diff --git a/packages/agent-bff/src/http/bff-http-error.ts b/packages/agent-bff/src/http/bff-http-error.ts
index dbba05fc4a..789db7c8bb 100644
--- a/packages/agent-bff/src/http/bff-http-error.ts
+++ b/packages/agent-bff/src/http/bff-http-error.ts
@@ -79,3 +79,12 @@ export function missingTimezone(
 export function invalidTimezone(value: string): BffHttpError {
   return new BffHttpError(400, 'invalid_timezone', `Invalid timezone: "${value}"`);
 }
+
+export function relationFieldNotSupported(fields: string[]): BffHttpError {
+  return new BffHttpError(
+    422,
+    'relation_field_not_supported',
+    'Nested relation field paths are not supported on top-level list and count',
+    { fields },
+  );
+}
diff --git a/packages/agent-bff/src/validation/relation-field-guard.ts b/packages/agent-bff/src/validation/relation-field-guard.ts
new file mode 100644
index 0000000000..8f6887f419
--- /dev/null
+++ b/packages/agent-bff/src/validation/relation-field-guard.ts
@@ -0,0 +1,15 @@
+import { relationFieldNotSupported } from '../http/bff-http-error';
+
+const RELATION_SEPARATOR = ':';
+
+export default function assertNoRelationFieldPaths(paths: string[]): void {
+  const offending: string[] = [];
+
+  for (const path of paths) {
+    if (path.includes(RELATION_SEPARATOR) && !offending.includes(path)) {
+      offending.push(path);
+    }
+  }
+
+  if (offending.length > 0) throw relationFieldNotSupported(offending);
+}
diff --git a/packages/agent-bff/test/validation/relation-field-guard.test.ts b/packages/agent-bff/test/validation/relation-field-guard.test.ts
new file mode 100644
index 0000000000..a88c79a331
--- /dev/null
+++ b/packages/agent-bff/test/validation/relation-field-guard.test.ts
@@ -0,0 +1,101 @@
+import { type BffHttpError, toErrorBody } from '../../src/http/bff-http-error';
+import assertNoRelationFieldPaths from '../../src/validation/relation-field-guard';
+
+function captureError(fn: () => void): unknown {
+  try {
+    fn();
+  } catch (error) {
+    return error;
+  }
+
+  throw new Error('expected to throw');
+}
+
+describe('assertNoRelationFieldPaths', () => {
+  it('rejects a nested relation path from a list projection', () => {
+    expect(() => assertNoRelationFieldPaths(['id', 'company:name'])).toThrow(
+      expect.objectContaining({
+        type: 'relation_field_not_supported',
+        status: 422,
+        details: { fields: ['company:name'] },
+      }),
+    );
+  });
+
+  it('rejects a nested relation path from a list filter field', () => {
+    expect(() => assertNoRelationFieldPaths(['owner:email'])).toThrow(
+      expect.objectContaining({
+        type: 'relation_field_not_supported',
+        status: 422,
+        details: { fields: ['owner:email'] },
+      }),
+    );
+  });
+
+  it('rejects a nested relation path from a list sort field', () => {
+    expect(() => assertNoRelationFieldPaths(['company:createdAt'])).toThrow(
+      expect.objectContaining({
+        type: 'relation_field_not_supported',
+        status: 422,
+        details: { fields: ['company:createdAt'] },
+      }),
+    );
+  });
+
+  it('rejects a nested relation path from a count filter field', () => {
+    expect(() => assertNoRelationFieldPaths(['status', 'company:tier'])).toThrow(
+      expect.objectContaining({
+        type: 'relation_field_not_supported',
+        status: 422,
+        details: { fields: ['company:tier'] },
+      }),
+    );
+  });
+
+  it('lists every offending path when several are present', () => {
+    expect(() =>
+      assertNoRelationFieldPaths(['id', 'company:name', 'title', 'owner:email']),
+    ).toThrow(
+      expect.objectContaining({
+        details: { fields: ['company:name', 'owner:email'] },
+      }),
+    );
+  });
+
+  it('reports a single offending path as an array of length 1', () => {
+    const error = captureError(() => assertNoRelationFieldPaths(['company:name']));
+
+    expect((error as BffHttpError).details).toEqual({ fields: ['company:name'] });
+  });
+
+  it('dedupes an offending path seen on two surfaces, keeping first-seen order', () => {
+    expect(() =>
+      assertNoRelationFieldPaths(['company:name', 'id', 'company:name', 'owner:email']),
+    ).toThrow(
+      expect.objectContaining({
+        details: { fields: ['company:name', 'owner:email'] },
+      }),
+    );
+  });
+
+  it('passes through direct field paths unchanged', () => {
+    expect(() => assertNoRelationFieldPaths(['id', 'title', 'createdAt'])).not.toThrow();
+  });
+
+  it('passes through an empty path list', () => {
+    expect(() => assertNoRelationFieldPaths([])).not.toThrow();
+  });
+
+  it('serializes to the wire error envelope with details.fields as an array', () => {
+    const error = captureError(() => assertNoRelationFieldPaths(['company:name', 'owner:email']));
+
+    expect(toErrorBody(error as BffHttpError)).toEqual({
+      error: {
+        type: 'relation_field_not_supported',
+        status: 422,
+        message: 'Nested relation field paths are not supported on top-level list and count',
+        details: { fields: ['company:name', 'owner:email'] },
+      },
+    });
+  });
+});