client
Unified MCP Client that wraps ClientSession with transport management.
ConnectMode
module-attribute
mode= value: "legacy" (initialize handshake), "auto" (discover, fall back to
initialize), or a modern protocol-version string (adopt directly). The str arm is for
forward-compat; Client.__post_init__ rejects anything outside that set at construction.
Client
dataclass
A high-level MCP client for connecting to MCP servers.
Supports in-memory transport for testing (pass a Server or MCPServer instance), Streamable HTTP transport (pass a URL string), or a custom Transport instance.
Example
from mcp.client import Client
from mcp.server.mcpserver import MCPServer
server = MCPServer("test")
@server.tool()
def add(a: int, b: int) -> int:
return a + b
async def main():
async with Client(server) as client:
result = await client.call_tool("add", {"a": 1, "b": 2})
asyncio.run(main())
Source code in src/mcp/client/client.py
263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462 463 464 465 466 467 468 469 470 471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487 488 489 490 491 492 493 494 495 496 497 498 499 500 501 502 503 504 505 506 507 508 509 510 511 512 513 514 515 516 517 518 519 520 521 522 523 524 525 526 527 528 529 530 531 532 533 534 535 536 537 538 539 540 541 542 543 544 545 546 547 548 549 550 551 552 553 554 555 556 557 558 559 560 561 562 563 564 565 566 567 568 569 570 571 572 573 574 575 576 577 578 579 580 581 582 583 584 585 586 587 588 589 590 591 592 593 594 595 596 597 598 599 600 601 602 603 604 605 606 607 608 609 610 611 612 613 614 615 616 617 618 619 620 621 622 623 624 625 626 627 628 629 630 631 632 633 634 635 636 637 638 639 640 641 642 643 644 645 646 647 648 649 650 651 652 653 654 655 656 657 658 659 660 661 662 663 664 665 666 667 668 669 670 671 672 673 674 675 676 677 678 679 680 681 682 683 684 685 686 687 688 689 690 691 692 693 694 695 696 697 698 699 700 701 702 703 704 705 706 707 708 709 710 711 712 713 714 715 716 717 718 719 720 721 722 723 724 725 726 727 728 729 730 731 732 733 734 735 736 737 738 739 740 741 742 743 744 745 746 747 748 749 750 751 752 753 754 755 756 757 758 759 760 761 762 763 764 765 766 767 768 769 770 771 772 773 774 775 776 777 778 779 780 781 782 783 784 785 786 787 788 789 790 791 792 793 794 795 796 797 798 799 800 801 802 803 804 805 806 807 808 809 810 811 812 813 814 815 816 817 818 819 820 821 822 823 824 825 826 827 828 829 830 831 832 833 834 835 836 837 838 839 840 841 842 843 844 845 846 847 848 849 850 851 852 853 854 855 856 857 858 859 860 861 862 863 864 865 866 867 868 869 870 871 872 873 874 875 876 877 878 879 880 881 882 883 884 885 886 887 888 889 890 891 892 893 894 895 896 897 898 899 900 901 902 903 904 905 906 907 908 909 910 911 912 913 914 915 916 917 918 919 920 921 922 923 924 925 926 927 928 929 930 931 932 933 934 | |
server
instance-attribute
The MCP server to connect to.
If the server is a Server or MCPServer instance, it will be connected in-process.
If the server is a URL string, it will be used as the URL for a streamable_http_client transport.
If the server is a Transport instance, it will be used directly.
raise_exceptions
class-attribute
instance-attribute
raise_exceptions: bool = False
Whether to raise exceptions from the server.
read_timeout_seconds
class-attribute
instance-attribute
read_timeout_seconds: float | None = None
Timeout for read operations.
sampling_callback
class-attribute
instance-attribute
sampling_callback: SamplingFnT | None = None
Callback for handling sampling requests.
sampling_capabilities
class-attribute
instance-attribute
sampling_capabilities: SamplingCapability | None = None
Sampling sub-capabilities (e.g. tools) declared alongside sampling_callback; no effect without it.
list_roots_callback
class-attribute
instance-attribute
list_roots_callback: ListRootsFnT | None = None
Callback for handling list roots requests.
logging_callback
class-attribute
instance-attribute
logging_callback: LoggingFnT | None = None
Callback for handling logging notifications.
message_handler
class-attribute
instance-attribute
message_handler: MessageHandlerFnT | None = None
Callback for handling raw messages.
client_info
class-attribute
instance-attribute
client_info: Implementation | None = None
Client implementation info to send to server.
mode
class-attribute
instance-attribute
mode: ConnectMode = 'auto'
How to negotiate the protocol version.
