diff --git a/features/scope_chain_resolver_extension.feature b/features/scope_chain_resolver_extension.feature
new file mode 100644
index 000000000..f9b82c9cf
--- /dev/null
+++ b/features/scope_chain_resolver_extension.feature
@@ -0,0 +1,60 @@
+Feature: Scope chain resolver extension registration and invocation
+  As a plugin developer
+  I want to register custom scope chain resolvers
+  So that I can extend entity name resolution with custom logic
+
+  Background:
+    Given the plugin manager is initialized
+    And the extension catalog is loaded
+
+  Scenario: ScopeChainResolverExtension protocol is defined
+    When I check the extension protocols
+    Then ScopeChainResolverExtension protocol should exist
+    And it should have a resolver_name property
+    And it should have a can_resolve method
+    And it should have a resolve method
+
+  Scenario: Scope chain resolver extension point is registered
+    When I get all extension point definitions
+    Then the extension point "scope.chain_resolver" should be registered
+    And its protocol type should be ScopeChainResolverExtension
+    And its description should mention custom scope chain resolution
+    And its registry key should be "scope"
+
+  Scenario: Custom scope resolver can be registered
+    Given a custom scope resolver implementation
+    When I register it with the plugin manager
+    Then the resolver should be available in the registry
+    And it should be retrievable by name
+
+  Scenario: Custom scope resolver can resolve names
+    Given a custom scope resolver that handles "custom:" prefix
+    And the resolver is registered
+    When I invoke the resolver with name "custom:entity"
+    And context data is provided
+    Then the resolver should return the resolved name
+    And the resolution should be correct
+
+  Scenario: Multiple scope resolvers can coexist
+    Given a resolver for "custom:" prefix
+    And a resolver for "remote:" prefix
+    When both are registered
+    Then both should be available
+    And each should handle its own prefix
+
+  Scenario: Scope resolver can decline to resolve
+    Given a scope resolver
+    When I ask it to resolve a name it doesn't handle
+    Then can_resolve should return False
+    And resolve should return None
+
+  Scenario: Scope resolver receives context
+    Given a scope resolver that uses context data
+    When I invoke it with context containing "namespace" key
+    Then the resolver should receive the context
+    And it should be able to use context values
+
+  Scenario: Extension point count is updated
+    When I count all extension points
+    Then the total should be 31
+    And scope.chain_resolver should be included
diff --git a/features/steps/scope_chain_resolver_steps.py b/features/steps/scope_chain_resolver_steps.py
index bac5936ba..858fc0a9e 100644
--- a/features/steps/scope_chain_resolver_steps.py
+++ b/features/steps/scope_chain_resolver_steps.py
@@ -1,4 +1,12 @@
-"""Step definitions for features/scope_chain_resolver.feature."""
+"""Step definitions for scope chain resolver features.
+
+Combines steps for two related feature files:
+
+- features/scope_chain_resolver.feature
+  (domain ScopeResolutionContext + ScopeResolverRegistry)
+- features/scope_chain_resolver_extension.feature
+  (plugin extension catalog + ScopeChainResolverExtension protocol)
+"""
 
 from __future__ import annotations
 
@@ -13,6 +21,20 @@ from cleveragents.domain.contexts.scope_chain_resolver import (
     ScopeResolutionContext,
     ScopeResolverRegistry,
 )
+from cleveragents.infrastructure.plugins.extension_catalog import (
+    TOTAL_EXTENSION_POINTS,
+    get_extension_point_definitions,
+)
+from cleveragents.infrastructure.plugins.extension_protocols import (
+    ALL_EXTENSION_PROTOCOLS,
+    ScopeChainResolverExtension,
+)
+
+
+# ===========================================================================
+# Steps for features/scope_chain_resolver.feature
+# (domain ScopeResolutionContext + ScopeResolverRegistry)
+# ===========================================================================
 
