diff --git a/desktop/src/features/mesh-compute/classifyModelRef.test.mjs b/desktop/src/features/mesh-compute/classifyModelRef.test.mjs
index 9876678c8a..448888def0 100644
--- a/desktop/src/features/mesh-compute/classifyModelRef.test.mjs
+++ b/desktop/src/features/mesh-compute/classifyModelRef.test.mjs
@@ -15,32 +15,38 @@ test("hf:// prefix → huggingface", () => {
   });
 });
 
-test("absolute path → local-path", () => {
+// The vendored mesh-llm SDK's `parse_exact_model_ref` has no local-path
+// variant — feeding any of these into `mesh_start_node` always fails with
+// "Expected an exact model ref" at runtime. We therefore classify them as
+// unknown so the UI keeps the Start button disabled with a clear signal
+// instead of letting the submit bubble up a cryptic SDK error.
+// See https://github.com/block/buzz/issues/4049.
+
+test("absolute path → unknown", () => {
   assert.deepEqual(classifyModelRef("/Users/me/models/qwen.gguf"), {
-    kind: "local-path",
-    path: "/Users/me/models/qwen.gguf",
+    kind: "unknown",
   });
 });
 
-test("relative path with ./ → local-path", () => {
-  assert.deepEqual(classifyModelRef("./models/qwen.gguf"), {
-    kind: "local-path",
-    path: "./models/qwen.gguf",
-  });
+test("relative path with ./ → unknown", () => {
+  assert.deepEqual(classifyModelRef("./models/qwen.gguf"), { kind: "unknown" });
 });
 
-test("home shortcut → local-path", () => {
-  assert.deepEqual(classifyModelRef("~/models/qwen.gguf"), {
-    kind: "local-path",
-    path: "~/models/qwen.gguf",
-  });
+test("relative path with ../ → unknown", () => {
+  assert.deepEqual(classifyModelRef("../models/qwen.gguf"), { kind: "unknown" });
+});
+
+test("home shortcut → unknown", () => {
+  assert.deepEqual(classifyModelRef("~/models/qwen.gguf"), { kind: "unknown" });
+});
+
+test(".gguf extension without path prefix → unknown", () => {
+  assert.deepEqual(classifyModelRef("my-model.gguf"), { kind: "unknown" });
 });
 
-test(".gguf extension without path prefix → local-path", () => {
-  // Bare filename ending in .gguf — user clearly means a file.
-  assert.deepEqual(classifyModelRef("my-model.gguf"), {
-    kind: "local-path",
-    path: "my-model.gguf",
+test("file:// URL → unknown", () => {
+  assert.deepEqual(classifyModelRef("file:/// Users/me/models/qwen.gguf"), {
+    kind: "unknown",
   });
 });
 
diff --git a/desktop/src/features/mesh-compute/classifyModelRef.ts b/desktop/src/features/mesh-compute/classifyModelRef.ts
index 666c56b309..84aee6865e 100644
--- a/desktop/src/features/mesh-compute/classifyModelRef.ts
+++ b/desktop/src/features/mesh-compute/classifyModelRef.ts
@@ -5,17 +5,21 @@
 export type ModelRefKind =
   | { kind: "catalog"; name: string }
   | { kind: "huggingface"; ref: string }
-  | { kind: "local-path"; path: string }
   | { kind: "unknown" };
 
 /**
  * Classify a model-ref string the way mesh-llm's runtime does:
  *  - `hf://…` → HuggingFace ref
- *  - starts with `/` or `./` or `~`, OR ends with `.gguf` → local file
  *  - otherwise non-empty → catalog name
  *  - empty/whitespace → unknown
  *
- * Source: mesh runtime/mod.rs:3390 ("local file, catalog name, or HuggingFace URL").
+ * Local filesystem paths are classified `unknown`, NOT `local-path`: the
+ * vendored mesh-llm SDK's `parse_exact_model_ref` has no local-path variant,
+ * so feeding a path into `mesh_start_node` always fails with "Expected an
+ * exact model ref". Treating paths as unknown keeps the Start button
+ * disabled with a clear UI-level signal instead of a runtime error at start.
+ * Reclassify-and-accept if/when the SDK gains a local-path branch.
+ * See https://github.com/block/buzz/issues/4049.
  *
  * This is validation-only — canonical resolution happens server-side via
  * `mesh_start_node`.
@@ -28,16 +32,17 @@ export function classifyModelRef(raw: string): ModelRefKind {
   if (trimmed.startsWith("hf://")) {
     return { kind: "huggingface", ref: trimmed };
   }
-  // Local path heuristics. Conservative: only mark as path when there are
-  // unambiguous signals (leading separator, home shortcut, .gguf extension).
+  // Local path heuristics — classify as unknown so the form refuses to
+  // submit. See the function-level doc-comment above and issue #4049.
   const looksLikePath =
     trimmed.startsWith("/") ||
     trimmed.startsWith("./") ||
     trimmed.startsWith("../") ||
     trimmed.startsWith("~") ||
-    trimmed.toLowerCase().endsWith(".gguf");
+    trimmed.toLowerCase().endsWith(".gguf") ||
+    trimmed.startsWith("file://");
   if (looksLikePath) {
-    return { kind: "local-path", path: trimmed };
+    return { kind: "unknown" };
   }
   return { kind: "catalog", name: trimmed };
 }
diff --git a/desktop/src/features/mesh-compute/ui/MeshComputeSettingsCard.tsx b/desktop/src/features/mesh-compute/ui/MeshComputeSettingsCard.tsx
index fd7550eff0..ef7552a986 100644
--- a/desktop/src/features/mesh-compute/ui/MeshComputeSettingsCard.tsx
+++ b/desktop/src/features/mesh-compute/ui/MeshComputeSettingsCard.tsx
@@ -301,8 +301,9 @@ export function MeshComputeSettingsCard() {
               value={modelInput}
             />
             <p className="text-sm font-normal text-muted-foreground">
-              Choose a suggested model below, or enter a model reference or
-              local file. Buzz downloads remote models when sharing starts.
+              Choose a suggested model below, or enter a catalog name or
+              HuggingFace ref. Buzz downloads remote models when sharing
+              starts; local files are not supported yet.
             </p>
             {catalog && catalog.entries.length > 0 ? (
               <CatalogPicker
diff --git a/desktop/tests/e2e/mesh-compute.spec.ts b/desktop/tests/e2e/mesh-compute.spec.ts
index b2e8fc28a9..4005e381ca 100644
--- a/desktop/tests/e2e/mesh-compute.spec.ts
+++ b/desktop/tests/e2e/mesh-compute.spec.ts
@@ -28,7 +28,7 @@ test("Share compute selects the curated default and starts and stops sharing", a
 
   await expect(card).toContainText("Not sharing right now");
   await expect(card).toContainText(
-    "Choose a suggested model below, or enter a model reference or local file",
+    "Choose a suggested model below, or enter a catalog name or HuggingFace ref",
   );
   await expect(model).toHaveValue("Gemma-4-E4B-it-Q4_K_M");
   await expect(toggle).toBeEnabled();