diff --git a/packages/git-sync/src/engine/stabilize.ts b/packages/git-sync/src/engine/stabilize.ts
index ce1acdcf..e6769cd0 100644
--- a/packages/git-sync/src/engine/stabilize.ts
+++ b/packages/git-sync/src/engine/stabilize.ts
@@ -72,7 +72,13 @@ export async function stabilizePageFile(
  * keeps re-pulls of an unchanged page byte-identical (no churn, loop-guard).
  */
 export async function stabilizePageBody(content: unknown): Promise<string> {
-  const md1 = convertProseMirrorToMarkdown(content);
+  // git-sync is the LOSSLESS mirror path, so run the serializer in `strict`
+  // mode: a node/mark type the converter has no case for (e.g. one added to the
+  // schema without a matching serializer arm) throws a ConverterLossError here
+  // rather than silently degrading — surfacing the loss loudly at write time
+  // instead of committing a lossy file. Valid content (every current schema type
+  // has a case) is unaffected.
+  const md1 = convertProseMirrorToMarkdown(content, { strict: true });
   const doc2 = await markdownToProseMirror(md1);
-  return convertProseMirrorToMarkdown(doc2);
+  return convertProseMirrorToMarkdown(doc2, { strict: true });
 }
diff --git a/packages/git-sync/test/stabilize.test.ts b/packages/git-sync/test/stabilize.test.ts
index c781546e..66191aa0 100644
--- a/packages/git-sync/test/stabilize.test.ts
+++ b/packages/git-sync/test/stabilize.test.ts
@@ -4,6 +4,7 @@ import { stabilizePageFile, type PageMeta } from '../src/engine/stabilize.js';
 // global DOM via jsdom at module load time (required for @tiptap/html under Node).
 import { markdownToProseMirror } from '@docmost/prosemirror-markdown';
 import { parseDocmostMarkdown } from '@docmost/prosemirror-markdown';
+import { ConverterLossError } from '@docmost/prosemirror-markdown';
 
 // stabilize.ts (SPEC §11 normalize-on-write) was 0% covered (only the gated e2e
 // touched it). stabilizePageFile is import-testable: build a small ProseMirror
@@ -66,6 +67,23 @@ describe('stabilizePageFile — normalize-on-write fixpoint (SPEC §11)', () =>
     expect(body1).toContain('data-src="/d.drawio"');
   });
 
+  it('runs the serializer in STRICT mode — an unmappable node throws, not a lossy write (#493)', async () => {
+    // git-sync is the lossless mirror path: a node type the converter has no
+    // case for (here a fabricated one, standing in for a schema type added
+    // without a matching serializer arm) must surface loudly at write time
+    // rather than being silently flattened into a lossy .md file.
+    const content = {
+      type: 'doc',
+      content: [
+        { type: 'paragraph', content: [{ type: 'text', text: 'ok' }] },
+        { type: 'quantumWidget', content: [{ type: 'text', text: 'lost?' }] },
+      ],
+    };
+    await expect(stabilizePageFile(content, meta)).rejects.toBeInstanceOf(
+      ConverterLossError,
+    );
+  });
+
   it('already-stable content is unchanged by the pass (idempotent)', async () => {
     // Plain prose is already a fixpoint; stabilizing it once and twice agree.
     const content = {
diff --git a/packages/prosemirror-markdown/src/lib/index.ts b/packages/prosemirror-markdown/src/lib/index.ts
index cbf8f462..062751a4 100644
--- a/packages/prosemirror-markdown/src/lib/index.ts
+++ b/packages/prosemirror-markdown/src/lib/index.ts
@@ -15,7 +15,10 @@ export {
 } from "./markdown-document.js";
 export type { DocmostMdMeta } from "./markdown-document.js";
 
-export { convertProseMirrorToMarkdown } from "./markdown-converter.js";
+export {
+  convertProseMirrorToMarkdown,
+  ConverterLossError,
+} from "./markdown-converter.js";
 export type { ConvertProseMirrorToMarkdownOptions } from "./markdown-converter.js";
 
 export {
diff --git a/packages/prosemirror-markdown/src/lib/markdown-converter.ts b/packages/prosemirror-markdown/src/lib/markdown-converter.ts
index 0ee31d39..db6a1520 100644
--- a/packages/prosemirror-markdown/src/lib/markdown-converter.ts
+++ b/packages/prosemirror-markdown/src/lib/markdown-converter.ts
@@ -33,6 +33,26 @@ import {
  */
 const MAX_NODE_DEPTH = 400;
 
