Logging, progress, and cancellation

Every handler receives a context as its second argument; the request-scoped helpers — progress, logging, and the cancellation signal — live on ctx.mcpReq.

Report progress from a handler

A client that wants progress puts a progressToken in the request's _meta. Read it from ctx.mcpReq._meta and send each update with ctx.mcpReq.notify.

server.registerTool(
    'process-files',
    {
        description: 'Process files with progress updates',
        inputSchema: z.object({ files: z.array(z.string()) })
    },
    async ({ files }, ctx) => {
        const progressToken = ctx.mcpReq._meta?.progressToken;

        for (let i = 0; i < files.length; i++) {
            // ... process files[i] ...

            if (progressToken !== undefined) {
                await ctx.mcpReq.notify({
                    method: 'notifications/progress',
                    params: { progressToken, progress: i + 1, total: files.length, message: `Processed ${files[i]}` }
                });
            }
        }

        return { content: [{ type: 'text', text: `Processed ${files.length} files` }] };
    }
);

Every call on this page comes from an in-memory Client connected to this server — Test a server shows that wiring. Pass an onprogress callback and the SDK puts the progressToken in _meta for you.

const result = await client.callTool(
    { name: 'process-files', arguments: { files: ['a.csv', 'b.csv', 'c.csv'] } },
    { onprogress: update => console.log(update) }
);
console.log(result.content);

The callback fires once per file, then the call resolves:

{ progress: 1, total: 3, message: 'Processed a.csv' }
{ progress: 2, total: 3, message: 'Processed b.csv' }
{ progress: 3, total: 3, message: 'Processed c.csv' }
[ { type: 'text', text: 'Processed 3 files' } ]

Skip progress when the client did not ask

Drop onprogress and the same request arrives with no progressToken, so the guard in the handler sends nothing.

const quiet = await client.callTool({ name: 'process-files', arguments: { files: ['d.csv', 'e.csv'] } });
console.log(quiet.content);

Only the result comes back:

[ { type: 'text', text: 'Processed 2 files' } ]

progress must increase on every notification for the same token; total and message are optional.

Log to the client

Deprecated — SEP-2577

Log to stderr (stdio servers) or use OpenTelemetry instead. MCP logging is deprecated as of protocol version 2026-07-28 (SEP-2577) and stays functional through the deprecation window (at least twelve months) — see the deprecated features registry.

Declare the logging capability when you construct the server.

const server = new McpServer({ name: 'file-processor', version: '1.0.0' }, { capabilities: { logging: {} } });

ctx.mcpReq.log(level, data) then sends a notifications/message from inside any handler — data is any JSON value.

server.registerTool(
    'validate-records',
    {
        description: 'Validate records before import',
        inputSchema: z.object({ records: z.array(z.string()) })
    },
    async ({ records }, ctx) => {
        await ctx.mcpReq.log('info', `Validating ${records.length} records`);
        const invalid = records.filter(record => !record.endsWith('.csv'));
        if (invalid.length > 0) {
            await ctx.mcpReq.log('warning', { invalid });
        }
        return { content: [{ type: 'text', text: `${records.length - invalid.length} of ${records.length} records are valid` }] };
    }
);

The connected client surfaces each one through its notifications/message handler.

client.setNotificationHandler('notifications/message', notification => {
    console.log(notification.params.level, notification.params.data);
});

Calling validate-records with one bad record delivers both log notifications before the result:

info Validating 2 records
warning { invalid: [ 'b.txt' ] }
[ { type: 'text', text: '1 of 2 records are valid' } ]

How the client's log level reaches ctx.mcpReq.log differs by protocol era — see Protocol versions.

Stop work when the request is cancelled

ctx.mcpReq.signal is an AbortSignal. The SDK aborts it when the client sends notifications/cancelled for the request, and when the connection closes — check it before each unit of work.

server.registerTool(
    'scan-archive',
    {
        description: 'Scan every page of the archive',
        inputSchema: z.object({ pages: z.number().int() })
    },
    async ({ pages }, ctx) => {
        let scanned = 0;
        for (let page = 0; page < pages; page++) {
            if (ctx.mcpReq.signal.aborted) {
                console.error(`Stopped after ${scanned} of ${pages} pages: ${ctx.mcpReq.signal.reason}`);
                break;
            }
            await new Promise(resolve => setTimeout(resolve, 100)); // ... scan one page ...
            scanned++;
        }
        return { content: [{ type: 'text', text: `Scanned ${scanned} pages` }] };
    }
);

Cancel from the client by aborting the signal you pass to the call.

const controller = new AbortController();
const scan = client.callTool({ name: 'scan-archive', arguments: { pages: 40 } }, { signal: controller.signal });

// the end user clicks Stop while the scan runs
setTimeout(() => controller.abort('the end user clicked Stop'), 5);

await scan.catch(error => console.log(String(error)));

The call rejects on the client and the handler stops at its next check; the abort reason travels in the notification and comes out as ctx.mcpReq.signal.reason:

SdkError: the end user clicked Stop
Stopped after 1 of 40 pages: the end user clicked Stop

The first line is the client's rejection, the second is the handler's console.error on the server. The SDK never sends a response for a cancelled request and discards whatever the handler still returns.

Pass the signal to your own I/O

Hand the same signal to fetch — or any API that accepts an AbortSignal — and cancellation propagates into the work the handler started.

const SOURCE_URLS = {
    readme: 'https://example.com/sources/readme.md',
    changelog: 'https://example.com/sources/changelog.md'
};

server.registerTool(
    'fetch-source',
    {
        description: 'Download one of the known source files',
        inputSchema: z.object({ source: z.enum(['readme', 'changelog']) })
    },
    async ({ source }, ctx) => {
        const response = await fetch(SOURCE_URLS[source], { signal: ctx.mcpReq.signal });
        return { content: [{ type: 'text', text: await response.text() }] };
    }
);

On cancellation fetch rejects mid-download and the handler unwinds with it, so no work outlives the request.

WARNING

Resolve an identifier against a fixed list, as fetch-source does. A tool that fetches a caller-supplied URL lets any connected client drive requests from your server's network position (server-side request forgery).

Recap

• Every handler receives a context as its second argument; the request-scoped helpers live on ctx.mcpReq.
• ctx.mcpReq.notify sends notifications/progress when the request carried a progressToken; progress must increase on each one.
• ctx.mcpReq.log(level, data) sends notifications/message once the logging capability is declared; MCP logging is deprecated (SEP-2577).
• ctx.mcpReq.signal aborts on cancellation and disconnect — check it in long loops and forward it to your own I/O.