diff --git a/crates/vite_migration/src/lib.rs b/crates/vite_migration/src/lib.rs
index c8ed9be841..95ab4c66f3 100644
--- a/crates/vite_migration/src/lib.rs
+++ b/crates/vite_migration/src/lib.rs
@@ -18,4 +18,4 @@ mod vite_config;
 pub use file_walker::{WalkResult, find_ts_files};
 pub use import_rewriter::{BatchRewriteResult, rewrite_imports_in_directory};
 pub use package::{rewrite_eslint, rewrite_prettier, rewrite_scripts};
-pub use vite_config::{MergeResult, merge_json_config, merge_tsdown_config};
+pub use vite_config::{MergeResult, has_config_key, merge_json_config, merge_tsdown_config};
diff --git a/crates/vite_migration/src/vite_config.rs b/crates/vite_migration/src/vite_config.rs
index 011a5ca4bb..2b67b610bd 100644
--- a/crates/vite_migration/src/vite_config.rs
+++ b/crates/vite_migration/src/vite_config.rs
@@ -1,6 +1,7 @@
 use std::{borrow::Cow, path::Path, sync::LazyLock};
 
 use ast_grep_config::{GlobalRules, RuleConfig, from_yaml_string};
+use ast_grep_core::{Doc, Node};
 use ast_grep_language::{LanguageExt, SupportLang};
 use regex::Regex;
 use vite_error::Error;
@@ -128,6 +129,93 @@ fn strip_schema_property(config: &str) -> Cow<'_, str> {
     RE_SCHEMA.replace_all(config, "")
 }
 
+/// Check whether `config_key` is already declared as a top-level property in
+/// the vite config's `defineConfig({...})` (or equivalent) object literal.
+///
+/// Mirrors the six shapes the merger understands (see `generate_merge_rule`):
+/// `defineConfig({...})`, `defineConfig((p) => ({...}))`, `return {...}`
+/// inside a `defineConfig` callback, `export default {...}`, and the
+/// `satisfies` export variant. The `return $VAR` variant cannot be inspected
+/// statically — for that shape we conservatively report `false`, which is
+/// safe because the merger uses object spread (`{ key: ..., ...$VAR }`) so
+/// duplicate keys are resolved at runtime by JS spread semantics.
+///
+/// Returns `true` only when the key appears as a **direct** member of one of
+/// those recognized object literals. Comments, string occurrences, nested
+/// keys (e.g. `plugins: [{ fmt: ... }]`), and unrelated objects are all
+/// ignored correctly.
+pub fn has_config_key(vite_config_content: &str, config_key: &str) -> Result<bool, Error> {
+    let grep = SupportLang::TypeScript.ast_grep(vite_config_content);
+    let root = grep.root();
+
+    for node in root.dfs() {
+        if node.kind() != "pair" {
+            continue;
+        }
+        let Some(key_node) = node.field("key") else { continue };
+        if !pair_key_matches(&key_node, config_key) {
+            continue;
+        }
+        let Some(parent_object) = node.parent() else { continue };
+        if parent_object.kind() != "object" {
+            continue;
+        }
+        if is_recognized_config_object(&parent_object) {
+            return Ok(true);
+        }
+    }
+
+    Ok(false)
+}
+
+fn pair_key_matches<D: Doc>(key_node: &Node<'_, D>, config_key: &str) -> bool {
+    let text = key_node.text();
+    match key_node.kind().as_ref() {
+        "property_identifier" => text == config_key,
+        "string" => text.trim_matches(|c| c == '"' || c == '\'' || c == '`') == config_key,
+        _ => false,
+    }
+}
+
+fn is_recognized_config_object<D: Doc>(object_node: &Node<'_, D>) -> bool {
+    let Some(parent) = object_node.parent() else { return false };
+    match parent.kind().as_ref() {
+        "export_statement" => true,
+        // `export default { ... } satisfies T` — hop past the satisfies wrapper.
