diff --git a/TODO.md b/TODO.md
index 11bf617..e8f88c9 100644
--- a/TODO.md
+++ b/TODO.md
@@ -1,4 +1,4 @@
-# TODO.md — go-help
+# TODO.md -- go-help
 
 Dispatched from core/go orchestration. Pick up tasks in order.
 
@@ -6,22 +6,22 @@ Dispatched from core/go orchestration. Pick up tasks in order.
 
 ## Phase 0: Hardening & Test Coverage
 
-- [x] **Expand parser tests** — Parser already at 100%. Existing tests cover: empty input, frontmatter-only, malformed YAML, all heading levels (H1-H6), Unicode content, path-derived IDs.
-- [x] **Expand search tests** — Added tests for: empty query, no results, case sensitivity, multi-word queries, special characters, overlapping matches, scoring boundary cases, nil-topic guard, snippet edge cases (headings-only, whitespace-only).
-- [x] **Add catalog tests** — Created `catalog_test.go` covering: DefaultCatalog, Add, List, Search, Get (found/not-found), score tie-breaking.
-- [x] **Benchmark search** — `BenchmarkSearch` with 150 topics. Baseline: ~745us/op, ~392KB/op, 4114 allocs/op (Ryzen 9 9950X).
-- [x] **`go vet ./...` clean** — No warnings.
-- [x] **Coverage: 100%** — Up from 92.1%.
+- [x] **Expand parser tests** -- Parser at 100%. Tests cover: empty input, frontmatter-only, malformed YAML (3 variants), frontmatter not at start, deeply nested headings (H4-H6 with content), Unicode (CJK, emoji, diacritics, mixed scripts), 10K+ line document, empty sections, headings without space, consecutive headings, GenerateID edge cases, path-derived IDs.
+- [x] **Expand search tests** -- Added tests for: empty query (4 variants), no results (3 variants), case sensitivity (4 variants), multi-word queries (4 variants), special characters (@, dots, underscores), overlapping matches, scoring boundary cases, nil-topic guard, snippet edge cases (headings-only, whitespace-only), duplicate topic IDs, catalog integration.
+- [x] **Add catalog tests** -- Created `catalog_test.go` covering: DefaultCatalog, Add, List, Search, Get (found/not-found), score tie-breaking.
+- [x] **Benchmark search** -- `search_bench_test.go` with 8 benchmarks: single word, multi-word, no results, partial match, 500-topic catalog, 1000-topic catalog, Add indexing, tokenize. Uses `b.Loop()` (Go 1.25+) and `b.ReportAllocs()`.
+- [x] **`go vet ./...` clean** -- No warnings.
+- [x] **Coverage: 100%** -- Up from 92.1%.
 
 ## Phase 1: Search Improvements
 
-- [x] **Fuzzy matching** — Levenshtein distance with max edit distance of 2. Words under 3 chars skip fuzzy. Score: +0.3 per fuzzy match (below prefix +0.5 and exact +1.0).
+- [x] **Fuzzy matching** -- Levenshtein distance with max edit distance of 2. Words under 3 chars skip fuzzy. Score: +0.3 per fuzzy match (below prefix +0.5 and exact +1.0).
 - [ ] Add stemming support for English search terms
-- [x] **Phrase search** — Quoted multi-word queries via `extractPhrases()`. Phrase boost: +8.0. Searches title, content, and section content.
-- [x] **Improved scoring weights** — Named constants: title +10, section +5, tag +3, phrase +8, all-words bonus +2, exact +1.0, prefix +0.5, fuzzy +0.3.
-- [x] **Tag boost** — Query words matching tags add +3.0 per matching tag.
-- [x] **Multi-word bonus** — All query words present in topic adds +2.0.
-- [x] **Tests for all new features** — Levenshtein, min3, extractPhrases, fuzzy search, phrase search, tag boost, multi-word bonus, scoring constants, phrase highlighting, section phrase matching.
+- [x] **Phrase search** -- Quoted multi-word queries via `extractPhrases()`. Phrase boost: +8.0. Searches title, content, and section content.
+- [x] **Improved scoring weights** -- Named constants: title +10, section +5, tag +3, phrase +8, all-words bonus +2, exact +1.0, prefix +0.5, fuzzy +0.3.
+- [x] **Tag boost** -- Query words matching tags add +3.0 per matching tag.
+- [x] **Multi-word bonus** -- All query words present in topic adds +2.0.
+- [x] **Tests for all new features** -- Levenshtein, min3, extractPhrases, fuzzy search, phrase search, tag boost, multi-word bonus, scoring constants, phrase highlighting, section phrase matching.
 
