feat: surface X-Logfire-Warning header

logfire/_internal/cli/__init__.py

@@ -25,6 +25,7 @@
 from ..client import LogfireClient
 from ..config import REGIONS, LogfireCredentials, get_base_url_from_token
 from ..config_params import ParamManager
+from ..server_response import install_logfire_response_hook
 from ..tracer import SDKTracerProvider
 from .auth import parse_auth, parse_logout
 from .prompt import parse_prompt
@@ -434,8 +435,9 @@ def log_trace_id(response: requests.Response, context: ContextCarrier, *args: An
     else:
         with tracer.start_as_current_span('logfire._internal.cli'), requests.Session() as session:
             context = get_context()
-            session.hooks = {'response': functools.partial(log_trace_id, context=context)}
+            session.hooks = {'response': [functools.partial(log_trace_id, context=context)]}
             session.headers.update(context)
+            install_logfire_response_hook(session)
             namespace._session = session
             namespace.func(namespace)
 

logfire/_internal/client.py

@@ -10,6 +10,7 @@
 from logfire.version import VERSION
 
 from .auth import UserToken, UserTokenCollection
+from .server_response import ServerResponseCallback, install_logfire_response_hook
 from .utils import UnexpectedResponse
 
 UA_HEADER = f'logfire/{VERSION}'
@@ -29,27 +30,43 @@ class LogfireClient:
 
     Args:
         user_token: The user token to use when authenticating against the API.
+        server_response_hook: Optional override for the API response hook (see
+            `AdvancedOptions.server_response_hook`).
     """
 
-    def __init__(self, user_token: UserToken) -> None:
+    def __init__(
+        self,
+        user_token: UserToken,
+        server_response_hook: ServerResponseCallback | None = None,
+    ) -> None:
         if user_token.is_expired:
             raise RuntimeError('The provided user token is expired')
         self.base_url = user_token.base_url
         self._token = user_token.token
         self._session = Session()
         self._session.headers.update({'Authorization': self._token, 'User-Agent': UA_HEADER})
+        install_logfire_response_hook(self._session, server_response_hook)
 
     @classmethod
-    def from_url(cls, base_url: str | None) -> Self:
+    def from_url(
+        cls,
+        base_url: str | None,
+        server_response_hook: ServerResponseCallback | None = None,
+    ) -> Self:
         """Create a client from the provided base URL.
 
         Args:
             base_url: The base URL to use when looking for a user token. If `None`, will prompt
                 the user into selecting a token from the token collection (or, if only one available,
                 use it directly). The token collection will be created from the `~/.logfire/default.toml`
                 file (or an empty one if no such file exists).
+            server_response_hook: Optional override for the API response hook (see
+                `AdvancedOptions.server_response_hook`).
         """
-        return cls(user_token=UserTokenCollection().get_token(base_url))
+        return cls(
+            user_token=UserTokenCollection().get_token(base_url),
+            server_response_hook=server_response_hook,
+        )
 
     def _get_raw(self, endpoint: str, params: dict[str, Any] | None = None) -> Response:
         response = self._session.get(urljoin(self.base_url, endpoint), params=params)

logfire/_internal/config.py

@@ -110,6 +110,7 @@
 from .logs import ProxyLoggerProvider
 from .metrics import ProxyMeterProvider
 from .scrubbing import NOOP_SCRUBBER, BaseScrubber, Scrubber, ScrubbingOptions
+from .server_response import ServerResponseCallback, install_logfire_response_hook
 from .stack_info import warn_at_user_stacklevel
 from .tracer import OPEN_SPANS, PendingSpanProcessor, ProxyTracerProvider
 from .utils import (
@@ -216,6 +217,32 @@ class AdvancedOptions:
     This log and configuration is experimental and may be modified or removed.
     """
 