+        "satisfies_expression" => parent.parent().is_some_and(|p| p.kind() == "export_statement"),
+        "arguments" => parent.parent().is_some_and(|c| is_define_config_call(&c)),
+        "parenthesized_expression" => is_define_config_arrow_body(&parent),
+        "return_statement" => is_inside_define_config_callback(&parent),
+        _ => false,
+    }
+}
+
+fn is_define_config_call<D: Doc>(call_node: &Node<'_, D>) -> bool {
+    call_node.kind() == "call_expression"
+        && call_node.field("function").is_some_and(|f| f.text() == "defineConfig")
+}
+
+fn is_define_config_arrow_body<D: Doc>(paren_node: &Node<'_, D>) -> bool {
+    paren_node
+        .parent()
+        .filter(|n| n.kind() == "arrow_function")
+        .and_then(|n| n.parent())
+        .filter(|n| n.kind() == "arguments")
+        .and_then(|n| n.parent())
+        .is_some_and(|c| is_define_config_call(&c))
+}
+
+fn is_inside_define_config_callback<D: Doc>(node: &Node<'_, D>) -> bool {
+    let mut current = node.parent();
+    while let Some(n) = current {
+        if is_define_config_call(&n) {
+            return true;
+        }
+        current = n.parent();
+    }
+    false
+}
+
 /// Check if the vite config uses a function callback pattern
 fn check_function_callback(vite_config_content: &str) -> Result<bool, Error> {
     // Match both sync and async arrow functions
@@ -358,6 +446,197 @@ mod tests {
 
     use super::*;
 
+    // ── has_config_key ────────────────────────────────────────────────────
+
+    #[test]
+    fn test_has_config_key_top_level_in_defineconfig() {
+        let cfg = r#"import { defineConfig } from 'vite-plus';
+
+export default defineConfig({
+  fmt: { singleQuote: true },
+  lint: { rules: {} },
+});
+"#;
+        assert!(has_config_key(cfg, "fmt").unwrap());
+        assert!(has_config_key(cfg, "lint").unwrap());
+        assert!(!has_config_key(cfg, "pack").unwrap());
+        assert!(!has_config_key(cfg, "staged").unwrap());
+    }
+
+    #[test]
+    fn test_has_config_key_quoted_key() {
+        let cfg = r#"import { defineConfig } from 'vite-plus';
+
+export default defineConfig({
+  'fmt': { singleQuote: true },
+  "lint": {},
+});
+"#;
+        assert!(has_config_key(cfg, "fmt").unwrap());
+        assert!(has_config_key(cfg, "lint").unwrap());
+    }
+
+    #[test]
+    fn test_has_config_key_ignores_comment_mentions() {
+        // The regex check was a false positive on these — AST check ignores them.
+        let cfg = r#"import { defineConfig } from 'vite-plus';
+
+// fmt: configure formatter here
+/* lint: TODO wire this up */
+export default defineConfig({
+  plugins: [],
+});
+"#;
+        assert!(!has_config_key(cfg, "fmt").unwrap());
+        assert!(!has_config_key(cfg, "lint").unwrap());
+    }
+
+    #[test]
+    fn test_has_config_key_ignores_string_literal_mentions() {
+        let cfg = r#"import { defineConfig } from 'vite-plus';
+
+export default defineConfig({
+  plugins: [],
+  description: 'has fmt: foo and lint: bar inside',
+});
+"#;
+        assert!(!has_config_key(cfg, "fmt").unwrap());
+        assert!(!has_config_key(cfg, "lint").unwrap());
+    }
+
+    #[test]
+    fn test_has_config_key_ignores_nested_keys() {
+        // `fmt:` is a nested property inside `plugins[0].options`, not top-level.
