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wenshao merged 2 commits into from
May 25, 2026
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fix(core): preserve duplicate object references in safeJsonStringify #4407

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e53a528
fix(core): preserve duplicate object references in safeJsonStringify
ihubanov May 21, 2026
dad73ac
test(core): cover deep unwinding and toJSON paths in safeJsonStringify
ihubanov May 21, 2026
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111 changes: 111 additions & 0 deletions packages/core/src/utils/safeJsonStringify.test.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -70,4 +70,115 @@ describe('safeJsonStringify', () => {
expect(safeJsonStringify(42)).toBe('42');
expect(safeJsonStringify(true)).toBe('true');
});

it('should preserve duplicate sibling references as full copies', () => {
// The same object referenced from two sibling properties is not a cycle:
// both branches must serialize in full, matching native JSON.stringify.
const shared = { name: 'shared', n: 1 };
const obj = { a: shared, b: shared };

const result = safeJsonStringify(obj);
expect(result).toBe(
'{"a":{"name":"shared","n":1},"b":{"name":"shared","n":1}}',
);
expect(result).not.toContain('[Circular]');
});

it('should preserve duplicate references repeated in an array', () => {
const shared = { id: 1 };
const arr = [shared, shared, shared];

const result = safeJsonStringify(arr);
expect(result).toBe('[{"id":1},{"id":1},{"id":1}]');
expect(result).not.toContain('[Circular]');
});

it('should preserve a shared leaf appearing on multiple branches', () => {
const leaf = { kind: 'leaf' };
const tree = { left: { sub: leaf }, right: { sub: leaf } };

const result = safeJsonStringify(tree);
expect(result).toBe(
'{"left":{"sub":{"kind":"leaf"}},"right":{"sub":{"kind":"leaf"}}}',
);
expect(result).not.toContain('[Circular]');
});

it('should detect indirect cycles via an intermediate object', () => {
// eslint-disable-next-line @typescript-eslint/no-explicit-any
const parent: any = { name: 'parent' };
// eslint-disable-next-line @typescript-eslint/no-explicit-any
const child: any = { name: 'child' };
parent.child = child;
child.parent = parent;

const result = safeJsonStringify(parent);
expect(result).toBe(
'{"name":"parent","child":{"name":"child","parent":"[Circular]"}}',
);
});

it('should preserve a shared subtree alongside a real cycle', () => {
const shared = { tag: 'shared' };
// eslint-disable-next-line @typescript-eslint/no-explicit-any
const root: any = { a: shared, b: shared };
root.self = root;

const result = safeJsonStringify(root);
expect(result).toBe(
'{"a":{"tag":"shared"},"b":{"tag":"shared"},"self":"[Circular]"}',
);
});

it('should preserve a shared leaf reached through deep ancestor chains', () => {
// Forces the unwind loop to pop five frames between the deep branch and
// the sibling branch. Without the pop, the second occurrence of `shared`
// would still see `shared` on the stack and emit [Circular].
const shared = { tag: 'shared' };
const root = {
l1: { l2: { l3: { l4: { l5: { leaf: shared } } } } },
sibling: { leaf: shared },
};

const result = safeJsonStringify(root);
expect(result).toBe(
'{"l1":{"l2":{"l3":{"l4":{"l5":{"leaf":{"tag":"shared"}}}}}},"sibling":{"leaf":{"tag":"shared"}}}',
);
expect(result).not.toContain('[Circular]');
});

it('should detect a real cycle through deep ancestor chains', () => {
// eslint-disable-next-line @typescript-eslint/no-explicit-any
const root: any = { l1: { l2: { l3: { l4: { l5: { back: null } } } } } };
root.l1.l2.l3.l4.l5.back = root;

const result = safeJsonStringify(root);
expect(result).toBe(
'{"l1":{"l2":{"l3":{"l4":{"l5":{"back":"[Circular]"}}}}}}',
);
});

it('should preserve a shared object returned by toJSON from sibling positions', () => {
// JSON.stringify calls toJSON() before invoking the replacer, so the
// replacer sees the post-toJSON value. Two siblings whose toJSON returns
// the same object are duplicate refs, not a cycle.
const shared = { tag: 'shared' };
const root = {
a: { toJSON: () => shared },
b: { toJSON: () => shared },
};