 ## Phase 2: core.help Integration
 
diff --git a/parser_test.go b/parser_test.go
index b95cadc..1ca0b46 100644
--- a/parser_test.go
+++ b/parser_test.go
@@ -1,9 +1,13 @@
+// SPDX-Licence-Identifier: EUPL-1.2
 package help
 
 import (
+	"fmt"
+	"strings"
 	"testing"
 
 	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
 )
 
 func TestGenerateID_Good(t *testing.T) {
@@ -337,3 +341,356 @@ func TestPathToTitle_Good(t *testing.T) {
 		})
 	}
 }
+
+// --- Phase 0: Expanded parser tests ---
+
+func TestParseTopic_Good_EmptyInput(t *testing.T) {
+	// Empty byte slice should produce a valid topic with no content
+	topic, err := ParseTopic("empty.md", []byte(""))
+
+	require.NoError(t, err)
+	assert.NotNil(t, topic)
+	assert.Equal(t, "empty", topic.ID)
+	assert.Equal(t, "", topic.Title)
+	assert.Equal(t, "", topic.Content)
+	assert.Empty(t, topic.Sections)
+	assert.Empty(t, topic.Tags)
+	assert.Empty(t, topic.Related)
+}
+
+func TestParseTopic_Good_FrontmatterOnly(t *testing.T) {
+	// Frontmatter with no body or sections
+	content := []byte(`---
+title: Metadata Only
+tags: [meta]
+order: 99
+---
+`)
+
+	topic, err := ParseTopic("meta.md", content)
+
+	require.NoError(t, err)
+	assert.Equal(t, "metadata-only", topic.ID)
+	assert.Equal(t, "Metadata Only", topic.Title)
+	assert.Equal(t, []string{"meta"}, topic.Tags)
+	assert.Equal(t, 99, topic.Order)
+	assert.Empty(t, topic.Sections)
+	// Body after frontmatter is just a newline
+	assert.Equal(t, "", strings.TrimSpace(topic.Content))
+}
+
+func TestExtractFrontmatter_Bad_MalformedYAML(t *testing.T) {
+	tests := []struct {
+		name    string
+		content string
+	}{
+		{
+			name: "unclosed bracket",
+			content: `---
+title: [broken
+tags: [also broken
+---
+
+# Content`,
+		},
+		{
+			name: "tab indentation error",
+			content: "---\ntitle: Good\n\t- bad indent\n---\n\n# Content",
+		},
+		{
+			name: "duplicate keys with conflicting types",
+			// YAML spec allows duplicate keys but implementations may vary;
+			// this tests that the parser does not panic regardless.
+			content: `---
+title: First
+title:
+  nested: value
+---
+
+# Content`,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			fm, body := ExtractFrontmatter(tt.content)
+			// Malformed YAML should return nil frontmatter without panic
+			if fm == nil {
+				// Body should be original content when YAML fails
+				assert.Equal(t, tt.content, body)
+			}
+			// No panic is the key assertion — test reaching here is success
+		})
+	}
+}
+
+func TestExtractFrontmatter_Bad_NotAtStart(t *testing.T) {
+	// Frontmatter delimiters that do not start at the beginning of the file
+	content := `Some preamble text.
+
+---
+title: Should Not Parse
+---
+
+# Content`
+
+	fm, body := ExtractFrontmatter(content)
+
+	assert.Nil(t, fm)
+	assert.Equal(t, content, body)
+}
+
+func TestExtractSections_Good_DeeplyNested(t *testing.T) {
+	content := `# Level 1
+
+Top-level content.
+
+## Level 2
+
+Second level.
+
+### Level 3
+
+Third level.
+
+#### Level 4
+
+Fourth level details.
+
+##### Level 5
+
+Fifth level fine print.
+
+###### Level 6
+
+Deepest heading level.
+`
+
+	sections := ExtractSections(content)
+
+	require.Len(t, sections, 6)
+
+	for i, expected := range []struct {
+		level int
+		title string
+	}{
+		{1, "Level 1"},
+		{2, "Level 2"},
+		{3, "Level 3"},
+		{4, "Level 4"},
+		{5, "Level 5"},
+		{6, "Level 6"},
+	} {
+		assert.Equal(t, expected.level, sections[i].Level, "section %d level", i)