+    server_response_hook: ServerResponseCallback | None = None
+    """Optional callback invoked for every HTTP response received from the Logfire API.
+
+    This is experimental and may be modified or removed.
+
+    This applies to OTLP exports, credential / project initialisation, and the remote
+    variables provider. The default surfaces the `X-Logfire-Warning` header as a
+    `LogfireServerWarning`.
+
+    Setting this replaces the default; pass `lambda response: None` to opt out entirely.
+
+    Example usage:
+
+    ```python skip-run="true" skip-reason="needs metric/logfire setup"
+    from logfire.types import ServerResponseCallbackHelper
+
+    def hook(helper: ServerResponseCallbackHelper):
+        my_metric.inc(helper.response.status_code)
+        helper.default_hook()  # call this to keep the default warning behavior
+
+    logfire.configure(advanced=AdvancedOptions(server_response_hook=hook))
+    ```
+
+    Raise from the hook to abort the calling code path.
+    """
+
     def generate_base_url(self, token: str) -> str:
         if self.base_url is not None:
             return self.base_url
@@ -1097,7 +1124,7 @@ def add_span_processor(span_processor: SpanProcessor) -> None:
                     # If we don't have tokens or credentials from a file,
                     # try initializing a new project and writing a new creds file.
                     # note, we only do this if `send_to_logfire` is explicitly `True`, not 'if-token-present'
-                    client = LogfireClient.from_url(self.advanced.base_url)
+                    client = LogfireClient.from_url(self.advanced.base_url, self.advanced.server_response_hook)
                     credentials = LogfireCredentials.initialize_project(client=client)
                     credentials.write_creds_file(self.data_dir)
 
@@ -1148,6 +1175,7 @@ def check_tokens():
                         base_url = self.advanced.generate_base_url(token)
                         headers = {'User-Agent': f'logfire/{VERSION}', 'Authorization': token}
                         session = OTLPExporterHttpSession()
+                        install_logfire_response_hook(session, self.advanced.server_response_hook)
                         span_exporter = BodySizeCheckingOTLPSpanExporter(
                             endpoint=urljoin(base_url, '/v1/traces'),
                             session=session,
@@ -1324,6 +1352,7 @@ def fix_pid():  # pragma: no cover
                     base_url=base_url,
                     token=self.api_key,
                     options=self.variables,
+                    server_response_hook=self.advanced.server_response_hook,
                 )
             multi_log_processor = SynchronousMultiLogRecordProcessor()
             for processor in log_record_processors:
@@ -1456,6 +1485,7 @@ def _lazy_init_variable_provider(self) -> VariableProvider:
                 base_url=base_url,
                 token=api_key,
                 options=options,
+                server_response_hook=self.advanced.server_response_hook,
             )
             self._variable_provider = provider
             provider.start(Logfire(config=self))
@@ -1472,7 +1502,9 @@ def warn_if_not_initialized(self, message: str):
             )
 
     def _initialize_credentials_from_token(self, token: str) -> LogfireCredentials | None:
-        return LogfireCredentials.from_token(token, requests.Session(), self.advanced.generate_base_url(token))
+        session = requests.Session()
+        install_logfire_response_hook(session, self.advanced.server_response_hook)
+        return LogfireCredentials.from_token(token, session, self.advanced.generate_base_url(token))
 
     def _ensure_flush_after_aws_lambda(self):
         """Ensure that `force_flush` is called after an AWS Lambda invocation.

logfire/_internal/server_response.py

@@ -0,0 +1,44 @@
+"""Surface out-of-band signals the Logfire backend wants every SDK request to know about.
+
+The server attaches the `X-Logfire-Warning` header to API responses to signal an
+out-of-band warning the server wants the user to see. It is surfaced via
+`warnings.warn(..., LogfireServerWarning)`. Python's standard "default" filter
+dedupes identical messages, so a chatty server only warns once.
+
+`install_logfire_response_hook(session)` wires this into a `requests.Session` as
+a response hook so every Logfire-bound HTTP response is inspected. Callers can
+pass a custom `hook` to replace the default behavior (see
+`AdvancedOptions.server_response_hook`).
+"""
+
+from __future__ import annotations
+
+from typing import Any
+
+import requests
+
+from logfire.types import ServerResponseCallback, ServerResponseCallbackHelper
+
+
+def install_logfire_response_hook(
+    session: requests.Session,
+    hook: ServerResponseCallback | None = None,
+) -> None:
+    """Install a `requests` response hook on `session` for every Logfire API response.
+
+    By default, calls `ServerResponseCallbackHelper.default_hook()`, which emits a warning
+    if the `X-Logfire-Warning` response header is present.
+
+    Pass a custom callable to replace the default behavior (e.g. opt out by passing `lambda _: None`).
+    """
+
+    def _hook(response: requests.Response, *args: Any, **kwargs: Any) -> requests.Response:
+        helper = ServerResponseCallbackHelper(response, args, kwargs)
+        if hook is not None:
+            hook(helper)
+        else:
+            helper.default_hook()
+        return response
+
+    response_hooks: list[Any] = session.hooks.setdefault('response', [])
+    response_hooks.append(_hook)

logfire/exceptions.py

@@ -3,3 +3,7 @@
 
 class LogfireConfigError(ValueError):
     """Error raised when there is a problem with the Logfire configuration."""
+
+
+class LogfireServerWarning(UserWarning):
+    """Warning emitted when the Logfire server returns an `X-Logfire-Warning` header on a response."""

