diff --git a/USAGE.md b/USAGE.md
index 12bef41..f48f9e6 100644
--- a/USAGE.md
+++ b/USAGE.md
@@ -226,6 +226,7 @@ These are invalid:
 |---|---|
 | Unbalanced parentheses | `(tag:bug`, `tag:bug)` |
 | Parentheses nested more than 64 levels deep | `((((…a…))))` with 65 `(` |
+| More than 256 search terms (words, phrases and qualifiers) | 257 words |
 | An operator with a missing side | `bug AND`, `OR bug` |
 | Consecutive operators | `a AND OR b` |
 | A qualifier without a value | `tag:` |
diff --git a/YKanBan.Tests/Search/SearchQueryParserTests.cs b/YKanBan.Tests/Search/SearchQueryParserTests.cs
index c28b77e..6ccd7c1 100644
--- a/YKanBan.Tests/Search/SearchQueryParserTests.cs
+++ b/YKanBan.Tests/Search/SearchQueryParserTests.cs
@@ -248,6 +248,28 @@ public class SearchQueryParserTests
         Assert.IsNotNull(SearchQueryParser.Parse(groups));
     }
 
+    [TestMethod]
+    public void AtomCountAtTheLimitParses()
+    {
+        Assert.IsNotNull(SearchQueryParser.Parse(string.Join(" ", Enumerable.Repeat("a", SearchQueryParser.MaxAtomCount))));
+    }
+
+    [TestMethod]
+    [DataRow(" ")]
+    [DataRow(" OR ")]
+    public void AtomCountBeyondTheLimitIsAnError(string separator)
+    {
+        AssertSyntaxError(string.Join(separator, Enumerable.Repeat("a", SearchQueryParser.MaxAtomCount + 1)));
+    }
+
+    [TestMethod]
+    public void QualifiersAndPhrasesCountTowardsTheAtomLimit()
+    {
+        int half = SearchQueryParser.MaxAtomCount / 2;
+        string text = string.Join(" ", Enumerable.Repeat("tag:\"a b\"", half).Concat(Enumerable.Repeat("\"c\"", half + 1)));
+        AssertSyntaxError(text);
+    }
+
     [TestMethod]
     public void VeryDeepNestingIsAnErrorRatherThanACrash()
     {
diff --git a/YKanBan.Tests/Search/SearchSqlCompilerTests.cs b/YKanBan.Tests/Search/SearchSqlCompilerTests.cs
index 9a81a1d..f643517 100644
--- a/YKanBan.Tests/Search/SearchSqlCompilerTests.cs
+++ b/YKanBan.Tests/Search/SearchSqlCompilerTests.cs
@@ -190,4 +190,20 @@ public class SearchSqlCompilerTests
         CollectionAssert.AreEqual(new[] { 5L }, fixture.Search("tag:äpfel"));
         CollectionAssert.AreEqual(new[] { 5L }, fixture.Search("column:érable"));
     }
+
+    [TestMethod]
+    public void LargestAcceptedQueriesRunWithinTheSqliteExpressionDepth()
+    {
+        using var fixture = new SearchFixture();
+        int count = SearchQueryParser.MaxAtomCount;
+        int depth = SearchQueryParser.MaxNestingDepth;
+        string nested = string.Concat(Enumerable.Repeat("(tag:x OR ", depth))
+            + string.Join(" ", Enumerable.Repeat("apple", count - depth))
+            + new string(')', depth);
+
+        CollectionAssert.AreEqual(new[] { 2L }, fixture.Search(string.Join(" ", Enumerable.Repeat("apple", count))));
+        CollectionAssert.AreEqual(new[] { 2L }, fixture.Search(string.Join(" OR ", Enumerable.Repeat("apple", count))));
+        CollectionAssert.AreEqual(new[] { 2L }, fixture.Search(string.Join(" ", Enumerable.Repeat("title:apple", count))));
+        CollectionAssert.AreEqual(new[] { 2L }, fixture.Search(nested));
+    }
 }
diff --git a/YKanBan/Search/SearchQueryParser.cs b/YKanBan/Search/SearchQueryParser.cs
index f1880c2..8d66453 100644
--- a/YKanBan/Search/SearchQueryParser.cs
+++ b/YKanBan/Search/SearchQueryParser.cs
@@ -9,8 +9,8 @@ namespace YKanBan.Search;
 /// phrases, invalid escapes, a backslash in a bare word, unknown qualifier
 /// keys, qualifiers without a value, unquoted values containing ':' or equal
 /// to AND/OR, id values that are not an unquoted positive integer with an
-/// optional '#', and parentheses nested deeper than <see cref="MaxNestingDepth"/>)
-/// raises <see cref="SearchSyntaxException"/>.
+/// optional '#', parentheses nested deeper than <see cref="MaxNestingDepth"/>,
+/// and more than <see cref="MaxAtomCount"/> atoms) raises <see cref="SearchSyntaxException"/>.
 ///
 /// Only uppercase AND/OR are operators (lowercase ones are plain words); AND —
 /// explicit or by adjacency — binds tighter than OR; parentheses group. Blank
@@ -25,6 +25,14 @@ public static class SearchQueryParser
     /// </summary>
     public const int MaxNestingDepth = 64;
 
