What causes "failed to encode source file: %w" in microsoft/typescript-go?

Update typescript-go to a build where parser and encoder are in sync

failed to encode source file: %w

Error message

failed to encode source file: %w

What it means

getSourceFile failed while serializing the AST: encoder.EncodeSourceFile returned an error, which is an internal encoder failure on this particular syntax tree (for example a node shape the wire encoder does not support), not a client-input problem. The error is unwrapped, so there is no ErrClientError to branch on.

Source

Thrown at internal/api/session.go:1361

		configSourceFile := tsoptions.NewTsconfigSourceFileFromFilePath(configFileName, requestedPath, configFileContent)
		return s.encodeSourceFileResponse(configSourceFile.SourceFile)
	}

	return s.encodeSourceFileResponse(nil)
}

func (s *Session) encodeSourceFileResponse(sourceFile *ast.SourceFile) (any, error) {
	if sourceFile == nil {
		if s.useBinaryResponses {
			return RawBinary(nil), nil
		}
		return nil, nil
	}

	// Encode the full source file.
	data, _, err := encoder.EncodeSourceFile(sourceFile)
	if err != nil {
		return nil, fmt.Errorf("failed to encode source file: %w", err)
	}

	if s.useBinaryResponses {
		return RawBinary(data), nil
	}
	return &SourceFileResponse{
		Data: base64.StdEncoding.EncodeToString(data),
	}, nil
}

// handleGetSourceFileNames returns file names of all source files in a project.
func (s *Session) handleGetSourceFileNames(ctx context.Context, params *GetSourceFileNamesParams) ([]string, error) {
	sd, err := s.getSnapshotData(params.Snapshot)
	if err != nil {
		return nil, err
	}

	program, err := sd.getProgram(params.Project)

View on GitHub (pinned to 1bcfa18d79)

Solutions

Update typescript-go to a build where parser and encoder are in sync
If you only need file-level facts, use getSourceFileMetadata instead of the full encoded AST
Reproduce with the single file and report it upstream with the input source

Example fix

// before
resp, err := getSourceFile(snapshot, project, file) // failed to encode source file

// after
meta, err := getSourceFileMetadata(snapshot, project, file) // metadata-only path avoids the encoder

Defensive patterns

Strategy: try-catch

Type guard

func isEncodeFailure(err error) bool {
    return err != nil && strings.Contains(err.Error(), "failed to encode source file")
}

Try / catch

resp, err := session.HandleRequest(ctx, string(api.MethodGetSourceFile), params)
if isEncodeFailure(err) {
    // encoder bug, not your input: fall back to metadata and report upstream
    meta, merr := session.HandleRequest(ctx, string(api.MethodGetSourceFileMetadata), params)
    if merr != nil { return merr }
    return meta
}
if err != nil { return err }

Prevention

Update typescript-go when parser/encoder versions can skew
Use getSourceFileMetadata when you do not need the full AST
Capture the offending file contents in logs (redacted as needed) to attach to an upstream issue

When it happens

Trigger: A source file containing AST shapes the encoder version cannot serialize; version skew where parser changes landed without matching encoder support; corrupted program state after unusual snapshot sequences.

Common situations: Nightly or mismatched typescript-go builds; exotic or generated TypeScript that exercises rarely-used syntax; clients relying on full AST dumps hitting encoder gaps.

Related errors

AI-assisted analysis of microsoft/typescript-go@1bcfa18d79 (2026-08-16). Data as JSON: /api/errors/08919d906f0645e2. Report an issue: GitHub.