Handle invalid assembly files with structured BAD_ASSEMBLY error

Wrap AssemblyBrowser construction in try-catch so corrupted or non-.NET
DLLs return a structured error instead of crashing the CLI.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: bouwkast

docs/development/InstrumentationGenerator.md

@@ -338,6 +338,7 @@ Machine-readable error codes:
 | `INVALID_CONFIG` | Bad JSON in `--config` or `--config-file` |
 | `UNKNOWN_KEY` | Unrecognized key in `--set` |
 | `GENERATION_ERROR` | Code generation failed |
+| `BAD_ASSEMBLY` | File is not a valid .NET assembly (corrupted, native, etc.) |
 
 The `AMBIGUOUS_OVERLOAD` error is notable: it includes the full overload list in `data`, so you can immediately retry with `--overload-index` without a separate inspect call.
 

docs/development/for-ai/InstrumentationGenerator-CLI.md

@@ -212,6 +212,7 @@ When `--json` is active, all errors are returned as structured JSON on stdout (n
 | `INVALID_CONFIG` | Bad JSON in `--config` or `--config-file` | Fix JSON syntax |
 | `UNKNOWN_KEY` | Bad key in `--set` | Use `--list-keys` to see valid keys |
 | `GENERATION_ERROR` | Code generation failed | Check error message for details |
+| `BAD_ASSEMBLY` | File is not a valid .NET assembly | Verify the file is a .NET DLL (not native, not corrupted) |
 
 ### AMBIGUOUS_OVERLOAD detail
 

tracer/src/Datadog.AutoInstrumentation.Generator.Cli/Commands/GenerateCommand.cs

@@ -186,7 +186,21 @@ private int Execute(ParseResult parseResult)
                 "Error: --config and --config-file are mutually exclusive. Use one or the other.");
         }
 
-        using var browser = new AssemblyBrowser(assemblyPath.FullName);
+        AssemblyBrowser browser;
+        try
+        {
+            browser = new AssemblyBrowser(assemblyPath.FullName);
+        }
+        catch (Exception ex)
+        {
+            return OutputHelper.WriteError(
+                jsonMode,
+                "generate",
+                ErrorCodes.BadAssembly,
+                $"Error: Failed to load assembly '{assemblyPath.Name}': {ex.Message}");
+        }
+
+        using var _ = browser;
         var methodDef = browser.ResolveMethod(
             type,
             method,

tracer/src/Datadog.AutoInstrumentation.Generator.Cli/Commands/InspectCommand.cs

@@ -168,7 +168,21 @@ private int Execute(ParseResult parseResult)
                 $"Error: Assembly file not found: {assemblyPath?.FullName ?? "<not specified>"}");
         }
 
-        using var browser = new AssemblyBrowser(assemblyPath.FullName);
+        AssemblyBrowser browser;
+        try
+        {
+            browser = new AssemblyBrowser(assemblyPath.FullName);
+        }
+        catch (Exception ex)
+        {
+            return OutputHelper.WriteError(
+                jsonMode,
+                "inspect",
+                ErrorCodes.BadAssembly,
+                $"Error: Failed to load assembly '{assemblyPath.Name}': {ex.Message}");
+        }
+
+        using var _ = browser;
 
         if (listTypes)
         {

tracer/src/Datadog.AutoInstrumentation.Generator.Cli/Output/ErrorCodes.cs

@@ -15,4 +15,5 @@ internal static class ErrorCodes
     public const string InvalidConfig = "INVALID_CONFIG";
     public const string UnknownKey = "UNKNOWN_KEY";
     public const string GenerationError = "GENERATION_ERROR";
+    public const string BadAssembly = "BAD_ASSEMBLY";
 }