+        let cfg = r#"import { defineConfig } from 'vite-plus';
+
+export default defineConfig({
+  plugins: [
+    somePlugin({
+      fmt: 'auto',
+      lint: { enabled: true },
+    }),
+  ],
+});
+"#;
+        assert!(!has_config_key(cfg, "fmt").unwrap());
+        assert!(!has_config_key(cfg, "lint").unwrap());
+    }
+
+    #[test]
+    fn test_has_config_key_arrow_callback() {
+        let cfg = r#"import { defineConfig } from 'vite-plus';
+
+export default defineConfig((env) => ({
+  fmt: { singleQuote: env.mode === 'production' },
+}));
+"#;
+        assert!(has_config_key(cfg, "fmt").unwrap());
+        assert!(!has_config_key(cfg, "lint").unwrap());
+    }
+
+    #[test]
+    fn test_has_config_key_return_block_callback() {
+        let cfg = r#"import { defineConfig } from 'vite-plus';
+
+export default defineConfig(({ mode }) => {
+  return {
+    fmt: { singleQuote: true },
+  };
+});
+"#;
+        assert!(has_config_key(cfg, "fmt").unwrap());
+        assert!(!has_config_key(cfg, "lint").unwrap());
+    }
+
+    #[test]
+    fn test_has_config_key_async_return_block_callback() {
+        let cfg = r#"
+export default defineConfig(async ({ command, mode }) => {
+  const data = await asyncFunction();
+  return {
+    lint: { rules: {} },
+  };
+});
+"#;
+        assert!(has_config_key(cfg, "lint").unwrap());
+        assert!(!has_config_key(cfg, "fmt").unwrap());
+    }
+
+    #[test]
+    fn test_has_config_key_plain_export() {
+        let cfg = r#"export default {
+  fmt: { singleQuote: true },
+};
+"#;
+        assert!(has_config_key(cfg, "fmt").unwrap());
+        assert!(!has_config_key(cfg, "lint").unwrap());
+    }
+
+    #[test]
+    fn test_has_config_key_satisfies_export() {
+        let cfg = r#"import type { UserConfig } from 'vite-plus';
+
+export default {
+  lint: { rules: {} },
+} satisfies UserConfig;
+"#;
+        assert!(has_config_key(cfg, "lint").unwrap());
+        assert!(!has_config_key(cfg, "fmt").unwrap());
+    }
+
+    #[test]
+    fn test_has_config_key_return_variable_is_unknown() {
+        // The merger handles this via object spread, so duplication is benign.
+        // We conservatively report `false`.
+        let cfg = r#"import { defineConfig } from 'vite-plus';
+
+export default defineConfig(({ mode }) => {
+  const configObject = { fmt: { singleQuote: true } };
+  return configObject;
+});
+"#;
+        assert!(!has_config_key(cfg, "fmt").unwrap());
+    }
+
+    #[test]
+    fn test_has_config_key_arrow_wrapper_around_defineconfig() {
+        // export default () => defineConfig({ ... }) — the wrapper is irrelevant;
+        // detection follows the defineConfig argument object.
+        let cfg = r#"import { defineConfig } from 'vite-plus';
+
+export default () =>
+  defineConfig({
+    fmt: { singleQuote: true },
+  });
+"#;
+        assert!(has_config_key(cfg, "fmt").unwrap());
+    }
+
+    #[test]
+    fn test_has_config_key_fate_template_shape() {
+        // Mirrors create-fate's drizzle template — the bug that motivated this fix.
