would hold for node attrs only, letting a
+ * new mark attr slip through unfuzzed and unallowlisted.
+ */
+export function allSchemaMarkAttrKeys(): string[] {
+  const keys: string[] = [];
+  for (const [name, mark] of Object.entries(schema.marks)) {
+    const attrs = (mark.spec?.attrs ?? {}) as Record<string, unknown>;
+    for (const attr of Object.keys(attrs)) keys.push(`mark:${name}.${attr}`);
+  }
+  return keys.sort();
+}
+
+export type AttrMode = 'p1' | 'fuzz';
+
+/**
+ * Build an arbitrary for ONE attribute's value (or the ABSENT sentinel) across
+ * the states legal for `mode`:
+ *   - p1   : absent / default / nonDefault (the round-trip-safe space).
+ *   - fuzz : the above PLUS degenerate (P2 tolerates the one-time
+ *            normalization; P3 only needs totality).
+ */
+export function attrValueArb(
+  type: string,
+  attr: string,
+  mode: AttrMode,
+): fc.Arbitrary<unknown | typeof ABSENT> {
+  const def = schemaAttrDefaults(type)[attr];
+  const p = policyFor(type, attr, def);
+
+  const states: fc.Arbitrary<unknown | typeof ABSENT>[] = [];
+  if (!p.always && !p.noDefault) states.push(fc.constant(ABSENT));
+  if (!p.noDefault) states.push(fc.constant(def));
+  if (!p.frozen && p.arb) states.push(p.arb);
+  if (mode === 'fuzz' && !p.frozen && p.degen !== undefined) {
+    states.push(fc.constant(p.degen));
+  }
+  if (states.length === 0) states.push(fc.constant(def));
+  return fc.oneof(...states);
+}
+
+/**
+ * Build an arbitrary for a node's full `attrs` object over all schema attrs.
+ * `base` pins caller-required attrs (e.g. a concrete `src`) verbatim; any attr
+ * present in `base` is NOT re-generated. Omitted (ABSENT) attrs are dropped.
+ */
+export function nodeAttrsArb(
+  type: string,
+  mode: AttrMode,
+  base: Record<string, unknown> = {},
+): fc.Arbitrary<Record<string, unknown>> {
+  const names = schemaAttrNames(type).filter((n) => !(n in base) && n !== 'id');
+  if (names.length === 0) return fc.constant({ ...base });
+  return fc
+    .tuple(...names.map((n) => attrValueArb(type, n, mode)))
+    .map((vals) => {
+      const attrs: Record<string, unknown> = { ...base };
+      names.forEach((n, i) => {
+        if (vals[i] !== ABSENT) attrs[n] = vals[i];
+      });
+      return attrs;
+    });
+}
diff --git a/packages/prosemirror-markdown/test/generative/counterexamples.test.ts b/packages/prosemirror-markdown/test/generative/counterexamples.test.ts
new file mode 100644
index 00000000..4d6afd40
--- /dev/null
+++ b/packages/prosemirror-markdown/test/generative/counterexamples.test.ts
@@ -0,0 +1,74 @@
+import { describe, expect, it } from 'vitest';
+import { readFileSync } from 'node:fs';
+import { fileURLToPath } from 'node:url';
+import path from 'node:path';
+import { convertProseMirrorToMarkdown } from '../../src/lib/markdown-converter.js';
+import { markdownToProseMirror } from '../../src/lib/markdown-to-prosemirror.js';
+import { docsCanonicallyEqual } from '../../src/lib/canonicalize.js';
+
+// ---------------------------------------------------------------------------
+// #351 committed counterexamples — REAL round-trip bugs surfaced by the flat
+// generative probing (attribute level). Each is pinned here as an `it.fails`
+// (vitest passes ONLY WHILE the assertion still fails), so that the day the
+// underlying src/ bug is fixed, the `it.fails` starts PASSING and vitest turns
+// this test RED — forcing us to delete the counterexample and (per the epic
+// guardrail) tighten the generator. A bare `it.fails` would ship silent
+// corruption, so every case below carries a loud `// BUG #351:` explanation.
+//
+// These bugs are NOT worked around by weakening any property: the offending
+// attribute is kept OUT of the P1/P2 generators (documented in
+// attr-arbitraries.ts), and the exact failing document lives here as the
+// regression pin. FIXING the bug is a separate, maintainer-approved src/ change.
+// ---------------------------------------------------------------------------
+
+const here = path.dirname(fileURLToPath(import.meta.url));
+const fixtureDir = path.resolve(here, '../fixtures/counterexamples');
+
+function loadDoc(file: string): any {
+  return JSON.parse(readFileSync(path.join(fixtureDir, file), 'utf8')).doc;
+}
+
+describe('#351 counterexamples (known round-trip bugs, pinned as it.fails)', () => {
+  // BUG #351: a `column` with a PERCENTAGE width ("50%") is not byte-stable.
+  // The column schema parses `data-width` with parseFloat, dropping the '%':
+  //   md1 = '...data-width="50%"...'   (first export)
+  //   re-import stores width = 50 (number)
+  //   md2 = '...data-width="50"...'    (second export)  => md2 !== md1
+  // A permanent GS-EDIT-REVERT churn on every git-sync pull. The editor stores
+  // column widths as percentages, so this is a genuine defect. The fix is in
+  // src/lib/docmost-schema.ts (column.width parseHTML must preserve the unit)
+  // and is out of scope for this test-only PR.
+  it.fails('column percentage width is byte-stable (P2)', async () => {
+    const doc = loadDoc('columns-column-width-percent.json');
+    const md1 = convertProseMirrorToMarkdown(doc);
+    const doc2 = await markdownToProseMirror(md1);
+    const md2 = convertProseMirrorToMarkdown(doc2);
+    // This assertion currently FAILS (md2 drops the '%'), which is exactly what
+    // `it.fails` expects. When the schema is fixed, it will PASS and flip this
+    // test red — our cue to remove the pin.
+    expect(md2).toBe(md1);
+  });
+
+  // BUG #351: an `orderedList` with a non-1 `start` loses its start number.
