diff --git a/packages/vitest/src/node/config/serializeConfig.ts b/packages/vitest/src/node/config/serializeConfig.ts
index b09e7fb39..3c2800450 100644
--- a/packages/vitest/src/node/config/serializeConfig.ts
+++ b/packages/vitest/src/node/config/serializeConfig.ts
@@ -1,3 +1,4 @@
+import type { SerializedDiffOptions } from '@vitest/utils/diff'
 import type { SerializedConfig } from '../../runtime/config'
 import type { TestProject } from '../project'
 import type { ApiConfig } from '../types/config'
@@ -42,8 +43,7 @@ export function serializeConfig(project: TestProject): SerializedConfig {
         allowWrite: api?.allowWrite,
       }
     })(project.isBrowserEnabled() ? config.browser.api : config.api),
-    // TODO: non serializable function?
-    diff: config.diff,
+    diff: serializeDiffOptions(config.diff),
     retry: config.retry,
     repeats: config.repeats,
     disableConsoleIntercept: config.disableConsoleIntercept,
@@ -162,3 +162,43 @@ export function serializeConfig(project: TestProject): SerializedConfig {
     disableColors: isAgent && !isForceColor(),
   }
 }
+
+const serializableDiffKeys = [
+  'aAnnotation',
+  'aIndicator',
+  'bAnnotation',
+  'bIndicator',
+  'commonIndicator',
+  'contextLines',
+  'emptyFirstOrLastLinePlaceholder',
+  'expand',
+  'includeChangeCounts',
+  'omitAnnotationLines',
+  'printBasicPrototype',
+  'maxDepth',
+  'truncateThreshold',
+  'truncateAnnotation',
+] satisfies (keyof SerializedDiffOptions)[]
+
+// `diff` can be an inline object containing color/compareKeys functions
+// (`DiffOptions`). Those functions are not structured-cloneable (threads pool)
+// and are silently dropped over `child_process` IPC (forks pool), so passing
+// the raw object to workers throws `DataCloneError`. Forward only the
+// serializable fields declared by `SerializedDiffOptions`, and only the ones
+// actually set — explicit `undefined` values would override the diff defaults
+// when the worker merges the options. The function-based options still
+// require the file-path form, which workers import locally.
+function serializeDiffOptions(
+  diff: string | SerializedDiffOptions | undefined,
+): string | SerializedDiffOptions | undefined {
+  if (diff == null || typeof diff === 'string') {
+    return diff
+  }
+  const result: SerializedDiffOptions = {}
+  for (const key of serializableDiffKeys) {
+    if (diff[key] !== undefined) {
+      (result as Record<string, unknown>)[key] = diff[key]
+    }
+  }
+  return result
+}
diff --git a/test/e2e/test/reporters/custom-diff-config.test.ts b/test/e2e/test/reporters/custom-diff-config.test.ts
index 96b6d2be3..ba3078635 100644
--- a/test/e2e/test/reporters/custom-diff-config.test.ts
+++ b/test/e2e/test/reporters/custom-diff-config.test.ts
@@ -21,3 +21,31 @@ test('invalid diff config file', async () => {
   expect(stderr).toContain('invalid diff config file')
   expect(stderr).toContain('Must have a default export with config object')
 })
+
+// https://github.com/vitest-dev/vitest/issues/8663
+// An inline `diff` object may carry non-serializable color functions. They must
+// be stripped before the config is sent to workers, otherwise the `threads`
+// pool throws a `DataCloneError` (structured clone) and the `forks` pool drops
+// them over IPC. The serializable annotations must still be applied.
+test.each(['forks', 'threads'] as const)(
+  'inline diff config object with color functions works in %s pool',
+  async (pool) => {
+    const filename = resolve('./fixtures/reporters/custom-diff-config.test.ts')
+    const { stderr } = await runVitest({
+      root: './fixtures/reporters',
+      pool,
+      diff: {
+        aAnnotation: 'Expected to be',
+        bAnnotation: 'But got',
+        // @ts-expect-error color functions are not part of the public diff type,
+        // but users pass them at runtime — this is what used to crash the worker.
+        aColor: (s: string) => s,
+      },
+    }, [filename])
+
+    expect(stderr).not.toContain('could not be cloned')
+    expect(stderr).not.toContain('DataCloneError')
+    expect(stderr).toContain('Expected to be')
+    expect(stderr).toContain('But got')
+  },
+)