+        let cfg = r#"import { defineConfig } from 'vite-plus';
+
+export default defineConfig({
+  fmt: {
+    experimentalSortImports: { newlinesBetween: false },
+    ignorePatterns: ['coverage/', 'dist/'],
+    singleQuote: true,
+  },
+  lint: {
+    extends: [nkzw],
+    options: { typeAware: true, typeCheck: true },
+    rules: { '@typescript-eslint/no-explicit-any': 'off' },
+  },
+  staged: { '*': 'vp check --fix' },
+});
+"#;
+        assert!(has_config_key(cfg, "fmt").unwrap());
+        assert!(has_config_key(cfg, "lint").unwrap());
+        assert!(has_config_key(cfg, "staged").unwrap());
+        assert!(!has_config_key(cfg, "pack").unwrap());
+    }
+
     #[test]
     fn test_check_function_callback() {
         let simple_config = r#"
diff --git a/packages/cli/binding/index.cjs b/packages/cli/binding/index.cjs
index be16457fe6..8abc4ebbef 100644
--- a/packages/cli/binding/index.cjs
+++ b/packages/cli/binding/index.cjs
@@ -767,6 +767,7 @@ if (!nativeBinding) {
 module.exports = nativeBinding;
 module.exports.detectWorkspace = nativeBinding.detectWorkspace;
 module.exports.downloadPackageManager = nativeBinding.downloadPackageManager;
+module.exports.hasConfigKey = nativeBinding.hasConfigKey;
 module.exports.mergeJsonConfig = nativeBinding.mergeJsonConfig;
 module.exports.mergeTsdownConfig = nativeBinding.mergeTsdownConfig;
 module.exports.rewriteEslint = nativeBinding.rewriteEslint;
diff --git a/packages/cli/binding/index.d.cts b/packages/cli/binding/index.d.cts
index 334e9b12f1..2050d90502 100644
--- a/packages/cli/binding/index.d.cts
+++ b/packages/cli/binding/index.d.cts
@@ -116,6 +116,16 @@ export interface DownloadPackageManagerResult {
   version: string;
 }
 
+/**
+ * Whether `config_key` is already declared as a top-level property in the
+ * vite config's `defineConfig({...})` (or equivalent) object literal.
+ *
+ * AST-based check covering the six shapes the merger understands; ignores
+ * comments, string literal occurrences, and nested keys. Returns `false`
+ * for unrecognized shapes (e.g. `return $VAR` from a callback).
+ */
+export declare function hasConfigKey(viteConfigPath: string, configKey: string): boolean;
+
 /** Result returned by JavaScript resolver functions. */
 export interface JsCommandResolvedResult {
   binPath: string;
diff --git a/packages/cli/binding/src/migration.rs b/packages/cli/binding/src/migration.rs
index 73d0553328..6b9e3af932 100644
--- a/packages/cli/binding/src/migration.rs
+++ b/packages/cli/binding/src/migration.rs
@@ -120,6 +120,18 @@ pub fn merge_json_config(
     })
 }
 
+/// Whether `config_key` is already declared as a top-level property in the
+/// vite config's `defineConfig({...})` (or equivalent) object literal.
+///
+/// AST-based check covering the six shapes the merger understands; ignores
+/// comments, string literal occurrences, and nested keys. Returns `false`
+/// for unrecognized shapes (e.g. `return $VAR` from a callback).
+#[napi]
+pub fn has_config_key(vite_config_path: String, config_key: String) -> Result<bool> {
+    let content = std::fs::read_to_string(&vite_config_path).map_err(anyhow::Error::from)?;
+    Ok(vite_migration::has_config_key(&content, &config_key).map_err(anyhow::Error::from)?)
+}
+
 /// Error from batch import rewriting
 #[napi(object)]
 pub struct BatchRewriteError {
diff --git a/packages/cli/snap-tests-global/migration-preserves-existing-fmt-and-lint/.oxfmtrc.jsonc b/packages/cli/snap-tests-global/migration-preserves-existing-fmt-and-lint/.oxfmtrc.jsonc
new file mode 100644