+  // CommonMark CAN express this ("5." starts the list at 5), but the converter
+  // always emits "1." and ignores `attrs.start` (markdown-converter.ts renders
+  // `${index + 1}.`; the <ol> HTML path also omits `start`):
+  //   doc.start = 5   ->   md1 = "1. alpha"   (start dropped on export)
+  //   re-import stores start = 1   =>   docsCanonicallyEqual(rt, doc) === false
+  // This is a P1 (semantic round-trip) loss of the SAME class as column.width:
+  // representable in markdown, silently dropped by the converter. It is pinned
+  // here as the LOUD counterexample rather than being masked as an "accepted
+  // normalization" in the generator — per the epic guardrail, deciding
+  // accept-vs-fix for a markdown-representable loss is a MAINTAINER call, so this
+  // stays a visible known-bug until the maintainer rules on it. The fix would be
+  // in src/lib/markdown-converter.ts (emit the start number on the first item)
+  // and is out of scope for this test-only PR.
+  it.fails('ordered list start number is preserved (P1)', async () => {
+    const doc = loadDoc('ordered-list-start.json');
+    const md1 = convertProseMirrorToMarkdown(doc);
+    const doc2 = await markdownToProseMirror(md1);
+    // Currently FAILS: doc2.start === 1 while doc.start === 5. When the converter
+    // preserves `start`, this PASSES and flips the test red — remove the pin then.
+    expect(docsCanonicallyEqual(doc2, doc)).toBe(true);
+  });
+});
diff --git a/packages/prosemirror-markdown/test/generative/flat-roundtrip.property.test.ts b/packages/prosemirror-markdown/test/generative/flat-roundtrip.property.test.ts
new file mode 100644
index 00000000..46621537
--- /dev/null
+++ b/packages/prosemirror-markdown/test/generative/flat-roundtrip.property.test.ts
@@ -0,0 +1,284 @@
+import { describe, expect, it, vi } from 'vitest';
+import fc from 'fast-check';
+// Real converter, imported the same way the sibling property test does.
+import { convertProseMirrorToMarkdown } from '../../src/lib/markdown-converter.js';
+// Importing markdownToProseMirror mutates the global DOM via jsdom at module
+// load (expected, required for @tiptap/html's generateJSON under Node).
+import { markdownToProseMirror } from '../../src/lib/markdown-to-prosemirror.js';
+import { docsCanonicallyEqual, canonicalizeContent } from '../../src/lib/index.js';
+import { firstDivergence } from '../roundtrip-helpers.js';
+import {
+  schema,
+  allSchemaAttrKeys,
+  allSchemaMarkAttrKeys,
+  attrIsValueFuzzed,
+} from './attr-arbitraries.js';
+import {
+  buildGenerators,
+  coveredTypes,
+  KNOWN_UNCOVERED,
+} from './node-generators.js';
+
+// ── Attribute-value coverage allowlist ──────────────────────────────────────
+// The node/mark completeness contract guarantees every TYPE is generated, but
+// NOT that every attribute is exercised at a NON-DEFAULT value. An attribute
+// with no `arb` in attr-arbitraries.ts is only ever tested at absent/default —
+// an INVISIBLE coverage hole (the reviewer's concern). This allowlist makes that
+// hole EXPLICIT: it is the exact set of attrs deliberately not value-fuzzed, so
+// a NEW attribute (or a newly-frozen one) that lands in this bucket flips the
+// snapshot test red and forces a reviewer to classify it. Each belongs to one of:
+//   - internal/opaque ids & placeholders (attachmentId, slugId, placeholder,
+//     creatorId, anchorId) — no meaningful non-default to assert;
+//   - dimensions/among the media family with no standalone md form here
+//     (aspectRatio, size, caption, drawio/excalidraw/pdf/video/youtube w/h/align)
+//     — round-trip candidates deferred to a later PR, not silently dropped;
+//   - ACCEPTED limitations with no md representation (indent, callout.icon,
+//     orderedList.type, table spans/bg/colwidth);
+//   - PINNED bugs (column.width, orderedList.start) tracked in
+//     counterexamples.test.ts.
+const ATTR_VALUE_FUZZ_ALLOWLIST = new Set<string>([
+  'attachment.attachmentId', 'attachment.mime', 'attachment.placeholder', 'attachment.size',
+  'audio.attachmentId', 'audio.placeholder', 'audio.size',
+  'callout.icon', 'column.width',
+  'drawio.align', 'drawio.alt', 'drawio.aspectRatio', 'drawio.attachmentId',
+  'drawio.height', 'drawio.size', 'drawio.title', 'drawio.width',
+  'embed.align', 'embed.height', 'embed.width',
+  'excalidraw.align', 'excalidraw.alt', 'excalidraw.aspectRatio', 'excalidraw.attachmentId',
+  'excalidraw.height', 'excalidraw.size', 'excalidraw.title', 'excalidraw.width',
+  'heading.indent',
+  'image.aspectRatio', 'image.attachmentId', 'image.caption', 'image.placeholder', 'image.size',
+  'mention.anchorId', 'mention.creatorId', 'mention.slugId',
+  'orderedList.start', 'orderedList.type', 'paragraph.indent',
+  'pdf.attachmentId', 'pdf.height', 'pdf.placeholder', 'pdf.size', 'pdf.width',
+  'tableCell.backgroundColor', 'tableCell.backgroundColorName', 'tableCell.colspan',
+  'tableCell.colwidth', 'tableCell.rowspan',
+  'tableHeader.backgroundColor', 'tableHeader.backgroundColorName', 'tableHeader.colspan',
+  'tableHeader.colwidth', 'tableHeader.rowspan',
+  'video.align', 'video.aspectRatio', 'video.attachmentId', 'video.placeholder', 'video.size',
+  'youtube.align', 'youtube.height', 'youtube.width',
+]);
+
+// ── MARK attribute-value coverage ───────────────────────────────────────────
+// Marks are fuzzed by the text generator (text-arbitraries.ts markedTextRunArb),
+// not the node OVERRIDES table, so their value-fuzz coverage is tracked with this
+// separate registry — otherwise the "no invisible coverage hole" guarantee would
+// hold for node attrs only, and a new mark attr (or a new attributed mark) would
+// silently escape the fuzz set. Every schema mark attr must be in exactly one of:
+//   MARK_ATTR_FUZZED    — actually driven at a non-default value by the generator;
+//   MARK_ATTR_ALLOWLIST — deliberately not value-fuzzed, with a reason.
