diff --git a/CHANGELOG.md b/CHANGELOG.md
index 5642421..f6eb72d 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -26,7 +26,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 - Logging: Fixed log flooding from repeated running state checks by only logging on state changes
 - Logging: Fixed log feedback loop caused by Output Channel document events triggering diagnostics
-
+- Diagnostics: Run diagnostics for Dev Proxy files already open before extension activation
 - Diagnostics: Language model diagnostic now correctly targets plugins that can use a local language model (OpenAIMockResponsePlugin, OpenApiSpecGeneratorPlugin, TypeSpecGeneratorPlugin) and shows as an informational hint instead of a warning
 
 ## [1.12.0] - 2026-01-29
diff --git a/src/documents.ts b/src/documents.ts
index 41890e5..f9beba7 100644
--- a/src/documents.ts
+++ b/src/documents.ts
@@ -76,4 +76,26 @@ export const registerDocumentListeners = (context: vscode.ExtensionContext, coll
             });
         })
     );
+
+    // Process already-open documents that were opened before the extension activated
+    for (const document of vscode.workspace.textDocuments) {
+        if (document.uri.scheme !== 'file') {
+            continue;
+        }
+        try {
+            if (isConfigFile(document)) {
+                updateConfigFileDiagnostics(context, document, collection);
+            } else if (isProxyFile(document)) {
+                updateFileDiagnostics(context, document, collection);
+            }
+        } catch (error) {
+            console.error('Error processing already-open document:', document.uri.fsPath, error);
+        }
+    }
+
+    // Set context for the active editor if it contains a config file
+    const activeEditor = vscode.window.activeTextEditor;
+    if (activeEditor && activeEditor.document.uri.scheme === 'file') {
+        vscode.commands.executeCommand('setContext', 'isDevProxyConfigFile', isConfigFile(activeEditor.document));
+    }
 };