diff --git a/packages/cordis/tool-cordis/tests/mount.spec.ts b/packages/cordis/tool-cordis/tests/mount.spec.ts
index 28ac6f2a5f..6bc2d5970d 100644
--- a/packages/cordis/tool-cordis/tests/mount.spec.ts
+++ b/packages/cordis/tool-cordis/tests/mount.spec.ts
@@ -392,6 +392,7 @@ describe('cordis_mount', () => {
     ['parameters: { value: { type: \'json\', default: (() => { const v = {}; v.self = v; return v })() } }', 'parameters.value.default.self must be lossless JSON data'],
     ['parameters: { value: { type: \'json\', default: Array(2) } }', 'parameters.value.default must be lossless JSON data'],
     ['parameters: { value: { type: \'json\', default: Object.assign([1], { extra: true }) } }', 'parameters.value.default must be lossless JSON data'],
+    ['parameters: { value: { type: \'json\', default: (() => { const v = Array(1); v.extra = true; return v })() } }', 'parameters.value.default must be lossless JSON data'],
     ['parameters: { value: { type: \'json\', default: new (class DefaultValue { constructor() { this.ok = true } })() } }', 'parameters.value.default must be lossless JSON data'],
     ['parameters: { value: { type: \'json\', default: new Date(0) } }', 'parameters.value.default must be lossless JSON data'],
   ])('rejects a malformed ParameterSchemaSpec (%s) with a teaching error', async (parameters, message) => {
diff --git a/packages/core/session/tests/json.spec.ts b/packages/core/session/tests/json.spec.ts
index 435126c22e..35d521f6d1 100644
--- a/packages/core/session/tests/json.spec.ts
+++ b/packages/core/session/tests/json.spec.ts
@@ -69,6 +69,8 @@ describe('snapshotJsonValue', () => {
     }
     class ExoticArray extends Array<number> {}
     const sparse = new Array<number>(1)
+    const compensatedSparse = new Array<number>(1)
+    Object.defineProperty(compensatedSparse, 'extra', { value: true })
     const decorated = [1]
     Object.defineProperty(decorated, 'extra', { value: true })
     const symbolDecorated = [1]
@@ -80,6 +82,7 @@ describe('snapshotJsonValue', () => {
     expect(snapshotJsonValue(new Map([['value', 1]]))).toBeUndefined()
     expect(snapshotJsonValue(new ExoticArray(1))).toBeUndefined()
     expect(snapshotJsonValue(sparse)).toBeUndefined()
+    expect(snapshotJsonValue(compensatedSparse)).toBeUndefined()
     expect(snapshotJsonValue(decorated)).toBeUndefined()
     expect(snapshotJsonValue(symbolDecorated)).toBeUndefined()
     expect(snapshotJsonValue(cyclic)).toBeUndefined()
@@ -145,6 +148,8 @@ describe('isJsonValue', () => {
     }
     class ExoticArray extends Array<number> {}
     const sparse = new Array<number>(1)
+    const compensatedSparse = new Array<number>(1)
+    Object.defineProperty(compensatedSparse, 'extra', { value: true })
     const decorated = Object.assign([1], { extra: true })
     const symbolDecorated = [1]
     Object.defineProperty(symbolDecorated, Symbol('extra'), { value: true })
@@ -152,6 +157,7 @@ describe('isJsonValue', () => {
     cyclic.self = cyclic
 
     expect(isJsonValue(sparse)).toBe(false)
+    expect(isJsonValue(compensatedSparse)).toBe(false)
     expect(isJsonValue(decorated)).toBe(false)
     expect(isJsonValue(symbolDecorated)).toBe(false)
     expect(isJsonValue(new ExoticArray(1))).toBe(false)
diff --git a/packages/core/tools/tests/properties.spec.ts b/packages/core/tools/tests/properties.spec.ts
index 54c4088909..e04e9f5c5b 100644
--- a/packages/core/tools/tests/properties.spec.ts
+++ b/packages/core/tools/tests/properties.spec.ts
@@ -7,6 +7,7 @@
 
 import { describe, expect, it } from 'vitest'
 import fc from 'fast-check'
+import { isJsonValue } from '@deepseek-ai/dsh-session'
 import { parameterSchemaSpecToJsonSchema, validateArgs } from '@deepseek-ai/dsh-tools'
 import type { ParameterPropertySpec, ParameterSchemaSpec, ValueSchemaSpec } from '@deepseek-ai/dsh-tools'
 
@@ -80,7 +81,7 @@ function valueForProp(prop: ParameterPropertySpec): fc.Arbitrary<unknown> {
     case 'null': return fc.constant(null)
     case 'object': return prop.properties ? validArgsForSpec(prop.properties) : fc.constant({})
     case 'array': return prop.items ? fc.array(valueForProp(prop.items), { maxLength: 3 }) : fc.constant([])
-    case 'json': return fc.jsonValue()
+    case 'json': return fc.jsonValue().filter(value => isJsonValue(value))
   }
 }