index 0000000000..aea6e47aad
--- /dev/null
+++ b/packages/cli/snap-tests-global/migration-preserves-existing-fmt-and-lint/.oxfmtrc.jsonc
@@ -0,0 +1,4 @@
+{
+  "singleQuote": false,
+  "printWidth": 80,
+}
diff --git a/packages/cli/snap-tests-global/migration-preserves-existing-fmt-and-lint/.oxlintrc.json b/packages/cli/snap-tests-global/migration-preserves-existing-fmt-and-lint/.oxlintrc.json
new file mode 100644
index 0000000000..232525913e
--- /dev/null
+++ b/packages/cli/snap-tests-global/migration-preserves-existing-fmt-and-lint/.oxlintrc.json
@@ -0,0 +1,8 @@
+{
+  "rules": {
+    "no-unused-vars": "error"
+  },
+  "options": {
+    "typeAware": false
+  }
+}
diff --git a/packages/cli/snap-tests-global/migration-preserves-existing-fmt-and-lint/package.json b/packages/cli/snap-tests-global/migration-preserves-existing-fmt-and-lint/package.json
new file mode 100644
index 0000000000..77272a6f34
--- /dev/null
+++ b/packages/cli/snap-tests-global/migration-preserves-existing-fmt-and-lint/package.json
@@ -0,0 +1,6 @@
+{
+  "devDependencies": {
+    "oxfmt": "1",
+    "oxlint": "1"
+  }
+}
diff --git a/packages/cli/snap-tests-global/migration-preserves-existing-fmt-and-lint/snap.txt b/packages/cli/snap-tests-global/migration-preserves-existing-fmt-and-lint/snap.txt
new file mode 100644
index 0000000000..33bf3d8916
--- /dev/null
+++ b/packages/cli/snap-tests-global/migration-preserves-existing-fmt-and-lint/snap.txt
@@ -0,0 +1,63 @@
+> vp migrate --no-interactive 2>&1 # should NOT duplicate fmt/lint blocks already in vite.config.ts (regression for vp create fate)
+◇ Migrated . to Vite+
+• Node <semver>  pnpm <semver>
+• 1 config update applied
+
+> cat vite.config.ts # exactly one fmt: and one lint: block, preserving template values
+import { defineConfig } from 'vite-plus';
+
+export default defineConfig({
+  staged: {
+    "*": "vp check --fix"
+  },
+  fmt: {
+    experimentalSortImports: {
+      newlinesBetween: false,
+    },
+    experimentalSortPackageJson: {
+      sortScripts: true,
+    },
+    experimentalTailwindcss: {
+      stylesheet: 'client/src/App.css',
+    },
+    ignorePatterns: [
+      'coverage/',
+      'dist/',
+      '.fate/',
+      'client/dist/',
+      'client/src/translations/',
+      'server/dist/',
+      'pnpm-lock.yaml',
+    ],
+    singleQuote: true,
+  },
+  lint: {
+    extends: ['@nkzw/oxlint-config'],
+    ignorePatterns: [
+      'coverage',
+      'dist',
+      '.fate',
+      'client/dist',
+      'server/dist',
+      'server/src/drizzle/migrations/**',
+    ],
+    options: { typeAware: true, typeCheck: true },
+    overrides: [
+      {
+        files: ['server/src/index.tsx', 'server/src/drizzle/seed.tsx', '**/__tests__/**'],
+        rules: {
+          'no-console': 'off',
+        },
+      },
+    ],
+    rules: {
+      '@typescript-eslint/no-explicit-any': 'off',
+    },
+  },
+});
+
+> test ! -f .oxfmtrc.jsonc && echo '.oxfmtrc.jsonc removed' # redundant standalone file removed
+.oxfmtrc.jsonc removed
+
+> test ! -f .oxlintrc.json && echo '.oxlintrc.json removed' # redundant standalone file removed
+.oxlintrc.json removed
diff --git a/packages/cli/snap-tests-global/migration-preserves-existing-fmt-and-lint/steps.json b/packages/cli/snap-tests-global/migration-preserves-existing-fmt-and-lint/steps.json
new file mode 100644
index 0000000000..cd04d1c41b
--- /dev/null
+++ b/packages/cli/snap-tests-global/migration-preserves-existing-fmt-and-lint/steps.json
@@ -0,0 +1,8 @@
+{
+  "commands": [