+const MARK_ATTR_FUZZED = new Set<string>([
+  'mark:link.href', // markedTextRunArb sets a random webUrl href
+  'mark:link.title', // ...and an optional letter-bearing title
+  'mark:highlight.color', // highlight mark carries a generated color
+  'mark:textStyle.color', // textStyle mark carries a generated color
+  'mark:comment.commentId', // comment anchor id (alphanumeric token)
+  'mark:comment.resolved', // comment resolved flag (rides only when true)
+]);
+const MARK_ATTR_ALLOWLIST = new Set<string>([
+  // link presentational/routing attrs: not part of the markdown link surface the
+  // converter emits (it round-trips href + title only), so there is no
+  // non-default value to assert here — a deferred concern for a link-specific
+  // fixture, not the flat generative pass.
+  'mark:link.internal',
+  'mark:link.target',
+  'mark:link.rel',
+  'mark:link.class',
+]);
+
+// Each run does a real convert + marked + jsdom parse (~ms). Give ample headroom
+// so the suite is deterministic regardless of parallel worker load (like the
+// sibling property file).
+vi.setConfig({ testTimeout: 30000 });
+
+// ---------------------------------------------------------------------------
+// #351 PR 1 — GENERATIVE (property-based) round-trip over FLAT (single-node)
+// documents at the ATTRIBUTE level.
+//
+// We assert three invariants for ANY generated valid flat document `d`
+// (pmToMd = convertProseMirrorToMarkdown, mdToPm = markdownToProseMirror):
+//
+//   P1 — semantic round-trip (nothing lost):
+//        docsCanonicallyEqual(await mdToPm(pmToMd(d)), d) === true
+//   P2 — byte fixpoint (anti "GS-EDIT-REVERT" churn):
+//        pmToMd(await mdToPm(pmToMd(d))) === pmToMd(d)
+//   P3 — totality: neither converter throws; bounded.
+//
+// The generators are schema-DERIVED (attribute lists come from
+// schema.nodes[type].spec.attrs) and stay inside the round-trip-supported space
+// proven empirically by probing the live converter (see attr-arbitraries.ts and
+// text-arbitraries.ts). P1 runs over the safe attribute space; P2/P3 run over
+// the wider 'fuzz' space that also injects degenerate attribute states, which
+// P2 tolerates via a one-time first-pass normalization and P3 via totality only.
+// ---------------------------------------------------------------------------
+
+// Fixed seed so every failure is reproducible; fast-check also prints the
+// shrunk counterexample. numRuns starts modest to keep CI under budget — the
+// issue's CI target is ~300-500 per property; the nightly / PR 3 will crank
+// this up further. Each property runs over the UNION (fc.oneof) of all flat
+// node generators, so the runs are shared across node types (one test per
+// property keeps the jsdom import cost and memory bounded — a per-generator ×
+// per-property matrix is ~200 heavy tests that OOMs the worker).
+const SEED = 20250705;
+const NUM_RUNS = 300;
+
+const P1_GENERATORS = buildGenerators('p1');
+const FUZZ_GENERATORS = buildGenerators('fuzz');
+
+// Union arbitraries: a single draw picks one node generator, then a document
+// from it. On failure fast-check prints the shrunk counterexample doc, which
+// names the offending node type directly.
+const p1Union = fc.oneof(...P1_GENERATORS.map((g) => g.arb));
+const fuzzUnion = fc.oneof(...FUZZ_GENERATORS.map((g) => g.arb));
+
+async function roundTrip(doc: unknown): Promise<{ md1: string; md2: string; doc2: any }> {
+  const md1 = convertProseMirrorToMarkdown(doc);
+  const doc2 = await markdownToProseMirror(md1);
+  const md2 = convertProseMirrorToMarkdown(doc2);
+  return { md1, md2, doc2 };
+}
+
+describe('#351 flat generative round-trip — completeness contract', () => {
+  it('every schema node and mark is covered by a generator or explicitly allowlisted', () => {
+    const covered = coveredTypes();
+    const uncovered: string[] = [];
+
+    for (const nodeType of Object.keys(schema.nodes)) {
+      if (covered.has(nodeType)) continue;
+      if (nodeType in KNOWN_UNCOVERED) continue;
+      uncovered.push(`node:${nodeType}`);
+    }
+    for (const markType of Object.keys(schema.marks)) {
+      if (covered.has(`mark:${markType}`)) continue;
+      if (markType in KNOWN_UNCOVERED) continue;
+      uncovered.push(`mark:${markType}`);
+    }
+
+    // A new node/mark added to the schema with no generator AND no allowlist
+    // entry MUST turn this test red — that is the whole point (no silent blind
+    // spots).
+    expect(
+      uncovered,
+      `these schema types have no generator and no KNOWN_UNCOVERED reason:\n  ${uncovered.join(
+        '\n  ',
+      )}`,
+    ).toEqual([]);
+  });
+
+  it('every KNOWN_UNCOVERED entry is a real schema type (no stale allowlist rows)', () => {
+    const all = new Set([...Object.keys(schema.nodes), ...Object.keys(schema.marks)]);
+    for (const t of Object.keys(KNOWN_UNCOVERED)) {
+      expect(all.has(t), `stale KNOWN_UNCOVERED entry: ${t}`).toBe(true);
+    }
+  });
+
+  it('every attribute is value-fuzzed OR explicitly allowlisted (no invisible hole)', () => {
+    // Makes the "generic-frozen" coverage hole VISIBLE: any schema attr not
+    // exercised at a non-default value must be a KNOWN entry in the allowlist.
+    // A new attr (or one that loses its `arb`) that falls into the not-fuzzed
+    // bucket without an allowlist row turns this red — no silent blind spots.
+    const unaccounted: string[] = [];
+    for (const key of allSchemaAttrKeys()) {
+      const i = key.indexOf('.');
+      const fuzzed = attrIsValueFuzzed(key.slice(0, i), key.slice(i + 1));
+      if (!fuzzed && !ATTR_VALUE_FUZZ_ALLOWLIST.has(key)) unaccounted.push(key);
+    }
+    expect(
+      unaccounted,
+      `these attrs are not value-fuzzed and not in ATTR_VALUE_FUZZ_ALLOWLIST:\n  ${unaccounted.join(
+        '\n  ',
+      )}`,
+    ).toEqual([]);
+  });
+
+  it('the attribute allowlist has no stale rows (every entry is really not-fuzzed)', () => {
+    const notFuzzed = new Set(
+      allSchemaAttrKeys().filter((key) => {
+        const i = key.indexOf('.');
+        return !attrIsValueFuzzed(key.slice(0, i), key.slice(i + 1));
+      }),
+    );
+    for (const key of ATTR_VALUE_FUZZ_ALLOWLIST) {
+      expect(
+        notFuzzed.has(key),
+        `stale allowlist row (attr is now value-fuzzed, remove it): ${key}`,
+      ).toBe(true);
+    }
+  });
+
+  it('every MARK attribute is value-fuzzed OR allowlisted (no invisible hole)', () => {
+    // The node guard above covers node attrs; marks are fuzzed by the text
+    // generator, so their coverage is tracked separately. A new mark attr (or a
+    // newly-attributed mark) that lands in neither set turns this red.
