diff --git a/crates/decodal-cli/src/main.rs b/crates/decodal-cli/src/main.rs
index 024e946..9809ba1 100644
--- a/crates/decodal-cli/src/main.rs
+++ b/crates/decodal-cli/src/main.rs
@@ -4,19 +4,22 @@ use std::{
     process::ExitCode,
 };
 
-use decodal_core::{Data, Diagnostic, DiagnosticKind, Engine, LoadedSource, SourceLoader, Span};
+use decodal_core::{
+    Data, Diagnostic, DiagnosticKind, Engine, LoadedSource, SourceId, SourceLoader, Span,
+    format_diagnostic_with,
+};
 
 fn main() -> ExitCode {
     match run() {
         Ok(()) => ExitCode::SUCCESS,
         Err(error) => {
-            print_diagnostic(&error);
+            eprintln!("{error}");
             ExitCode::FAILURE
         }
     }
 }
 
-fn run() -> Result<(), Diagnostic> {
+fn run() -> Result<(), String> {
     let mut args = env::args().skip(1);
     let first = args.next().unwrap_or_else(|| String::from("--help"));
     match first.as_str() {
@@ -51,12 +54,19 @@ fn run() -> Result<(), Diagnostic> {
     }
 }
 
-fn materialize_path(path: &str) -> Result<Data, Diagnostic> {
-    let root = read_root_source(path)?;
+fn materialize_path(path: &str) -> Result<Data, String> {
+    let root = read_root_source(path).map_err(format_raw_diagnostic)?;
+    let root_name = root.name.clone();
     let mut engine = Engine::new(FsLoader);
-    let module = engine.add_root_source(root.key, root.name, &root.source)?;
-    let value = engine.eval_module(module)?;
-    engine.materialize(&value)
+    let module = engine
+        .add_root_source(root.key, root.name, &root.source)
+        .map_err(|error| format_diagnostic_with_root(&error, &root_name))?;
+    let value = engine
+        .eval_module(module)
+        .map_err(|error| engine.format_diagnostic(&error))?;
+    engine
+        .materialize(&value)
+        .map_err(|error| engine.format_diagnostic(&error))
 }
 
 fn read_root_source(path: &str) -> Result<LoadedSource, Diagnostic> {
@@ -139,24 +149,12 @@ fn print_help() {
     println!("  decodal -                  Read DCDL source from stdin");
 }
 
-fn print_diagnostic(error: &Diagnostic) {
-    eprintln!(
-        "error[{kind:?}] {source}:{start}..{end}: {message}",
-        kind = error.kind,
-        source = error.span.source.0,
-        start = error.span.start,
-        end = error.span.end,
-        message = error.message,
-    );
-    for label in &error.labels {
-        eprintln!(
-            "  note {source}:{start}..{end}: {message}",
-            source = label.span.source.0,
-            start = label.span.start,
-            end = label.span.end,
-            message = label.message,
-        );
-    }
+fn format_diagnostic_with_root(error: &Diagnostic, root_name: &str) -> String {
+    format_diagnostic_with(error, |source| (source == SourceId(0)).then_some(root_name))
+}
+
+fn format_raw_diagnostic(error: Diagnostic) -> String {
+    format_diagnostic_with(&error, |_| None)
 }
 
 fn print_data(data: &Data, indent: usize) {
diff --git a/crates/decodal-core/src/eval.rs b/crates/decodal-core/src/eval.rs
index 6220ad6..dc52280 100644
--- a/crates/decodal-core/src/eval.rs
+++ b/crates/decodal-core/src/eval.rs
@@ -87,6 +87,17 @@ impl<L: SourceLoader> Engine<L> {
         self.materialize_with_path(value, &mut Vec::new())
     }
 
+    pub fn source_name(&self, source: SourceId) -> Option<&str> {
+        self.modules
+            .iter()
+            .find(|module| module.source == source)
+            .map(|module| module.name.as_str())
+    }
+
+    pub fn format_diagnostic(&self, diagnostic: &Diagnostic) -> String {
+        format_diagnostic_with(diagnostic, |source| self.source_name(source))
+    }
+
     fn materialize_with_path(
         &mut self,
         value: &RuntimeValue,
@@ -1163,6 +1174,39 @@ fn primitive_type(name: &str) -> Option<PrimitiveType> {
     }
 }
 