+		assert.Equal(t, expected.title, sections[i].Title, "section %d title", i)
+	}
+
+	// Verify content is associated with correct sections
+	assert.Contains(t, sections[0].Content, "Top-level content.")
+	assert.Contains(t, sections[3].Content, "Fourth level details.")
+	assert.Contains(t, sections[5].Content, "Deepest heading level.")
+}
+
+func TestExtractSections_Good_DeeplyNestedWithContent(t *testing.T) {
+	// H4, H5, H6 with meaningful content under each
+	content := `#### Configuration Options
+
+Set these in your config file.
+
+##### Advanced Options
+
+Only for power users.
+
+###### Experimental Flags
+
+These may change without notice.
+`
+
+	sections := ExtractSections(content)
+
+	require.Len(t, sections, 3)
+	assert.Equal(t, 4, sections[0].Level)
+	assert.Equal(t, "Configuration Options", sections[0].Title)
+	assert.Contains(t, sections[0].Content, "Set these in your config file.")
+
+	assert.Equal(t, 5, sections[1].Level)
+	assert.Equal(t, "Advanced Options", sections[1].Title)
+	assert.Contains(t, sections[1].Content, "Only for power users.")
+
+	assert.Equal(t, 6, sections[2].Level)
+	assert.Equal(t, "Experimental Flags", sections[2].Title)
+	assert.Contains(t, sections[2].Content, "These may change without notice.")
+}
+
+func TestParseTopic_Good_Unicode(t *testing.T) {
+	tests := []struct {
+		name    string
+		content string
+		title   string
+	}{
+		{
+			name: "CJK characters",
+			content: `---
+title: 日本語ドキュメント
+tags: [日本語, ドキュメント]
+---
+
+# 日本語ドキュメント
+
+はじめにの内容です。
+
+## インストール
+
+インストール手順はこちら。
+`,
+			title: "日本語ドキュメント",
+		},
+		{
+			name: "emoji in title and content",
+			content: `---
+title: Rocket Launch 🚀
+tags: [emoji, fun]
+---
+
+# Rocket Launch 🚀
+
+This topic has emoji 🎉 in the content.
+
+## Features ✨
+
+- Fast ⚡
+- Reliable 🔒
+`,
+			title: "Rocket Launch 🚀",
+		},
+		{
+			name: "diacritics and accented characters",
+			content: `---
+title: Présentation Générale
+tags: [français]
+---
+
+# Présentation Générale
+
+Bienvenue à la documentation. Les données sont protégées.
+
+## Résumé
+
+Aperçu des fonctionnalités clés.
+`,
+			title: "Présentation Générale",
+		},
+		{
+			name: "mixed scripts",
+			content: `---
+title: Mixed Скрипты 混合
+---
+
+# Mixed Скрипты 混合
+
+Content with Кириллица, 中文, العربية, and हिन्दी.
+`,
+			title: "Mixed Скрипты 混合",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			topic, err := ParseTopic("unicode.md", []byte(tt.content))
+
+			require.NoError(t, err)
+			assert.Equal(t, tt.title, topic.Title)
+			assert.NotEmpty(t, topic.ID)
+			assert.True(t, len(topic.Sections) > 0, "should extract sections from unicode content")
+		})
+	}
+}
+
+func TestParseTopic_Good_VeryLongDocument(t *testing.T) {
+	// Build a document with 10,000+ lines
+	var b strings.Builder
+
+	b.WriteString("---\ntitle: Massive Document\ntags: [large, stress]\n---\n\n")
+
+	// Generate 100 sections, each with ~100 lines of content
+	for i := range 100 {
+		b.WriteString(fmt.Sprintf("## Section %d\n\n", i+1))
+		for j := range 100 {
+			b.WriteString(fmt.Sprintf("Line %d of section %d: Lorem ipsum dolor sit amet.\n", j+1, i+1))
+		}
+		b.WriteString("\n")
+	}
+
+	content := b.String()
+	lineCount := strings.Count(content, "\n")
+	assert.Greater(t, lineCount, 10000, "document should exceed 10K lines")
+
+	topic, err := ParseTopic("massive.md", []byte(content))
+
+	require.NoError(t, err)
+	assert.Equal(t, "Massive Document", topic.Title)
