github/github-mcp-server · warning

failed to marshal response: %w

Error message

failed to marshal response: %w

What it means

After fetching and (under lockdown) filtering sub-issues, json.Marshal of the []*github.SubIssue slice failed. These are plain go-github structs whose fields are all stdlib-JSON-safe types, so in production this is nearly unreachable; it fires only with exotic data shapes — a forked go-github with custom MarshalJSON that errors, NaN/Inf values in custom numeric types, or cyclic structures injected by mocks in tests.

Source

Thrown at pkg/github/issues.go:951

			}
			login := user.GetLogin()
			if login == "" {
				continue
			}
			isSafeContent, err := cache.IsSafeContent(ctx, login, owner, repo)
			if err != nil {
				return utils.NewToolResultError(fmt.Sprintf("failed to check lockdown mode: %v", err)), nil
			}
			if isSafeContent {
				filteredSubIssues = append(filteredSubIssues, subIssue)
			}
		}
		subIssues = filteredSubIssues
	}

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

	return utils.NewToolResultText(string(r)), nil
}

// GetIssueParent returns the parent issue of the given issue, or a null
// parent when the issue is not a sub-issue. It reads the GraphQL
// Issue.parent field, the upward counterpart to get_sub_issues.
//
// The parent title is always sanitized (it may be cross-repo). Under
// lockdown mode the parent is only returned when its author has push
// access to the parent repo (mirroring GetIssue); otherwise it is omitted.
func GetIssueParent(ctx context.Context, client *githubv4.Client, deps ToolDependencies, owner string, repo string, issueNumber int) (*mcp.CallToolResult, error) {
	cache, err := deps.GetRepoAccessCache(ctx)
	if err != nil {
		return nil, fmt.Errorf("failed to get repo access cache: %w", err)
	}
	flags := deps.GetFlags(ctx)

View on GitHub (pinned to 0ea1f775a7)

Solutions

1. Unwrap the error — json.UnsupportedTypeError/UnsupportedValueError names the exact offending type and field path
2. If a forked go-github is in use, audit its custom MarshalJSON implementations
3. Sanitize non-encodable values (NaN/Inf) before they reach response structs
4. Verify go-github version matches the one in go.sum (dependency confusion check)

func isMarshalError(err error) bool {
    var te *json.UnsupportedTypeError
    var ve *json.UnsupportedValueError
    return errors.As(err, &te) || errors.As(err, &ve)
}

res, err := githubpkg.GetSubIssues(ctx, client, deps, owner, repo, num, pagination)
if err != nil {
    var te *json.UnsupportedTypeError
    if errors.As(err, &te) {
        log.Error("unencodable field in sub-issue payload", "type", te.Type.String(), "path", fmt.Sprint(te.Value))
    }
    return err
}

When it happens

Trigger: json.Marshal encounters a value it cannot encode: NaN/Inf passed through a custom float type, a type with a MarshalJSON method that itself returns an error, or a reference cycle. The standard github.SubIssue shape cannot produce any of these.

Common situations: Vendored/forked go-github versions adding custom marshalers; test fixtures replacing API structs with cyclic fakes; dependency confusion pulling a different github.com/google/go-github version.

Related errors

• 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
• failed to marshal workflow: %w

AI-assisted analysis of github/github-mcp-server@0ea1f775a7 (2026-08-15). Data as JSON: /api/errors/5810621b5ba07b86. Report an issue: GitHub.