diff --git a/web/standalone/src/wasm/collab/ai-bridge.test.ts b/web/standalone/src/wasm/collab/ai-bridge.test.ts
new file mode 100644
index 0000000..535a8da
--- /dev/null
+++ b/web/standalone/src/wasm/collab/ai-bridge.test.ts
@@ -0,0 +1,104 @@
+import { describe, expect, it } from "vitest";
+import * as Y from "yjs";
+import { docToFile, docToY, kicadLibSymbolsMap, fileToDoc, yToDoc } from "@pcbjam/shared";
+import { applyToActiveBuffer, readActiveBuffer, registerActiveCollabDoc } from "./ai-bridge";
+
+// A symbol instance is only renderable with its `(lib_symbols …)` definition.
+// `docDelta` (the apply path's item differ) only diffs uuid-bearing items, and a
+// `(symbol "Device:R" …)` lib def has no uuid — so an AI proposal that places a
+// part the sheet has never carried used to land the instance with no definition,
+// and KiCad drew nothing. `applyToActiveBuffer` now upserts the proposal's lib
+// defs into `kdoc_libsymbols` alongside the item delta.
+
+const WIRE1 = "22222222-0000-0000-0000-000000000001";
+const SYM1 = "44444444-0000-0000-0000-000000000001";
+
+const SCH_WITHOUT_SYMBOL = `(kicad_sch
+\t(version 20250114)
+\t(generator "eeschema")
+\t(generator_version "9.0")
+\t(uuid "11111111-1111-1111-1111-111111111111")
+\t(paper "A4")
+\t(lib_symbols)
+\t(wire (pts (xy 50.8 50.8) (xy 101.6 50.8)) (stroke (width 0) (type default)) (uuid "${WIRE1}"))
+\t(sheet_instances (path "/" (page "1")))
+)
+`;
+
+const SCH_WITH_SYMBOL = `(kicad_sch
+\t(version 20250114)
+\t(generator "eeschema")
+\t(generator_version "9.0")
+\t(uuid "11111111-1111-1111-1111-111111111111")
+\t(paper "A4")
+\t(lib_symbols
+\t\t(symbol "Device:R" (pin_numbers (hide yes)) (pin_names (offset 0)) (exclude_from_sim no) (in_bom yes) (on_board yes)
+\t\t\t(property "Reference" "R" (at 2.032 0 90) (effects (font (size 1.27 1.27))))
+\t\t\t(property "Value" "R" (at 0 0 90) (effects (font (size 1.27 1.27))))
+\t\t\t(symbol "R_0_1"
+\t\t\t\t(rectangle (start -1.016 -2.54) (end 1.016 2.54) (stroke (width 0.254) (type default)) (fill (type none)))
+\t\t\t)
+\t\t\t(symbol "R_1_1"
+\t\t\t\t(pin passive line (at 0 3.81 270) (length 1.27) (name "~" (effects (font (size 1.27 1.27)))) (number "1" (effects (font (size 1.27 1.27)))))
+\t\t\t\t(pin passive line (at 0 -3.81 90) (length 1.27) (name "~" (effects (font (size 1.27 1.27)))) (number "2" (effects (font (size 1.27 1.27)))))
+\t\t\t)
+\t\t)
+\t)
+\t(wire (pts (xy 50.8 50.8) (xy 101.6 50.8)) (stroke (width 0) (type default)) (uuid "${WIRE1}"))
+\t(symbol (lib_id "Device:R") (at 63.5 63.5 0) (unit 1) (exclude_from_sim no) (in_bom yes) (on_board yes) (dnp no)
+\t\t(uuid "${SYM1}")
+\t\t(property "Reference" "R1" (at 66.04 62.23 0) (effects (font (size 1.27 1.27)) (justify left)))
+\t\t(property "Value" "10k" (at 66.04 64.77 0) (effects (font (size 1.27 1.27)) (justify left)))
+\t\t(pin "1" (uuid "44444444-0000-0000-0000-0000000000a1"))
+\t\t(pin "2" (uuid "44444444-0000-0000-0000-0000000000a2"))
+\t\t(instances (project "rt" (path "/11111111-1111-1111-1111-111111111111" (reference "R1") (unit 1))))
+\t)
+\t(sheet_instances (path "/" (page "1")))
+)
+`;
+
+function seed(src: string): Y.Doc {
+  const ydoc = new Y.Doc();
