What causes "failed to add vcs generated file: %w" in dagger/dagger?

Inspect the wrapped inner error for the concrete dagql/IO failure and fix that.

failed to add vcs generated file: %w

Error message

failed to add vcs generated file: %w

What it means

Thrown by `runCodegen` when appending the updated `.gitattributes` (marking SDK-generated paths as `linguist-generated`) into the generated code directory fails via the `withNewFile` dagql select. The codegen result directory could not be modified.

Source

Thrown at core/schema/modulesource.go:2656

			}
			fileName := strings.TrimPrefix(fileName, "/")
			gitAttrsContents = append(gitAttrsContents,
				fmt.Appendf(nil, "/%s linguist-generated\n", fileName)...,
			)
		}

		err = dag.Select(ctx, genDirInst, &genDirInst,
			dagql.Selector{
				Field: "withNewFile",
				Args: []dagql.NamedInput{
					{Name: "path", Value: dagql.String(gitAttrsPath)},
					{Name: "contents", Value: dagql.String(gitAttrsContents)},
					{Name: "permissions", Value: dagql.Int(0o600)},
				},
			},
		)
		if err != nil {
			return res, fmt.Errorf("failed to add vcs generated file: %w", err)
		}
	}

	// update .gitignore in the generated context directory
	writeGitignore := true // default to true if not set
	if srcInst.Self().CodegenConfig != nil && srcInst.Self().CodegenConfig.AutomaticGitignore != nil {
		writeGitignore = *srcInst.Self().CodegenConfig.AutomaticGitignore
	}
	vcsIgnoredPaths := generatedCode.VCSIgnoredPaths
	if srcInst.Self().ConfigFilename == modules.Filename {
		// toml modules build from committed generated files: gitignoring them
		// would filter them out of the local module context.
		vcsIgnoredPaths = slices.DeleteFunc(slices.Clone(vcsIgnoredPaths), func(ignore string) bool {
			return ignoresGeneratedPath(ignore, generatedCode.VCSGeneratedPaths)
		})
	}
	if writeGitignore && len(vcsIgnoredPaths) > 0 {
		gitIgnorePath := filepath.Join(srcInst.Self().SourceSubpath, ".gitignore")

View on GitHub (pinned to 82ba2681db)

Solutions

Inspect the wrapped inner error for the concrete dagql/IO failure and fix that.
Check that no directory conflicts with the `.gitattributes` path inside the module source subpath.
Validate the SDK's returned VCSGeneratedPaths (no empty/invalid names) if using a custom SDK.
Retry after restarting the engine if the error indicates transient storage failure.

Example fix

// before: SDK returns generated paths with invalid characters
VCSGeneratedPaths: ["sdk/../evil"]
// after
VCSGeneratedPaths: ["sdk/**"]
$ dagger develop

Defensive patterns

Strategy: try-catch

Validate before calling

# ensure no directory occupies the .gitattributes path in the module source
ls -la $(grep -q source dagger.json && echo source/ || echo .)/.gitattributes

Try / catch

res, err := s.runCodegen(ctx, srcInst)
if err != nil && strings.Contains(err.Error(), "failed to add vcs generated file") {
    return fmt.Errorf("check .gitattributes path conflicts / engine storage: %w", err)
}

Prevention

Don't create a directory named .gitattributes inside the module source subpath.
For custom SDKs, return clean relative VCSGeneratedPaths entries.
Restart the engine if directory-mutation errors appear transiently.

When it happens

Trigger: Codegen produced VCSGeneratedPaths and the `withNewFile` select on the generated directory errored — engine-side directory mutation failure, invalid path for the generated file, or an inner dagql error from the select chain.

Common situations: Engine storage/IO failures; path conflicts (e.g. a directory exists where the .gitattributes file should be written); bugs in custom SDKs returning invalid VCSGeneratedPaths entries.

Related errors

AI-assisted analysis of dagger/dagger@82ba2681db (2026-09-05). Data as JSON: /api/errors/dfcae7f69d2299de. Report an issue: GitHub.