fix(deps): remove mermaid as hard runtime dependency

.changeset/fix-mermaid-hard-dep.md

@@ -0,0 +1,18 @@
+---
+"streamdown": patch
+---
+
+fix(deps): remove `mermaid` as a hard runtime dependency
+
+`mermaid` was listed in `dependencies` but the core `streamdown` package
+only imports it as a type (`import type { MermaidConfig }`). The actual
+mermaid runtime is exclusively used by the optional `@streamdown/mermaid`
+plugin — pulling ~75 MB into every install unnecessarily.
+
+This patch replaces the type import with a local structural type for
+`MermaidConfig` so no type-level coupling to the `mermaid` package remains
+in the distributed typings. Users who want fully-typed mermaid config can
+still `import type { MermaidConfig } from 'mermaid'` themselves; the
+structural type is compatible.
+
+Fixes #501

packages/streamdown/__tests__/mermaid.test.tsx

@@ -1,12 +1,11 @@
 import { render, waitFor } from "@testing-library/react";
-import type { MermaidConfig } from "mermaid";
 import { act } from "react";
 import { beforeEach, describe, expect, it, vi } from "vitest";
 import { Mermaid } from "../lib/mermaid";
 import { MermaidDownloadDropdown } from "../lib/mermaid/download-button";
 import { MermaidFullscreenButton } from "../lib/mermaid/fullscreen-button";
 import { PluginContext } from "../lib/plugin-context";
-import type { DiagramPlugin } from "../lib/plugin-types";
+import type { DiagramPlugin, MermaidConfig } from "../lib/plugin-types";
 
 const { saveMock, mockInitialize, mockRender, mockMermaid } = vi.hoisted(() => {
   const mockInitialize = vi.fn();

packages/streamdown/index.tsx

@@ -1,6 +1,5 @@
 "use client";
 
-import type { MermaidConfig } from "mermaid";
 import {
   type ComponentProps,
   type CSSProperties,
@@ -33,7 +32,11 @@ import { hasIncompleteCodeFence, hasTable } from "./lib/incomplete-code-utils";
 import { type ExtraProps, Markdown, type Options } from "./lib/markdown";
 import { parseMarkdownIntoBlocks } from "./lib/parse-blocks";
 import { PluginContext } from "./lib/plugin-context";
-import type { PluginConfig, ThemeInput } from "./lib/plugin-types";
+import type {
+  MermaidConfig,
+  PluginConfig,
+  ThemeInput,
+} from "./lib/plugin-types";
 import { PrefixContext } from "./lib/prefix-context";
 import { preprocessCustomTags } from "./lib/preprocess-custom-tags";
 import { preprocessLiteralTagContent } from "./lib/preprocess-literal-tag-content";

packages/streamdown/lib/mermaid/download-button.tsx

@@ -1,8 +1,8 @@
-import type { MermaidConfig } from "mermaid";
 import { useContext, useEffect, useRef, useState } from "react";
 import { StreamdownContext } from "../../index";
 import { useIcons } from "../icon-context";
 import { useMermaidPlugin } from "../plugin-context";
+import type { MermaidConfig } from "../plugin-types";
 import { useCn } from "../prefix-context";
 import { useTranslations } from "../translations-context";
 import { save } from "../utils";

packages/streamdown/lib/mermaid/fullscreen-button.tsx

@@ -1,8 +1,8 @@
-import type { MermaidConfig } from "mermaid";
 import { type ComponentProps, useContext, useEffect, useState } from "react";
 import { createPortal } from "react-dom";
 import { StreamdownContext } from "../../index";
 import { useIcons } from "../icon-context";
+import type { MermaidConfig } from "../plugin-types";
 import { useCn } from "../prefix-context";
 import { lockBodyScroll, unlockBodyScroll } from "../scroll-lock";
 import { useTranslations } from "../translations-context";

packages/streamdown/lib/mermaid/index.tsx

@@ -1,8 +1,8 @@
-import type { MermaidConfig } from "mermaid";
 import { useContext, useEffect, useState } from "react";
 import { useDeferredRender } from "../../hooks/use-deferred-render";
 import { StreamdownContext } from "../../index";
 import { useMermaidPlugin } from "../plugin-context";
+import type { MermaidConfig } from "../plugin-types";
 import { useCn } from "../prefix-context";
 import { PanZoom } from "./pan-zoom";
 

packages/streamdown/lib/plugin-types.ts

@@ -1,4 +1,12 @@
-import type { MermaidConfig } from "mermaid";
+/**
+ * Structural type for Mermaid configuration.
+ * Avoids a hard dependency on the "mermaid" package in the core bundle.
+ * Users who need full type safety can import MermaidConfig from "mermaid"
+ * directly — it is structurally compatible with this type.
+ */
+// biome-ignore lint/suspicious/noExplicitAny: structural pass-through for mermaid config
+export type MermaidConfig = Record<string, any>;
+
 import type React from "react";
 import type {
   BundledLanguage,

packages/streamdown/package.json

@@ -74,7 +74,6 @@
     "remend": "workspace:*",
     "tailwind-merge": "^3.4.0",
     "unified": "^11.0.5",
-    "mermaid": "^11.12.2",
     "unist-util-visit": "^5.0.0",
     "unist-util-visit-parents": "^6.0.0"
   }