The main entry point uses print() to stderr for usage instructions. Consider using logging.error() for consistency with the rest of the script's logging setup.

The logic for validating session_idle_timeout is duplicated in the constructor. Consider moving this validation to a helper method or using a property setter to ensure consistency.

The error response construction is repeated. Consider defining a helper method to generate standard JSON-RPC error responses to reduce code duplication.

The regex generated for the MQTT multi-level wildcard # is incorrect when it is the only segment in the pattern. If pattern is #, regex_parts will be empty, and the resulting regex will be ^(/.*)?$. This regex only matches strings starting with a forward slash (or an empty string), failing to match common topics like foo. Per MQTT specification, a root # should match all topics.

The regex for the topic pattern is compiled on every call to get_matching. For better performance, especially when many retained events are stored, consider caching the compiled regex patterns, similar to how SubscriptionRegistry does it.

The MQTT topic matching logic is duplicated here and in _handle_event. It also contains the same bug regarding the root # wildcard as found in src/mcp/server/events.py. Consider moving the pattern-to-regex conversion logic to a shared utility module (e.g., src/mcp/shared/topic_utils.py) to ensure consistency and maintainability.

Importing re inside a method that may be called frequently (on every incoming event) is inefficient. Move the import to the top level of the module.

Since python-ulid is now a required dependency in pyproject.toml, these imports should be moved to the top level of the module instead of being inside the method.

The logic for converting an MQTT-style topic pattern to a regular expression and matching it against a topic is duplicated here in _topic_matches_subscriptions and _handle_event. A similar function, _pattern_to_regex, also exists in src/mcp/server/events.py.

To improve maintainability and reduce code duplication, consider extracting this logic into a shared utility function. This function could take a pattern and a topic and return whether they match. You could place this utility in a shared module, for example mcp.shared.events_utils.

For example, you could have a function like:

# in a new mcp/shared/events_utils.py
import re

def topic_matches_pattern(topic: str, pattern: str) -> bool:
    # ... implementation of regex conversion and matching ...

This would simplify both _topic_matches_subscriptions and _handle_event significantly.

The topic_filter provided to set_event_handler should be pre-compiled into a regex pattern during registration rather than being reconstructed and recompiled on every incoming event in _handle_event. This will improve performance when processing high volumes of events.

The subscription enforcement logic here contradicts the documentation and creates inconsistent behavior.

According to docs/events.md:247, events that do not match any active subscription should be dropped. However, the current implementation if self._subscribed_patterns and ... only performs the check if the client has at least one subscription. This means a client with zero subscriptions will receive all events pushed by the server, but as soon as it subscribes to a single topic, it becomes restricted.

Removing the self._subscribed_patterns and check will ensure that events are only processed if they match an active subscription, aligning the code with the documented behavior.

The MQTT-style topic matching logic in _topic_matches_subscriptions is inefficient and duplicated.

1. Performance: The code repeatedly splits pattern strings and constructs regex patterns for every incoming event. This will become a significant CPU bottleneck for clients with multiple subscriptions or high event volume. Compiled regexes should be cached.
2. Duplication: The logic for converting MQTT wildcards (+, #) to regex is duplicated in _handle_event (lines 316-336) and is also present in src/mcp/server/events.py.
3. Validation: This implementation lacks the validation enforced on the server side (e.g., ensuring # is the last segment).

Consider refactoring the conversion logic into a shared utility function and caching the compiled regex patterns within the ClientSession (e.g., using a dict[str, re.Pattern]).

The validation logic for session_idle_timeout should be consistent with other timeout configurations in the SDK. Consider using a more descriptive error message or a standard validation pattern if available.

The error response for a missing session ID uses a hardcoded 'server-error' ID. Consider using a more dynamic or standard identifier if the protocol allows, or ensure this ID is documented as a specific error case.

Using list(self._response_streams.items()) creates a full copy of the dictionary items. If this dictionary is large, consider using an iterator or a more memory-efficient approach to avoid unnecessary allocations during connection closure.

The match method performs a linear scan over all sessions and all their patterns. While acceptable for a reference implementation or small-scale deployments, this $O(N \times M)$ complexity (where $N$ is the number of sessions and $M$ is the average number of subscriptions per session) could become a performance bottleneck in a server with many active sessions and high event volume. Consider using a trie-based structure or a specialized topic tree for more efficient matching if high throughput is required.

The current implementation only supports a single global event handler. If a user uses the @on_event decorator multiple times, each subsequent call will overwrite the previous handler. This is likely unexpected behavior for a PUB/SUB system. Consider maintaining a list of handlers and their associated filters.

Renaming this to add_event_handler and appending to a list would allow multiple handlers to coexist, fixing the overwriting issue when using the decorator or multiple manual registrations.

Awaiting the event handler directly in the reader loop means that a slow or blocking handler will stop the client from receiving any further messages (including responses to pending requests). Consider spawning a background task for each handler execution, though be mindful of event ordering if that is a requirement.

The + wildcard in MQTT is intended to match exactly one topic level, which can be an empty string (e.g., a//c matches a/+/c). The current regex [^/]+ requires at least one character, which will fail to match empty levels. Using [^/]* is more compliant with MQTT semantics.

Holding the asyncio.Lock during the entire matching loop (which involves synchronous regex execution) can lead to significant performance bottlenecks and block other operations like add or remove if there are many sessions or patterns. It is better to snapshot the subscriptions under the lock and perform the matching outside of it.

The datetime.fromisoformat method in Python 3.10 (which this project supports) does not handle the 'Z' suffix for UTC. This will cause a ValueError for valid ISO 8601 strings ending in 'Z', leading _is_expired to incorrectly return False. Additionally, accepting an optional now parameter allows for optimization in batch operations like get_matching.

In get_matching, _is_expired is called for every topic in the store. Since _is_expired calls datetime.now(timezone.utc) internally, this results in redundant system calls. It is more efficient to capture the current time once at the start of get_matching and pass it to _is_expired.

The regex for the + wildcard currently uses [^/]+, which requires at least one character in the topic segment. However, according to the MQTT specification (which this implementation aims to follow), the + wildcard matches exactly one topic level, and that level can be empty. For example, sport/+ should match sport/. Changing the regex to [^/]* would align with standard MQTT behavior by allowing empty segments to match.

The _compiled dictionary in SubscriptionRegistry acts as a cache for compiled regular expressions but is never pruned. If clients subscribe to many unique topic patterns (e.g., patterns containing unique IDs), this dictionary will grow indefinitely, potentially leading to a memory leak. Consider using a size-limited cache (like functools.lru_cache on a helper function or a custom eviction policy) to prevent unbounded memory growth.