diff --git a/packages/core/src/tool/html-markdown.ts b/packages/core/src/tool/html-markdown.ts
index ea18530f17e..057d08b6738 100644
--- a/packages/core/src/tool/html-markdown.ts
+++ b/packages/core/src/tool/html-markdown.ts
@@ -95,7 +95,7 @@ export function convertHTMLToMarkdown(html: string) {
     const remaining = limit - outputBytes
     const next = bytes.byteLength <= remaining ? value : sliceBytes(value, remaining)
     output.push(next)
-    outputBytes += encoder.encode(next).byteLength
+    outputBytes += bytes.byteLength <= remaining ? bytes.byteLength : encoder.encode(next).byteLength
     last = next.at(-1) ?? last
   }
   const appendRaw = (value: string) => {
diff --git a/packages/core/test/tool-webfetch.test.ts b/packages/core/test/tool-webfetch.test.ts
index 1d5d73c1762..dcc8895fd9f 100644
--- a/packages/core/test/tool-webfetch.test.ts
+++ b/packages/core/test/tool-webfetch.test.ts
@@ -128,6 +128,15 @@ describe("WebFetchTool helpers", () => {
     expect(output).toHaveLength(WebFetchTool.MAX_RESPONSE_BYTES - 64 * 1024)
   })
 
+  test.each(["x", "\u00e9", "\u{1f600}"])("preserves UTF-8 boundaries at the content limit for %s", (character) => {
+    const budget = WebFetchTool.MAX_RESPONSE_BYTES - 64 * 1024
+    const fitting = "aa" + character.repeat(Math.floor((budget - 2) / Buffer.byteLength(character)))
+    expect(WebFetchTool.convertHTMLToMarkdown(fitting)).toBe(fitting)
+    const truncated = WebFetchTool.convertHTMLToMarkdown(fitting + character)
+    expect(truncated).toBe(fitting)
+    expect(Buffer.byteLength(truncated)).toBe(Buffer.byteLength(fitting))
+  })
+
   test("bounds deeply nested list output and fragmented code fences", () => {
     const lists = `${"<ul><li>item".repeat(2_000)}${"</li></ul>".repeat(2_000)}`
     const quotes = `${"<blockquote><p>item".repeat(2_000)}${"</p></blockquote>".repeat(2_000)}`