+    const unaccounted: string[] = [];
+    for (const key of allSchemaMarkAttrKeys()) {
+      if (!MARK_ATTR_FUZZED.has(key) && !MARK_ATTR_ALLOWLIST.has(key)) {
+        unaccounted.push(key);
+      }
+    }
+    expect(
+      unaccounted,
+      `these mark attrs are neither in MARK_ATTR_FUZZED nor MARK_ATTR_ALLOWLIST:\n  ${unaccounted.join(
+        '\n  ',
+      )}`,
+    ).toEqual([]);
+  });
+
+  it('the MARK fuzz/allowlist sets have no stale rows (every entry is a real schema mark attr)', () => {
+    const all = new Set(allSchemaMarkAttrKeys());
+    for (const key of [...MARK_ATTR_FUZZED, ...MARK_ATTR_ALLOWLIST]) {
+      expect(all.has(key), `stale mark-attr registry row: ${key}`).toBe(true);
+    }
+  });
+});
+
+describe('#351 flat generative round-trip — properties', () => {
+  it('generator validity: every generated doc passes schema.check()', () => {
+    // A generator that emits an invalid ProseMirror document is a GENERATOR bug.
+    fc.assert(
+      fc.property(fuzzUnion, (doc) => {
+        schema.nodeFromJSON(doc).check(); // throws on an invalid doc
+        return true;
+      }),
+      { numRuns: NUM_RUNS, seed: SEED },
+    );
+  });
+
+  it('P1 — semantic round-trip: docsCanonicallyEqual(mdToPm(pmToMd(d)), d)', async () => {
+    await fc.assert(
+      fc.asyncProperty(p1Union, async (doc) => {
+        const { doc2 } = await roundTrip(doc);
+        if (!docsCanonicallyEqual(doc2, doc)) {
+          // Surface the precise divergence in the failure message.
+          const div = firstDivergence(
+            JSON.parse(JSON.stringify(canonicalizeContent(doc2))),
+            JSON.parse(JSON.stringify(canonicalizeContent(doc))),
+          );
+          throw new Error(
+            `P1 divergence @ ${div?.path}: got=${JSON.stringify(div?.a)} want=${JSON.stringify(div?.b)}`,
+          );
+        }
+      }),
+      { numRuns: NUM_RUNS, seed: SEED },
+    );
+  });
+
+  it('P2 — byte fixpoint: pmToMd(mdToPm(pmToMd(d))) === pmToMd(d)', async () => {
+    await fc.assert(
+      fc.asyncProperty(fuzzUnion, async (doc) => {
+        const { md1, md2 } = await roundTrip(doc);
+        expect(md2).toBe(md1);
+      }),
+      { numRuns: NUM_RUNS, seed: SEED },
+    );
+  });
+
+  it('P3 — totality: neither converter throws', async () => {
+    await fc.assert(
+      fc.asyncProperty(fuzzUnion, async (doc) => {
+        // Throwing here fails the property; fast-check shrinks to a minimal doc.
+        await roundTrip(doc);
+      }),
+      { numRuns: NUM_RUNS, seed: SEED },
+    );
+  });
+});
diff --git a/packages/prosemirror-markdown/test/generative/node-generators.ts b/packages/prosemirror-markdown/test/generative/node-generators.ts
new file mode 100644
index 00000000..f3d72881
--- /dev/null
+++ b/packages/prosemirror-markdown/test/generative/node-generators.ts
@@ -0,0 +1,310 @@
+/**
+ * Flat single-node document generators (#351, PR 1).
+ *
+ * For every schema node type that can stand alone, a fast-check arbitrary
+ * producing `{ type:'doc', content:[ <the target node> ] }` with generated attrs
+ * (via nodeAttrsArb) and the minimal REQUIRED immediate children the schema
+ * demands (a heading's inline text, a listItem's one paragraph, a table's
+ * minimal rows, details' summary+content, a callout's one paragraph). Kept
+ * FLAT: a single target node, no deep nesting — nested structural generation is
+ * PR 2.
+ *
+ * The `mode` threads through to the attribute arbitraries:
+ *   - 'p1'   : the round-trip-safe attribute space (P1 semantic round-trip).
+ *   - 'fuzz' : adds degenerate attribute states (P2 byte-fixpoint tolerates the
+ *              one-time normalization; P3 only needs totality).
+ *
+ * A COMPLETENESS CONTRACT (see flat-roundtrip.property.test.ts) enumerates the
+ * whole schema and asserts every node/mark is EITHER produced by a generator
+ * here OR listed in KNOWN_UNCOVERED with a reason — so a new schema type with no
+ * generator turns the suite RED.
+ */
+import fc from 'fast-check';
+import { type AttrMode, nodeAttrsArb } from './attr-arbitraries.js';
+import {
+  inlineContentArb,
+  headingInlineContentArb,
+  plainInlineContentArb,
+  phraseArb,
+  markedTextRunArb,
+} from './text-arbitraries.js';
+
+const doc = (node: any) => ({ type: 'doc', content: [node] });
+const para = (content: any[]) => ({ type: 'paragraph', content });
+
+/** A named flat-document generator. */
+export interface NamedGen {
+  name: string;
+  arb: fc.Arbitrary<any>;
+}
+
+// ---------------------------------------------------------------------------
+// Per-target generators, each a function of mode.