+pub fn format_diagnostic_with<'a>(
+    diagnostic: &Diagnostic,
+    mut source_name: impl FnMut(SourceId) -> Option<&'a str>,
+) -> String {
+    let mut out = format!(
+        "{:?} at {}:{}..{}: {}",
+        diagnostic.kind,
+        format_source(diagnostic.span.source, &mut source_name),
+        diagnostic.span.start,
+        diagnostic.span.end,
+        diagnostic.message,
+    );
+    for label in &diagnostic.labels {
+        out.push_str(&format!(
+            "\nnote at {}:{}..{}: {}",
+            format_source(label.span.source, &mut source_name),
+            label.span.start,
+            label.span.end,
+            label.message,
+        ));
+    }
+    out
+}
+
+fn format_source<'a>(
+    source: SourceId,
+    source_name: &mut impl FnMut(SourceId) -> Option<&'a str>,
+) -> String {
+    source_name(source)
+        .map(String::from)
+        .unwrap_or_else(|| format!("source {}", source.0))
+}
+
 fn constraint_primary_span(constraints: &[ConstraintEntry]) -> Span {
     constraints
         .first()
diff --git a/crates/decodal-core/src/lib.rs b/crates/decodal-core/src/lib.rs
index 25504ab..b81c058 100644
--- a/crates/decodal-core/src/lib.rs
+++ b/crates/decodal-core/src/lib.rs
@@ -17,7 +17,7 @@ pub use ast::{Ast, BinaryOp, CompareOp, Expr, ExprId, Field, Literal, Param};
 pub use constraints::normalize_constraints;
 pub use diagnostic::{Diagnostic, DiagnosticKind, Result};
 pub use embedding::{HostField, HostValue};
-pub use eval::Engine;
+pub use eval::{Engine, format_diagnostic_with};
 pub use lexer::{Lexer, Token, TokenKind};
 pub use module::{EmptyLoader, LoadedSource, Module, SourceLoader};
 pub use parser::{ParseOutput, Parser, SourceForm, parse_source, parse_source_with_source_id};
diff --git a/crates/decodal-wasm/src/lib.rs b/crates/decodal-wasm/src/lib.rs
index a67d12f..bf465d3 100644
--- a/crates/decodal-wasm/src/lib.rs
+++ b/crates/decodal-wasm/src/lib.rs
@@ -1,7 +1,8 @@
 use std::collections::BTreeMap;
 
 use decodal_core::{
-    Data, Diagnostic, DiagnosticKind, EmptyLoader, Engine, LoadedSource, SourceLoader, Span,
+    Data, Diagnostic, DiagnosticKind, EmptyLoader, Engine, LoadedSource, SourceId, SourceLoader,
+    Span, format_diagnostic_with,
 };
 use wasm_bindgen::prelude::*;
 
@@ -24,11 +25,18 @@ fn encode_result(result: Result<String, String>) -> String {
 
 fn evaluate_inner(source: &str) -> Result<String, String> {
     let mut engine = Engine::new(EmptyLoader);
-    let module = engine
-        .add_root_source("playground", "playground", source)
-        .map_err(format_diagnostic)?;
-    let value = engine.eval_module(module).map_err(format_diagnostic)?;
-    let data = engine.materialize(&value).map_err(format_diagnostic)?;
+    let module = match engine.add_root_source("playground", "playground", source) {
+        Ok(module) => module,
+        Err(error) => return Err(format_diagnostic_with_root(&error, "playground")),
+    };
+    let value = match engine.eval_module(module) {
+        Ok(value) => value,
+        Err(error) => return Err(engine.format_diagnostic(&error)),
+    };
+    let data = match engine.materialize(&value) {
+        Ok(data) => data,
+        Err(error) => return Err(engine.format_diagnostic(&error)),
+    };
     Ok(format_data(&data, 0))
 }
 
