diff --git a/crates/pyrefly_config/src/error_kind.rs b/crates/pyrefly_config/src/error_kind.rs
index 79cd48749f..b4a7433e22 100644
--- a/crates/pyrefly_config/src/error_kind.rs
+++ b/crates/pyrefly_config/src/error_kind.rs
@@ -257,6 +257,8 @@ pub enum ErrorKind {
     UnannotatedAttribute,
     /// A function parameter is missing a type annotation.
     UnannotatedParameter,
+    /// A protocol member is assigned a value in the class body without an explicit type annotation.
+    UnannotatedProtocolMember,
     /// A function is missing a return type annotation.
     UnannotatedReturn,
     /// Attempting to use a name that may be unbound or uninitialized
diff --git a/pyrefly/lib/alt/class/class_field.rs b/pyrefly/lib/alt/class/class_field.rs
index 92bddea4d0..b74aab5899 100644
--- a/pyrefly/lib/alt/class/class_field.rs
+++ b/pyrefly/lib/alt/class/class_field.rs
@@ -1588,6 +1588,20 @@ impl<'a, Ans: LookupAnswer> AnswersSolver<'a, Ans> {
                 ..
             } => {
                 let direct_annotation = annot.map(|a| self.get_idx(a).annotation.clone());
+                if metadata.is_protocol()
+                    && direct_annotation.is_none()
+                    && !is_dunder(name.as_str())
+                {
+                    self.error(
+                        errors,
+                        range,
+                        ErrorInfo::Kind(ErrorKind::UnannotatedProtocolMember),
+                        format!(
+                            "Protocol member `{}` must have an explicit type annotation",
+                            name,
+                        ),
+                    );
+                }
                 let initialization = if let ExprOrBinding::Expr(e) = value.as_ref()
                     && let Some(dm) = metadata.dataclass_metadata()
                     && let Expr::Call(call) = e
diff --git a/pyrefly/lib/test/protocol.rs b/pyrefly/lib/test/protocol.rs
index 5adb9fc39a..ea8b6600a9 100644
--- a/pyrefly/lib/test/protocol.rs
+++ b/pyrefly/lib/test/protocol.rs
@@ -1008,15 +1008,16 @@ def to_foo() -> Foo[MySeries]:
 
 // https://github.com/facebook/pyrefly/issues/2925
 testcase!(
-    bug = "Should detect ambiguous protocol members with value assignments",
     test_protocol_ambiguous_member,
     r#"
 from typing import Protocol
 
 class Ambiguous(Protocol):
-    # Assigning a value in a Protocol body is ambiguous: is it declaring
-    # a member with a type, or providing a default value?
-    x = None
-    y = ...
+    x = None  # E: Protocol member `x` must have an explicit type annotation
+    y = ...  # E: Protocol member `y` must have an explicit type annotation
+
+class Ok(Protocol):
+    x: int
+    y: str = "default"
 "#,
 );
diff --git a/website/docs/error-kinds.mdx b/website/docs/error-kinds.mdx
index 19f9878af4..b7a559d894 100644
--- a/website/docs/error-kinds.mdx
+++ b/website/docs/error-kinds.mdx
@@ -1254,6 +1254,21 @@ def process_data(x: int, y: int) -> int:
     return x + y
 ```
 
+## unannotated-protocol-member
+
+This error is raised when a protocol member is assigned a value in the class body without an explicit type annotation. Protocol members must have explicitly declared types so that implementations know exactly what type to provide.
+
+```python
+from typing import Protocol
+
+class MyProto(Protocol):
+    x = None  # error: Protocol member `x` must have an explicit type annotation
+
+# Fixed version:
+class MyProto(Protocol):
+    x: int | None = None
+```
+
 ## unannotated-return
 
 This error is raised when a function is missing a return type annotation. This helps enforce fully-typed codebases by ensuring all functions declare their return types explicitly. To fix it, add a return type annotation to the function.