+	assert.Equal(t, "massive-document", topic.ID)
+	assert.Len(t, topic.Sections, 100)
+
+	// Verify first and last sections have correct titles
+	assert.Equal(t, "Section 1", topic.Sections[0].Title)
+	assert.Equal(t, "Section 100", topic.Sections[99].Title)
+
+	// Verify content is captured in sections
+	assert.Contains(t, topic.Sections[0].Content, "Line 1 of section 1")
+	assert.Contains(t, topic.Sections[99].Content, "Line 100 of section 100")
+}
+
+func TestExtractSections_Bad_EmptyString(t *testing.T) {
+	sections := ExtractSections("")
+	assert.Empty(t, sections)
+}
+
+func TestExtractSections_Bad_HeadingWithoutSpace(t *testing.T) {
+	// "#NoSpace" is not a valid markdown heading (needs space after #)
+	content := `#NoSpace
+##AlsoNoSpace
+Some text.
+`
+
+	sections := ExtractSections(content)
+	assert.Empty(t, sections, "headings without space after # should not be parsed")
+}
+
+func TestExtractSections_Good_ConsecutiveHeadings(t *testing.T) {
+	// Headings with no content between them
+	content := `# Title
+## Subtitle
+### Sub-subtitle
+`
+
+	sections := ExtractSections(content)
+
+	require.Len(t, sections, 3)
+	// First two sections should have empty content
+	assert.Equal(t, "", sections[0].Content)
+	assert.Equal(t, "", sections[1].Content)
+	assert.Equal(t, "", sections[2].Content)
+}
+
+func TestGenerateID_Ugly_EmptyString(t *testing.T) {
+	result := GenerateID("")
+	assert.Equal(t, "", result)
+}
+
+func TestGenerateID_Good_OnlySpecialChars(t *testing.T) {
+	result := GenerateID("!@#$%^&*()")
+	assert.Equal(t, "", result)
+}
+
+func TestGenerateID_Good_CJK(t *testing.T) {
+	result := GenerateID("日本語テスト")
+	assert.NotEmpty(t, result)
+	assert.NotContains(t, result, " ")
+}
+
+func TestGenerateID_Good_Emoji(t *testing.T) {
+	result := GenerateID("Hello 🌍 World")
+	// Emoji are not letters or digits, so they are dropped
+	assert.Equal(t, "hello-world", result)
+}
diff --git a/search_bench_test.go b/search_bench_test.go
new file mode 100644
index 0000000..8d6fe3c
--- /dev/null
+++ b/search_bench_test.go
@@ -0,0 +1,176 @@
+// SPDX-Licence-Identifier: EUPL-1.2
+package help
+
+import (
+	"fmt"
+	"strings"
+	"testing"
+)
+
+// titleCase capitalises the first letter of a string.
+// Used in benchmarks to avoid deprecated strings.Title.
+func titleCase(s string) string {
+	if len(s) == 0 {
+		return s
+	}
+	return strings.ToUpper(s[:1]) + s[1:]
+}
+
+// buildLargeCatalog creates a search index with n topics for benchmarking.
+// Each topic has a title, content with multiple paragraphs, sections, and tags.
+func buildLargeCatalog(n int) *searchIndex {
+	idx := newSearchIndex()
+
+	// Word pools for generating varied content
+	subjects := []string{
+		"configuration", "deployment", "monitoring", "testing", "debugging",
+		"authentication", "authorisation", "networking", "storage", "logging",
+		"caching", "scheduling", "routing", "migration", "backup",
+		"encryption", "compression", "validation", "serialisation", "templating",
+	}
+	verbs := []string{
+		"install", "configure", "deploy", "monitor", "debug",
+		"authenticate", "authorise", "connect", "store", "analyse",
+		"cache", "schedule", "route", "migrate", "restore",
+	}
+	adjectives := []string{
+		"advanced", "basic", "custom", "distributed", "encrypted",
+		"federated", "graceful", "hybrid", "incremental", "just-in-time",
+	}
+
+	for i := range n {
+		subj := subjects[i%len(subjects)]
+		verb := verbs[i%len(verbs)]
