diff --git a/.changeset/fix-bigint-json-schema.md b/.changeset/fix-bigint-json-schema.md
new file mode 100644
index 0000000..aa5cde8
--- /dev/null
+++ b/.changeset/fix-bigint-json-schema.md
@@ -0,0 +1,5 @@
+---
+"incur": patch
+---
+
+Fixed `z.bigint()`, `z.coerce.bigint()`, `z.date()`, and `z.coerce.date()` schemas failing during skill sync by representing them as `{ type: "string" }` in JSON Schema output.
diff --git a/src/Schema.test.ts b/src/Schema.test.ts
index 09d8afc..593a147 100644
--- a/src/Schema.test.ts
+++ b/src/Schema.test.ts
@@ -97,6 +97,35 @@ describe('toJsonSchema', () => {
     })
   })
 
+  test('converts z.bigint() as string', () => {
+    expect(Schema.toJsonSchema(z.bigint())).toEqual({ type: 'string' })
+  })
+
+  test('converts z.coerce.bigint() as string', () => {
+    expect(Schema.toJsonSchema(z.coerce.bigint())).toEqual({ type: 'string' })
+  })
+
+  test('converts z.object() with bigint field', () => {
+    expect(
+      Schema.toJsonSchema(z.object({ amount: z.coerce.bigint().describe('Token amount') })),
+    ).toEqual({
+      type: 'object',
+      properties: {
+        amount: { type: 'string', description: 'Token amount' },
+      },
+      required: ['amount'],
+      additionalProperties: false,
+    })
+  })
+
+  test('converts z.date() as string', () => {
+    expect(Schema.toJsonSchema(z.date())).toEqual({ type: 'string' })
+  })
+
+  test('converts z.coerce.date() as string', () => {
+    expect(Schema.toJsonSchema(z.coerce.date())).toEqual({ type: 'string' })
+  })
+
   test('full object with optional, default, and describe', () => {
     const result = Schema.toJsonSchema(
       z.object({
diff --git a/src/Schema.ts b/src/Schema.ts
index 85704c4..e0dd404 100644
--- a/src/Schema.ts
+++ b/src/Schema.ts
@@ -1,8 +1,18 @@
 import { z } from 'zod'
 
-/** Converts a Zod schema to a JSON Schema object. Strips the `$schema` meta-property. */
+/**
+ * Converts a Zod schema to a JSON Schema object. Strips the `$schema`
+ * meta-property. Represents bigints and dates as `{ type: "string" }`
+ * since JSON lacks native types for them.
+ */
 export function toJsonSchema(schema: z.ZodType): Record<string, unknown> {
-  const result = z.toJSONSchema(schema) as Record<string, unknown>
+  const result = z.toJSONSchema(schema, {
+    unrepresentable: 'any',
+    override: (ctx) => {
+      const type = ctx.zodSchema._zod?.def?.type
+      if (type === 'bigint' || type === 'date') ctx.jsonSchema.type = 'string'
+    },
+  }) as Record<string, unknown>
   delete result.$schema
   return result
 }