diff --git a/.cspell/custom-words.txt b/.cspell/custom-words.txt
index ce73c361..55507e83 100644
--- a/.cspell/custom-words.txt
+++ b/.cspell/custom-words.txt
@@ -19,6 +19,7 @@ celerybeat
 classpath
 CLASSPATH
 CMSPI
+codegen
 cmwallet
 cncf
 Cobo
@@ -35,6 +36,7 @@ davecgh
 dcql
 Dcql
 DCQL
+datamodel
 deviceauth
 Dfile
 dmypy
@@ -149,6 +151,7 @@ ropeproject
 RPCURL
 Rulebook
 screenreaders
+sepolia
 setlocal
 sharedpref
 Shopcider
@@ -169,6 +172,7 @@ Truelayer
 Trulioo
 udpa
 unmarshal
+usdc
 viewmodel
 vulnz
 Wallex
diff --git a/code/sdk/python/ap2/sdk/generated/types/payment_instrument.py b/code/sdk/python/ap2/sdk/generated/types/payment_instrument.py
index 81ff9568..df146b7a 100644
--- a/code/sdk/python/ap2/sdk/generated/types/payment_instrument.py
+++ b/code/sdk/python/ap2/sdk/generated/types/payment_instrument.py
@@ -1,10 +1,10 @@
 # generated by datamodel-codegen:
 #   filename:  types/payment_instrument.json
-#   timestamp: 2026-04-28T00:39:38+00:00
+#   timestamp: 2026-08-13T10:40:58+00:00
 
 from __future__ import annotations
 
-from pydantic import BaseModel, Field
+from pydantic import BaseModel, ConfigDict, Field
 
 
 class PaymentInstrument(BaseModel):
@@ -12,6 +12,9 @@ class PaymentInstrument(BaseModel):
     Instrument used for payment.
     """
 
+    model_config = ConfigDict(
+        extra='allow',
+    )
     id: str = Field(..., description='unique identifier for this instrument')
     type: str = Field(
         ..., description='unique string identifying this category of instrument'
diff --git a/code/sdk/python/ap2/tests/payment_instrument_tests.py b/code/sdk/python/ap2/tests/payment_instrument_tests.py
new file mode 100644
index 00000000..b63a4b35
--- /dev/null
+++ b/code/sdk/python/ap2/tests/payment_instrument_tests.py
@@ -0,0 +1,74 @@
+"""Tests for extensible payment instrument fields."""
+
+from ap2.sdk.generated.open_payment_mandate import (
+    AllowedPaymentInstruments,
+    OpenPaymentMandate,
+)
+from ap2.sdk.generated.types.payment_instrument import PaymentInstrument
+from ap2.sdk.payment_mandate_chain import PaymentMandateChain
+from ap2.tests.conftest import make_cnf, sample_payment_mandate
+
+
+def test_type_specific_fields_survive_signed_payment_mandate_roundtrip(
+    user_key,
+    user_public_key,
+    agent_key,
+    holder,
+):
+    """Type-specific instrument fields remain signed and typed after verify."""
+    instrument = PaymentInstrument(
+        id='x402-base-sepolia-usdc',
+        type='x402',
+        payee_address='0x1111111111111111111111111111111111111111',
+        facilitator='https://facilitator.example',
+    )
+    expected_extension = {
+        'payee_address': '0x1111111111111111111111111111111111111111',
+        'facilitator': 'https://facilitator.example',
+    }
+
+    instrument_dump = instrument.model_dump(exclude_none=True)
+    assert {
+        key: instrument_dump[key] for key in expected_extension
+    } == expected_extension
+
+    closed_mandate = sample_payment_mandate(payment_instrument=instrument)
+    nested_dump = closed_mandate.model_dump(exclude_none=True)
+    assert {
+        key: nested_dump['payment_instrument'][key]
+        for key in expected_extension
+    } == expected_extension
+
+    open_token = holder.create(
+        payloads=[
+            OpenPaymentMandate(
+                constraints=[
+                    AllowedPaymentInstruments(allowed=[instrument]),
+                ],
+                cnf=make_cnf(agent_key),
+            )
+        ],
+        issuer_key=user_key,
+    )
+    signed_chain = holder.present(
+        holder_key=agent_key,
+        mandate_token=open_token,
+        payloads=[closed_mandate],
+        aud='merchant',
+        nonce='merchant-nonce',
+    )
+
+    verified_payloads = holder.verify(
+        token=signed_chain,
+        key_or_provider=lambda _token: user_public_key,
+    )
+    parsed_chain = PaymentMandateChain.parse(verified_payloads)
+
+    assert parsed_chain.verify() == []
+    parsed_instrument = parsed_chain.closed_mandate.payment_instrument
+    assert {
+        key: parsed_instrument.model_dump(exclude_none=True)[key]
+        for key in expected_extension
+    } == expected_extension
+    assert parsed_instrument.payee_address == expected_extension['payee_address']
+    assert parsed_instrument.facilitator == expected_extension['facilitator']
diff --git a/code/sdk/schemas/ap2/types/payment_instrument.json b/code/sdk/schemas/ap2/types/payment_instrument.json
index 99857b37..ac885556 100644
--- a/code/sdk/schemas/ap2/types/payment_instrument.json
+++ b/code/sdk/schemas/ap2/types/payment_instrument.json
@@ -4,6 +4,7 @@
   "title": "Payment Instrument",
   "description": "Instrument used for payment.",
   "type": "object",
+  "additionalProperties": true,
   "properties": {
     "id": {
       "type": "string",
@@ -22,4 +23,4 @@
     "id",
     "type"
   ]
-}
\ No newline at end of file
+}