diff --git a/.changeset/nervous-clocks-invite.md b/.changeset/nervous-clocks-invite.md
new file mode 100644
index 000000000000..b4cbc504a948
--- /dev/null
+++ b/.changeset/nervous-clocks-invite.md
@@ -0,0 +1,5 @@
+---
+"@langchain/core": patch
+---
+
+feat: replace btoa with toBase64Url for encoding in drawMermaidImage
diff --git a/libs/langchain-core/src/runnables/graph_mermaid.ts b/libs/langchain-core/src/runnables/graph_mermaid.ts
index bf909f600ce3..42bedd7336bd 100644
--- a/libs/langchain-core/src/runnables/graph_mermaid.ts
+++ b/libs/langchain-core/src/runnables/graph_mermaid.ts
@@ -1,4 +1,5 @@
 import { Edge, Node } from "./types.js";
+import { toBase64Url } from "./utils.js";
 
 function _escapeNodeLabel(nodeLabel: string): string {
   // Escapes the node label for Mermaid syntax.
@@ -201,8 +202,8 @@ export async function drawMermaidImage(
   let backgroundColor = config?.backgroundColor ?? "white";
   const imageType = config?.imageType ?? "png";
 
-  // Use btoa for compatibility, assume ASCII
-  const mermaidSyntaxEncoded = btoa(mermaidSyntax);
+  const mermaidSyntaxEncoded = toBase64Url(mermaidSyntax);
+
   // Check if the background color is a hexadecimal color code using regex
   if (backgroundColor !== undefined) {
     const hexColorPattern = /^#(?:[0-9a-fA-F]{3}){1,2}$/;
diff --git a/libs/langchain-core/src/runnables/tests/graph_mermaid.test.ts b/libs/langchain-core/src/runnables/tests/graph_mermaid.test.ts
index 43742293426e..3daec17f2ba3 100644
--- a/libs/langchain-core/src/runnables/tests/graph_mermaid.test.ts
+++ b/libs/langchain-core/src/runnables/tests/graph_mermaid.test.ts
@@ -8,6 +8,7 @@ import {
   type MockedFunction,
 } from "vitest";
 import { drawMermaidImage } from "../graph_mermaid.js";
+import { toBase64Url } from "../utils.js";
 
 // Mock global fetch
 const mockFetch = vi.fn() as MockedFunction<typeof fetch>;
@@ -36,7 +37,7 @@ describe("drawMermaidImage", () => {
     expect(mockFetch).toHaveBeenCalledTimes(1);
 
     // Check the URL construction
-    const expectedEncodedSyntax = btoa(mermaidSyntax);
+    const expectedEncodedSyntax = toBase64Url(mermaidSyntax);
     const expectedUrl = `https://mermaid.ink/img/${expectedEncodedSyntax}?bgColor=!white&type=png`;
     expect(mockFetch).toHaveBeenCalledWith(expectedUrl);
   });
@@ -54,7 +55,7 @@ describe("drawMermaidImage", () => {
     });
 
     expect(result).toBe(mockBlob);
-    const expectedEncodedSyntax = btoa(mermaidSyntax);
+    const expectedEncodedSyntax = toBase64Url(mermaidSyntax);
     const expectedUrl = `https://mermaid.ink/img/${expectedEncodedSyntax}?bgColor=!white&type=jpeg`;
     expect(mockFetch).toHaveBeenCalledWith(expectedUrl);
   });
@@ -72,7 +73,7 @@ describe("drawMermaidImage", () => {
     });
 
     expect(result).toBe(mockBlob);
-    const expectedEncodedSyntax = btoa(mermaidSyntax);
+    const expectedEncodedSyntax = toBase64Url(mermaidSyntax);
     const expectedUrl = `https://mermaid.ink/img/${expectedEncodedSyntax}?bgColor=!white&type=webp`;
     expect(mockFetch).toHaveBeenCalledWith(expectedUrl);
   });
@@ -90,7 +91,7 @@ describe("drawMermaidImage", () => {
     });
 
     expect(result).toBe(mockBlob);
-    const expectedEncodedSyntax = btoa(mermaidSyntax);
+    const expectedEncodedSyntax = toBase64Url(mermaidSyntax);
     const expectedUrl = `https://mermaid.ink/img/${expectedEncodedSyntax}?bgColor=#FF5733&type=png`;
     expect(mockFetch).toHaveBeenCalledWith(expectedUrl);
   });
@@ -108,7 +109,7 @@ describe("drawMermaidImage", () => {
     });
 
     expect(result).toBe(mockBlob);
-    const expectedEncodedSyntax = btoa(mermaidSyntax);
+    const expectedEncodedSyntax = toBase64Url(mermaidSyntax);
     const expectedUrl = `https://mermaid.ink/img/${expectedEncodedSyntax}?bgColor=#FFF&type=png`;
     expect(mockFetch).toHaveBeenCalledWith(expectedUrl);
   });
@@ -126,7 +127,7 @@ describe("drawMermaidImage", () => {
     });
 
     expect(result).toBe(mockBlob);
-    const expectedEncodedSyntax = btoa(mermaidSyntax);
+    const expectedEncodedSyntax = toBase64Url(mermaidSyntax);
     const expectedUrl = `https://mermaid.ink/img/${expectedEncodedSyntax}?bgColor=!transparent&type=png`;
     expect(mockFetch).toHaveBeenCalledWith(expectedUrl);
   });
@@ -176,7 +177,7 @@ describe("drawMermaidImage", () => {
     const result = await drawMermaidImage(complexSyntax);
 
     expect(result).toBe(mockBlob);
-    const expectedEncodedSyntax = btoa(complexSyntax);
+    const expectedEncodedSyntax = toBase64Url(complexSyntax);
     const expectedUrl = `https://mermaid.ink/img/${expectedEncodedSyntax}?bgColor=!white&type=png`;
     expect(mockFetch).toHaveBeenCalledWith(expectedUrl);
   });
@@ -194,7 +195,7 @@ describe("drawMermaidImage", () => {
     });
 
     expect(result).toBe(mockBlob);
-    const expectedEncodedSyntax = btoa(mermaidSyntax);
+    const expectedEncodedSyntax = toBase64Url(mermaidSyntax);
     const expectedUrl = `https://mermaid.ink/img/${expectedEncodedSyntax}?bgColor=!white&type=png`;
     expect(mockFetch).toHaveBeenCalledWith(expectedUrl);
   });
diff --git a/libs/langchain-core/src/runnables/utils.ts b/libs/langchain-core/src/runnables/utils.ts
index 2193fff6694d..a7fbc737f98b 100644
--- a/libs/langchain-core/src/runnables/utils.ts
+++ b/libs/langchain-core/src/runnables/utils.ts
@@ -74,3 +74,9 @@ export class _RootEventFilter {
     return include;
   }
 }
+
+export const toBase64Url = (str: string): string => {
+  // Use btoa for compatibility, assume ASCII
+  const encoded = btoa(str);
+  return encoded.replace(/\+/g, "-").replace(/\//g, "_").replace(/=+$/, "");
+};