+// ---------------------------------------------------------------------------
+
+const gen = {
+  paragraph: (m: AttrMode) =>
+    fc.tuple(nodeAttrsArb('paragraph', m), inlineContentArb).map(([attrs, content]) =>
+      doc({ type: 'paragraph', attrs, content }),
+    ),
+
+  heading: (m: AttrMode) =>
+    fc.tuple(nodeAttrsArb('heading', m), headingInlineContentArb).map(([attrs, content]) =>
+      doc({ type: 'heading', attrs, content }),
+    ),
+
+  blockquote: (_m: AttrMode) =>
+    inlineContentArb.map((content) => doc({ type: 'blockquote', content: [para(content)] })),
+
+  bulletList: (_m: AttrMode) =>
+    fc
+      .array(inlineContentArb, { minLength: 1, maxLength: 3 })
+      .map((items) =>
+        doc({
+          type: 'bulletList',
+          content: items.map((c) => ({ type: 'listItem', content: [para(c)] })),
+        }),
+      ),
+
+  orderedList: (m: AttrMode) =>
+    fc
+      .tuple(nodeAttrsArb('orderedList', m), fc.array(inlineContentArb, { minLength: 1, maxLength: 3 }))
+      .map(([attrs, items]) =>
+        doc({
+          type: 'orderedList',
+          attrs,
+          content: items.map((c) => ({ type: 'listItem', content: [para(c)] })),
+        }),
+      ),
+
+  taskList: (m: AttrMode) =>
+    fc
+      .array(fc.tuple(nodeAttrsArb('taskItem', m), inlineContentArb), { minLength: 1, maxLength: 3 })
+      .map((items) =>
+        doc({
+          type: 'taskList',
+          content: items.map(([attrs, c]) => ({ type: 'taskItem', attrs, content: [para(c)] })),
+        }),
+      ),
+
+  codeBlock: (m: AttrMode) =>
+    fc
+      .tuple(
+        nodeAttrsArb('codeBlock', m),
+        // A fenced code block always re-imports with a TRAILING NEWLINE in its
+        // text (empirically confirmed). Author the newline so the doc is already
+        // at the round-trip fixpoint (supported-space shaping, not a masked bug).
+        fc.array(phraseArb, { minLength: 1, maxLength: 3 }).map((lines) => lines.join('\n') + '\n'),
+      )
+      .map(([attrs, code]) =>
+        doc({ type: 'codeBlock', attrs, content: [{ type: 'text', text: code }] }),
+      ),
+
+  horizontalRule: (_m: AttrMode) => fc.constant(doc({ type: 'horizontalRule' })),
+
+  pageBreak: (_m: AttrMode) => fc.constant(doc({ type: 'pageBreak' })),
+
+  image: (m: AttrMode) => nodeAttrsArb('image', m).map((attrs) => doc({ type: 'image', attrs })),
+
+  callout: (m: AttrMode) =>
+    fc.tuple(nodeAttrsArb('callout', m), inlineContentArb).map(([attrs, content]) =>
+      doc({ type: 'callout', attrs, content: [para(content)] }),
+    ),
+
+  mathBlock: (m: AttrMode) =>
+    nodeAttrsArb('mathBlock', m).map((attrs) => doc({ type: 'mathBlock', attrs })),
+
+  details: (m: AttrMode) =>
+    fc
+      .tuple(nodeAttrsArb('details', m), plainInlineContentArb, inlineContentArb)
+      .map(([attrs, summary, body]) =>
+        doc({
+          type: 'details',
+          attrs,
+          content: [
+            { type: 'detailsSummary', content: summary },
+            { type: 'detailsContent', content: [para(body)] },
+          ],
+        }),
+      ),
+
+  table: (_m: AttrMode) =>
+    fc.integer({ min: 1, max: 3 }).chain((cols) => {
+      // GFM alignment is column-wide (encoded in the header separator), so a
+      // column's alignment must be identical on the header and every body cell,
+      // else the second export re-aligns and churns. Pick ONE align per column.
+      const alignsArb = fc.array(fc.constantFrom(undefined, 'left', 'center', 'right'), {
+        minLength: cols,
+        maxLength: cols,
+      });
+      const cell = (header: boolean, align?: string) =>
+        phraseArb.map((t) => ({
+          type: header ? 'tableHeader' : 'tableCell',
+          // colspan/rowspan pinned to 1 (GFM cannot express spans); optional
+          // column-consistent align.
+          attrs: { colspan: 1, rowspan: 1, ...(align ? { align } : {}) },
+          content: [para([{ type: 'text', text: t }])],
+        }));
+      return alignsArb.chain((aligns) => {
+        const headerRow = fc
+          .tuple(...aligns.map((a) => cell(true, a)))
+          .map((cells) => ({ type: 'tableRow', content: cells }));
+        const bodyRow = fc
+          .tuple(...aligns.map((a) => cell(false, a)))
+          .map((cells) => ({ type: 'tableRow', content: cells }));
+        return fc
+          .tuple(headerRow, fc.array(bodyRow, { minLength: 1, maxLength: 2 }))
+          .map(([h, body]) => doc({ type: 'table', content: [h, ...body] }));
+      });
+    }),
+
+  columns: (m: AttrMode) =>
+    // Couple the column count to the layout so the two stay consistent
+    // (two_equal/left_sidebar/right_sidebar -> 2, three_equal -> 3).