 
 # ---------------------------------------------------------------------------
@@ -257,3 +279,370 @@ def step_registry_empty(context: Context) -> None:
     assert context.registry.get_resolvers() == {}, (
         f"Expected empty registry, got {list(context.registry.get_resolvers())}"
     )
+
+
+# ===========================================================================
+# Steps for features/scope_chain_resolver_extension.feature
+# (plugin extension catalog + ScopeChainResolverExtension protocol)
+# ===========================================================================
+
+
+@given("the plugin manager is initialized")
+def step_plugin_manager_initialized(context: Any) -> None:
+    """Initialize the plugin manager context."""
+    context.plugin_manager = None  # Will be set up as needed
+
+
+@given("the extension catalog is loaded")
+def step_extension_catalog_loaded(context: Any) -> None:
+    """Load the extension catalog."""
+    context.extension_points = get_extension_point_definitions()
+
+
+@when("I check the extension protocols")
+def step_check_extension_protocols(context: Any) -> None:
+    """Check that extension protocols are available."""
+    context.protocols = ALL_EXTENSION_PROTOCOLS
+
+
+@then("ScopeChainResolverExtension protocol should exist")
+def step_scope_chain_resolver_protocol_exists(context: Any) -> None:
+    """Verify ScopeChainResolverExtension protocol exists."""
+    assert ScopeChainResolverExtension is not None
+    assert "scope.chain_resolver" in context.protocols
+    assert context.protocols["scope.chain_resolver"] is ScopeChainResolverExtension
+
+
+@then("it should have a resolver_name property")
+def step_has_resolver_name_property(context: Any) -> None:
+    """Verify the protocol has resolver_name property."""
+    protocol = ScopeChainResolverExtension
+    assert hasattr(protocol, "resolver_name")
+
+
+@then("it should have a can_resolve method")
+def step_has_can_resolve_method(context: Any) -> None:
+    """Verify the protocol has can_resolve method."""
+    protocol = ScopeChainResolverExtension
+    assert hasattr(protocol, "can_resolve")
+
+
+@then("it should have a resolve method")
+def step_has_resolve_method(context: Any) -> None:
+    """Verify the protocol has resolve method."""
+    protocol = ScopeChainResolverExtension
+    assert hasattr(protocol, "resolve")
+
+
+@when("I get all extension point definitions")
+def step_get_extension_point_definitions(context: Any) -> None:
+    """Get all extension point definitions."""
+    context.extension_points = get_extension_point_definitions()
+
+
+@then('the extension point "scope.chain_resolver" should be registered')
+def step_scope_chain_resolver_registered(context: Any) -> None:
+    """Verify scope.chain_resolver extension point is registered."""
+    ep_names = [ep.name for ep in context.extension_points]
+    assert "scope.chain_resolver" in ep_names
+
+
+@then("its protocol type should be ScopeChainResolverExtension")
+def step_protocol_type_is_scope_chain_resolver(context: Any) -> None:
+    """Verify the protocol type is correct."""
+    ep = next(
+        (ep for ep in context.extension_points if ep.name == "scope.chain_resolver"),
+        None,
+    )
+    assert ep is not None
+    assert ep.protocol_type is ScopeChainResolverExtension
+
+
+@then("its description should mention custom scope chain resolution")
+def step_description_mentions_scope_chain(context: Any) -> None:
+    """Verify the description mentions scope chain resolution."""
+    ep = next(
+        (ep for ep in context.extension_points if ep.name == "scope.chain_resolver"),
+        None,
+    )
+    assert ep is not None
+    assert "scope" in ep.description.lower() or "resolver" in ep.description.lower()
+
+
+@then('its registry key should be "scope"')
+def step_registry_key_is_scope(context: Any) -> None:
+    """Verify the registry key is 'scope'."""
+    ep = next(
+        (ep for ep in context.extension_points if ep.name == "scope.chain_resolver"),
+        None,
+    )
+    assert ep is not None
+    assert ep.registry_key == "scope"
+
+
+@given("a custom scope resolver implementation")
+def step_custom_scope_resolver_implementation(context: Any) -> None:
+    """Create a custom scope resolver implementation."""
+
+    class CustomScopeResolver:
+        """Test implementation of ScopeChainResolverExtension."""
+
+        @property
+        def resolver_name(self) -> str:
+            return "test_resolver"
+
+        def can_resolve(self, name: str) -> bool:
+            return name.startswith("test:")
+
+        def resolve(self, name: str, context: dict[str, Any]) -> str | None:
+            if self.can_resolve(name):
+                return f"resolved_{name}"
+            return None
+
+    context.custom_resolver = CustomScopeResolver()
+
+
+@when("I register it with the plugin manager")
+def step_register_with_plugin_manager(context: Any) -> None:
+    """Register the custom resolver."""
+    context.registered_resolver = context.custom_resolver
+
+
+@then("the resolver should be available in the registry")
+def step_resolver_available_in_registry(context: Any) -> None:
+    """Verify the resolver is available."""
+    assert context.registered_resolver is not None
+    assert hasattr(context.registered_resolver, "resolver_name")
+
+
+@then("it should be retrievable by name")
+def step_retrievable_by_name(context: Any) -> None:
+    """Verify the resolver can be retrieved by name."""
+    resolver = context.registered_resolver
+    assert resolver.resolver_name == "test_resolver"
+
+
+@given('a custom scope resolver that handles "custom:" prefix')
+def step_custom_resolver_with_prefix(context: Any) -> None:
+    """Create a resolver that handles custom: prefix."""
+
+    class CustomPrefixResolver:
+        @property
+        def resolver_name(self) -> str:
+            return "custom_prefix_resolver"
+
+        def can_resolve(self, name: str) -> bool:
+            return name.startswith("custom:")
+
+        def resolve(self, name: str, context: dict[str, Any]) -> str | None:
+            if self.can_resolve(name):
+                # Remove prefix and return resolved name
+                entity = name[7:]  # Remove "custom:"
+                return f"resolved_{entity}"
+            return None
+
+    context.custom_resolver = CustomPrefixResolver()
+
+
+@given("the resolver is registered")
+def step_resolver_is_registered(context: Any) -> None:
+    """Register the resolver."""
+    context.registered_resolver = context.custom_resolver
+
+
+@when('I invoke the resolver with name "custom:entity"')
+def step_invoke_resolver_with_name(context: Any) -> None:
+    """Invoke the resolver with a test name."""
+    resolver = context.registered_resolver
+    context.resolution_result = resolver.resolve("custom:entity", {})
+
+
+@when("context data is provided")
+def step_context_data_provided(context: Any) -> None:
+    """Provide context data."""
+    context.context_data = {"namespace": "test_namespace"}
+
+
+@then("the resolver should return the resolved name")
+def step_resolver_returns_resolved_name(context: Any) -> None:
+    """Verify the resolver returns a resolved name."""
+    assert context.resolution_result is not None
+
+
+@then("the resolution should be correct")
+def step_resolution_is_correct(context: Any) -> None:
+    """Verify the resolution is correct."""
+    assert context.resolution_result == "resolved_entity"
+
+
+@given('a resolver for "custom:" prefix')
+def step_resolver_for_custom_prefix(context: Any) -> None:
+    """Create a resolver for custom: prefix."""
+
+    class CustomResolver:
+        @property
+        def resolver_name(self) -> str:
+            return "custom_resolver"
+
+        def can_resolve(self, name: str) -> bool:
+            return name.startswith("custom:")
+
+        def resolve(self, name: str, context: dict[str, Any]) -> str | None:
+            if self.can_resolve(name):
+                return f"custom_{name[7:]}"
+            return None
+
+    context.custom_resolver = CustomResolver()
+
+
+@given('a resolver for "remote:" prefix')
+def step_resolver_for_remote_prefix(context: Any) -> None:
+    """Create a resolver for remote: prefix."""
+
+    class RemoteResolver:
+        @property
+        def resolver_name(self) -> str:
+            return "remote_resolver"
+
+        def can_resolve(self, name: str) -> bool:
+            return name.startswith("remote:")
+
+        def resolve(self, name: str, context: dict[str, Any]) -> str | None:
+            if self.can_resolve(name):
+                return f"remote_{name[7:]}"
+            return None
+
+    context.remote_resolver = RemoteResolver()
+
+
+@when("both are registered")
+def step_both_registered(context: Any) -> None:
+    """Register both resolvers."""
+    context.resolvers = {
+        "custom": context.custom_resolver,
+        "remote": context.remote_resolver,
+    }
+
+
+@then("both should be available")
+def step_both_available(context: Any) -> None:
+    """Verify both resolvers are available."""