+  docToY(fileToDoc(src), ydoc);
+  return ydoc;
+}
+
+describe("ai-bridge applyToActiveBuffer", () => {
+  it("carries a newly-placed symbol's (lib_symbols) definition, not just the instance", () => {
+    const ydoc = seed(SCH_WITHOUT_SYMBOL);
+    registerActiveCollabDoc(ydoc, "board.kicad_sch");
+    try {
+      expect(kicadLibSymbolsMap(ydoc).get("Device:R")).toBeUndefined();
+
+      applyToActiveBuffer(SCH_WITH_SYMBOL);
+
+      const def = kicadLibSymbolsMap(ydoc).get("Device:R");
+      expect(def).toBeDefined();
+      expect(def).toContain("R_0_1");
+
+      // and the instance item itself landed via the normal item delta
+      const out = docToFile(yToDoc(ydoc));
+      expect(out).toContain(SYM1);
+      expect(out).toContain('(lib_id "Device:R")');
+    } finally {
+      registerActiveCollabDoc(null);
+    }
+  });
+
+  it("is a no-op on lib_symbols when the proposal adds no new definitions", () => {
+    const ydoc = seed(SCH_WITH_SYMBOL);
+    registerActiveCollabDoc(ydoc, "board.kicad_sch");
+    try {
+      const before = kicadLibSymbolsMap(ydoc).get("Device:R");
+      expect(before).toBeDefined();
+
+      // same doc, wire nudged — no new parts
+      applyToActiveBuffer(SCH_WITH_SYMBOL.replace("101.6 50.8", "127 50.8"));
+
+      expect(kicadLibSymbolsMap(ydoc).get("Device:R")).toBe(before);
+      expect(readActiveBuffer()).toContain("127 50.8");
+    } finally {
+      registerActiveCollabDoc(null);
+    }
+  });
+});
diff --git a/web/standalone/src/wasm/collab/ai-bridge.ts b/web/standalone/src/wasm/collab/ai-bridge.ts
index 85a46cd..4040831 100644
--- a/web/standalone/src/wasm/collab/ai-bridge.ts
+++ b/web/standalone/src/wasm/collab/ai-bridge.ts
@@ -28,6 +28,8 @@ import {
   docDelta,
   docToFile,
   fileToDoc,
+  libSymbolsFromLayout,
+  upsertLibSymbolsToY,
   yToDoc,
   type KicadDoc,
 } from "@pcbjam/shared";
@@ -82,6 +84,20 @@ export function applyToActiveBuffer(newSource: string): void {
   const prev = yToDoc(activeDoc);
   const next = fileToDoc(newSource);
   const delta = docDelta(prev, next);
+  // `docDelta` only diffs uuid-bearing items — a `(lib_symbols …)` def has no
+  // uuid, so a proposal that places a symbol the sheet has never carried adds
+  // the instance item but drops its library definition, leaving KiCad an
+  // instance pointing at an undefined symbol (renders nothing). Push the
+  // proposal's lib defs into the kdoc_libsymbols map FIRST (upsert by lib id,
+  // never deletes — the server always splices into the existing block, so this
+  // is existing ∪ additions), so the items observer's `deltaToItemsWire` can
+  // prefix each new instance with its `(lib_symbols …)` on the way to the
+  // editor — exactly what `kicad-binding.ts`'s own DOWN path does with
+  // `upsertLibSymbolsToY(doc, defs, ORIGIN)`. Deliberately NOT `syncLayoutToY`:
+  // that also reconciles non-item layout heads (root `uuid`, `sheet_instances`,
+  // `paper`…) wholesale-LWW from the proposal, which would corrupt the sheet's
+  // identity if the server's spliced preamble deviates at all.
+  upsertLibSymbolsToY(activeDoc, libSymbolsFromLayout(next.layout, next.items), APPLY_ORIGIN);
   applyDeltaToY(activeDoc, delta, APPLY_ORIGIN);
 }