diff --git a/Cargo.lock b/Cargo.lock
index a50f11f..1899373 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -803,6 +803,7 @@ dependencies = [
  "serde",
  "serde_json",
  "syn",
+ "tempfile",
  "walkdir",
 ]
 
diff --git a/crates/notgraph/Cargo.toml b/crates/notgraph/Cargo.toml
index d60407d..76fb13c 100644
--- a/crates/notgraph/Cargo.toml
+++ b/crates/notgraph/Cargo.toml
@@ -12,6 +12,9 @@ path = "src/lib.rs"
 name = "notgraph"
 path = "src/main.rs"
 
+[dev-dependencies]
+tempfile = { workspace = true }
+
 [dependencies]
 anyhow         = { workspace = true }
 clap           = { workspace = true }
diff --git a/crates/notgraph/src/emit.rs b/crates/notgraph/src/emit.rs
index a6d3155..75bc88d 100644
--- a/crates/notgraph/src/emit.rs
+++ b/crates/notgraph/src/emit.rs
@@ -194,12 +194,15 @@ fn write_html(
     isolated.sort();
     roots.extend(isolated);
 
+    // Sanitize crate name for use as a Mermaid node ID (hyphens are not valid)
+    let mermaid_id = |n: &str| n.replace('-', "_");
+
     let node_label = |n: &str| -> String {
         let (fi, fo) = fan_map.get(n).copied().unwrap_or((0, 0));
         let syms = sym_count.get(n).copied().unwrap_or(0);
         format!(
             "{}[\"{}<br/>in:{} out:{} | {} pub\"]",
-            n, n, fi, fo, syms
+            mermaid_id(n), n, fi, fo, syms
         )
     };
 
@@ -223,7 +226,7 @@ fn write_html(
     }
 
     for (from, to) in &crate_graph.edges {
-        mermaid.push_str(&format!("  {} --> {}\n", to, from));
+        mermaid.push_str(&format!("  {} --> {}\n", mermaid_id(to), mermaid_id(from)));
     }
 
     // Heatmap styles
@@ -231,7 +234,7 @@ fn write_html(
         let color = heatmap_color(fs.fan_in);
         mermaid.push_str(&format!(
             "  style {} fill:{},stroke:#4a9eff,color:#e0e0e0\n",
-            fs.name, color
+            mermaid_id(&fs.name), color
         ));
     }
 
@@ -239,7 +242,7 @@ fn write_html(
     for n in &cycle_nodes {
         mermaid.push_str(&format!(
             "  style {} fill:#7a1a1a,stroke:#f44336,color:#ffffff\n",
-            n
+            mermaid_id(n)
         ));
     }
 
@@ -263,7 +266,7 @@ fn write_html(
             .flat_map(|c| c.iter().map(|n| n.as_str()))
             .collect();
 
-        let mut d = format!("flowchart TD\n");
+        let mut d = "flowchart TD\n".to_string();
 
         // Nodes: strip crate prefix for readable labels, skip ::tests modules
         for node in &mg.nodes {
diff --git a/crates/notgraph/tests/integration.rs b/crates/notgraph/tests/integration.rs
index 0f581d7..4a32493 100644
--- a/crates/notgraph/tests/integration.rs
+++ b/crates/notgraph/tests/integration.rs
@@ -1,7 +1,8 @@
-use notgraph_lib::{analysis, module_graph, symbols};
+use notgraph_lib::{analysis, emit, module_graph, symbols, types};
 use std::path::Path;
+use tempfile::TempDir;
 
-fn fixtures(name: &str) -> std::path::PathBuf {
+fn fixture_src(name: &str) -> std::path::PathBuf {
     Path::new(env!("CARGO_MANIFEST_DIR"))
         .join("tests/fixtures")
         .join(name)
@@ -10,7 +11,7 @@ fn fixtures(name: &str) -> std::path::PathBuf {
 
 #[test]
 fn clean_fixture_has_no_cycles() {
-    let mg = module_graph::build("clean".to_string(), &fixtures("clean")).unwrap();
+    let mg = module_graph::build("clean".to_string(), &fixture_src("clean")).unwrap();
     assert!(
         analysis::detect_cycles(&mg.nodes, &mg.edges).is_empty(),
         "expected no cycles; nodes={:?} edges={:?}",
@@ -34,9 +35,167 @@ fn detect_cycles_finds_cycles_in_synthetic_graph() {
     );
 }
 
