diff --git a/docs/http-mcp.md b/docs/http-mcp.md
index c44933f..5dcdf9c 100644
--- a/docs/http-mcp.md
+++ b/docs/http-mcp.md
@@ -37,6 +37,11 @@ External tools are stateless and do not share the internal research turn's
 12-request budget. No tool writes to an SNS account. Tool failures return
 `isError: true` and a structured error in text content.
 
+Tool input schemas must also work with non-JavaScript JSON Schema validators.
+Mastodon hashtag character restrictions remain enforced by the server and are
+described in the schema, without exporting JavaScript Unicode property escapes
+as a `pattern` that Python JSON Schema validators reject.
+
 Activity publication uses the same service as Home and Messages. Exact retries
 return the original receipt before revision checks; conflicting publications
 make no changes. Browser clients refresh on committed changes through the
diff --git a/src/features/platforms/mastodon-source.test.ts b/src/features/platforms/mastodon-source.test.ts
new file mode 100644
index 0000000..a1b27e0
--- /dev/null
+++ b/src/features/platforms/mastodon-source.test.ts
@@ -0,0 +1,34 @@
+import { expect, it } from "vitest";
+import { z } from "zod";
+import { mastodonSourceSchema } from "./mastodon-source";
+
+it.each(["日本語_123", "café", "ＡＢ１２", " hello_world "])(
+  "accepts Unicode hashtag %s",
+  (target) => {
+    expect(mastodonSourceSchema.parse({ platform: "mastodon", kind: "hashtag", target })).toEqual({
+      platform: "mastodon",
+      kind: "hashtag",
+      target: target.trim(),
+    });
+  },
+);
+
+it.each(["", " ", "#tag", "two words", "tag-name", "😀", "a".repeat(129)])(
+  "rejects invalid hashtag %s",
+  (target) => {
+    expect(
+      mastodonSourceSchema.safeParse({ platform: "mastodon", kind: "hashtag", target }).success,
+    ).toBe(false);
+  },
+);
+
+it("describes hashtag constraints without requiring Unicode regex support in MCP clients", () => {
+  const schema = z.toJSONSchema(mastodonSourceSchema.options[2].shape.target, { io: "input" });
+  expect(schema).toEqual({
+    $schema: "https://json-schema.org/draft/2020-12/schema",
+    type: "string",
+    minLength: 1,
+    maxLength: 128,
+    description: "Hashtag without #, containing only Unicode letters, numbers, and underscores.",
+  });
+});
diff --git a/src/features/platforms/mastodon-source.ts b/src/features/platforms/mastodon-source.ts
index c2c2100..b9353e9 100644
--- a/src/features/platforms/mastodon-source.ts
+++ b/src/features/platforms/mastodon-source.ts
@@ -22,8 +22,13 @@ export const mastodonSourceSchema = z.discriminatedUnion("kind", [
       target: z
         .string()
         .trim()
-        .regex(/^[\p{L}\p{N}_]+$/u)
-        .max(128),
+        .min(1)
+        .max(128)
+        // JSON Schema consumers do not all support Unicode property escapes.
+        .refine((value) => /^[\p{L}\p{N}_]+$/u.test(value), {
+          message: "Use only letters, numbers, and underscores in a hashtag.",
+        })
+        .describe("Hashtag without #, containing only Unicode letters, numbers, and underscores."),
     })
     .strict(),
   z