logfire/types.py

@@ -2,7 +2,9 @@
 
 from collections.abc import Callable
 from dataclasses import dataclass
-from typing import TYPE_CHECKING
+from typing import TYPE_CHECKING, Any
+
+import requests
 
 from logfire._internal.constants import (
     ATTRIBUTES_LOG_LEVEL_NUM_KEY,
@@ -11,8 +13,10 @@
     LevelName,
     log_level_attributes,
 )
+from logfire._internal.stack_info import warn_at_user_stacklevel
 from logfire._internal.tracer import get_parent_span
 from logfire._internal.utils import canonicalize_exception_traceback
+from logfire.exceptions import LogfireServerWarning
 
 if TYPE_CHECKING:
     from opentelemetry.sdk.trace import ReadableSpan, Span
@@ -260,3 +264,40 @@ def my_callback(helper: logfire.types.ExceptionCallbackHelper):
 
     helper.no_record_exception()
 """
+
+
+@dataclass
+class ServerResponseCallbackHelper:
+    """Helper object passed to the server response callback.
+
+    This is experimental and may change significantly in future releases.
+    """
+
+    response: requests.Response
+    """The raw HTTP response from the Logfire API."""
+
+    args: tuple[Any, ...]
+    """Positional arguments passed to the response hook by `requests`."""
+
+    kwargs: dict[str, Any]
+    """Keyword arguments passed to the response hook by `requests`."""
+
+    WARNING_HEADER_NAME = 'X-Logfire-Warning'
+
+    @property
+    def warning_header(self) -> str | None:
+        """Value of the Logfire warning header, or `None` if not present."""
+        return self.response.headers.get(self.WARNING_HEADER_NAME)
+
+    def default_hook(self) -> None:
+        """The default hook behavior: emit a `LogfireServerWarning` if the warning header is present."""
+        warning_message = self.warning_header
+        if warning_message:
+            warn_at_user_stacklevel(warning_message, LogfireServerWarning)
+
+
+ServerResponseCallback = Callable[[ServerResponseCallbackHelper], None]
+"""Callable invoked for every Logfire API response received by the SDK.
+
+This is experimental and may change significantly in future releases.
+"""

logfire/variables/remote.py

@@ -17,6 +17,7 @@
 
 from logfire._internal.client import UA_HEADER
 from logfire._internal.config import VariablesOptions
+from logfire._internal.server_response import ServerResponseCallback, install_logfire_response_hook
 from logfire._internal.utils import UnexpectedResponse
 from logfire.variables.abstract import (
     ResolvedVariable,
@@ -54,21 +55,31 @@ class LogfireRemoteVariableProvider(VariableProvider):
     The threading implementation draws heavily from opentelemetry.sdk._shared_internal.BatchProcessor.
     """
 
-    def __init__(self, base_url: str, token: str, options: VariablesOptions):
+    def __init__(
+        self,
+        base_url: str,
+        token: str,
+        options: VariablesOptions,
+        server_response_hook: ServerResponseCallback | None = None,
+    ):
         """Create a new remote variable provider.
 
         Args:
             base_url: The base URL of the Logfire API.
             token: Authentication token for the Logfire API.
             options: Options for retrieving remote variables.
+            server_response_hook: Optional override for the API response hook
+                (see `AdvancedOptions.server_response_hook`).
         """
         block_before_first_resolve = options.block_before_first_resolve
         polling_interval = options.polling_interval
 
         self._base_url = base_url
         self._token = token
+        self._server_response_hook = server_response_hook
         self._session = Session()
         self._session.headers.update({'Authorization': f'bearer {token}', 'User-Agent': UA_HEADER})
+        install_logfire_response_hook(self._session, server_response_hook)
         self._timeout = options.timeout
         self._block_before_first_fetch = block_before_first_resolve
         self._polling_interval: timedelta = (
@@ -197,6 +208,7 @@ def _sse_listener(self):  # pragma: no cover
                             'Cache-Control': 'no-cache',
                         }
                     )
+                    install_logfire_response_hook(sse_session, self._server_response_hook)
 
                     # Open streaming connection
                     response = sse_session.get(sse_url, stream=True, timeout=(10, None))