@@ -46,7 +46,7 @@ Agents: Security, Architecture, Code Quality, Refactor/Dead Code.
 
 - [x] **M1** Async client (`AsyncHyperpingClient`) — shipped in PR #13 (feature/sdk-py-03-async-client).
 - [x] **M2** Pagination (`page` param, `hasNextPage` auto-pagination via `collect_all_pages` / `collect_all_pages_async`) — shipped in PR #12 (merged) and PR #13.
-- [ ] **M3** Per-endpoint circuit breaker (`per_endpoint_circuit_breaker: bool = False` option). **Deferred — moderate scope, add in follow-up.**
+- [x] **M3** Per-endpoint circuit breaker (`per_endpoint_circuit_breaker: bool = False` option) on `HyperpingClient` and `AsyncHyperpingClient`. Per-path state via `circuit_breaker_state_for(path)`. Default off; existing single-shared-breaker behaviour preserved.
 - [x] **M4** `MonitorCreate` now has `@model_validator(mode="after")` that raises `ValueError` if DNS fields are set on non-DNS monitors.
 - [x] **M5** `MonitorListResponse` is in `__all__` — retained but documented as not returned by any client method. Will be used once pagination lands.
 - [x] **M6** `APIErrorResponse` removed from `__all__` (documented as intentionally internal in comment).
@@ -106,7 +106,7 @@ The following items require either a semver bump, a separate PR, or manual work:
 
 - ~~**M1** Async client~~ — shipped
 - ~~**M2** Pagination~~ — shipped
-- **M3** Per-endpoint circuit breaker option
+- ~~**M3** Per-endpoint circuit breaker option~~ — shipped
 - **M11** URL validation for HTTP-protocol monitors (cross-field, needs discriminated union work)
 - **M12** DateTime coercion (breaking change — v0.2.0)
 - **H11** Pin all GitHub Actions `uses:` to 40-char commit SHAs (requires per-tag SHA lookup)
 
@@ -5,6 +5,20 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [Unreleased]
+
+### Added
+
+- Per-endpoint circuit breaker option (`per_endpoint_circuit_breaker: bool = False`) on
+  `HyperpingClient` and `AsyncHyperpingClient`. When enabled, each `Endpoint` gets its own
+  breaker state so a single flaky endpoint no longer blocks traffic to healthy ones.
+  Sub-resource paths (e.g. `/v1/monitors/{uuid}`, `/v1/monitors/{uuid}/reports`) are
+  bucketed under their parent `Endpoint` prefix so the breaker set stays bounded; pass a
+  custom `breaker_key_fn` to change that. The OPEN-state error message now identifies
+  which endpoint tripped. State for a given path is readable via
+  `client.circuit_breaker_state_for(path)` in either mode. Default behaviour is
+  unchanged. See README for details.
+
 ## [1.5.0] - 2026-04-20
 
