diff --git a/CHANGELOG.md b/CHANGELOG.md
index 684fed8..343217e 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -4,6 +4,10 @@ Notable changes to this project, following [Keep a Changelog](https://keepachang
 
 ## [Unreleased]
 
+### Fixed
+
+- The editor plugin stops reporting `ambiguous column` on the branches of a set operation. It pooled every FROM/JOIN in the statement into one scope, so the columns a `union` shares - which is all of them, since branches have to be column-compatible - looked like they came from two tables at once, putting a squiggle on nearly every UNION. Diagnostics now cover the first branch, which is also the branch the core takes the row shape from ([#294](https://github.com/tiagolauer/OwlSQL/issues/294)).
+
 ## [0.2.0] - 2026-07-29
 
 ### Changed
diff --git a/ts-plugin/src/diagnostics.cts b/ts-plugin/src/diagnostics.cts
index 02733b1..164772e 100644
--- a/ts-plugin/src/diagnostics.cts
+++ b/ts-plugin/src/diagnostics.cts
@@ -285,6 +285,40 @@ function columnTokenFromEntry(entry: ColumnEntry): { token: string; offset: numb
   return { token, offset: leadingWhitespace };
 }
 
+// Only the first branch of a set operation is diagnosed. The scanner pools
+// every FROM/JOIN in the statement into one scope, so both branches of a
+// `union` shared a source list and any column name they had in common - which
+// is every column, since branches have to be compatible - came back as
+// `ambiguous column` (issue #294). The core types a set operation by its first
+// branch, and truncating here matches that: later branches are not reported
+// on, instead of being reported wrongly. Truncation keeps every offset before
+// the cut, so spans stay aligned with the source.
+const SET_OPERATOR = /\b(?:union|intersect|except)\b/gi;
+
+function parenDepthAt(text: string, index: number): number {
+  let depth = 0;
+  for (let i = 0; i < index; i += 1) {
+    const char = text[i];
+    if (char === '(') {
+      depth += 1;
+    } else if (char === ')') {
+      depth -= 1;
+    }
+  }
+  return depth;
+}
+
+function firstBranchEnd(stripped: string): number {
+  SET_OPERATOR.lastIndex = 0;
+  let match: RegExpExecArray | null;
+  while ((match = SET_OPERATOR.exec(stripped)) !== null) {
+    if (parenDepthAt(stripped, match.index) === 0) {
+      return match.index;
+    }
+  }
+  return stripped.length;
+}
+
 function getQueryDiagnostics(
   typescript: typeof ts,
   checker: ts.TypeChecker,
@@ -298,7 +332,9 @@ function getQueryDiagnostics(
   // computed against it drift out of alignment with literalStart (a raw
   // source position) by one character per preceding line break on a CRLF
   // file. Mirrors the same fix already applied to hover in index.cts.
-  const text = sourceFile.text.slice(literalStart, literal.getEnd() - 1);
+  const fullText = sourceFile.text.slice(literalStart, literal.getEnd() - 1);
+  const branchEnd = firstBranchEnd(stripStringLiterals(fullText).stripped);
+  const text = fullText.slice(0, branchEnd);
   const { stripped } = stripStringLiterals(text);
   // A CTE query's outer statement doesn't start at offset 0 - skip the
   // WITH-clause prefix (mirroring ParseWithClause in src/cte.ts) so the
diff --git a/ts-plugin/tests/diagnostics.test.ts b/ts-plugin/tests/diagnostics.test.ts
index 4b516c9..7ba49dd 100644
--- a/ts-plugin/tests/diagnostics.test.ts
+++ b/ts-plugin/tests/diagnostics.test.ts
@@ -284,4 +284,37 @@ describe('ts-plugin diagnostics: getQueryDiagnostics', () => {
       diagnosticsFor('select posts.titel from users join posts on users.id = posts.user_id'),
     ).toEqual([{ message: 'unknown column: titel', text: 'titel' }]);
   });
+
+  // Regression for #294: the scanner pooled both branches of a set operation
+  // into one scope, so a column name they share - which is every column, since
+  // branches have to be compatible - came back as ambiguous.
+  describe('set operations', () => {
+    it('does not report a shared column name across UNION branches', () => {
+      expect(diagnosticsFor('select id from users union select id from posts')).toEqual([]);
+    });
+
+    it('does not report across UNION ALL, INTERSECT or EXCEPT either', () => {
+      expect(diagnosticsFor('select id from users union all select id from posts')).toEqual([]);
+      expect(diagnosticsFor('select id from users intersect select id from posts')).toEqual([]);
+      expect(diagnosticsFor('select id from users except select id from posts')).toEqual([]);
+    });
+
+    it('still reports a typo in the first branch', () => {
+      expect(diagnosticsFor('select nope from users union select id from posts')).toEqual([
+        { message: 'unknown column: nope', text: 'nope' },
+      ]);
+    });
+
+    it('still reports an ambiguous column inside the first branch itself', () => {
+      expect(
+        diagnosticsFor('select id from users join posts on users.id = posts.user_id union select id from posts'),
+      ).toEqual([{ message: 'ambiguous column: id', text: 'id' }]);
+    });
+
+    it('does not cut the statement at a union inside a subquery', () => {
+      expect(
+        diagnosticsFor('select nope from users where id in (select id from posts union select id from posts)'),
+      ).toEqual([{ message: 'unknown column: nope', text: 'nope' }]);
+    });
+  });
 });