feat: add MCP_HIDE_INPUT_IN_ERRORS env var to redact payloads from validation errors

Pydantic's ValidationError repr includes the raw input_value by default.
When the SDK calls model_validate_json() on untrusted data (SSE messages,
OAuth responses) and validation fails, logger.exception() dumps the entire
payload into logs. This can leak sensitive tool output or OAuth tokens.

Setting MCP_HIDE_INPUT_IN_ERRORS=1 before importing the SDK applies
hide_input_in_errors=True to the jsonrpc_message_adapter TypeAdapter and
the OAuth models (OAuthToken, OAuthClientMetadata, OAuthMetadata,
ProtectedResourceMetadata). The error type and location remain in the
message; only the raw input is omitted.

Opt-in via env var to preserve the current debugging-friendly default.

Author: maxisbey

src/mcp/shared/auth.py

@@ -1,11 +1,15 @@
 from typing import Any, Literal
 
-from pydantic import AnyHttpUrl, AnyUrl, BaseModel, Field, field_validator
+from pydantic import AnyHttpUrl, AnyUrl, BaseModel, ConfigDict, Field, field_validator
+
+from mcp.types.jsonrpc import HIDE_INPUT_IN_ERRORS
 
 
 class OAuthToken(BaseModel):
     """See https://datatracker.ietf.org/doc/html/rfc6749#section-5.1"""
 
+    model_config = ConfigDict(hide_input_in_errors=HIDE_INPUT_IN_ERRORS)
+
     access_token: str
     token_type: Literal["Bearer"] = "Bearer"
     expires_in: int | None = None
@@ -37,6 +41,8 @@ class OAuthClientMetadata(BaseModel):
     See https://datatracker.ietf.org/doc/html/rfc7591#section-2
     """
 
+    model_config = ConfigDict(hide_input_in_errors=HIDE_INPUT_IN_ERRORS)
+
     redirect_uris: list[AnyUrl] | None = Field(..., min_length=1)
     # supported auth methods for the token endpoint
     token_endpoint_auth_method: (
@@ -105,6 +111,8 @@ class OAuthMetadata(BaseModel):
     See https://datatracker.ietf.org/doc/html/rfc8414#section-2
     """
 
+    model_config = ConfigDict(hide_input_in_errors=HIDE_INPUT_IN_ERRORS)
+
     issuer: AnyHttpUrl
     authorization_endpoint: AnyHttpUrl
     token_endpoint: AnyHttpUrl
@@ -134,6 +142,8 @@ class ProtectedResourceMetadata(BaseModel):
     See https://datatracker.ietf.org/doc/html/rfc9728#section-2
     """
 
+    model_config = ConfigDict(hide_input_in_errors=HIDE_INPUT_IN_ERRORS)
+
     resource: AnyHttpUrl
     authorization_servers: list[AnyHttpUrl] = Field(..., min_length=1)
     jwks_uri: AnyHttpUrl | None = None

src/mcp/types/jsonrpc.py

@@ -2,9 +2,18 @@
 
 from __future__ import annotations
 
+import os
 from typing import Annotated, Any, Literal
 
-from pydantic import BaseModel, Field, TypeAdapter
+from pydantic import BaseModel, ConfigDict, Field, TypeAdapter
+
+HIDE_INPUT_IN_ERRORS = os.environ.get("MCP_HIDE_INPUT_IN_ERRORS", "").lower() in ("1", "true")
+"""When True, pydantic ValidationError reprs omit the ``input_value`` field.
+
+Set the ``MCP_HIDE_INPUT_IN_ERRORS`` environment variable to ``1`` or ``true`` before
+importing the SDK to prevent raw request/response payloads from appearing in logs
+when JSON parsing or validation fails. The error type and location are still shown.
+"""
 
 RequestId = Annotated[int, Field(strict=True)] | str
 """The ID of a JSON-RPC request."""
@@ -80,4 +89,6 @@ class JSONRPCError(BaseModel):
 
 
 JSONRPCMessage = JSONRPCRequest | JSONRPCNotification | JSONRPCResponse | JSONRPCError
-jsonrpc_message_adapter: TypeAdapter[JSONRPCMessage] = TypeAdapter(JSONRPCMessage)
+jsonrpc_message_adapter: TypeAdapter[JSONRPCMessage] = TypeAdapter(
+    JSONRPCMessage, config=ConfigDict(hide_input_in_errors=HIDE_INPUT_IN_ERRORS)
+)

tests/test_types.py

@@ -1,6 +1,9 @@
+import subprocess
+import sys
 from typing import Any
 
 import pytest
+from pydantic import ValidationError
 
 from mcp.types import (
     LATEST_PROTOCOL_VERSION,
@@ -360,3 +363,51 @@ def test_list_tools_result_preserves_json_schema_2020_12_fields():
     assert tool.input_schema["$schema"] == "https://json-schema.org/draft/2020-12/schema"
     assert "$defs" in tool.input_schema
     assert tool.input_schema["additionalProperties"] is False
+
+
+def test_validation_error_shows_input_by_default():
+    """By default, ValidationError repr includes input_value (pydantic's default behavior)."""
+    with pytest.raises(ValidationError) as exc_info:
+        jsonrpc_message_adapter.validate_json('{"result":{"content":"SECRET-PAYLOAD')
+    assert "input_value" in repr(exc_info.value)
+    assert "SECRET-PAYLOAD" in repr(exc_info.value)
+
+
+_HIDE_INPUT_CHECK_SCRIPT = """
+import pydantic
+from mcp.types import jsonrpc_message_adapter
+from mcp.shared.auth import OAuthToken, OAuthMetadata, ProtectedResourceMetadata, OAuthClientMetadata
+
+def check(fn, name):
+    try:
+        fn()
+    except pydantic.ValidationError as e:
+        err = repr(e)
+        assert "input_value" not in err, f"{name}: input_value leaked: {err!r}"
+        assert "SECRET" not in err, f"{name}: payload leaked: {err!r}"
+        # still useful: error type/location present
+        assert "json_invalid" in err or "validation error" in err
+    else:
+        raise AssertionError(f"{name}: expected ValidationError")
+
+check(lambda: jsonrpc_message_adapter.validate_json('{"result":"SECRET'), "jsonrpc_message_adapter")
+check(lambda: OAuthToken.model_validate_json('{"access_token":"SECRET'), "OAuthToken")
+check(lambda: OAuthMetadata.model_validate_json('{"issuer":"SECRET'), "OAuthMetadata")
+check(lambda: ProtectedResourceMetadata.model_validate_json('{"resource":"SECRET'), "ProtectedResourceMetadata")
+check(lambda: OAuthClientMetadata.model_validate_json('{"redirect_uris":"SECRET'), "OAuthClientMetadata")
+print("OK")
+"""
+
+
+@pytest.mark.parametrize("env_value", ["1", "true", "True", "TRUE"])
+def test_hide_input_in_errors_env_var(env_value: str):
+    """When MCP_HIDE_INPUT_IN_ERRORS is set, ValidationError repr omits input_value."""
+    result = subprocess.run(
+        [sys.executable, "-c", _HIDE_INPUT_CHECK_SCRIPT],
+        env={"MCP_HIDE_INPUT_IN_ERRORS": env_value},
+        capture_output=True,
+        text=True,
+        check=False,
+    )
+    assert result.returncode == 0, f"stdout={result.stdout!r} stderr={result.stderr!r}"
+    assert result.stdout.strip() == "OK"