@@ -48,11 +56,18 @@ fn evaluate_project_inner(entry: &str, files_json: &str) -> Result<String, Strin
         .ok_or_else(|| format!("entry file `{entry}` was not found"))?;
 
     let mut engine = Engine::new(VirtualLoader { files });
-    let module = engine
-        .add_root_source(entry.clone(), entry.clone(), &source)
-        .map_err(format_diagnostic)?;
-    let value = engine.eval_module(module).map_err(format_diagnostic)?;
-    let data = engine.materialize(&value).map_err(format_diagnostic)?;
+    let module = match engine.add_root_source(entry.clone(), entry.clone(), &source) {
+        Ok(module) => module,
+        Err(error) => return Err(format_diagnostic_with_root(&error, &entry)),
+    };
+    let value = match engine.eval_module(module) {
+        Ok(value) => value,
+        Err(error) => return Err(engine.format_diagnostic(&error)),
+    };
+    let data = match engine.materialize(&value) {
+        Ok(data) => data,
+        Err(error) => return Err(engine.format_diagnostic(&error)),
+    };
     Ok(format_data(&data, 0))
 }
 
@@ -124,22 +139,10 @@ fn normalize_path(path: &str) -> Option<String> {
     }
 }
 
-fn format_diagnostic(diagnostic: decodal_core::Diagnostic) -> String {
-    let mut out = format!(
-        "{:?} at {}:{}..{}: {}",
-        diagnostic.kind,
-        diagnostic.span.source.0,
-        diagnostic.span.start,
-        diagnostic.span.end,
-        diagnostic.message,
-    );
-    for label in diagnostic.labels {
-        out.push_str(&format!(
-            "\nnote at {}:{}..{}: {}",
-            label.span.source.0, label.span.start, label.span.end, label.message,
-        ));
-    }
-    out
+fn format_diagnostic_with_root(diagnostic: &decodal_core::Diagnostic, root_name: &str) -> String {
+    format_diagnostic_with(diagnostic, |source| {
+        (source == SourceId(0)).then_some(root_name)
+    })
 }
 
 fn format_data(data: &Data, indent: usize) -> String {
@@ -237,4 +240,17 @@ mod tests {
         let output = evaluate_project_inner("main.dcdl", files).unwrap();
         assert!(output.contains("\"port\": 9443"));
     }
+
+    #[test]
+    fn project_diagnostics_use_virtual_file_names() {
+        let files = r#"{
+            "main.dcdl":"let dep = import \"./schemas/service.dcdl\"; in dep.Service & { port = 80; }",
+            "schemas/service.dcdl":"Service = { port = Int & > 443 default 8443; }"
+        }"#;
+        let error = evaluate_project_inner("main.dcdl", files).unwrap_err();
+        assert!(error.contains("main.dcdl:"));
+        assert!(error.contains("schemas/service.dcdl:"));
+        assert!(!error.contains("source 0:"));
+        assert!(!error.contains("source 1:"));
+    }
 }
diff --git a/doc/manual/souce/design/diagnostics-and-fallback.md b/doc/manual/souce/design/diagnostics-and-fallback.md
index 40323b4..871bbf1 100644
--- a/doc/manual/souce/design/diagnostics-and-fallback.md
+++ b/doc/manual/souce/design/diagnostics-and-fallback.md
@@ -20,6 +20,7 @@ DiagnosticLabel {
 ```
 
 `span` は primary location を示す。
+表示時には `Span.source` を source id のまま出すのではなく、可能な限り file path や virtual file name に解決する。
 `labels` は同じ error に関係する追加 location を示す。
 合成や materialize の失敗では、衝突した constraint、value、default、または処理中 field path を label に含める。
 
diff --git a/site/decodal-site/src/wasm/decodal_wasm_bg.wasm b/site/decodal-site/src/wasm/decodal_wasm_bg.wasm
index 25635b8..7387400 100644
Binary files a/site/decodal-site/src/wasm/decodal_wasm_bg.wasm and b/site/decodal-site/src/wasm/decodal_wasm_bg.wasm differ