gildas-lormeau/SingleFile · error · Error

writeResponse.error || "Failed to write file via MCP"

Error message

writeResponse.error || "Failed to write file via MCP"

What it means

The MCP library's upload calls writeFile on the configured MCP server; if the response indicates success === false, it throws writeResponse.error if present, otherwise the generic "Failed to write file via MCP". This means the server accepted the request but could not write the file — the reason is server-defined and only available via writeResponse.error.

Source

Thrown at src/lib/mcp/mcp.js:108

                    path = await prompt(path);
                    if (path) {
                        return await upload(path, content, options);
                    } else {
                        return { url: path, skipped: true };
                    }
                } else {
                    options.filenameConflictAction = CONFLICT_ACTION_UNIQUIFY;
                    return await upload(path, content, options);
                }
            }
        }

        const writeResponse = await writeFile(serverUrl, authToken, path, content, signal, getRequestId);

        if (writeResponse.success) {
            return { url: path };
        } else {
            throw new Error(writeResponse.error || "Failed to write file via MCP");
        }
    } catch (error) {
        if (error.name != ABORT_ERROR_NAME) {
            throw error;
        }
    }
}

async function checkFileExists(serverUrl, authToken, path, signal, getRequestId) {
    const requestBody = {
        jsonrpc: MCP_JSONRPC_VERSION,
        id: getRequestId(),
        method: "tools/call",
        params: {
            name: "get_file_info",
            arguments: {
                path: path
            }

View on GitHub (pinned to 517fb7c5cf)

Solutions

1. Inspect the full writeResponse (log it before throwing) to capture any server-provided error detail.
2. Verify the target path is inside the MCP server's configured allowed directories.
3. Confirm the server's write tool is reachable and its result shape matches {success: boolean, error?: string}.
4. Test the same write with an MCP inspector/client against serverUrl to isolate client vs server.

Example fix

// before
await mcp.upload(url, token, path, content); // Error: Failed to write file via MCP
// after
const res = await writeFile(serverUrl, token, path, content, signal, getRequestId);
if (!res.success) throw new Error(`MCP write failed for ${path}: ${res.error ?? JSON.stringify(res)}`);

function isWritableMcpPath(serverConfig, path) {
  return serverConfig.allowedRoots.some(root => path.startsWith(root));
}
if (!isWritableMcpPath(cfg, path)) throw new Error("path outside MCP server allowed roots");

function isWriteSuccess(r) {
  return r !== null && typeof r === "object" && r.success === true;
}
// usage: const res = await writeFile(...); if (!isWriteSuccess(res)) handle(res.error);

try { await mcp.upload(url, token, path, content); }
catch (e) {
  // e.message is writeResponse.error or the generic fallback
  logMcpWriteFailure(path, e.message);
  throw e;
}

When it happens

Trigger: writeFile resolves with {success: false} and no error field — e.g. the MCP tool returned an error result, the path is not writable on the server, the server-side tool rejected the content, or the server returned an empty/unexpected success payload.

Common situations: MCP server filesystem tool lacks permission for the target directory; path outside the server's allowed roots; server tool returning isError results without a message; misconfigured serverUrl pointing at a server whose write tool has a different contract.

Related errors

• MCP server error: ${response.status} ${response.statusText}
• error.message + " (MCP)"
• data.error.message

AI-assisted analysis of gildas-lormeau/SingleFile@517fb7c5cf (2026-09-01). Data as JSON: /api/errors/180c1d098cbb692b. Report an issue: GitHub.