diff --git a/crates/application/src/use_cases/check_freshness.rs b/crates/application/src/use_cases/check_freshness.rs
index d640b4d..615a993 100644
--- a/crates/application/src/use_cases/check_freshness.rs
+++ b/crates/application/src/use_cases/check_freshness.rs
@@ -1,18 +1,17 @@
 use archlens_domain::{DomainError, NormalizedGraph, ports::DiagramRenderer};
 
-/// Compares the current rendered output against an on-disk file.
+/// Compares the current rendered output against provided file content.
 /// Returns `Ok(true)` if up to date, `Ok(false)` if stale.
 pub struct CheckFreshness<'a> {
     pub graph: &'a NormalizedGraph,
     pub renderer: &'a dyn DiagramRenderer,
-    pub existing_path: &'a std::path::Path,
+    pub existing_content: &'a str,
 }
 
 impl<'a> CheckFreshness<'a> {
     pub fn execute(&self) -> Result<bool, DomainError> {
         let rendered = self.renderer.render(self.graph.as_graph())?;
         let current = rendered.files().first().map(|f| f.content()).unwrap_or("");
-        let existing = std::fs::read_to_string(self.existing_path).unwrap_or_default();
-        Ok(current == existing)
+        Ok(current == self.existing_content)
     }
 }
diff --git a/crates/application/src/use_cases/diff_diagram.rs b/crates/application/src/use_cases/diff_diagram.rs
index 31dd518..6ae4b37 100644
--- a/crates/application/src/use_cases/diff_diagram.rs
+++ b/crates/application/src/use_cases/diff_diagram.rs
@@ -12,22 +12,21 @@ impl DiffResult {
     }
 }
 
-/// Compares the rendered current graph against an existing diagram file and
+/// Compares the rendered current graph against provided diagram content and
 /// returns which lines were added or removed.
 pub struct DiffDiagram<'a> {
     pub graph: &'a NormalizedGraph,
     pub renderer: &'a dyn DiagramRenderer,
-    pub existing_path: &'a std::path::Path,
+    pub existing_content: &'a str,
 }
 
 impl<'a> DiffDiagram<'a> {
     pub fn execute(&self) -> Result<DiffResult, DomainError> {
         let rendered = self.renderer.render(self.graph.as_graph())?;
         let current = rendered.files().first().map(|f| f.content()).unwrap_or("");
-        let existing = std::fs::read_to_string(self.existing_path).unwrap_or_default();
 
         let current_lines: std::collections::HashSet<&str> = current.lines().collect();
-        let existing_lines: std::collections::HashSet<&str> = existing.lines().collect();
+        let existing_lines: std::collections::HashSet<&str> = self.existing_content.lines().collect();
 
         let added: Vec<String> = current_lines
             .difference(&existing_lines)
diff --git a/crates/application/tests/check_freshness_tests.rs b/crates/application/tests/check_freshness_tests.rs
new file mode 100644
index 0000000..08a1458
--- /dev/null
+++ b/crates/application/tests/check_freshness_tests.rs
@@ -0,0 +1,43 @@
+mod fakes;
+
+use archlens_domain::{CodeGraph, NormalizedGraph, ports::DiagramRenderer};
+use archlens_application::use_cases::check_freshness::CheckFreshness;
+use fakes::FakeDiagramRenderer;
+
+fn empty_graph() -> NormalizedGraph {
+    NormalizedGraph::from_project(CodeGraph::new())
+}
+
+#[test]
+fn returns_true_when_content_matches() {
+    let graph = empty_graph();
+    let renderer = FakeDiagramRenderer::new();
+    let rendered = renderer.render(graph.as_graph()).unwrap();
+    let existing = rendered.files().first().unwrap().content().to_string();
+
+    let result = CheckFreshness {
+        graph: &graph,
+        renderer: &renderer,
+        existing_content: &existing,
+    }
+    .execute()
+    .unwrap();
+
+    assert!(result);
+}
+
+#[test]
+fn returns_false_when_content_differs() {
+    let graph = empty_graph();
+    let renderer = FakeDiagramRenderer::new();
+
+    let result = CheckFreshness {
+        graph: &graph,
+        renderer: &renderer,
+        existing_content: "stale content that does not match",
+    }
+    .execute()
+    .unwrap();
+
+    assert!(!result);
+}
diff --git a/crates/application/tests/diff_diagram_tests.rs b/crates/application/tests/diff_diagram_tests.rs
new file mode 100644
index 0000000..6aab1db
--- /dev/null
+++ b/crates/application/tests/diff_diagram_tests.rs
@@ -0,0 +1,44 @@
+mod fakes;
+
+use archlens_domain::{CodeGraph, NormalizedGraph, ports::DiagramRenderer};
+use archlens_application::use_cases::diff_diagram::DiffDiagram;
+use fakes::FakeDiagramRenderer;
+
+fn empty_graph() -> NormalizedGraph {
+    NormalizedGraph::from_project(CodeGraph::new())
+}
+
+#[test]
+fn no_diff_when_content_matches() {
+    let graph = empty_graph();
+    let renderer = FakeDiagramRenderer::new();
+    let rendered = renderer.render(graph.as_graph()).unwrap();
+    let existing = rendered.files().first().unwrap().content().to_string();
+
+    let result = DiffDiagram {
+        graph: &graph,
+        renderer: &renderer,
+        existing_content: &existing,
+    }
+    .execute()
+    .unwrap();
+
+    assert!(result.is_empty());
+}
+
+#[test]
+fn detects_added_lines() {
+    let graph = empty_graph();
+    let renderer = FakeDiagramRenderer::new();
+
+    let result = DiffDiagram {
+        graph: &graph,
+        renderer: &renderer,
+        existing_content: "old line that will be removed",
+    }
+    .execute()
+    .unwrap();
+
+    assert!(!result.added.is_empty());
+    assert!(!result.removed.is_empty());
+}
diff --git a/crates/presentation/src/lib.rs b/crates/presentation/src/lib.rs
index 1b0b37e..4449f3a 100644
--- a/crates/presentation/src/lib.rs
+++ b/crates/presentation/src/lib.rs
@@ -46,10 +46,12 @@ pub fn run(args: Cli) -> Result<()> {
         let existing_path = args.output.as_ref().ok_or_else(|| {
             anyhow::anyhow!("--check requires --output to specify the file to check against")
         })?;
+        let existing_content = std::fs::read_to_string(existing_path)
+            .map_err(|e| anyhow::anyhow!("cannot read {existing_path}: {e}"))?;
         let up_to_date = CheckFreshness {
             graph: &graph,
             renderer: &*renderer,
-            existing_path: std::path::Path::new(existing_path),
+            existing_content: &existing_content,
         }
         .execute()?;
         if up_to_date {
@@ -222,10 +224,12 @@ fn run_diff(args: &Cli, existing_path: &std::path::Path) -> Result<()> {
     let graph = build_graph(args, level)?;
     let renderer = create_renderer(&args.format, level, !args.no_weights)?;
 
+    let existing_content = std::fs::read_to_string(existing_path)
+        .map_err(|e| anyhow::anyhow!("cannot read {}: {e}", existing_path.display()))?;
     let diff = DiffDiagram {
         graph: &graph,
         renderer: &*renderer,
-        existing_path,
+        existing_content: &existing_content,
     }
     .execute()?;