+/**
+ * Thrown by {@link convertProseMirrorToMarkdown} in `strict` mode when it hits a
+ * node or mark type it has no lossless markdown form for (the serializer would
+ * otherwise silently degrade it — drop an unknown mark, flatten an unknown node
+ * to its children). Carries the offending kind/name so a caller (git-sync) can
+ * surface exactly what would have been lost.
+ */
+export class ConverterLossError extends Error {
+  readonly kind: "node" | "mark";
+  readonly typeName: string;
+  constructor(kind: "node" | "mark", typeName: string) {
+    super(
+      `convertProseMirrorToMarkdown: unknown ${kind} type "${typeName}" has no lossless markdown representation (strict mode)`,
+    );
+    this.name = "ConverterLossError";
+    this.kind = kind;
+    this.typeName = typeName;
+  }
+}
+
 /**
  * Options for {@link convertProseMirrorToMarkdown}.
  */
@@ -46,6 +66,23 @@ export interface ConvertProseMirrorToMarkdownOptions {
    * path where resolved anchors MUST be preserved for round-tripping.
    */
   dropResolvedCommentAnchors?: boolean;
+  /**
+   * Optional sink for LOSS warnings. When the serializer reaches a node or mark
+   * type it has no dedicated case for, it degrades gracefully (flattens an
+   * unknown node to its children, drops an unknown mark) — historically a SILENT
+   * data loss. When this array is provided, one human-readable message per such
+   * event is pushed here so the caller can observe (and log) what was degraded.
+   * Not provided by default -> behavior is byte-identical to before for existing
+   * callers.
+   */
+  warnings?: string[];
+  /**
+   * When true, THROW a {@link ConverterLossError} on the FIRST unknown node/mark
+   * instead of degrading silently — a warning becomes a hard error. Used by the
+   * lossless git-sync export path and the converter tests, where an unmapped
+   * type is a bug to surface, not data to quietly drop.
+   */
+  strict?: boolean;
 }
 
 /**
@@ -157,6 +194,26 @@ export function convertProseMirrorToMarkdown(
   // callers (mcp getPage / in-app AI chat) pass it true.
   const dropResolvedCommentAnchors = options.dropResolvedCommentAnchors === true;
 
+  // Loss reporting for node/mark types with no dedicated serializer case. In
+  // `strict` mode the FIRST such type throws (git-sync, tests); otherwise the
+  // serializer degrades gracefully (as it always has) but records one warning
+  // per unmapped type into the optional sink so the loss is observable, not
+  // silent. Deduped per type so a document with many unknown nodes of one type
+  // produces one message.
+  const strict = options.strict === true;
+  const warningsSink = options.warnings;
+  const seenLossTypes = new Set<string>();
+  const warnLoss = (kind: "node" | "mark", typeName: string): void => {
+    if (strict) throw new ConverterLossError(kind, typeName);
+    if (!warningsSink) return;
+    const key = `${kind}:${typeName}`;
+    if (seenLossTypes.has(key)) return;
+    seenLossTypes.add(key);
+    warningsSink.push(
+      `Unknown ${kind} type "${typeName}" has no lossless markdown form; it was degraded on export.`,
+    );
+  };
+
   // Escape a value interpolated into an HTML double-quoted attribute value
   // (textAlign, colors, image src, math `text`, all data-* attrs, etc.). In the
   // ATTRIBUTE context only the quote that delimits the value and the ampersand
@@ -646,6 +703,12 @@ export function convertProseMirrorToMarkdown(
                 }
                 break;
               }
+              default:
+                // Unknown mark: no dedicated case, so it has no markdown form and
+                // is dropped from the run. Report the loss (throws in strict
+                // mode) then leave the text unwrapped — the historical behavior.
+                warnLoss("mark", String(mark.type));
+                break;
             }
           }
         }
@@ -1224,7 +1287,11 @@ export function convertProseMirrorToMarkdown(
       }
 