+		adj := adjectives[i%len(adjectives)]
+
+		title := fmt.Sprintf("%s %s Guide %d", titleCase(adj), titleCase(subj), i)
+		content := fmt.Sprintf(
+			"This guide covers how to %s %s %s systems. "+
+				"It includes step-by-step instructions for setting up %s "+
+				"in both development and production environments. "+
+				"The %s process requires careful planning and %s tools. "+
+				"Make sure to review the prerequisites before starting.",
+			verb, adj, subj, subj, subj, adj,
+		)
+
+		sections := []Section{
+			{
+				ID:      fmt.Sprintf("overview-%d", i),
+				Title:   "Overview",
+				Content: fmt.Sprintf("An overview of %s %s patterns and best practices.", adj, subj),
+			},
+			{
+				ID:      fmt.Sprintf("setup-%d", i),
+				Title:   fmt.Sprintf("%s Setup", titleCase(subj)),
+				Content: fmt.Sprintf("Detailed setup instructions for %s. Run the %s command to begin.", subj, verb),
+			},
+			{
+				ID:      fmt.Sprintf("troubleshooting-%d", i),
+				Title:   "Troubleshooting",
+				Content: fmt.Sprintf("Common issues when working with %s and how to resolve them.", subj),
+			},
+		}
+
+		idx.Add(&Topic{
+			ID:       fmt.Sprintf("%s-%s-%d", adj, subj, i),
+			Title:    title,
+			Content:  content,
+			Sections: sections,
+			Tags:     []string{subj, adj, verb, "guide"},
+		})
+	}
+
+	return idx
+}
+
+func BenchmarkSearch_SingleWord(b *testing.B) {
+	idx := buildLargeCatalog(200)
+	b.ReportAllocs()
+	b.ResetTimer()
+
+	for b.Loop() {
+		idx.Search("configuration")
+	}
+}
+
+func BenchmarkSearch_MultiWord(b *testing.B) {
+	idx := buildLargeCatalog(200)
+	b.ReportAllocs()
+	b.ResetTimer()
+
+	for b.Loop() {
+		idx.Search("advanced deployment guide")
+	}
+}
+
+func BenchmarkSearch_NoResults(b *testing.B) {
+	idx := buildLargeCatalog(200)
+	b.ReportAllocs()
+	b.ResetTimer()
+
+	for b.Loop() {
+		idx.Search("xylophone")
+	}
+}
+
+func BenchmarkSearch_PartialMatch(b *testing.B) {
+	idx := buildLargeCatalog(200)
+	b.ReportAllocs()
+	b.ResetTimer()
+
+	for b.Loop() {
+		idx.Search("config")
+	}
+}
+
+func BenchmarkSearch_LargeCatalog500(b *testing.B) {
+	idx := buildLargeCatalog(500)
+	b.ReportAllocs()
+	b.ResetTimer()
+
+	for b.Loop() {
+		idx.Search("deployment monitoring")
+	}
+}
+
+func BenchmarkSearch_LargeCatalog1000(b *testing.B) {
+	idx := buildLargeCatalog(1000)
+	b.ReportAllocs()
+	b.ResetTimer()
+
+	for b.Loop() {
+		idx.Search("testing guide")
+	}
+}
+
+func BenchmarkSearchIndex_Add(b *testing.B) {
+	// Benchmark the indexing/add path
+	topic := &Topic{
+		ID:      "bench-topic",
+		Title:   "Benchmark Topic Title",
+		Content: "This is benchmark content with several words for indexing purposes.",
+		Tags:    []string{"bench", "performance"},
+		Sections: []Section{
+			{ID: "s1", Title: "First Section", Content: "Section content for benchmarking."},
+			{ID: "s2", Title: "Second Section", Content: "More section content here."},
+		},
+	}
+
+	b.ReportAllocs()
+	b.ResetTimer()
+
+	for b.Loop() {
+		idx := newSearchIndex()
+		idx.Add(topic)
+	}
+}
+
+func BenchmarkTokenize(b *testing.B) {
+	text := "The quick brown fox jumps over the lazy dog. Configuration and deployment are covered in detail."
+	b.ReportAllocs()
+	b.ResetTimer()
+
+	for b.Loop() {
+		tokenize(text)
+	}
+}
diff --git a/search_test.go b/search_test.go
index f6fd074..f122e4b 100644
--- a/search_test.go
+++ b/search_test.go
@@ -1,3 +1,4 @@
+// SPDX-Licence-Identifier: EUPL-1.2
 package help
 