 ### Added
 
@@ -252,6 +252,48 @@ client = HyperpingClient(
 )
 ```
 
+### Per-endpoint circuit breaker
+
+By default a single shared circuit breaker covers every request. If one endpoint flakes, every other endpoint is also blocked. Enable `per_endpoint_circuit_breaker=True` to keep one breaker per *endpoint* so a failing endpoint does not punish healthy ones:
+
+```python
+client = HyperpingClient(
+    api_key="sk_...",
+    per_endpoint_circuit_breaker=True,
+)
+
+# Inspect state for an endpoint. The breaker key is canonicalised to the
+# matching `Endpoint` prefix, so all sub-resource paths share a bucket:
+from hyperping import CircuitState, Endpoint
+
+state = client.circuit_breaker_state_for(str(Endpoint.MONITORS))
+# /v1/monitors, /v1/monitors/mon_abc and /v1/monitors/mon_abc/reports all
+# report the same state — they share the `/v1/monitors` breaker.
+assert client.circuit_breaker_state_for(f"{Endpoint.MONITORS}/mon_abc") == state
+assert state in {CircuitState.CLOSED, CircuitState.HALF_OPEN, CircuitState.OPEN}
+```
+
+If you need different bucketing (e.g. one breaker per resource UUID, or a single breaker per HTTP verb), pass a `breaker_key_fn`:
+
+```python
+def per_resource(path: str) -> str:
+    # one breaker per literal request path
+    return path.split("?", 1)[0]
+
+client = HyperpingClient(
+    api_key="sk_...",
+    per_endpoint_circuit_breaker=True,
+    breaker_key_fn=per_resource,
+)
+```
+
+| Option | Type | Default | Description |
+| --- | --- | --- | --- |
+| `per_endpoint_circuit_breaker` | `bool` | `False` | When `True`, maintain a separate circuit breaker keyed by request endpoint instead of using one shared breaker. The same `circuit_breaker_config` applies to every per-endpoint breaker. The shared breaker remains accessible via `client.circuit_breaker`. |
+| `breaker_key_fn` | `Callable[[str], str] \| None` | `None` | Override the default endpoint-prefix bucketing. Receives the request path and returns the breaker key. Default behaviour collapses every path under the matching `Endpoint` prefix so the breaker set stays bounded (one per `Endpoint`); a custom function takes responsibility for keeping the key set bounded. Ignored unless `per_endpoint_circuit_breaker=True`. |
+
+State for any path is readable via `client.circuit_breaker_state_for(path)`. In the default (single-breaker) mode this returns the shared breaker's state for any path, so the call is always safe regardless of the flag. The same options and method are available on `AsyncHyperpingClient`.
+
 ## Type Safety
 
 This package ships a `py.typed` marker (PEP 561) and is fully typed. Works out of the box with mypy and pyright.
 
@@ -14,7 +14,10 @@
 import asyncio
 import logging
 import random
+import threading
+from collections.abc import Callable
 from typing import Any
+from urllib.parse import urlsplit
 
 import httpx
 from pydantic import SecretStr
@@ -28,10 +31,11 @@
 from hyperping._circuit_breaker import (
     CircuitBreaker,
     CircuitBreakerConfig,
+    CircuitState,
 )
 from hyperping._internals import DEFAULT_USER_AGENT, RETRY_AFTER_MAX, sanitize_for_log
 from hyperping.client import DEFAULT_RETRY_CONFIG, RetryConfig
-from hyperping.endpoints import API_BASE
+from hyperping.endpoints import API_BASE, Endpoint
 from hyperping.exceptions import (
     HyperpingAPIError,
     HyperpingAuthError,
@@ -73,6 +77,8 @@ def __init__(
         retry_config: RetryConfig | None = None,
         circuit_breaker_config: CircuitBreakerConfig | None = None,
         user_agent: str | None = None,
+        per_endpoint_circuit_breaker: bool = False,
+        breaker_key_fn: Callable[[str], str] | None = None,
     ) -> None:
         """Initialize the async Hyperping API client.
 
@@ -82,8 +88,19 @@ def __init__(
             base_url: Override the default API base URL.
             timeout: HTTP request timeout in seconds.
             retry_config: Retry behaviour configuration.
-            circuit_breaker_config: Circuit breaker configuration.
+            circuit_breaker_config: Circuit breaker configuration. When
+                ``per_endpoint_circuit_breaker`` is ``True`` the same config is
+                applied to each per-endpoint breaker.
             user_agent: Custom ``User-Agent`` header value.
+            per_endpoint_circuit_breaker: When ``True``, maintain an
+                independent breaker per :class:`~hyperping.endpoints.Endpoint`
+                prefix (sub-resources inherit the parent endpoint's breaker,
+                so the breaker set stays bounded). Default ``False``
+                preserves the original single-shared-breaker behaviour.
+            breaker_key_fn: Override the default endpoint-prefix bucketing.
+                Receives the request path and must return the breaker key.
+                Ignored unless ``per_endpoint_circuit_breaker`` is ``True``.
+                Caller is responsible for keeping the key set bounded.
         """
         raw_key = api_key.get_secret_value() if isinstance(api_key, SecretStr) else api_key
         if not raw_key or not raw_key.strip():
@@ -92,7 +109,12 @@ def __init__(
         self.base_url = (base_url or self.DEFAULT_BASE_URL).rstrip("/")
         self.timeout = timeout
         self.retry_config = retry_config or DEFAULT_RETRY_CONFIG
+        self._circuit_breaker_config = circuit_breaker_config
         self._circuit_breaker = CircuitBreaker(circuit_breaker_config)
+        self._per_endpoint_circuit_breaker = per_endpoint_circuit_breaker
+        self._breaker_key_fn = breaker_key_fn
+        self._endpoint_breakers: dict[str, CircuitBreaker] = {}