       default:
-        // Fallback: process children
+        // Unknown node type: no dedicated case, so the node's identity + attrs
+        // have no lossless markdown form. Report the loss (throws in strict
+        // mode) then degrade by flattening to its children — the historical
+        // graceful fallback.
+        warnLoss("node", String(type));
         return nodeContent.map(processNode).join("");
     }
   };
@@ -1348,6 +1415,12 @@ export function convertProseMirrorToMarkdown(
                 t = `<span data-comment-id="${escapeAttr(mark.attrs.commentId)}"${r}>${t}</span>`;
               }
               break;
+            default:
+              // Unknown mark on the raw-HTML path: dropped (no HTML form). Report
+              // the loss (throws in strict mode) — same policy as the markdown
+              // path's marks loop above.
+              warnLoss("mark", String(mark.type));
+              break;
           }
         }
         return t;
diff --git a/packages/prosemirror-markdown/test/converter-loss-warnings.test.ts b/packages/prosemirror-markdown/test/converter-loss-warnings.test.ts
new file mode 100644
index 00000000..fc6e22b8
--- /dev/null
+++ b/packages/prosemirror-markdown/test/converter-loss-warnings.test.ts
@@ -0,0 +1,112 @@
+import { describe, expect, it } from "vitest";
+import {
+  convertProseMirrorToMarkdown,
+  ConverterLossError,
+} from "../src/lib/markdown-converter.js";
+
+/**
+ * #493 commit 3 — a node/mark type the serializer has no dedicated case for used
+ * to be degraded SILENTLY (an unknown node flattened to its children, an unknown
+ * mark dropped from the run). The serializer now REPORTS the loss:
+ *   - default (non-strict): unchanged graceful degradation, but one warning per
+ *     unmapped type is pushed into an optional `warnings` sink so callers can
+ *     observe it;
+ *   - strict: the FIRST unmapped type throws a ConverterLossError (git-sync +
+ *     tests), turning a silent loss into a hard, surfaced error.
+ *
+ * Exercised through the REAL converter (no mock): the observable properties are
+ * the emitted markdown, the warnings collected, and the thrown error.
+ */
+
+const doc = (...nodes: any[]) => ({ type: "doc", content: nodes });
+
+describe("converter loss reporting — unknown node types", () => {
+  const unknownNode = doc({
+    type: "quantumWidget",
+    content: [{ type: "text", text: "inner text" }],
+  });
+
+  it("degrades to children AND records a warning (non-strict, sink provided)", () => {
+    const warnings: string[] = [];
+    const md = convertProseMirrorToMarkdown(unknownNode, { warnings });
+    // Graceful degrade: the child text still survives (historical behavior).
+    expect(md).toContain("inner text");
+    // The loss is now observable.
+    expect(warnings).toHaveLength(1);
+    expect(warnings[0]).toContain("quantumWidget");
+    expect(warnings[0]).toContain("node");
+  });
+
+  it("stays byte-identical for callers that pass no sink (zero behavior change)", () => {
+    const withSink: string[] = [];
+    const a = convertProseMirrorToMarkdown(unknownNode, { warnings: withSink });
+    const b = convertProseMirrorToMarkdown(unknownNode);
+    expect(b).toBe(a); // the sink does not alter the produced markdown
+  });
+
+  it("throws ConverterLossError in strict mode", () => {
+    try {
+      convertProseMirrorToMarkdown(unknownNode, { strict: true });
+      expect.unreachable("strict mode must throw on an unknown node");
+    } catch (e) {
+      expect(e).toBeInstanceOf(ConverterLossError);
+      expect((e as ConverterLossError).kind).toBe("node");
+      expect((e as ConverterLossError).typeName).toBe("quantumWidget");
+    }
+  });
+
+  it("dedupes the warning per type (many unknown nodes -> one message)", () => {
+    const warnings: string[] = [];
+    convertProseMirrorToMarkdown(
+      doc(
+        { type: "quantumWidget", content: [{ type: "text", text: "a" }] },
+        { type: "quantumWidget", content: [{ type: "text", text: "b" }] },
+      ),
+      { warnings },
+    );
+    expect(warnings).toHaveLength(1);
+  });
+});
+
+describe("converter loss reporting — unknown mark types", () => {
+  const unknownMark = doc({
+    type: "paragraph",
+    content: [{ type: "text", text: "glowing", marks: [{ type: "glow" }] }],
+  });
+
+  it("drops the mark but keeps the text AND records a warning (non-strict)", () => {
+    const warnings: string[] = [];
+    const md = convertProseMirrorToMarkdown(unknownMark, { warnings });
+    expect(md).toBe("glowing"); // text survives, mark silently had no form
+    expect(warnings).toHaveLength(1);
+    expect(warnings[0]).toContain("glow");
+    expect(warnings[0]).toContain("mark");
+  });
+
+  it("throws ConverterLossError in strict mode", () => {
+    expect(() =>
+      convertProseMirrorToMarkdown(unknownMark, { strict: true }),
+    ).toThrow(ConverterLossError);
+  });
+});
+
+describe("converter loss reporting — known content is never flagged", () => {
+  it("a fully-mapped document produces no warnings and does not throw in strict mode", () => {
+    const d = doc(
+      { type: "heading", attrs: { level: 2 }, content: [{ type: "text", text: "Title" }] },
+      {
+        type: "paragraph",
+        content: [
+          { type: "text", text: "bold", marks: [{ type: "bold" }] },
+          { type: "text", text: " and " },
+          { type: "text", text: "link", marks: [{ type: "link", attrs: { href: "https://x.y" } }] },
+        ],
+      },
+      { type: "bulletList", content: [{ type: "listItem", content: [{ type: "paragraph", content: [{ type: "text", text: "item" }] }] }] },
+    );
+    const warnings: string[] = [];
+    const md = convertProseMirrorToMarkdown(d, { warnings, strict: true });
+    expect(warnings).toEqual([]);
+    expect(md).toContain("## Title");
+  });
+});