+    fc
+      .constantFrom('two_equal', 'three_equal', 'left_sidebar', 'right_sidebar')
+      .chain((layout) => {
+        const count = layout === 'three_equal' ? 3 : 2;
+        return fc
+          .tuple(
+            nodeAttrsArb('columns', m, { layout, widthMode: 'normal' }),
+            fc.array(inlineContentArb, { minLength: count, maxLength: count }),
+          )
+          .map(([attrs, bodies]) =>
+            doc({
+              type: 'columns',
+              attrs,
+              content: bodies.map((c) => ({ type: 'column', content: [para(c)] })),
+            }),
+          );
+      }),
+
+  subpages: (m: AttrMode) =>
+    nodeAttrsArb('subpages', m).map((attrs) => doc({ type: 'subpages', attrs })),
+
+  audio: (m: AttrMode) => nodeAttrsArb('audio', m).map((attrs) => doc({ type: 'audio', attrs })),
+  video: (m: AttrMode) => nodeAttrsArb('video', m).map((attrs) => doc({ type: 'video', attrs })),
+  pdf: (m: AttrMode) => nodeAttrsArb('pdf', m).map((attrs) => doc({ type: 'pdf', attrs })),
+  youtube: (m: AttrMode) => nodeAttrsArb('youtube', m).map((attrs) => doc({ type: 'youtube', attrs })),
+  embed: (m: AttrMode) => nodeAttrsArb('embed', m).map((attrs) => doc({ type: 'embed', attrs })),
+  drawio: (m: AttrMode) => nodeAttrsArb('drawio', m).map((attrs) => doc({ type: 'drawio', attrs })),
+  excalidraw: (m: AttrMode) =>
+    nodeAttrsArb('excalidraw', m).map((attrs) => doc({ type: 'excalidraw', attrs })),
+  attachment: (m: AttrMode) =>
+    nodeAttrsArb('attachment', m).map((attrs) => doc({ type: 'attachment', attrs })),
+  htmlEmbed: (m: AttrMode) =>
+    nodeAttrsArb('htmlEmbed', m).map((attrs) => doc({ type: 'htmlEmbed', attrs })),
+  pageEmbed: (m: AttrMode) =>
+    nodeAttrsArb('pageEmbed', m).map((attrs) => doc({ type: 'pageEmbed', attrs })),
+  transclusionReference: (m: AttrMode) =>
+    nodeAttrsArb('transclusionReference', m).map((attrs) =>
+      doc({ type: 'transclusionReference', attrs }),
+    ),
+
+  transclusionSource: (m: AttrMode) =>
+    fc.tuple(nodeAttrsArb('transclusionSource', m), inlineContentArb).map(([attrs, content]) =>
+      doc({ type: 'transclusionSource', attrs, content: [para(content)] }),
+    ),
+
+  // A footnote reference PLUS its definition (the reference has no standalone
+  // markdown form without its definition — see KNOWN_UNCOVERED note for the
+  // bare reference). Both carry the same id. The definition body uses
+  // headingInlineContentArb (NO hard breaks): a footnote is serialized inline as
+  // `^[...]`, so a hard break inside it collapses to a single space on re-parse
+  // (empirically confirmed) — that is the container's markdown limitation, not
+  // an attribute-level concern. The reference-bearing paragraph is a NORMAL
+  // paragraph and keeps the full inline corpus.
+  footnotes: (m: AttrMode) =>
+    fc.tuple(fc.constantFrom('fn1', 'fn2', 'note'), inlineContentArb, headingInlineContentArb).map(
+      ([id, refText, noteBody]) => ({
+        type: 'doc',
+        content: [
+          para([...refText, { type: 'footnoteReference', attrs: { id } }]),
+          {
+            type: 'footnotesList',
+            content: [{ type: 'footnoteDefinition', attrs: { id }, content: [para(noteBody)] }],
+          },
+        ],
+      }),
+    ),
+
+  // ── inline targets wrapped in a paragraph ────────────────────────────────
+  mention: (m: AttrMode) =>
+    nodeAttrsArb('mention', m).map((attrs) => doc(para([{ type: 'mention', attrs }]))),
+
+  mathInline: (m: AttrMode) =>
+    fc.tuple(phraseArb, nodeAttrsArb('mathInline', m)).map(([t, attrs]) =>
+      doc(para([{ type: 'text', text: t }, { type: 'mathInline', attrs }])),
+    ),
+
+  status: (m: AttrMode) =>
+    nodeAttrsArb('status', m).map((attrs) => doc(para([{ type: 'status', attrs }]))),
+
+  hardBreak: (_m: AttrMode) =>
+    fc.tuple(phraseArb, phraseArb).map(([a, b]) =>
+      doc(para([{ type: 'text', text: a }, { type: 'hardBreak' }, { type: 'text', text: b }])),
+    ),
+
+  // ── marks: a paragraph of marked runs (covers every mark type) ───────────
+  marksOnText: (_m: AttrMode) =>
+    fc.array(markedTextRunArb, { minLength: 1, maxLength: 5 }).map((runs) => {
+      // Merge adjacent same-mark runs (see text-arbitraries.normalizeInline).
+      const out: any[] = [];
+      for (const r of runs) {
+        const prev = out[out.length - 1];
+        if (prev && JSON.stringify(prev.marks ?? []) === JSON.stringify(r.marks ?? [])) {
+          prev.text += r.text;
+        } else out.push({ ...r });
+      }
+      return doc(para(out));
+    }),
+};
+
+/** Build the full list of named generators for a given mode. */
+export function buildGenerators(mode: AttrMode): NamedGen[] {
+  return Object.entries(gen).map(([name, f]) => ({ name, arb: f(mode) }));
+}
+
+// ---------------------------------------------------------------------------
+// Completeness contract support.
+// ---------------------------------------------------------------------------
+
+/**
+ * Schema node/mark types deliberately NOT covered by a P1/P2 generator, each
+ * with a one-line reason. Excluding a type means it is kept OUT of the round-
+ * trip generators — it does NOT weaken any property.
+ *
+ * NOTE (empirical): the candidates the issue flagged for review — pageEmbed,
+ * subpages, transclusionSource/Reference, mention, status — were PROBED against
+ * the live converter and DO round-trip P1/P2 with placeholder ids, so they are
+ * COVERED by real generators rather than allowlisted here. The allowlist below
+ * holds only types with no standalone flat generator by construction.
+ */
+export const KNOWN_UNCOVERED: Record<string, string> = {
+  // The root node; it is the wrapper every generated doc already is, never a
+  // "target" content node, so it has no standalone generator of its own.
+  doc: 'the document root wrapper, not a content node with a standalone generator',
+};
+
+/** Recursively collect every node type and `mark:<type>` under a tree. */
+export function collectTypes(node: any, seen = new Set<string>()): Set<string> {
+  if (!node || typeof node !== 'object') return seen;
+  if (node.type) seen.add(node.type);
+  for (const m of node.marks ?? []) if (m?.type) seen.add(`mark:${m.type}`);
+  for (const c of node.content ?? []) collectTypes(c, seen);
+  return seen;
+}
+
+/**
+ * Sample every generator and return the union of node/mark types they produce.
+ * Deterministic (fixed seed) so the completeness contract is stable.
+ */
+export function coveredTypes(seed = 12345, perGen = 60): Set<string> {
+  const seen = new Set<string>();
+  for (const { arb } of buildGenerators('p1')) {
+    for (const sample of fc.sample(arb, { numRuns: perGen, seed })) {
+      collectTypes(sample, seen);
+    }
+  }
+  return seen;
+}
diff --git a/packages/prosemirror-markdown/test/generative/text-arbitraries.ts b/packages/prosemirror-markdown/test/generative/text-arbitraries.ts
new file mode 100644
index 00000000..76353b76
--- /dev/null
+++ b/packages/prosemirror-markdown/test/generative/text-arbitraries.ts
@@ -0,0 +1,258 @@
+/**
+ * Hostile inline-text corpus for the generative flat-document round-trip suite
+ * (#351, PR 1).