 import (
@@ -7,6 +8,7 @@ import (
 	"unicode/utf8"
 
 	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
 )
 
 func TestTokenize_Good(t *testing.T) {
@@ -339,6 +341,8 @@ func TestSearchResult_Score_Good(t *testing.T) {
 	assert.Greater(t, results[0].Score, results[1].Score)
 }
 
+// --- Upstream Phase 0 tests (100% coverage) ---
+
 func TestExtractSnippet_Good_HeadingsOnly(t *testing.T) {
 	// Content with only headings and no body text should return empty snippet
 	// when no regexes are provided. Covers the empty-return branch.
@@ -545,7 +549,7 @@ func TestSearchIndex_Search_Good_FuzzyMatching(t *testing.T) {
 	})
 
 	t.Run("two-edit typo still matches", func(t *testing.T) {
-		// "deplymnt" is within 2 edits of "deployment" — but first check
+		// "deplymnt" is within 2 edits of "deployment" -- but first check
 		// that "deploymnt" (1 edit) works.
 		results := idx.Search("deploymnt")
 		assert.NotEmpty(t, results, "fuzzy match should find results for 1-edit typo")
@@ -792,3 +796,374 @@ func TestSearchIndex_Search_Good_PhraseHighlighting(t *testing.T) {
 			"phrase should be highlighted in snippet")
 	}
 }
+
+// --- Phase 0 additional tests: expanded edge cases ---
+
+func TestSearchIndex_Search_Bad_EmptyQuery(t *testing.T) {
+	idx := newSearchIndex()
+	idx.Add(&Topic{ID: "test", Title: "Test Topic", Content: "Some content."})
+
+	t.Run("empty string", func(t *testing.T) {
+		results := idx.Search("")
+		assert.Nil(t, results)
+	})
+
+	t.Run("whitespace only", func(t *testing.T) {
+		results := idx.Search("   ")
+		assert.Nil(t, results)
+	})
+
+	t.Run("single character", func(t *testing.T) {
+		// Single chars are filtered by tokenize (min 2 chars)
+		results := idx.Search("a")
+		assert.Nil(t, results)
+	})
+
+	t.Run("punctuation only", func(t *testing.T) {
+		results := idx.Search("!@#$%")
+		assert.Nil(t, results)
+	})
+}
+
+func TestSearchIndex_Search_Bad_NoResults(t *testing.T) {
+	idx := newSearchIndex()
+
+	idx.Add(&Topic{
+		ID:      "golang",
+		Title:   "Golang Programming",
+		Content: "Building applications with Go and goroutines.",
+	})
+
+	t.Run("completely unrelated query", func(t *testing.T) {
+		results := idx.Search("quantum physics")
+		assert.Empty(t, results)
+	})
+
+	t.Run("empty index", func(t *testing.T) {
+		emptyIdx := newSearchIndex()
+		results := emptyIdx.Search("anything")
+		assert.Empty(t, results)
+	})
+}
+
+func TestSearchIndex_Search_Good_CaseSensitivity(t *testing.T) {
+	idx := newSearchIndex()
+
+	idx.Add(&Topic{
+		ID:      "case-test",
+		Title:   "PostgreSQL Configuration",
+		Content: "Configure POSTGRESQL settings. The postgresql.conf file controls everything.",
+	})
+
+	t.Run("lowercase query matches uppercase content", func(t *testing.T) {
+		results := idx.Search("postgresql")
+		require.NotEmpty(t, results)
+		assert.Equal(t, "case-test", results[0].Topic.ID)
+	})
+
+	t.Run("uppercase query matches lowercase content", func(t *testing.T) {
+		results := idx.Search("POSTGRESQL")
+		require.NotEmpty(t, results)
+		assert.Equal(t, "case-test", results[0].Topic.ID)
+	})
+
+	t.Run("mixed case query matches", func(t *testing.T) {
+		results := idx.Search("PostgreSQL")
+		require.NotEmpty(t, results)
+		assert.Equal(t, "case-test", results[0].Topic.ID)
+	})
+
+	t.Run("title case sensitivity", func(t *testing.T) {
+		results := idx.Search("configuration")
+		require.NotEmpty(t, results)
+		assert.Equal(t, "case-test", results[0].Topic.ID)
+	})
+}
+
+func TestSearchIndex_Search_Good_MultiWord(t *testing.T) {
+	idx := newSearchIndex()
+
+	idx.Add(&Topic{
+		ID:      "docker-compose",
+		Title:   "Docker Compose Setup",
+		Content: "Learn how to use Docker Compose for container orchestration.",
+	})
+	idx.Add(&Topic{
+		ID:      "docker-basics",
+		Title:   "Docker Basics",
+		Content: "Introduction to Docker containers and images.",
+	})
+	idx.Add(&Topic{
+		ID:      "kubernetes",
+		Title:   "Kubernetes Setup",
+		Content: "Setting up a Kubernetes cluster for production.",
+	})
+
+	t.Run("both words match same topic", func(t *testing.T) {
+		results := idx.Search("docker compose")
+		require.NotEmpty(t, results)
