diff --git a/.changeset/big-integer-search-params.md b/.changeset/big-integer-search-params.md
new file mode 100644
index 0000000000..bdc1ba40c1
--- /dev/null
+++ b/.changeset/big-integer-search-params.md
@@ -0,0 +1,5 @@
+---
+'@tanstack/router-core': patch
+---
+
+fix: preserve integer search param literals beyond Number.MAX_SAFE_INTEGER as strings — JSON.parse silently rounds them (e.g. an 18-digit ad id `120247103250460643` becomes `120247103250460640`), corrupting the value on any redirect or link re-serialization
diff --git a/packages/router-core/src/searchParams.ts b/packages/router-core/src/searchParams.ts
index 740d36441c..a33d12a903 100644
--- a/packages/router-core/src/searchParams.ts
+++ b/packages/router-core/src/searchParams.ts
@@ -1,12 +1,30 @@
 import { decode, encode } from './qss'
 import type { AnySchema } from './validators'
 
+const integerLiteralRegex = /^-?\d+$/
+
+/**
+ * `JSON.parse`, except integer literals beyond `Number.MAX_SAFE_INTEGER` are
+ * rejected (kept as strings by the callers below): parsing them to a float64
+ * silently rounds the value (e.g. "120247103250460643" becomes
+ * 120247103250460640), so any re-serialization corrupts the original — a
+ * common hazard with 18-digit ad/click ids in marketing URLs.
+ */
+function parseJsonPreservingUnsafeIntegers(str: string): any {
+  if (integerLiteralRegex.test(str) && !Number.isSafeInteger(Number(str))) {
+    throw new Error('Integer literal exceeds safe integer range')
+  }
+  return JSON.parse(str)
+}
+
 /** Default `parseSearch` that strips leading '?' and JSON-parses values. */
-export const defaultParseSearch = parseSearchWith(JSON.parse)
+export const defaultParseSearch = parseSearchWith(
+  parseJsonPreservingUnsafeIntegers,
+)
 /** Default `stringifySearch` using JSON.stringify for complex values. */
 export const defaultStringifySearch = stringifySearchWith(
   JSON.stringify,
-  JSON.parse,
+  parseJsonPreservingUnsafeIntegers,
 )
 
 /**
diff --git a/packages/router-core/tests/searchParams.test.ts b/packages/router-core/tests/searchParams.test.ts
index 006e119be5..3133c345ef 100644
--- a/packages/router-core/tests/searchParams.test.ts
+++ b/packages/router-core/tests/searchParams.test.ts
@@ -99,3 +99,29 @@ describe('Search Params serialization and deserialization', () => {
     )
   })
 })
+
+describe('Unsafe integer literals', () => {
+  /*
+   * Integer literals beyond Number.MAX_SAFE_INTEGER cannot survive a trip
+   * through float64 — JSON.parse would silently round them (e.g. an 18-digit
+   * ad id "120247103250460643" becomes 120247103250460640). They are kept as
+   * strings so they round-trip losslessly.
+   */
+  test.each([
+    ['120247103250460643'],
+    ['-120247103250460643'],
+    ['9007199254740993'],
+  ])('unsafe integer literal %s is preserved as a string', (literal) => {
+    const parsed = defaultParseSearch(`?foo=${literal}`)
+    expect(parsed).toEqual({ foo: literal })
+    const restringified = defaultStringifySearch(parsed)
+    expect(defaultParseSearch(restringified)).toEqual({ foo: literal })
+  })
+
+  test('safe integer literals still parse as numbers', () => {
+    expect(defaultParseSearch('?foo=123')).toEqual({ foo: 123 })
+    expect(defaultParseSearch(`?foo=${Number.MAX_SAFE_INTEGER}`)).toEqual({
+      foo: Number.MAX_SAFE_INTEGER,
+    })
+  })
+})