+    "vp migrate --no-interactive 2>&1 # should NOT duplicate fmt/lint blocks already in vite.config.ts (regression for vp create fate)",
+    "cat vite.config.ts # exactly one fmt: and one lint: block, preserving template values",
+    "test ! -f .oxfmtrc.jsonc && echo '.oxfmtrc.jsonc removed' # redundant standalone file removed",
+    "test ! -f .oxlintrc.json && echo '.oxlintrc.json removed' # redundant standalone file removed"
+  ]
+}
diff --git a/packages/cli/snap-tests-global/migration-preserves-existing-fmt-and-lint/vite.config.ts b/packages/cli/snap-tests-global/migration-preserves-existing-fmt-and-lint/vite.config.ts
new file mode 100644
index 0000000000..c06fce4147
--- /dev/null
+++ b/packages/cli/snap-tests-global/migration-preserves-existing-fmt-and-lint/vite.config.ts
@@ -0,0 +1,48 @@
+import { defineConfig } from 'vite-plus';
+
+export default defineConfig({
+  fmt: {
+    experimentalSortImports: {
+      newlinesBetween: false,
+    },
+    experimentalSortPackageJson: {
+      sortScripts: true,
+    },
+    experimentalTailwindcss: {
+      stylesheet: 'client/src/App.css',
+    },
+    ignorePatterns: [
+      'coverage/',
+      'dist/',
+      '.fate/',
+      'client/dist/',
+      'client/src/translations/',
+      'server/dist/',
+      'pnpm-lock.yaml',
+    ],
+    singleQuote: true,
+  },
+  lint: {
+    extends: ['@nkzw/oxlint-config'],
+    ignorePatterns: [
+      'coverage',
+      'dist',
+      '.fate',
+      'client/dist',
+      'server/dist',
+      'server/src/drizzle/migrations/**',
+    ],
+    options: { typeAware: true, typeCheck: true },
+    overrides: [
+      {
+        files: ['server/src/index.tsx', 'server/src/drizzle/seed.tsx', '**/__tests__/**'],
+        rules: {
+          'no-console': 'off',
+        },
+      },
+    ],
+    rules: {
+      '@typescript-eslint/no-explicit-any': 'off',
+    },
+  },
+});
diff --git a/packages/cli/src/init-config.ts b/packages/cli/src/init-config.ts
index 15c4ad7e87..7991a0fa62 100644
--- a/packages/cli/src/init-config.ts
+++ b/packages/cli/src/init-config.ts
@@ -1,7 +1,7 @@
 import fs from 'node:fs';
 import path from 'node:path';
 
-import { mergeJsonConfig } from '../binding/index.js';
+import { hasConfigKey, mergeJsonConfig } from '../binding/index.js';
 import { createDefaultVitePlusLintConfig } from './oxlint-plugin-config.ts';
 import { fmt as resolveFmt } from './resolve-fmt.ts';
 import { runCommandSilently } from './utils/command.ts';
@@ -134,11 +134,6 @@ export default defineConfig({});
   return viteConfigPath;
 }
 
-function hasConfigKey(viteConfigPath: string, configKey: string): boolean {
-  const viteConfig = fs.readFileSync(viteConfigPath, 'utf8');
-  return new RegExp(`\\b${configKey}\\s*:`).test(viteConfig);
-}
-
 async function vpFmt(cwd: string, filePath: string): Promise<void> {
   const { binPath, envs } = await resolveFmt();
   const result = await runCommandSilently({
diff --git a/packages/cli/src/migration/__tests__/migrator.spec.ts b/packages/cli/src/migration/__tests__/migrator.spec.ts
index 45b094caa7..c710d0b1d3 100644
--- a/packages/cli/src/migration/__tests__/migrator.spec.ts
+++ b/packages/cli/src/migration/__tests__/migrator.spec.ts
@@ -1441,3 +1441,50 @@ describe('rewriteStandaloneProject — tsconfig types rewriting', () => {
     );
   });
 });
+
+// Regression: templates such as `create-fate` ship a populated vite.config.ts
+// alongside a standalone `.oxfmtrc.jsonc` / `.oxlintrc.json`. The merge step
+// must not insert a second `fmt:` / `lint:` block when one is already present.
