diff --git a/internal/difc/sink_server_ids_test.go b/internal/difc/sink_server_ids_test.go
new file mode 100644
index 00000000..ea6eca59
--- /dev/null
+++ b/internal/difc/sink_server_ids_test.go
@@ -0,0 +1,223 @@
+package difc
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+// resetSinkServerIDs resets the global sinkServerIDs to a clean state for test isolation.
+func resetSinkServerIDs(t *testing.T) {
+	t.Helper()
+	sinkServerIDsMu.Lock()
+	sinkServerIDs = []string{}
+	sinkServerIDsMu.Unlock()
+}
+
+func TestSetSinkServerIDs(t *testing.T) {
+	tests := []struct {
+		name     string
+		input    []string
+		expected []string
+	}{
+		{
+			name:     "nil clears configuration",
+			input:    nil,
+			expected: nil,
+		},
+		{
+			name:     "empty slice clears configuration",
+			input:    []string{},
+			expected: nil,
+		},
+		{
+			name:     "single server ID",
+			input:    []string{"github"},
+			expected: []string{"github"},
+		},
+		{
+			name:     "multiple server IDs stored sorted",
+			input:    []string{"slack", "github", "jira"},
+			expected: []string{"github", "jira", "slack"},
+		},
+		{
+			name:     "duplicate IDs are deduplicated",
+			input:    []string{"github", "slack", "github"},
+			expected: []string{"github", "slack"},
+		},
+		{
+			name:     "whitespace is trimmed",
+			input:    []string{"  github  ", "\tslack\t"},
+			expected: []string{"github", "slack"},
+		},
+		{
+			name:     "empty strings are skipped",
+			input:    []string{"github", "", "   ", "slack"},
+			expected: []string{"github", "slack"},
+		},
+		{
+			// All entries are blank so none are added to normalized; sinkServerIDs
+			// is set to the non-nil but empty normalized slice (not nil, because the
+			// early-nil path only triggers when len(serverIDs)==0).
+			name:     "all empty strings results in empty slice",
+			input:    []string{"", "   ", "\t"},
+			expected: []string{},
+		},
+		{
+			name:     "duplicate whitespace-trimmed IDs are deduplicated",
+			input:    []string{"github", "  github  "},
+			expected: []string{"github"},
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Cleanup(func() { resetSinkServerIDs(t) })
+
+			SetSinkServerIDs(tt.input)
+
+			sinkServerIDsMu.RLock()
+			result := sinkServerIDs
+			sinkServerIDsMu.RUnlock()
+
+			assert.Equal(t, tt.expected, result)
+		})
+	}
+}
+
+func TestSetSinkServerIDs_OverwritesPreviousConfiguration(t *testing.T) {
+	t.Cleanup(func() { resetSinkServerIDs(t) })
+
+	SetSinkServerIDs([]string{"server-a", "server-b"})
+
+	sinkServerIDsMu.RLock()
+	first := make([]string, len(sinkServerIDs))
+	copy(first, sinkServerIDs)
+	sinkServerIDsMu.RUnlock()
+
+	require.Equal(t, []string{"server-a", "server-b"}, first)
+
+	SetSinkServerIDs([]string{"server-c"})
+
+	sinkServerIDsMu.RLock()
+	second := sinkServerIDs
+	sinkServerIDsMu.RUnlock()
+
+	assert.Equal(t, []string{"server-c"}, second, "second call should overwrite first configuration")
+}
+
+func TestSetSinkServerIDs_ClearWithEmpty(t *testing.T) {
+	t.Cleanup(func() { resetSinkServerIDs(t) })
+
+	SetSinkServerIDs([]string{"github", "slack"})
+	SetSinkServerIDs([]string{})
+
+	sinkServerIDsMu.RLock()
+	result := sinkServerIDs
+	sinkServerIDsMu.RUnlock()
+
+	assert.Nil(t, result, "empty input should clear sink server IDs")
+}
+
+func TestIsSinkServerID(t *testing.T) {
+	tests := []struct {
+		name        string
+		configured  []string
+		queryID     string
+		expected    bool
+	}{
+		{
+			name:       "matching server ID returns true",
+			configured: []string{"github"},
+			queryID:    "github",
+			expected:   true,
+		},
+		{
+			name:       "non-matching server ID returns false",
+			configured: []string{"github"},
+			queryID:    "slack",
+			expected:   false,
+		},
+		{
+			name:       "empty configuration returns false",
+			configured: []string{},
+			queryID:    "github",
+			expected:   false,
+		},
+		{
+			name:       "nil configuration returns false",
+			configured: nil,
+			queryID:    "github",
+			expected:   false,
+		},
+		{
+			name:       "matches one of multiple configured IDs",
+			configured: []string{"github", "slack", "jira"},
+			queryID:    "slack",
+			expected:   true,
+		},
+		{
+			name:       "does not match any of multiple configured IDs",
+			configured: []string{"github", "slack", "jira"},
+			queryID:    "notion",
+			expected:   false,
+		},
+		{
+			name:       "case-sensitive: uppercase does not match lowercase",
+			configured: []string{"github"},
+			queryID:    "GitHub",
+			expected:   false,
+		},
+		{
+			name:       "empty query ID does not match non-empty configured IDs",
+			configured: []string{"github"},
+			queryID:    "",
+			expected:   false,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Cleanup(func() { resetSinkServerIDs(t) })
+
+			SetSinkServerIDs(tt.configured)
+			result := IsSinkServerID(tt.queryID)
+			assert.Equal(t, tt.expected, result)
+		})
+	}
+}
+
+func TestIsSinkServerID_AfterClear(t *testing.T) {
+	t.Cleanup(func() { resetSinkServerIDs(t) })
+
+	SetSinkServerIDs([]string{"github"})
+	require.True(t, IsSinkServerID("github"), "should match before clearing")
+
+	SetSinkServerIDs(nil)
+	assert.False(t, IsSinkServerID("github"), "should not match after clearing")
+}
+
+func TestSetSinkServerIDs_Concurrency(t *testing.T) {
+	t.Cleanup(func() { resetSinkServerIDs(t) })
+
+	done := make(chan struct{}, 20)
+
+	for i := 0; i < 10; i++ {
+		go func() {
+			defer func() { done <- struct{}{} }()
+			SetSinkServerIDs([]string{"github", "slack"})
+		}()
+	}
+
+	for i := 0; i < 10; i++ {
+		go func() {
+			defer func() { done <- struct{}{} }()
+			_ = IsSinkServerID("github")
+		}()
+	}
+
+	for i := 0; i < 20; i++ {
+		<-done
+	}
+}