Method Write

mcp/streamable.go:1781–1929

Write implements the [Connection] interface.

(ctx context.Context, msg jsonrpc.Message)

// Write implements the [Connection] interface.
func (c *streamableClientConn) Write(ctx context.Context, msg jsonrpc.Message) error {
	if err := c.failure(); err != nil {
		return err
	}

	var requestSummary string
	var forCall *jsonrpc.Request
	switch msg := msg.(type) {
	case *jsonrpc.Request:
		requestSummary = fmt.Sprintf("sending %q", msg.Method)
		if msg.IsCall() {
			forCall = msg
		}
	case *jsonrpc.Response:
		requestSummary = fmt.Sprintf("sending jsonrpc response #%d", msg.ID)
	default:
		panic("unreachable")
	}

	data, err := jsonrpc.EncodeMessage(msg)
	if err != nil {
		return fmt.Errorf("%s: %v", requestSummary, err)
	}

	doRequest := func() (*http.Request, *http.Response, error) {
		req, err := http.NewRequestWithContext(ctx, http.MethodPost, c.url, bytes.NewReader(data))
		if err != nil {
			return nil, nil, err
		}
		req.Header.Set("Content-Type", "application/json")
		req.Header.Set("Accept", "application/json, text/event-stream")
		if err := c.setMCPHeaders(req); err != nil {
			// Failure to set headers means that the request was not sent.
			// Wrap with ErrRejected so the jsonrpc2 connection doesn't set writeErr
			// and permanently break the connection.
			return nil, nil, fmt.Errorf("%s: %w: %w", requestSummary, jsonrpc2.ErrRejected, err)
		}
		// Keep this after the setMCPHeaders call to ensure that the
		// protocol version header is set.
		setStandardHeaders(req.Header, msg)
		resp, err := c.client.Do(req)
		if err != nil {
			// Any error from client.Do means the request didn't reach the server.
			// Wrap with ErrRejected so the jsonrpc2 connection doesn't set writeErr
			// and permanently break the connection.
			err = fmt.Errorf("%s: %w: %w", requestSummary, jsonrpc2.ErrRejected, err)
		}
		return req, resp, err
	}

	req, resp, err := doRequest()
	if err != nil {
		return err
	}

	if (resp.StatusCode == http.StatusUnauthorized || resp.StatusCode == http.StatusForbidden) && c.oauthHandler != nil {
		if err := c.oauthHandler.Authorize(ctx, req, resp); err != nil {
			// If the caller's context was cancelled while we were running the