+    /// <summary>
+    /// Most atoms (words, phrases and qualifiers) accepted in one expression.
+    /// The compiled SQL chains every atom into one expression, and SQLite
+    /// rejects expressions nested deeper than 1000 levels; together with
+    /// <see cref="MaxNestingDepth"/> this keeps every valid query well below that.
+    /// </summary>
+    public const int MaxAtomCount = 256;
+
     /// <summary>The recognized qualifier keys; add new entries here to extend the grammar.</summary>
     private static readonly (string Key, QualifierKind Kind)[] QualifierTable =
     [
@@ -122,7 +130,9 @@ public static class SearchQueryParser
         {
             List<SearchToken> tokens = SearchLexer.Tokenize(text).ToList();
             EnsureNestingWithinLimit(tokens);
-            return Query.ParseOrThrow(tokens);
+            SearchNode query = Query.ParseOrThrow(tokens);
+            EnsureAtomCountWithinLimit(query);
+            return query;
         }
         catch (ParseException exception)
         {
@@ -152,6 +162,31 @@ public static class SearchQueryParser
         }
     }
 
+    /// <summary>
+    /// Rejects expressions with more than <see cref="MaxAtomCount"/> atoms.
+    /// </summary>
+    /// <param name="query">The parsed query.</param>
+    /// <exception cref="SearchSyntaxException">The expression has too many atoms.</exception>
+    private static void EnsureAtomCountWithinLimit(SearchNode query)
+    {
+        if (CountAtoms(query) > MaxAtomCount)
+        {
+            throw new SearchSyntaxException($"The expression has more than {MaxAtomCount} search terms.");
+        }
+    }
+
+    /// <summary>
+    /// Counts the atoms (leaves) of a parsed query.
+    /// </summary>
+    /// <param name="node">The node to count.</param>
+    /// <returns>The number of atoms under the node.</returns>
+    private static int CountAtoms(SearchNode node) => node switch
+    {
+        SearchNode.And and => and.Terms.Sum(CountAtoms),
+        SearchNode.Or or => or.Terms.Sum(CountAtoms),
+        _ => 1,
+    };
+
     #region Atom construction
 
     /// <summary>
diff --git a/docs/PLAN.md b/docs/PLAN.md
index 92ced0d..7a273d5 100644
--- a/docs/PLAN.md
+++ b/docs/PLAN.md
@@ -16,7 +16,7 @@
 - 网络共享明确不支持；命令行参数边界；macOS 无参启动行为写入文档（§2.3、§4.1、附录）
 - 补充：导出转义规则、卡片列表虚拟化、模态窗口 FlowDirection、`Resources` 类名使用规则、CI osx-x64 运行器（§5、§7、§8）
 - 依赖：Avalonia 11.3.6 → 11.3.22，Semi.Avalonia 11.2.1.10 → 11.3.22（§7）
-- 实现回补：搜索括号嵌套上限 64 层，空短语、短语紧贴文本、`key:` 后空白均为非法（§6.1、§6.2）；导出表格单元格转义 `\`（§5.8）；初始化中途锁冲突进锁冲突页、数据库文件缺失不静默新建（§4.4）
+- 实现回补：搜索括号嵌套上限 64 层、单个表达式至多 256 个原子，空短语、短语紧贴文本、`key:` 后空白均为非法（§6.1、§6.2）；导出表格单元格转义 `\`（§5.8）；初始化中途锁冲突进锁冲突页、数据库文件缺失不静默新建（§4.4）
 - 里程碑：标注 M0–M4 已完成，新增 **M4R（v1.9 回补）**（§9）
 
 ## 1. 项目概述
@@ -295,6 +295,7 @@ phrase  := '"' 一个或多个字符 '"'   （内部支持转义：\" → 字面
 - **严格校验**：非法表达式弹出错误对话框，不执行搜索。非法集合：
   - 括号不配对
   - 括号嵌套超过 64 层（防止递归解析栈溢出；顺序并列的括号组不计入深度）
+  - 原子（裸词、短语、限定符）超过 256 个（编译出的 SQL 把全部原子串成一个表达式，SQLite 表达式深度上限为 1000）
   - 运算符悬空（`bug AND`）、连续运算符（`a AND OR b`）
   - 限定符无值（`tag:`）、限定符值为裸 `AND`/`OR`（`tag:AND`，须写 `tag:"AND"`）
   - 短语引号未闭合、无效转义（`tag:"a\x"`）、裸词中出现 `\`