+		// docker-compose should rank highest (both words in title + content)
+		assert.Equal(t, "docker-compose", results[0].Topic.ID)
+	})
+
+	t.Run("one word matches multiple topics", func(t *testing.T) {
+		results := idx.Search("docker")
+		require.Len(t, results, 2)
+		// Both docker topics should appear
+		ids := []string{results[0].Topic.ID, results[1].Topic.ID}
+		assert.Contains(t, ids, "docker-compose")
+		assert.Contains(t, ids, "docker-basics")
+	})
+
+	t.Run("words from different topics", func(t *testing.T) {
+		results := idx.Search("docker kubernetes")
+		require.NotEmpty(t, results)
+		// All three topics should match (docker matches 2, kubernetes matches 1)
+		assert.GreaterOrEqual(t, len(results), 3)
+	})
+
+	t.Run("three word query narrows results", func(t *testing.T) {
+		results := idx.Search("docker compose setup")
+		require.NotEmpty(t, results)
+		// docker-compose has all three words, should rank first
+		assert.Equal(t, "docker-compose", results[0].Topic.ID)
+	})
+}
+
+func TestSearchIndex_Search_Good_SpecialCharsExpanded(t *testing.T) {
+	idx := newSearchIndex()
+
+	idx.Add(&Topic{
+		ID:      "email-config",
+		Title:   "Email Configuration",
+		Content: "Set SMTP_HOST to smtp.example.com and PORT to 587.",
+	})
+	idx.Add(&Topic{
+		ID:      "dotfiles",
+		Title:   "Dotfile Management",
+		Content: "Manage your .bashrc and .zshrc files across machines.",
+	})
+	idx.Add(&Topic{
+		ID:      "at-mentions",
+		Title:   "User Mentions",
+		Content: "Use @username to mention users in comments.",
+	})
+
+	t.Run("query with at symbol", func(t *testing.T) {
+		// "@username" tokenises to "username" (@ is stripped)
+		results := idx.Search("@username")
+		require.NotEmpty(t, results)
+		assert.Equal(t, "at-mentions", results[0].Topic.ID)
+	})
+
+	t.Run("query with dots", func(t *testing.T) {
+		// "smtp.example.com" tokenises to "smtp", "example", "com"
+		results := idx.Search("smtp.example.com")
+		require.NotEmpty(t, results)
+		assert.Equal(t, "email-config", results[0].Topic.ID)
+	})
+
+	t.Run("query with underscores", func(t *testing.T) {
+		// "SMTP_HOST" tokenises to "smtp", "host"
+		results := idx.Search("SMTP_HOST")
+		require.NotEmpty(t, results)
+		assert.Equal(t, "email-config", results[0].Topic.ID)
+	})
+}
+
+func TestSearchIndex_Search_Good_OverlappingMatches(t *testing.T) {
+	idx := newSearchIndex()
+
+	idx.Add(&Topic{
+		ID:      "search-guide",
+		Title:   "Searching and Search Results",
+		Content: "The search function searches through searchable content to find search results.",
+	})
+
+	// "search" should match: "searching", "search", "searches", "searchable"
+	results := idx.Search("search")
+	require.NotEmpty(t, results)
+	assert.Equal(t, "search-guide", results[0].Topic.ID)
+	// Score should be boosted since "search" appears in the title
+	assert.Greater(t, results[0].Score, 10.0)
+}
+
+func TestSearchIndex_Search_Good_ScoringBoundary(t *testing.T) {
+	idx := newSearchIndex()
+
+	// Topic A: exact title match
+	idx.Add(&Topic{
+		ID:      "exact-title",
+		Title:   "Installation",
+		Content: "Basic content without the query word repeated.",
+	})
+
+	// Topic B: no title match but heavy body usage
+	idx.Add(&Topic{
+		ID:      "heavy-body",
+		Title:   "Getting Started Guide",
+		Content: "Installation steps: First install the package. Then install dependencies. The installation is straightforward. Install everything.",
+		Sections: []Section{
+			{
+				ID:      "install-section",
+				Title:   "Install Steps",
+				Content: "Detailed installation instructions for every platform.",
+			},
+		},
+	})
+
+	results := idx.Search("installation")
+	require.Len(t, results, 2)
+
+	// Title match gets +10 boost, so "exact-title" should rank first
+	assert.Equal(t, "exact-title", results[0].Topic.ID, "exact title match should rank above body-heavy match")
+	assert.Greater(t, results[0].Score, results[1].Score)
+}
+
+func TestSearchIndex_Search_Good_TagMatching(t *testing.T) {
+	idx := newSearchIndex()
+
+	idx.Add(&Topic{
+		ID:      "tagged-topic",
+		Title:   "Workflow Automation",
+		Content: "Automate your CI/CD pipeline.",