const result = safeJsonStringify(root);
expect(result).toBe('{"a":{"tag":"shared"},"b":{"tag":"shared"}}');
expect(result).not.toContain('[Circular]');
});

it('should detect a cycle when toJSON returns an ancestor', () => {
// eslint-disable-next-line @typescript-eslint/no-explicit-any
const root: any = { name: 'root' };
root.child = { toJSON: () => root };

const result = safeJsonStringify(root);
expect(result).toBe('{"name":"root","child":"[Circular]"}');
});
});
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ihubanov marked this conversation as resolved.
26 changes: 19 additions & 7 deletions packages/core/src/utils/safeJsonStringify.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -7,6 +7,11 @@
/**
* Safely stringifies an object to JSON, handling circular references by replacing them with [Circular].
*
* Only true cycles (an object reachable from itself along the current ancestor
* path) are replaced. Duplicate references (the same object appearing in
* multiple sibling positions) are preserved as full copies, matching the
* behavior of `JSON.stringify` on acyclic graphs.
*
* @param obj - The object to stringify
* @param space - Optional space parameter for formatting (defaults to no formatting)
* @returns JSON string with circular references replaced by [Circular]
Expand All @@ -15,16 +20,23 @@ export function safeJsonStringify(
obj: unknown,
space?: string | number,
): string {
const seen = new WeakSet();
const ancestors: object[] = [];
return JSON.stringify(
obj,
(key, value) => {
if (typeof value === 'object' && value !== null) {
if (seen.has(value)) {
return '[Circular]';
}
seen.add(value);
function (this: unknown, _key, value) {
if (typeof value !== 'object' || value === null) {
return value;
}
// `this` is the parent of `value`. As JSON.stringify's DFS walk unwinds
// back up the tree, pop any ancestors that are no longer on the path
// to `this` so the stack reflects only the current chain of ancestors.
while (ancestors.length > 0 && ancestors[ancestors.length - 1] !== this) {
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ancestors.pop();
}

@wenshao wenshao May 21, 2026

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[Suggestion] DAG with shared refs can produce exponentially large output. The old WeakSet collapsed all revisited objects to [Circular] (O(n) output). The new algorithm fully expands every duplicate reference — correct per JSON.stringify semantics, but it removes the implicit output-size guard. A DAG with 50 levels where each node has 2 children referencing the same leaf produces ~2⁵⁰ copies of the leaf in the output, blocking the event loop until OOM.

This matters because safeJsonStringify is called on MCP tool params (mcp-tool.ts:452) and tool registry params (tool-registry.ts:48), which originate from external MCP servers (attacker-controlled input).

Consider adding a node-count cap as a safety valve:

let nodeCount = 0;
const MAX_NODES = 10_000;
// in replacer, before ancestors.push:
if (++nodeCount > MAX_NODES) return '[Truncated]';

— qwen-latest-series-invite-beta-v36 via Qwen Code /review

@ihubanov ihubanov May 21, 2026 •
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Fair concern, but I think this is the cost of correctness over the old behavior.

Native JSON.stringify already produces the 2^N output on the same DAG shape, so what the helper does now matches what callers would get if they removed the safeJsonStringify wrapper. The old WeakSet doubled as a hidden size guard, but it did so by silently corrupting the output for any acyclic graph with shared refs.

I looked at the actual callers (loggers.ts, file-exporters.ts, qwen-logger.ts, message-bus.ts, mcp-tool.ts, tool-registry.ts). They all stringify request/response payloads, tool args, error messages. Flat-ish JSON from the model or typed message structs. None of them currently fan out into a DAG shape that would blow up.

If size-guarded serialization for arbitrary user input becomes a real concern, I think that's a separate enhancement (a max output size or max duplicate count knob) rather than reintroducing the false [Circular]s here. Can revisit as a follow-up if telemetry ever surfaces a caller that needs it.

if (ancestors.includes(value as object)) {
return '[Circular]';
}
ancestors.push(value as object);
return value;
},
space,
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