+/// The cyclic fixture's module graph builds without error and has the expected
+/// cross-module containment structure. The cycle detection test using synthetic
+/// data covers the algorithm; this test confirms the builder handles mutual
+/// `pub mod` declarations without panicking.
+#[test]
+fn cyclic_fixture_module_graph_builds() {
+    let mg = module_graph::build("cyclic".to_string(), &fixture_src("cyclic")).unwrap();
+    assert!(
+        mg.nodes.contains(&"cyclic".to_string()),
+        "expected root module node 'cyclic'"
+    );
+    // foo and bar are declared at the top level in lib.rs
+    assert!(
+        mg.nodes.contains(&"cyclic::foo".to_string()),
+        "expected cyclic::foo node"
+    );
+    assert!(
+        mg.nodes.contains(&"cyclic::bar".to_string()),
+        "expected cyclic::bar node"
+    );
+}
+
+/// write_all produces report.html, report.md, and report.json in the output dir.
+#[test]
+fn write_all_creates_expected_files() {
+    use types::{CrateGraph, GraphStats, ModuleGraph};
+
+    let out = TempDir::new().unwrap();
+    let crate_graph = CrateGraph {
+        nodes: vec!["alpha".to_string(), "beta".to_string()],
+        edges: vec![("beta".to_string(), "alpha".to_string())],
+    };
+    let module_graphs: Vec<ModuleGraph> = vec![];
+    let stats = analysis::analyse(&crate_graph, &module_graphs, 3);
+    let symbol_tables: Vec<types::SymbolTable> = vec![];
+
+    emit::write_all(out.path(), &crate_graph, &module_graphs, &stats, &symbol_tables).unwrap();
+
+    assert!(
+        out.path().join("report.html").exists(),
+        "report.html missing"
+    );
+    assert!(out.path().join("report.md").exists(), "report.md missing");
+    assert!(
+        out.path().join("report.json").exists(),
+        "report.json missing"
+    );
+}
+
+/// Crate names with hyphens must not produce malformed Mermaid output — the
+/// generated HTML must contain a sanitized node ID (hyphen replaced with _).
+#[test]
+fn write_all_sanitizes_hyphenated_crate_names() {
+    use types::{CrateGraph, ModuleGraph};
+
+    let out = TempDir::new().unwrap();
+    let crate_graph = CrateGraph {
+        nodes: vec!["my-core".to_string(), "my-app".to_string()],
+        edges: vec![("my-app".to_string(), "my-core".to_string())],
+    };
+    let module_graphs: Vec<ModuleGraph> = vec![];
+    let stats = analysis::analyse(&crate_graph, &module_graphs, 3);
+    let symbol_tables: Vec<types::SymbolTable> = vec![];
+
+    emit::write_all(out.path(), &crate_graph, &module_graphs, &stats, &symbol_tables).unwrap();
+
+    let html = std::fs::read_to_string(out.path().join("report.html")).unwrap();
+    // Raw hyphenated names must not appear as unquoted node IDs
+    assert!(
+        !html.contains("my-core["),
+        "hyphenated ID 'my-core[' must not appear in Mermaid — use my_core"
+    );
+    assert!(
+        html.contains("my_core["),
+        "sanitized ID 'my_core[' must appear in Mermaid output"
+    );
+}
+
+/// Heatmap: a single-crate graph (max_fan_in = 0 or 1) must not panic and must
+/// produce a style directive in the HTML output.
+#[test]
+fn write_all_heatmap_does_not_panic_for_single_crate() {
+    use types::{CrateGraph, ModuleGraph};
+
+    let out = TempDir::new().unwrap();
+    let crate_graph = CrateGraph {
+        nodes: vec!["solo".to_string()],
+        edges: vec![],
+    };
+    let module_graphs: Vec<ModuleGraph> = vec![];
+    let stats = analysis::analyse(&crate_graph, &module_graphs, 3);
+    let symbol_tables: Vec<types::SymbolTable> = vec![];
+
+    emit::write_all(out.path(), &crate_graph, &module_graphs, &stats, &symbol_tables).unwrap();
+
+    let html = std::fs::read_to_string(out.path().join("report.html")).unwrap();
+    assert!(
+        html.contains("style solo fill:"),
+        "heatmap style directive missing for solo crate"
+    );
+}
+
+/// Per-crate module graphs: write_all renders a <h3> section for each
+/// ModuleGraph entry in the HTML output.
+#[test]
+fn write_all_renders_per_crate_module_graphs() {
+    use types::{CrateGraph, ModuleGraph};
+
+    let out = TempDir::new().unwrap();
+    let mg = module_graph::build("clean".to_string(), &fixture_src("clean")).unwrap();
+    let crate_graph = CrateGraph {
+        nodes: vec!["clean".to_string()],
+        edges: vec![],
+    };
+    let stats = analysis::analyse(&crate_graph, &[mg.clone()], 3);
+    let symbol_tables: Vec<types::SymbolTable> = vec![];
+
+    emit::write_all(out.path(), &crate_graph, &[mg], &stats, &symbol_tables).unwrap();
+
+    let html = std::fs::read_to_string(out.path().join("report.html")).unwrap();
+    assert!(
+        html.contains("<h3>clean</h3>"),
+        "expected per-crate <h3>clean</h3> section in HTML"
+    );
+    assert!(
+        html.contains("flowchart TD"),
+        "expected flowchart TD directive for module graph"
+    );
+}
+
+/// Cycle callouts: when the crate graph contains cycles, the HTML must include
+/// the cycle-entry element with the cycle nodes.
+#[test]
+fn write_all_cycle_callout_appears_in_html() {
+    use types::{CrateGraph, ModuleGraph};
+
+    let out = TempDir::new().unwrap();
+    // Build a two-node crate graph with a synthetic cycle
+    let crate_graph = CrateGraph {
+        nodes: vec!["alpha".to_string(), "beta".to_string()],
+        edges: vec![
+            ("alpha".to_string(), "beta".to_string()),
+            ("beta".to_string(), "alpha".to_string()),
+        ],
+    };
+    let module_graphs: Vec<ModuleGraph> = vec![];
+    let stats = analysis::analyse(&crate_graph, &module_graphs, 3);
+    let symbol_tables: Vec<types::SymbolTable> = vec![];
+
+    emit::write_all(out.path(), &crate_graph, &module_graphs, &stats, &symbol_tables).unwrap();
+
+    let html = std::fs::read_to_string(out.path().join("report.html")).unwrap();
+    assert!(
+        html.contains("cycle-entry"),
+        "expected cycle-entry element in HTML when cycles are present"
+    );
+}
+
 #[test]
 fn symbols_fixture_has_six_public_symbols() {
-    let table = symbols::build("symbols".to_string(), &fixtures("symbols")).unwrap();
+    let table = symbols::build("symbols".to_string(), &fixture_src("symbols")).unwrap();
     let pub_count = table.symbols.iter().filter(|s| s.is_pub).count();
     assert_eq!(
         pub_count,