+		Tags:    []string{"devops", "cicd", "automation"},
+	})
+
+	// Search for a tag that does not appear in title or content
+	results := idx.Search("devops")
+	require.NotEmpty(t, results)
+	assert.Equal(t, "tagged-topic", results[0].Topic.ID)
+}
+
+func TestSearchIndex_Search_Good_SectionTitleBoost(t *testing.T) {
+	idx := newSearchIndex()
+
+	idx.Add(&Topic{
+		ID:      "section-match",
+		Title:   "Complete Reference",
+		Content: "Overview of all features.",
+		Sections: []Section{
+			{ID: "deployment", Title: "Deployment", Content: "How to deploy your application."},
+			{ID: "monitoring", Title: "Monitoring", Content: "Set up health checks."},
+		},
+	})
+
+	idx.Add(&Topic{
+		ID:      "body-match",
+		Title:   "Quick Tips",
+		Content: "Deployment can be tricky, here are some tips.",
+	})
+
+	results := idx.Search("deployment")
+	require.Len(t, results, 2)
+
+	// Section title match gives +5 boost (in addition to other scoring)
+	sectionResult := results[0]
+	assert.Equal(t, "section-match", sectionResult.Topic.ID)
+	if sectionResult.Section != nil {
+		assert.Equal(t, "deployment", sectionResult.Section.ID)
+	}
+}
+
+func TestTokenize_Good_SpecialCases(t *testing.T) {
+	t.Run("only special characters", func(t *testing.T) {
+		result := tokenize("!@#$%^&*()")
+		assert.Nil(t, result)
+	})
+
+	t.Run("unicode tokens", func(t *testing.T) {
+		result := tokenize("日本語 テスト")
+		assert.NotEmpty(t, result, "CJK characters should tokenise as words")
+	})
+
+	t.Run("mixed unicode and ascii", func(t *testing.T) {
+		result := tokenize("hello 世界 world")
+		assert.Contains(t, result, "hello")
+		assert.Contains(t, result, "world")
+	})
+
+	t.Run("numbers only", func(t *testing.T) {
+		result := tokenize("12345 67890")
+		assert.Equal(t, []string{"12345", "67890"}, result)
+	})
+
+	t.Run("hyphenated words split", func(t *testing.T) {
+		result := tokenize("pre-commit")
+		assert.Equal(t, []string{"pre", "commit"}, result)
+	})
+}
+
+func TestHighlight_Good_NoMatches(t *testing.T) {
+	result := highlight("no matches here", compileRegexes([]string{"xyz"}))
+	assert.Equal(t, "no matches here", result)
+}
+
+func TestHighlight_Good_AdjacentMatches(t *testing.T) {
+	// Two words right next to each other
+	result := highlight("foobar", compileRegexes([]string{"foo", "bar"}))
+	// "foo" and "bar" are adjacent, should be merged into one highlight
+	assert.Equal(t, "**foobar**", result)
+}
+
+func TestExtractSnippet_Good_HeadingsSkipped(t *testing.T) {
+	// When no regex is given, snippet should skip heading lines
+	content := "# Heading\n\nActual content here."
+	snippet := extractSnippet(content, nil)
+	assert.Contains(t, snippet, "Actual content here.")
+	assert.NotContains(t, snippet, "# Heading")
+}
+
+func TestSearchIndex_Search_Good_DuplicateTopicIDs(t *testing.T) {
+	idx := newSearchIndex()
+
+	// Adding the same topic twice should not cause duplicate results
+	topic := &Topic{
+		ID:      "deduplicated",
+		Title:   "Unique Topic",
+		Content: "Unique content about testing.",
+	}
+	idx.Add(topic)
+	idx.Add(topic)
+
+	results := idx.Search("unique")
+	assert.Len(t, results, 1)
+}
+
+func TestCatalog_Search_Good_Integration(t *testing.T) {
+	// Test the full Catalog.Search path (integration through catalog -> index)
+	cat := &Catalog{
+		topics: make(map[string]*Topic),
+		index:  newSearchIndex(),
+	}
+
+	cat.Add(&Topic{
+		ID:      "alpha",
+		Title:   "Alpha Feature",
+		Content: "This is the alpha version of the feature.",
+		Tags:    []string{"experimental"},
+	})
+	cat.Add(&Topic{
+		ID:      "beta",
+		Title:   "Beta Release Notes",
+		Content: "Improvements and bug fixes in the beta.",
+		Tags:    []string{"release"},
+	})
+
+	t.Run("search via catalog", func(t *testing.T) {
+		results := cat.Search("alpha")
+		require.NotEmpty(t, results)
+		assert.Equal(t, "alpha", results[0].Topic.ID)
+	})
+
+	t.Run("search by tag via catalog", func(t *testing.T) {
+		results := cat.Search("experimental")
+		require.NotEmpty(t, results)
+		assert.Equal(t, "alpha", results[0].Topic.ID)
+	})
+
+	t.Run("empty query via catalog", func(t *testing.T) {
+		results := cat.Search("")
+		assert.Nil(t, results)
+	})
+}