diff --git a/packages/core/src/files.test.ts b/packages/core/src/files.test.ts
new file mode 100644
index 00000000..1a353c7c
--- /dev/null
+++ b/packages/core/src/files.test.ts
@@ -0,0 +1,90 @@
+import { describe, expect, it } from "vitest";
+
+import { hasOversizedPathSegment, MAX_PATH_SEGMENT_BYTES, writeFile } from "./files.js";
+import type { StorageAdapter } from "./storage.js";
+
+describe("hasOversizedPathSegment", () => {
+  it("accepts a path whose every segment is at the byte limit", () => {
+    const segment = "a".repeat(MAX_PATH_SEGMENT_BYTES - ".json".length);
+    expect(hasOversizedPathSegment(`/reddit/subreddits/localllama/posts/${segment}.json`)).toBe(false);
+  });
+
+  it("rejects a path with one segment over the byte limit", () => {
+    const segment = "a".repeat(MAX_PATH_SEGMENT_BYTES + 1);
+    expect(hasOversizedPathSegment(`/reddit/subreddits/localllama/posts/${segment}.json`)).toBe(true);
+  });
+
+  it("counts UTF-8 encoded bytes, not JS string length", () => {
+    // Each "é" is 1 UTF-16 code unit but 2 UTF-8 bytes.
+    const segment = "é".repeat(MAX_PATH_SEGMENT_BYTES); // 200 chars, 400 bytes
+    expect(hasOversizedPathSegment(`/x/${segment}`)).toBe(true);
+  });
+
+  it("ignores empty segments from a normalized path", () => {
+    expect(hasOversizedPathSegment("/reddit/subreddits/localllama.json")).toBe(false);
+  });
+
+  it("reproduces the real oversized r/LocalLLaMA post slug that broke the mount", () => {
+    const badPath =
+      "/reddit/subreddits/localllama/posts/i-just-don-t-get-it-these-big-tech-companies-can-illegally-" +
+      "scrape-the-entire-internet-and-gatekeep-their-better-models-behind-higher-prices-so-it-s-natural-" +
+      "that-people-look-for-affordable-options-and-there-will-be-providers-who-apparently-distill-models-" +
+      "from-them__1uvwn9q.json";
+    expect(hasOversizedPathSegment(badPath)).toBe(true);
+  });
+});
+
+describe("writeFile", () => {
+  function storageStub(overrides: Partial<StorageAdapter> = {}): StorageAdapter {
+    return {
+      getFile: () => null,
+      listFiles: () => [],
+      putFile: () => {},
+      deleteFile: () => {},
+      appendEvent: () => {},
+      listEvents: () => ({ items: [], nextCursor: null }),
+      getRecentEvents: () => [],
+      getOperation: () => null,
+      putOperation: () => {},
+      listOperations: () => ({ items: [], nextCursor: null }),
+      nextRevision: () => "rev_1",
+      nextOperationId: () => "op_1",
+      nextEventId: () => "evt_1",
+      enqueueWriteback: () => {},
+      getPendingWritebacks: () => [],
+      getWorkspaceId: () => "ws_test",
+      ...overrides,
+    } as StorageAdapter;
+  }
+
+  it("rejects a write whose path has an oversized segment before touching storage", () => {
+    const segment = "a".repeat(MAX_PATH_SEGMENT_BYTES + 1);
+    let storageTouched = false;
+    const storage = storageStub({
+      getFile: () => {
+        storageTouched = true;
+        return null;
+      },
+    });
+
+    const result = writeFile(storage, {
+      path: `/reddit/subreddits/localllama/posts/${segment}.json`,
+      ifMatch: "*",
+      content: "{}",
+    });
+
+    expect(result).toEqual({ ok: false, error: "invalid_input" });
+    expect(storageTouched).toBe(false);
+  });
+
+  it("allows a write with normal-length path segments to proceed past the guard", () => {
+    const storage = storageStub();
+    const result = writeFile(storage, {
+      path: "/reddit/subreddits/localllama/posts/a-normal-title__abc123.json",
+      ifMatch: "0",
+      content: "{}",
+    });
+
+    expect(result.ok).toBe(true);
+  });
+});
diff --git a/packages/core/src/files.ts b/packages/core/src/files.ts
index ec0e4cbc..fdb06775 100644
--- a/packages/core/src/files.ts
+++ b/packages/core/src/files.ts
@@ -94,6 +94,9 @@ export function writeFile(storage: StorageAdapter, req: WriteFileRequest): Write
   }
 
   const path = normalizePath(req.path);
+  if (hasOversizedPathSegment(path)) {
+    return { ok: false, error: "invalid_input" };
+  }
   const ifMatch = normalizeIfMatch(req.ifMatch);
   if (!ifMatch) {
     return { ok: false, error: "missing_precondition" };
@@ -274,6 +277,27 @@ export function deleteFile(storage: StorageAdapter, req: DeleteFileRequest): Del
 export const DEFAULT_CONTENT_TYPE = "text/markdown";
 export const MAX_FILE_BYTES = 10 * 1024 * 1024;
 