+describe('rewriteStandaloneProject — preserves existing fmt/lint blocks', () => {
+  let tmpDir: string;
+
+  beforeEach(() => {
+    tmpDir = fs.mkdtempSync(path.join(os.tmpdir(), 'vp-test-merge-existing-'));
+    fs.writeFileSync(
+      path.join(tmpDir, 'package.json'),
+      JSON.stringify({ name: 'test', devDependencies: { vite: '^7.0.0' } }),
+    );
+  });
+
+  afterEach(() => {
+    fs.rmSync(tmpDir, { recursive: true, force: true });
+  });
+
+  it('does not duplicate fmt block when vite.config.ts already has one and .oxfmtrc.jsonc exists', () => {
+    fs.writeFileSync(
+      path.join(tmpDir, 'vite.config.ts'),
+      `import { defineConfig } from 'vite-plus';
+
+export default defineConfig({
+  fmt: {
+    singleQuote: true,
+  },
+});
+`,
+    );
+    fs.writeFileSync(
+      path.join(tmpDir, '.oxfmtrc.jsonc'),
+      JSON.stringify({ singleQuote: false }, null, 2),
+    );
+
+    rewriteStandaloneProject(tmpDir, makeWorkspaceInfo(tmpDir, PackageManager.pnpm), true, true);
+
+    const viteConfig = fs.readFileSync(path.join(tmpDir, 'vite.config.ts'), 'utf8');
+    expect(viteConfig.match(/\bfmt\s*:/g)?.length).toBe(1);
+    // Template-authored value wins (singleQuote: true) — standalone config dropped.
+    expect(viteConfig).toContain('singleQuote: true');
+    expect(viteConfig).not.toContain('singleQuote: false');
+    // Redundant standalone file removed.
+    expect(fs.existsSync(path.join(tmpDir, '.oxfmtrc.jsonc'))).toBe(false);
+  });
+});
diff --git a/packages/cli/src/migration/migrator.ts b/packages/cli/src/migration/migrator.ts
index a771fbe7e3..bb891296fc 100644
--- a/packages/cli/src/migration/migrator.ts
+++ b/packages/cli/src/migration/migrator.ts
@@ -9,6 +9,7 @@ import semver from 'semver';
 import { Scalar, YAMLMap, YAMLSeq } from 'yaml';
 
 import {
+  hasConfigKey,
   mergeJsonConfig,
   mergeTsdownConfig,
   rewriteEslint,
@@ -2065,11 +2066,8 @@ function injectConfigDefaults(
   report?: MigrationReport,
 ): void {
   const configs = detectConfigs(projectPath);
-  if (configs.viteConfig) {
-    const content = fs.readFileSync(path.join(projectPath, configs.viteConfig), 'utf8');
-    if (new RegExp(`\\b${configKey}\\s*:`).test(content)) {
-      return;
-    }
+  if (configs.viteConfig && hasConfigKey(path.join(projectPath, configs.viteConfig), configKey)) {
+    return;
   }
 
   const viteConfig = ensureViteConfig(projectPath, configs, silent, report);
@@ -2097,6 +2095,21 @@ function mergeAndRemoveJsonConfig(
 ): void {
   const fullViteConfigPath = path.join(projectPath, viteConfigPath);
   const fullJsonConfigPath = path.join(projectPath, jsonConfigPath);
+  // Skip merge when the key is already present in vite.config.ts — the Rust
+  // merge step always prepends, so without this guard a template that ships
+  // both an inline `${configKey}:` block and a standalone JSON file (e.g.
+  // create-fate's vite.config.ts + .oxfmtrc.jsonc) ends up with two of them.
+  // AST-based check ignores comments, string-literal occurrences, and nested
+  // keys (e.g. `plugins: [{ fmt: ... }]`).
+  if (hasConfigKey(fullViteConfigPath, configKey)) {
+    fs.unlinkSync(fullJsonConfigPath);
+    if (!silent) {
+      prompts.log.info(
+        `${configKey} config already present in ${displayRelative(fullViteConfigPath)} — removed redundant ${displayRelative(fullJsonConfigPath)}`,
+      );
+    }
+    return;
+  }
   const result = mergeJsonConfig(fullViteConfigPath, fullJsonConfigPath, configKey);
   if (result.updated) {
     fs.writeFileSync(fullViteConfigPath, result.content);