Repository navigation
[BUG]: json_schema_builder throws on a property named $id, so it can't validate a catalog against catalog_definition.json #4188
Description
- I have searched the existing issues to make sure this bug has not already been reported.
Describe the Bug
json_schema_builder throws a TypeError when a schema has a property named $id, $anchor or $dynamicAnchor and validation has to look up an anchor. That's a $ref to an $anchor, or any $dynamicRef.
I ran into this validating a catalog against the official v1.0 catalog_definition.json. That schema declares a property named $id (line 9), and it checks each component against the JSON Schema 2020-12 meta-schema (lines 229-235), which uses $dynamicRef. So any catalog where a component has properties throws instead of returning validation errors, and I ended up dropping that check from my tests.
The package lives in flutter/genui, which routes issues here.
Steps to Reproduce
Run this with json_schema_builder: 0.1.7. It doesn't need any files or network.
import 'package:json_schema_builder/json_schema_builder.dart';
void main() {
for (final name in ['name', r'$id', r'$anchor', r'$dynamicAnchor']) {
final schema = Schema.fromMap({
'type': 'object',
'properties': {
name: {'type': 'string'},
'child': {r'$ref': '#node'},
},
r'$defs': {
'node': {r'$anchor': 'node', 'type': 'object'},
},
});
try {
print('$name: ${schema.validateSync({'child': <String, Object?>{}})}');
} catch (error) {
print('$name: $error');
}
}
}Expected Behavior
All four lines print [], like name does.
Under properties, the keys are instance property names, not keywords (JSON Schema Core 10.3.2.1), so a property can be called $id. ajv accepts these schemas, and it accepts the catalog below under catalog_definition.json.
Screenshots / Video / Logs
Output of the program:
name: []
$id: type '_Map<String, String>' is not a subtype of type 'String?' in type cast
$anchor: type '_Map<String, String>' is not a subtype of type 'String?' in type cast
$dynamicAnchor: type '_Map<String, String>' is not a subtype of type 'String?' in type cast
The A2UI case. The third catalog is {"catalogId":"https://example.com/c","protocolVersion":"1.0","components":{"X":{"type":"object","properties":{"component":{"const":"X"}}}}}:
component {type: object} -> valid
component {description: "x"} -> valid
component with one property (the shape every real component has) -> THROWS _TypeError: type '_Map<String, dynamic>' is not a subtype of type 'String?' in type cast
at #0 Schema.$id (package:json_schema_builder/src/schema/schema.dart:458:37) | #1 SchemaValidation._findDynamicAnchorInSchema.visit (package:json_schema_builder/src/schema_validation.dart:1421:48) | #2 SchemaValidation._findDynamicAnchorInSchema.visit (package:json_schema_builder/src/schema_validation.dart:1431:11)
Environment Details
- OS: macOS 27.0.1
- Browser/Platform: Dart SDK 3.13.4
- SDK/Package Name & Version: json_schema_builder 0.1.7 from pub.dev, and
flutter/genuimain (27f5846). 0.1.0 and 0.1.6 throw too. - Protocol Version: v1.0 for the catalog case (
a2uimain, 8bf644a). The program above doesn't depend on a protocol version. - Agent Framework & LLM Model: n/a
Additional Context
Two functions search a schema for an anchor, and both walk every map in the document as if it were a schema. That includes the map under properties:
SchemaRegistry._findAnchor, which is what the program above hitsSchemaValidation._findDynamicAnchorInSchema, which is what the A2UI case hits
They read $id, $anchor and $dynamicAnchor through getters that cast to String?. Under properties the value is a schema, so the cast throws.
The JSON Schema Test Suite has this case for $ref ("property named $ref that is not a reference"), but nothing for these three names, so the package's tests don't cover it. The a2ui_core test that checks catalogs against this schema (_catalogDefinitionErrors) strips the external $refs first, so it doesn't reach this either.
I think the fix is for both functions to only descend into the keywords that hold subschemas. Happy to open a PR on flutter/genui with that and a regression test if that works for you.
Activity
polina-c commentedon Nov 12, 2026
From triage bot:
Triage suggestion:
For humans
Summary:
json_schema_builder, which lives in flutter/genui and whicha2ui_coredepends on, throws aTypeErrorwhen a schema has a property named$id,$anchoror$dynamicAnchorand validation looks up an anchor. The v1.0 catalog_definition.json declares a property named$id, so validating a catalog against it throws. The report has a short reproduction and offers a fix.Why flagged for triage:
This issue has external input but no priority label yet.
Suggestion:
Set P2, and accept the reporter's offer to fix it in flutter/genui. I read the code at the reported commit and the diagnosis holds:
_findAnchorand_findDynamicAnchorInSchemavisit every map in the schema, including the map underproperties, and read these keywords through getters that cast toString?. P2 rather than P1: the reported impact is on validating a catalog against the component definition in tests and tools, and the reporter has a workaround (dropping that check).gspencergoogis the best reviewer for the genui PR: they wrote 10 of the last 20 commits tojson_schema_builder.type: bug,P2,lang: dart,status: first-line-handledReply:
To adjust and apply this suggestion, run in your a2ui checkout:
For software
🤖 Agent-generated
Varun-S10 commentedon Nov 12, 2026
Hi @leoafarias, Thank you for raising the issue. We have reproduced the issue and we appreciate you have raised pr to fix the issue. The pr will be reviewed. Thank you for your patience and understanding.