diff --git a/.chloggen/mx-psi_deprecate-func-methods.yaml b/.chloggen/mx-psi_deprecate-func-methods.yaml
new file mode 100644
index 00000000000..90908efe992
--- /dev/null
+++ b/.chloggen/mx-psi_deprecate-func-methods.yaml
@@ -0,0 +1,25 @@
+# Use this changelog template to create an entry for release notes.
+
+# One of 'breaking', 'deprecation', 'new_component', 'enhancement', 'bug_fix'
+change_type: deprecation
+
+# The name of the component, or a single word describing the area of concern, (e.g. otlpreceiver)
+component: extension/extensionauth
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: Deprecate methods on `*Func` types.
+
+# One or more tracking issues or pull requests related to the change
+issues: [12480]
+
+# (Optional) One or more lines of additional information to render under the primary note.
+# These lines will be padded with 2 spaces and then inserted directly into the document.
+# Use pipe (|) for multiline entries.
+subtext:
+
+# Optional: The change log or logs in which this entry should be included.
+# e.g. '[user]' or '[user, api]'
+# Include 'user' if the change is relevant to end users.
+# Include 'api' if there is a change to a library API.
+# Default: '[user]'
+change_logs: [api]
diff --git a/extension/extensionauth/client.go b/extension/extensionauth/client.go
index 2ea8653cc23..a303082cb3d 100644
--- a/extension/extensionauth/client.go
+++ b/extension/extensionauth/client.go
@@ -39,6 +39,7 @@ func (of clientOptionFunc) apply(e *defaultClient) {
 // ClientRoundTripperFunc specifies the function that returns a RoundTripper that can be used to authenticate HTTP requests.
 type ClientRoundTripperFunc func(base http.RoundTripper) (http.RoundTripper, error)
 
+// Deprecated: [v0.121.0] No longer used, will be removed.
 func (f ClientRoundTripperFunc) RoundTripper(base http.RoundTripper) (http.RoundTripper, error) {
 	if f == nil {
 		return base, nil
@@ -49,6 +50,7 @@ func (f ClientRoundTripperFunc) RoundTripper(base http.RoundTripper) (http.Round
 // ClientPerRPCCredentialsFunc specifies the function that returns a PerRPCCredentials that can be used to authenticate gRPC requests.
 type ClientPerRPCCredentialsFunc func() (credentials.PerRPCCredentials, error)
 
+// Deprecated: [v0.121.0] No longer used, will be removed.
 func (f ClientPerRPCCredentialsFunc) PerRPCCredentials() (credentials.PerRPCCredentials, error) {
 	if f == nil {
 		return nil, nil
@@ -56,11 +58,29 @@ func (f ClientPerRPCCredentialsFunc) PerRPCCredentials() (credentials.PerRPCCred
 	return f()
 }
 
+var _ Client = (*defaultClient)(nil)
+
 type defaultClient struct {
 	component.StartFunc
 	component.ShutdownFunc
-	ClientRoundTripperFunc
-	ClientPerRPCCredentialsFunc
+	clientRoundTripperFunc      ClientRoundTripperFunc
+	clientPerRPCCredentialsFunc ClientPerRPCCredentialsFunc
+}
+
+// PerRPCCredentials implements Client.
+func (d *defaultClient) PerRPCCredentials() (credentials.PerRPCCredentials, error) {
+	if d.clientPerRPCCredentialsFunc == nil {
+		return nil, nil
+	}
+	return d.clientPerRPCCredentialsFunc()
+}
+
+// RoundTripper implements Client.
+func (d *defaultClient) RoundTripper(base http.RoundTripper) (http.RoundTripper, error) {
+	if d.clientRoundTripperFunc == nil {
+		return base, nil
+	}
+	return d.clientRoundTripperFunc(base)
 }
 
 // WithClientStart overrides the default `Start` function for a component.Component.
@@ -83,7 +103,7 @@ func WithClientShutdown(shutdownFunc component.ShutdownFunc) ClientOption {
 // The default round tripper is no-op.
 func WithClientRoundTripper(roundTripperFunc ClientRoundTripperFunc) ClientOption {
 	return clientOptionFunc(func(o *defaultClient) {
-		o.ClientRoundTripperFunc = roundTripperFunc
+		o.clientRoundTripperFunc = roundTripperFunc
 	})
 }
 
@@ -91,7 +111,7 @@ func WithClientRoundTripper(roundTripperFunc ClientRoundTripperFunc) ClientOptio
 // There's no default.
 func WithClientPerRPCCredentials(perRPCCredentialsFunc ClientPerRPCCredentialsFunc) ClientOption {
 	return clientOptionFunc(func(o *defaultClient) {
-		o.ClientPerRPCCredentialsFunc = perRPCCredentialsFunc
+		o.clientPerRPCCredentialsFunc = perRPCCredentialsFunc
 	})
 }
 
diff --git a/extension/extensionauth/server.go b/extension/extensionauth/server.go
index b9055ee94b1..d5fc2dcd50b 100644
--- a/extension/extensionauth/server.go
+++ b/extension/extensionauth/server.go
@@ -29,12 +29,22 @@ type Server interface {
 	Authenticate(ctx context.Context, sources map[string][]string) (context.Context, error)
 }
 
+var _ Server = (*defaultServer)(nil)
+
 type defaultServer struct {
-	ServerAuthenticateFunc
+	serverAuthenticateFunc ServerAuthenticateFunc
 	component.StartFunc
 	component.ShutdownFunc
 }
 
+// Authenticate implements Server.
+func (d *defaultServer) Authenticate(ctx context.Context, sources map[string][]string) (context.Context, error) {
+	if d.serverAuthenticateFunc == nil {
+		return ctx, nil
+	}
+	return d.serverAuthenticateFunc(ctx, sources)
+}
+
 // ServerOption represents the possible options for NewServer.
 type ServerOption interface {
 	apply(*defaultServer)
@@ -50,6 +60,7 @@ func (of serverOptionFunc) apply(e *defaultServer) {
 // on the given sources map. See Server.Authenticate.
 type ServerAuthenticateFunc func(ctx context.Context, sources map[string][]string) (context.Context, error)
 
+// Deprecated: [v0.121.0] No longer used, will be removed.
 func (f ServerAuthenticateFunc) Authenticate(ctx context.Context, sources map[string][]string) (context.Context, error) {
 	if f == nil {
 		return ctx, nil
@@ -60,7 +71,7 @@ func (f ServerAuthenticateFunc) Authenticate(ctx context.Context, sources map[st
 // WithServerAuthenticate specifies which function to use to perform the authentication.
 func WithServerAuthenticate(authFunc ServerAuthenticateFunc) ServerOption {
 	return serverOptionFunc(func(o *defaultServer) {
-		o.ServerAuthenticateFunc = authFunc
+		o.serverAuthenticateFunc = authFunc
 	})
 }