+// Local mounts materialize each path segment as a real filesystem entry, and
+// most filesystems (ext4, APFS, ...) reject names over 255 bytes with
+// ENAMETOOLONG -- worse once a mount's own atomic-write suffix (e.g.
+// `.tmp-<random>`) is appended to the leaf segment. A provider adapter that
+// builds a path segment from unbounded user content (a long title, etc.) can
+// produce a canonical path no mount can ever materialize, which then fails
+// forever on every sync cycle since nothing else revisits an already-written
+// path. Reject that at write time instead of a bounded content-derived slug
+// having to be probabilistically clean everywhere it's used.
+export const MAX_PATH_SEGMENT_BYTES = 200;
+
+function encodedByteLength(value: string): number {
+  return new TextEncoder().encode(value).length;
+}
+
+export function hasOversizedPathSegment(path: string): boolean {
+  return path
+    .split("/")
+    .some((segment) => segment.length > 0 && encodedByteLength(segment) > MAX_PATH_SEGMENT_BYTES);
+}
+
 function normalizeEncoding(value?: string): "utf-8" | "base64" | null {
   const encoding = value?.trim().toLowerCase() ?? "";
   if (!encoding || encoding === "utf-8" || encoding === "utf8") {
diff --git a/packages/core/src/webhooks.test.ts b/packages/core/src/webhooks.test.ts
index d461af97..9c8b88b1 100644
--- a/packages/core/src/webhooks.test.ts
+++ b/packages/core/src/webhooks.test.ts
@@ -208,6 +208,33 @@ describe("webhook Slack path canonicalization", () => {
       "/slack/channels/C123/messages/1711111111_000100/meta.json",
     );
   });
+
+  it("rejects an envelope write whose path has an oversized segment instead of writing it", () => {
+    const storage = new MemoryStorage();
+    const oversizedSlug = "a".repeat(250);
+
+    const queued = ingestWebhook(storage, {
+      provider: "reddit",
+      eventType: "file.created",
+      path: `/reddit/subreddits/localllama/posts/${oversizedSlug}__1uvwn9q.json`,
+      deliveryId: "delivery_1",
+      timestamp: "2026-07-15T09:45:00.000Z",
+      correlationId: "corr_1",
+    }, {
+      generateEnvelopeId: () => "env_1",
+      coalesceWindowMs: 10_000,
+    });
+    expect(queued.status).toBe("queued");
+
+    const envelope = storage.envelopes.get("env_1");
+    if (!envelope) throw new Error("envelope not queued");
+
+    const result = applyWebhookEnvelope(storage, envelope);
+
+    expect(result.status).toBe("rejected");
+    expect(result.reason).toBe("path_too_long");
+    expect(storage.files.size).toBe(0);
+  });
 });
 
 function fileRow(path: string, overrides: Partial<FileRow> = {}): FileRow {
diff --git a/packages/core/src/webhooks.ts b/packages/core/src/webhooks.ts
index 6d4fe7cb..ef384a3e 100644
--- a/packages/core/src/webhooks.ts
+++ b/packages/core/src/webhooks.ts
@@ -21,7 +21,7 @@ import type {
   Paginated,
   FileSemantics,
 } from "./storage.js";
-import { normalizePath, DEFAULT_CONTENT_TYPE, MAX_FILE_BYTES, encodedSize } from "./files.js";
+import { normalizePath, DEFAULT_CONTENT_TYPE, MAX_FILE_BYTES, encodedSize, hasOversizedPathSegment } from "./files.js";
 
 export interface IngestWebhookInput {
   provider: string;
@@ -397,6 +397,16 @@ export function applyWebhookEnvelope(
       };
     }
 
+    if (hasOversizedPathSegment(event.path)) {
+      return {
+        status: "rejected",
+        eventType: event.type,
+        path: event.path,
+        revision: null,
+        reason: "path_too_long",
+      };
+    }
+
     const content =
       typeof event.content === "string"
         ? event.content