+    assert len(context.resolvers) == 2
+    assert "custom" in context.resolvers
+    assert "remote" in context.resolvers
+
+
+@then("each should handle its own prefix")
+def step_each_handles_own_prefix(context: Any) -> None:
+    """Verify each resolver handles its own prefix."""
+    custom = context.resolvers["custom"]
+    remote = context.resolvers["remote"]
+
+    assert custom.can_resolve("custom:test")
+    assert not custom.can_resolve("remote:test")
+
+    assert remote.can_resolve("remote:test")
+    assert not remote.can_resolve("custom:test")
+
+
+@given("a scope resolver")
+def step_scope_resolver(context: Any) -> None:
+    """Create a basic scope resolver."""
+
+    class BasicResolver:
+        @property
+        def resolver_name(self) -> str:
+            return "basic_resolver"
+
+        def can_resolve(self, name: str) -> bool:
+            return name.startswith("basic:")
+
+        def resolve(self, name: str, context: dict[str, Any]) -> str | None:
+            if self.can_resolve(name):
+                return f"basic_{name[6:]}"
+            return None
+
+    context.resolver = BasicResolver()
+
+
+@when("I ask it to resolve a name it doesn't handle")
+def step_ask_to_resolve_unhandled_name(context: Any) -> None:
+    """Ask the resolver to handle a name it doesn't support."""
+    resolver = context.resolver
+    context.can_resolve_result = resolver.can_resolve("other:test")
+    context.resolve_result = resolver.resolve("other:test", {})
+
+
+@then("can_resolve should return False")
+def step_can_resolve_returns_false(context: Any) -> None:
+    """Verify can_resolve returns False."""
+    assert context.can_resolve_result is False
+
+
+@then("resolve should return None")
+def step_resolve_returns_none(context: Any) -> None:
+    """Verify resolve returns None."""
+    assert context.resolve_result is None
+
+
+@given("a scope resolver that uses context data")
+def step_resolver_uses_context(context: Any) -> None:
+    """Create a resolver that uses context data."""
+
+    class ContextAwareResolver:
+        @property
+        def resolver_name(self) -> str:
+            return "context_aware_resolver"
+
+        def can_resolve(self, name: str) -> bool:
+            return name.startswith("ctx:")
+
+        def resolve(self, name: str, context: dict[str, Any]) -> str | None:
+            if self.can_resolve(name):
+                namespace = context.get("namespace", "default")
+                entity = name[4:]  # Remove "ctx:"
+                return f"{namespace}:{entity}"
+            return None
+
+    context.resolver = ContextAwareResolver()
+
+
+@when('I invoke it with context containing "namespace" key')
+def step_invoke_with_namespace_context(context: Any) -> None:
+    """Invoke the resolver with namespace context."""
+    resolver = context.resolver
+    ctx = {"namespace": "myapp"}
+    context.resolve_result = resolver.resolve("ctx:entity", ctx)
+
+
+@then("the resolver should receive the context")
+def step_resolver_receives_context(context: Any) -> None:
+    """Verify the resolver received the context."""
+    assert context.resolve_result is not None
+
+
+@then("it should be able to use context values")
+def step_can_use_context_values(context: Any) -> None:
+    """Verify the resolver used context values."""
+    assert context.resolve_result == "myapp:entity"
+
+
+@when("I count all extension points")
+def step_count_extension_points(context: Any) -> None:
+    """Count all extension points."""
+    context.extension_points = get_extension_point_definitions()
+    context.total_count = len(context.extension_points)
+
+
+@then("the total should be 31")
+def step_total_is_31(context: Any) -> None:
+    """Verify the total is 31."""
+    assert context.total_count == 31
+    assert TOTAL_EXTENSION_POINTS == 31
+
+
+@then("scope.chain_resolver should be included")
+def step_scope_chain_resolver_included(context: Any) -> None:
+    """Verify scope.chain_resolver is included."""
+    ep_names = [ep.name for ep in context.extension_points]
+    assert "scope.chain_resolver" in ep_names
diff --git a/src/cleveragents/infrastructure/plugins/extension_catalog.py b/src/cleveragents/infrastructure/plugins/extension_catalog.py
index fa49e6120..bca6d6222 100644
--- a/src/cleveragents/infrastructure/plugins/extension_catalog.py
+++ b/src/cleveragents/infrastructure/plugins/extension_catalog.py
@@ -1,4 +1,4 @@
-"""Extension point catalog for all 30 spec-defined extension points.
+"""Extension point catalog for all 31 spec-defined extension points.
 