+ *
+ * These arbitraries are a DIRECT PORT of the "supported space" guardrails that
+ * `test/markdown-roundtrip.property.test.ts` proved empirically against the live
+ * converter. That file's long header documents WHY each guardrail exists; rather
+ * than re-derive them, we reuse the exact same shapes here so the attribute-level
+ * generative suite inherits the same byte-stable text space. Each guardrail is
+ * cited back to that file below.
+ *
+ * The corpus deliberately spans the CommonMark / canon hostile alphabet
+ * (`* _ [ ] ( ) { } | < > & # ! ~ = + -`), unicode / emoji / RTL, and the legal
+ * mark combinations on runs (including the `code` mark, which the schema's
+ * `excludes: "_"` makes suppress every co-occurring mark — so it is never
+ * combined with another mark in the byte-stable space).
+ */
+import fc from 'fast-check';
+
+// ---------------------------------------------------------------------------
+// Words and the hostile special-character alphabet.
+// (Ported from markdown-roundtrip.property.test.ts, "Inline text arbitraries".)
+// ---------------------------------------------------------------------------
+
+/** Alphanumeric "word" (no markdown-significant characters). Length 1..6. */
+export const wordArb = fc
+  .stringMatching(/^[A-Za-z0-9]{1,6}$/)
+  .filter((w) => w.length > 0);
+
+/**
+ * A SINGLE markdown-significant character, emitted only as an isolated,
+ * space-flanked token. Every char the task calls out plus a few more; each was
+ * verified byte-stable in this position by the sibling property test.
+ *
+ * NOTE: the backtick (`) is DELIBERATELY excluded from free-floating plain text
+ * (it is a code-span delimiter that re-pairs globally). It is exercised only via
+ * the `code` mark and code blocks — see markdown-roundtrip.property.test.ts.
+ */
+export const specialCharArb = fc.constantFrom(
+  '*', '_', '[', ']', '(', ')', '{', '}', '|', '<', '>', '&', '#', '!', '~', '=', '+', '-',
+);
+
+// A pinch of unicode / emoji / RTL, always word-like (no markdown specials) so
+// it stays inside the space-flanked corpus. Kept letter/emoji-bearing so it is
+// never coerced to a number (see letterPhraseArb rationale).
+export const unicodeWordArb = fc.constantFrom(
+  'café', 'naïve', 'Zürich', 'Москва', 'こんにちは', '你好', '😀', '🚀x', 'مرحبا', 'שלום',
+);
+
+/**
+ * A "safe special" text string: a space-joined sequence of tokens that always
+ * BEGINS and ENDS with an alphanumeric word, with any isolated special chars (or
+ * unicode words) confined to the MIDDLE, each space-flanked by words.
+ *
+ * Both boundary guarantees matter (verbatim from the sibling test):
+ *   * Leading word: the line never opens with a block/inline trigger
+ *     (">", "*", "-", "#", "1." ...).
+ *   * Trailing word: adjacent text runs CONCATENATE with no separator, so a run
+ *     ending in a bare "<" beside a run starting with a letter would form a fake
+ *     HTML tag. Ending every run with a word keeps every special internal and
+ *     space-flanked even after concatenation.
+ */
+export const safeTextArb: fc.Arbitrary<string> = fc
+  .tuple(
+    wordArb,
+    fc.array(fc.oneof(wordArb, specialCharArb, unicodeWordArb), {
+      minLength: 0,
+      maxLength: 3,
+    }),
+    wordArb,
+  )
+  .map(([first, middle, last]) => [first, ...middle, last].join(' '));
+
+/**
+ * A plain alphanumeric phrase (1..3 words) for places where even isolated
+ * specials are not wanted (e.g. code-block language, mention labels, status
+ * text, table cells rendered on the plain-markdown path).
+ */
+export const phraseArb: fc.Arbitrary<string> = fc
+  .array(wordArb, { minLength: 1, maxLength: 3 })
+  .map((ws) => ws.join(' '));
+
+/**
+ * A phrase guaranteed to contain at least one letter. Used for image/media alt
+ * text and link titles: a PURELY numeric alt/title (e.g. "0") is parsed back as
+ * a NUMBER and then dropped by the converter's `value || ""` coercion — not
+ * byte-stable. A letter anywhere keeps it a string. (Ported verbatim.)
+ */
+export const letterPhraseArb: fc.Arbitrary<string> = fc
+  .tuple(
+    fc.stringMatching(/^[A-Za-z]{1,4}$/),
+    fc.array(wordArb, { minLength: 0, maxLength: 2 }),
+  )
+  .map(([head, rest]) => [head, ...rest].join(' '));
+
+/** A paren/space-free URL — safe inside markdown link/image `(...)` syntax. */
+export const urlArb: fc.Arbitrary<string> = fc
+  .webUrl()
+  .filter((u) => !/[()\s]/.test(u));
+
+// ---------------------------------------------------------------------------
+// Marked inline runs.
+// (Ported from markdown-roundtrip.property.test.ts "markedTextRunArb".)
+// ---------------------------------------------------------------------------
+
+/**
+ * A text run with an OPTIONAL single non-code formatting mark (bold/italic/
+ * strike/underline/superscript/subscript/spoiler), or a SOLE `code` mark, or a
+ * link, or an inline comment anchor. `code` is NEVER combined with another mark
+ * in the byte-stable space (that combination is a documented converter
+ * limitation — the schema's `code` mark declares `excludes: "_"`). Marks wrap
+ * `safeTextArb`, which stays stable even when it contains isolated specials.
+ *
+ * The mark set here is broadened past the sibling test's {bold,italic,strike}
+ * to also cover underline / superscript / subscript / spoiler / textStyle /
+ * highlight (all single, non-code marks), so the marks-on-text generator
+ * exercises every mark the schema declares except the deliberately-excluded
+ * `code`+other combination.
+ */
+export const markedTextRunArb: fc.Arbitrary<any> = fc.oneof(
+  // Plain text.
+  safeTextArb.map((t) => ({ type: 'text', text: t })),
+  // Single formatting mark (attribute-free marks).