+        self._endpoint_breakers_lock = threading.Lock()
 
         self._client = httpx.AsyncClient(
             base_url=self.base_url,
@@ -120,9 +142,74 @@ async def __aexit__(self, *args: Any) -> None:
 
     @property
     def circuit_breaker(self) -> CircuitBreaker:
-        """Access the circuit breaker state (for monitoring)."""
+        """Access the (shared) circuit breaker state (for monitoring).
+
+        In per-endpoint mode this returns the original shared breaker, kept
+        for backward compatibility; the per-path breakers are exposed via
+        :meth:`circuit_breaker_state_for`.
+        """
         return self._circuit_breaker
 
+    def _resolve_breaker_key(self, path: str) -> str:
+        """Map a request path to its circuit-breaker key.
+
+        Default bucketing strips query/fragment and collapses the path under
+        the longest matching :class:`Endpoint` prefix; a custom
+        ``breaker_key_fn`` wins outright.
+        """
+        if self._breaker_key_fn is not None:
+            return self._breaker_key_fn(path)
+        pure = urlsplit(path).path
+        for ep in Endpoint:
+            ep_value = ep.value
+            if pure == ep_value or pure.startswith(ep_value + "/"):
+                return ep_value
+        return pure
+
+    def _breaker_for(self, path: str) -> CircuitBreaker:
+        """Return the breaker that governs ``path`` (shared, or per-endpoint)."""
+        if not self._per_endpoint_circuit_breaker:
+            return self._circuit_breaker
+        key = self._resolve_breaker_key(path)
+        # threading.Lock here is intentional: see HyperpingClient._breaker_for
+        # for the rationale (works under both pure-asyncio and mixed-thread use).
+        with self._endpoint_breakers_lock:
+            breaker = self._endpoint_breakers.get(key)
+            if breaker is None:
+                breaker = CircuitBreaker(self._circuit_breaker_config)
+                self._endpoint_breakers[key] = breaker
+            return breaker
+
+    def circuit_breaker_state_for(self, path: str) -> CircuitState:
+        """Return the circuit state of the breaker governing ``path``.
+
+        In per-endpoint mode the path is canonicalised the same way as during
+        a request; untouched buckets report :attr:`CircuitState.CLOSED`
+        without allocating a breaker. In default mode the shared breaker's
+        state is returned for any path.
+        """
+        if not self._per_endpoint_circuit_breaker:
+            return self._circuit_breaker.state
+        key = self._resolve_breaker_key(path)
+        with self._endpoint_breakers_lock:
+            breaker = self._endpoint_breakers.get(key)
+        return breaker.state if breaker is not None else CircuitState.CLOSED
+
+    def _circuit_open_message(self, breaker: CircuitBreaker, path: str) -> str:
+        """Build the error message raised when a request is rejected by an OPEN breaker."""
+        if self._per_endpoint_circuit_breaker:
+            key = self._resolve_breaker_key(path)
+            return (
+                f"Circuit breaker OPEN for {key!r} - API calls to this endpoint suspended. "
+                f"Consecutive failures: {breaker.failure_count}. "
+                f"Will recover after {breaker.recovery_timeout}s."
+            )
+        return (
+            f"Circuit breaker OPEN - API calls suspended. "
+            f"Consecutive failures: {breaker.failure_count}. "
+            f"Will recover after {breaker.recovery_timeout}s."
+        )
+
     # ==================== Error Handling ====================
 
     def _parse_error_body(self, response: httpx.Response) -> dict[str, Any]:
@@ -236,7 +323,7 @@ async def _execute_single_attempt(
         if response.status_code >= 400:
             return response
 
-        self._circuit_breaker.record_success()
+        self._breaker_for(path).record_success()
         if response.status_code == 204:
             return {}
         return response.json()  # type: ignore[no-any-return]
@@ -262,13 +349,9 @@ async def _request(
         Raises:
             HyperpingAPIError: On API errors after retries exhausted
         """
-        if not self._circuit_breaker.call_allowed():
-            cb = self._circuit_breaker
-            raise HyperpingAPIError(
-                f"Circuit breaker OPEN - API calls suspended. "
-                f"Consecutive failures: {cb.failure_count}. "
-                f"Will recover after {cb.recovery_timeout}s."
-            )
+        breaker = self._breaker_for(path)
+        if not breaker.call_allowed():
+            raise HyperpingAPIError(self._circuit_open_message(breaker, path))
 
         last_exception: Exception | None = None
         delay = self.retry_config.initial_delay
@@ -299,7 +382,7 @@ async def _request(
                     continue
 
                 if response.status_code >= 500:
-                    self._circuit_breaker.record_failure()
+                    breaker.record_failure()
                 self._handle_response_error(response)
 
             except (httpx.TimeoutException, httpx.RequestError) as e:
@@ -320,7 +403,7 @@ async def _request(
                         self.retry_config.max_delay,
                     )
                     continue
-                self._circuit_breaker.record_failure()
+                breaker.record_failure()
                 if isinstance(e, httpx.TimeoutException):
                     raise HyperpingAPIError(f"Request timeout after {max_attempts} attempts") from e
                 raise HyperpingAPIError(f"Request failed: {e}") from e