 Provides :func:`register_all_extension_points` which registers every
 extension point defined in ``docs/specification.md`` into the
@@ -10,7 +10,7 @@ category derived from its dotted name prefix.
 Based on ``docs/specification.md`` Extension Points Summary and
 issue #939.
 
-ISSUES CLOSED: #939
+ISSUES CLOSED: #939, #5705
 """
 
 from __future__ import annotations
@@ -33,6 +33,7 @@ from cleveragents.infrastructure.plugins.extension_protocols import (
     ResourceResolverExtension,
     ResourceTypeHandlerExtension,
     SafetyGuardrailExtension,
+    ScopeChainResolverExtension,
     SkillProviderExtension,
     SkillTemplateExtension,
     ToolMiddlewareExtension,
@@ -89,7 +90,7 @@ def _category_from_name(name: str) -> str:
     return name.split(".")[0]
 
 
-# The authoritative list of all 30 spec-defined extension points.
+# The authoritative list of all 31 spec-defined extension points.
 _EXTENSION_POINT_DEFS: tuple[_ExtensionPointDef, ...] = (
     # --- Context (12) ---
     _ExtensionPointDef(
@@ -245,6 +246,12 @@ _EXTENSION_POINT_DEFS: tuple[_ExtensionPointDef, ...] = (
         ConfigValidatorExtension,
         "Validates configuration values against rules",
     ),
+    # --- Scope Chain (1) ---
+    _ExtensionPointDef(
+        "scope.chain_resolver",
+        ScopeChainResolverExtension,
+        "Custom scope chain resolver for entity name resolution",
+    ),
     # --- Safety (1) ---
     _ExtensionPointDef(
         "safety.guardrail",
@@ -254,11 +261,11 @@ _EXTENSION_POINT_DEFS: tuple[_ExtensionPointDef, ...] = (
 )
 
 # Public constant: number of spec-defined extension points.
-TOTAL_EXTENSION_POINTS: int = 30
+TOTAL_EXTENSION_POINTS: int = 31
 
 
 def get_extension_point_definitions() -> tuple[ExtensionPoint, ...]:
-    """Return all 30 spec-defined extension points as ExtensionPoint models.
+    """Return all 31 spec-defined extension points as ExtensionPoint models.
 
     Each entry includes the point's name, Protocol type, description,
     and a ``registry_key`` equal to its category prefix.
@@ -288,7 +295,7 @@ def get_extension_points_by_category() -> dict[str, list[ExtensionPoint]]:
 
 
 def register_all_extension_points(manager: ExtensionPointRegistrar) -> int:
-    """Register all 30 spec-defined extension points on *manager*.
+    """Register all 31 spec-defined extension points on *manager*.
 
     Args:
         manager: Object with a ``register_extension_point`` method
diff --git a/src/cleveragents/infrastructure/plugins/extension_protocols.py b/src/cleveragents/infrastructure/plugins/extension_protocols.py
index 2c30e1e2f..47323041c 100644
--- a/src/cleveragents/infrastructure/plugins/extension_protocols.py
+++ b/src/cleveragents/infrastructure/plugins/extension_protocols.py
@@ -1,13 +1,13 @@
 """Typed Protocol interfaces for all spec-defined extension points.
 
-Defines ``@runtime_checkable`` Protocol classes for each of the 30
+Defines ``@runtime_checkable`` Protocol classes for each of the 31
 extension point categories.  Each Protocol specifies the minimum
 interface that a plugin implementation must satisfy.
 
 Based on ``docs/specification.md`` Extension Points Summary and
 issue #939.
 
-ISSUES CLOSED: #939
+ISSUES CLOSED: #939, #5705
 """
 
 from __future__ import annotations
@@ -286,6 +286,28 @@ class ConfigValidatorExtension(Protocol):
     def validate(self, config: Mapping[str, Any]) -> Sequence[str]: ...
 
 
+# ---------------------------------------------------------------------------
+# Scope chain extension protocols (1 extension point)
+# ---------------------------------------------------------------------------
+
+
+@runtime_checkable
+class ScopeChainResolverExtension(Protocol):
+    """Protocol for custom scope chain resolver plugins.
+
+    Allows third-party plugins to extend how entity names are resolved
+    during name resolution (e.g., custom namespace prefixes, remote
+    server lookups, alias resolution strategies).
+    """
+
+    @property
+    def resolver_name(self) -> str: ...
+
+    def can_resolve(self, name: str) -> bool: ...
+
+    def resolve(self, name: str, context: Mapping[str, Any]) -> str | None: ...
+
+
 # ---------------------------------------------------------------------------
 # Safety extension protocols (1 extension point)
 # ---------------------------------------------------------------------------
@@ -337,5 +359,6 @@ ALL_EXTENSION_PROTOCOLS: dict[str, type[Any]] = {
     "event.filter": EventFilterExtension,
     "config.source": ConfigSourceExtension,
     "config.validator": ConfigValidatorExtension,
+    "scope.chain_resolver": ScopeChainResolverExtension,
     "safety.guardrail": SafetyGuardrailExtension,
 }