+  fc
+    .tuple(
+      safeTextArb,
+      fc.constantFrom('bold', 'italic', 'strike', 'underline', 'superscript', 'subscript', 'spoiler'),
+    )
+    .map(([t, m]) => ({ type: 'text', text: t, marks: [{ type: m }] })),
+  // highlight with a color attr.
+  fc
+    .tuple(safeTextArb, fc.constantFrom('#ffcc00', '#a0e0ff', 'yellow'))
+    .map(([t, color]) => ({ type: 'text', text: t, marks: [{ type: 'highlight', attrs: { color } }] })),
+  // textStyle with a color attr.
+  fc
+    .tuple(safeTextArb, fc.constantFrom('#123456', '#ff0000', '#00aa88'))
+    .map(([t, color]) => ({ type: 'text', text: t, marks: [{ type: 'textStyle', attrs: { color } }] })),
+  // Sole code mark (backtick span). safeTextArb is backtick-free, so the span
+  // content cannot contain an inner backtick.
+  safeTextArb.map((t) => ({ type: 'text', text: t, marks: [{ type: 'code' }] })),
+  // Link with safe text, a paren/space-free href, optionally a letter-bearing
+  // title (a purely numeric title is coerced to a number and dropped).
+  fc
+    .tuple(phraseArb, urlArb, fc.option(letterPhraseArb, { nil: undefined }))
+    .map(([t, href, title]) => ({
+      type: 'text',
+      text: t,
+      marks: [{ type: 'link', attrs: title ? { href, title } : { href } }],
+    })),
+  // Inline comment anchor: a span[data-comment-id] that must survive byte-for-
+  // byte. commentId is an alphanumeric token; `resolved` rides only when true.
+  fc
+    .tuple(safeTextArb, fc.stringMatching(/^[A-Za-z0-9]{4,10}$/), fc.boolean())
+    .map(([t, commentId, resolved]) => ({
+      type: 'text',
+      text: t,
+      marks: [
+        { type: 'comment', attrs: resolved ? { commentId, resolved: true } : { commentId } },
+      ],
+    })),
+);
+
+// ---------------------------------------------------------------------------
+// Inline atoms and inline-content assembly.
+// (Ported from markdown-roundtrip.property.test.ts.)
+// ---------------------------------------------------------------------------
+
+/** Inline math node carrying LaTeX that includes the `a < b` the task asks for. */
+export const mathInlineArb: fc.Arbitrary<any> = fc
+  .constantFrom('a < b', 'x^2 + y^2', 'a < b < c', '\\frac{1}{2}', 'E = mc^2')
+  .map((text) => ({ type: 'mathInline', attrs: { text } }));
+
+/** Mention node; label/id/entity are plain phrases / uuids. */
+export const mentionArb: fc.Arbitrary<any> = fc
+  .tuple(phraseArb, fc.uuid(), fc.uuid())
+  .map(([label, id, entityId]) => ({
+    type: 'mention',
+    attrs: { id, label, entityType: 'user', entityId },
+  }));
+
+export const hardBreakArb: fc.Arbitrary<any> = fc.constant({ type: 'hardBreak' });
+
+const sameMarks = (a: any[] | undefined, b: any[] | undefined): boolean =>
+  JSON.stringify(a ?? []) === JSON.stringify(b ?? []);
+
+/**
+ * Canonicalize a generated inline-content array the way ProseMirror stores it,
+ * then trim the markdown-fragile edges. (Ported verbatim from
+ * markdown-roundtrip.property.test.ts "normalizeInline":)
+ *   1) MERGE adjacent text runs with IDENTICAL marks (the editor coalesces
+ *      them; split same-mark runs export to ambiguous "**a****b**").
+ *   2) Collapse CONSECUTIVE hard breaks (two render a blank line marked eats).
+ *   3) Drop a TRAILING hard break (removed by the converter's .trim()).
+ */
+export function normalizeInline(nodes: any[]): any[] {
+  const out: any[] = [];
+  for (const node of nodes) {
+    const prev = out[out.length - 1];
+    if (node.type === 'hardBreak' && prev && prev.type === 'hardBreak') continue;
+    if (
+      node.type === 'text' &&
+      prev &&
+      prev.type === 'text' &&
+      sameMarks(prev.marks, node.marks)
+    ) {
+      prev.text += node.text;
+      continue;
+    }
+    out.push(node.type === 'text' ? { ...node } : node);
+  }
+  while (out.length > 1 && out[out.length - 1].type === 'hardBreak') out.pop();
+  return out;
+}
+
+/**
+ * Inline content for a paragraph: at least one marked text run, optionally with
+ * inline atoms (math/mention) and hard breaks interspersed. Always starts with a
+ * text run so the paragraph never opens with a block trigger. (Ported.)
+ */
+export const inlineContentArb: fc.Arbitrary<any[]> = fc
+  .tuple(
+    markedTextRunArb,
+    fc.array(
+      fc.oneof(
+        { weight: 5, arbitrary: markedTextRunArb },
+        { weight: 1, arbitrary: mathInlineArb },
+        { weight: 1, arbitrary: mentionArb },
+        { weight: 1, arbitrary: hardBreakArb },
+      ),
+      { minLength: 0, maxLength: 4 },
+    ),
+  )
+  .map(([first, rest]) => normalizeInline([first, ...rest]));
+
+/**
+ * Inline content for a HEADING — identical to a paragraph's, but WITHOUT hard
+ * breaks. A hard break inside an ATX heading is not byte-stable (marked splits
+ * the heading). (Ported.)
+ */
+export const headingInlineContentArb: fc.Arbitrary<any[]> = fc
+  .tuple(
+    markedTextRunArb,
+    fc.array(
+      fc.oneof(
+        { weight: 5, arbitrary: markedTextRunArb },
+        { weight: 1, arbitrary: mathInlineArb },
+        { weight: 1, arbitrary: mentionArb },
+      ),
+      { minLength: 0, maxLength: 4 },
+    ),
+  )
+  .map(([first, rest]) => normalizeInline([first, ...rest]));
+
+/** Simple plain-text inline content (single run) for containers rendered on the
+ * raw-HTML path (table cells / column bodies) where fancy inline is undesirable. */
+export const plainInlineContentArb: fc.Arbitrary<any[]> = phraseArb.map((t) => [
+  { type: 'text', text: t },
+]);