github/github-mcp-server · warning

failed to marshal response: %w

Error message

failed to marshal response: %w

What it means

After a successful GetCommit, the tool converts the commit to convertToMinimalCommit(commit, detail) and json.Marshals it. Plain data structs cannot make Marshal fail, so this branch is effectively unreachable defensive code; hitting it indicates a type regression (func/chan field, cyclic pointer) in the minimal-commit types rather than bad API data.

Source

Thrown at pkg/github/repositories.go:121

					err,
				), nil, nil
			}
			defer func() { _ = resp.Body.Close() }()

			if resp.StatusCode != 200 {
				body, err := io.ReadAll(resp.Body)
				if err != nil {
					return nil, nil, fmt.Errorf("failed to read response body: %w", err)
				}
				return ghErrors.NewGitHubAPIStatusErrorResponse(ctx, "failed to get commit", resp, body), nil, nil
			}

			// Convert to minimal commit
			minimalCommit := convertToMinimalCommit(commit, detail)

			r, err := json.Marshal(minimalCommit)
			if err != nil {
				return nil, nil, fmt.Errorf("failed to marshal response: %w", err)
			}

			result := utils.NewToolResultText(string(r))
			// Commit content is reachable from the repo's history; in public
			// repos anyone can land it via a PR (untrusted), in private repos
			// only collaborators can (trusted). Confidentiality follows repo
			// visibility.
			result = attachRepoVisibilityIFCLabel(ctx, deps, client, owner, repo, result, ifc.LabelCommitContents)
			return result, nil, nil
		},
	)
}

// ListCommits creates a tool to get the list of commits of a branch in a GitHub
// repository.
func ListCommits(t translations.TranslationHelperFunc) inventory.ServerTool {
	schema := &jsonschema.Schema{
		Type: "object",

View on GitHub (pinned to 0ea1f775a7)

Solutions

1. Review recent edits to convertToMinimalCommit and its return type for non-JSON-safe fields
2. Add a CI test marshaling a converted commit fixture
3. Model derived values as precomputed strings/ints, never closures

Example fix

// before
type MinimalCommit struct {
	SHA        string      `json:"sha"`
	Message    string      `json:"message"`
	DiffFn     func() string // no json tag, Marshal fails on it
}

// after
type MinimalCommit struct {
	SHA        string `json:"sha"`
	Message    string `json:"message"`
	DiffPreview string `json:"diff_preview,omitempty"` // precomputed
}

null

var unsupportedType *json.UnsupportedTypeError
if errors.As(err, &unsupportedType) {
	// regression in MinimalCommit types — file a bug, no operational fix
}

r, err := json.Marshal(minimalCommit)
if err != nil {
	log.Error("commit marshal regression", "err", err)
	return nil, nil, fmt.Errorf("internal serialization failure: %w", err)
}

When it happens

Trigger: A field of unsupported type (func, chan, complex, cyclic pointer) is added to the minimal commit struct family; every get_commit call then fails at marshal time despite the API call succeeding.

Common situations: Refactors embedding raw go-github types that carry func fields; computed helpers accidentally stored as fields instead of methods.

Related errors

• failed to marshal response: %w
• GitHub App authentication and GITHUB_PERSONAL_ACCESS_TOKEN a
• GitHub App ID or client ID is required (GITHUB_APP_ID)
• repo is required
• failed to marshal response: %w

AI-assisted analysis of github/github-mcp-server@0ea1f775a7 (2026-08-15). Data as JSON: /api/errors/09f3b2307ea5fd56. Report an issue: GitHub.