> ## Documentation Index
> Fetch the complete documentation index at: https://docs.apitally.io/llms.txt
> Use this file to discover all available pages before exploring further.

# Masking

> Mask sensitive data with the Apitally SDK for Python.

The Apitally SDK provides mechanisms for masking sensitive data in captured requests, responses, and application logs.

## Default masking

The SDK automatically masks common sensitive query parameters, headers, and request/response body fields based on built-in patterns. For example, fields named `password`, `token`, `secret`, or headers like `Authorization` are masked by default.

See the [data privacy](/data-privacy#data-masking) page for more information about default masking.

## Custom masking

You can extend the default masking rules by providing additional regular expressions via the `mask_query_params`, `mask_headers`, and `mask_body_fields` parameters. Patterns are case-insensitive and match anywhere within the name. Use `^` and `$` anchors for exact matches.

Body field patterns recursively match keys in JSON objects and replace the corresponding values only when they are strings.

The following example uses FastAPI. The same masking options are available for all supported frameworks.

```python FastAPI example {13-16} theme={null}
import apitally
from fastapi import FastAPI

app = FastAPI()

apitally.init(
    app,
    write_token="your-write-token",
    env="dev",
    capture_request_headers=True,
    capture_request_body=True,
    capture_response_body=True,
    # Mask specific query parameters, headers and body fields
    mask_query_params=[r"^card_number$", r"^account_id$"],
    mask_headers=[r"^X-Custom-Key$", r"^X-Internal-"],
    mask_body_fields=[r"^credit_card$", r"social_security"],
)
```

### Body masking callbacks

For more control over body masking, you can provide callback functions via the `mask_request_body` and `mask_response_body` parameters. Each function receives the ended request [`ReadableSpan`](https://opentelemetry-python.readthedocs.io/en/stable/sdk/trace.html#opentelemetry.sdk.trace.ReadableSpan) and the captured body as `bytes`. Request metadata is available through [`span.attributes`](/sdk-reference/python/v1/attributes). Each function should return the masked body as `bytes`, or `None` to mask the entire body.

```python Callback function examples {47-49} theme={null}
import json

import apitally
from fastapi import FastAPI
from opentelemetry.sdk.trace import ReadableSpan


def mask_request_body(span: ReadableSpan, body: bytes) -> bytes | None:
    # Mask entire request body for admin endpoints
    path = (span.attributes or {}).get("url.path")
    if isinstance(path, str) and path.startswith("/admin/"):
        return None
    # Otherwise, return the original request body
    return body


def mask_response_body(span: ReadableSpan, body: bytes) -> bytes | None:
    path = (span.attributes or {}).get("url.path")
    # Mask entire response body for admin endpoints
    if isinstance(path, str) and path.startswith("/admin/"):
        return None
    # Mask specific fields in user profile responses
    if isinstance(path, str) and path.startswith("/users/"):
        try:
            data = json.loads(body)
            if isinstance(data, dict):
                if "email" in data:
                    data["email"] = "******"
                if "phone" in data:
                    data["phone"] = "******"
                return json.dumps(data).encode()
        except (json.JSONDecodeError, UnicodeDecodeError):
            pass
    # Otherwise, return the original response body
    return body


app = FastAPI()

apitally.init(
    app,
    write_token="your-write-token",
    env="dev",
    capture_request_headers=True,
    capture_request_body=True,
    capture_response_body=True,
    # Mask request and response bodies using custom logic
    mask_request_body=mask_request_body,
    mask_response_body=mask_response_body,
)
```

<Note>
  Callbacks are applied before pattern-based field masking. If the returned body contains JSON, it is still masked using the default and custom `mask_body_fields` patterns.
</Note>

### Log record masking callback

To mask sensitive data in application logs, provide a callback function via the `mask_log_record` parameter. The function receives an OpenTelemetry [`ReadWriteLogRecord`](https://opentelemetry-python.readthedocs.io/en/stable/sdk/_logs.html#opentelemetry.sdk._logs.ReadWriteLogRecord). You can modify the log message through `record.log_record.body` and structured fields through `record.log_record.attributes`. Return the same record to keep it, or `None` to drop it.

```python Callback function example {25} theme={null}
import re

import apitally
from fastapi import FastAPI
from opentelemetry.sdk._logs import ReadWriteLogRecord


def mask_log_record(record: ReadWriteLogRecord) -> ReadWriteLogRecord | None:
    # Drop logs from the httpx logger
    if record.instrumentation_scope and record.instrumentation_scope.name == "httpx":
        return None
    # Mask tokens
    body = record.log_record.body
    if isinstance(body, str):
        record.log_record.body = re.sub(r"token=\S+", "token=******", body)
    return record


app = FastAPI()

apitally.init(
    app,
    write_token="your-write-token",
    env="dev",
    mask_log_record=mask_log_record,
)
```