'auto' (the default) probes server/discover and falls back to the initialize handshake on legacy servers;
for an in-process Server/MCPServer it dispatches directly without JSON-RPC framing. 'legacy' forces the
initialize handshake (byte-identical pre-2026 behavior). A modern protocol-version string (e.g. '2026-07-28')
adopts that version directly without a probe — supply prior_discover to reuse a known DiscoverResult, or
omit it to synthesize a minimal one.
prior_discover
class-attribute
instance-attribute
prior_discover: DiscoverResult | None = None
A previously-obtained DiscoverResult to install via .adopt() when mode is a version pin. Ignored when mode='legacy'.
elicitation_callback
class-attribute
instance-attribute
elicitation_callback: ElicitationFnT | None = None
Callback for handling elicitation requests.
input_required_max_rounds
class-attribute
instance-attribute
input_required_max_rounds: int = (
DEFAULT_INPUT_REQUIRED_MAX_ROUNDS
)
Cap on InputRequiredResult retry rounds before call_tool / get_prompt /
read_resource give up. Use client.session.<method>(..., allow_input_required=True)
to drive the loop manually instead.
extensions
class-attribute
instance-attribute
extensions: Sequence[ClientExtension] | None = None
Opt-in client extensions (SEP-2133).
Each instance contributes its capability ad, its result claims (resolved
transparently by call_tool), and its notification bindings. For an
ad-only entry use mcp.client.advertise(identifier, settings).
cache
class-attribute
instance-attribute
cache: CacheConfig | Literal[False] | None = None
Client-side response caching for the SEP-2549 cacheable methods (2026-07-28).
None (the default) honors server ttlMs/cacheScope hints with a per-client
in-memory store; pass a CacheConfig to customize, or False to disable. The
cacheable verbs take a per-call cache_mode (see CacheMode); calls carrying
meta always reach the server. A CacheConfig with a custom store requires
target_id when the server is not a URL (no identity can be derived).
__aenter__
async
__aenter__() -> Client
Enter the async context manager.
Source code in src/mcp/client/client.py
438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 | |
__aexit__
async
__aexit__(
exc_type: type[BaseException] | None,
exc_val: BaseException | None,
exc_tb: Any,
) -> None
Exit the async context manager.
Source code in src/mcp/client/client.py
462 463 464 465 466 | |
session
property
session: ClientSession
Get the underlying ClientSession.
This provides access to the full ClientSession API for advanced use cases.
Raises:
| Type | Description |
|---|---|
RuntimeError
|
If accessed before entering the context manager. |
protocol_version
property
protocol_version: str
Negotiated protocol version (set by initialize/discover/adopt during __aenter__).
server_info
property
server_info: Implementation
Server name/version (set by initialize/discover/adopt during __aenter__).
server_capabilities
property
server_capabilities: ServerCapabilities
Server capabilities (set by initialize/discover/adopt during __aenter__).
send_ping
async
send_ping(
*, meta: RequestParamsMeta | None = None
) -> EmptyResult
Send a ping request to the server.
Source code in src/mcp/client/client.py
505 506 507 508 509 510 511 | |
send_progress_notification
async
send_progress_notification(
progress_token: str | int,
progress: float,
total: float | None = None,
message: str | None = None,
) -> None
Send a progress notification to the server.
Source code in src/mcp/client/client.py
513 514 515 516 517 518 519 520 521 522 523 524 525 526 527 528 529 530 | |
set_logging_level
async
set_logging_level(
level: LoggingLevel,
*,
meta: RequestParamsMeta | None = None
) -> EmptyResult
Set the logging level on the server.
Source code in src/mcp/client/client.py
532 533 534 535 | |
list_resources
async
list_resources(
*,
cursor: str | None = None,
meta: RequestParamsMeta | None = None,
cache_mode: CacheMode = "use"
) -> ListResourcesResult
List available resources from the server.
Source code in src/mcp/client/client.py
576 577 578 579 580 581 582 583 584 585 586 587 588 589 590 | |
list_resource_templates
async
list_resource_templates(
*,
cursor: str | None = None,
meta: RequestParamsMeta | None = None,
cache_mode: CacheMode = "use"
) -> ListResourceTemplatesResult
List available resource templates from the server.
Source code in src/mcp/client/client.py
592 593 594 595 596 597 598 599 600 601 602 603 604 605 606 | |
read_resource
async
read_resource(
uri: str,
*,
input_responses: InputResponses | None = None,
request_state: str | None = None,
meta: RequestParamsMeta | None = None,
cache_mode: CacheMode = "use"
) -> ReadResourceResult
Read a resource from the server.
If the server returns an InputRequiredResult, the embedded input
requests are dispatched to this client's sampling / elicitation / roots
callbacks and the read is retried automatically (up to
input_required_max_rounds).
Parameters:
| Name | Type | Description | Default |
|---|---|---|---|
uri
|
str
|
The URI of the resource to read. |
required |
input_responses
|
InputResponses | None
|
Responses to seed the first call with (e.g. when
resuming from a persisted |
None
|
request_state
|
str | None
|
Opaque state to seed the first call with. |
None
|
meta
|
RequestParamsMeta | None
|
Additional metadata for the request. |
None
|
cache_mode
|
CacheMode
|
Cache behavior for this call (see |
'use'
|
Returns:
| Type | Description |
|---|---|
ReadResourceResult
|
The resource content. |
Raises:
| Type | Description |
|---|---|
InputRequiredRoundsExceededError
|
|
MCPError
|
A callback returned |
ValidationError
|
The server returned a result that does not conform to the negotiated protocol version. |
Source code in src/mcp/client/client.py
608 609 610 611 612 613 614 615 616 617 618 619 620 621 622 623 624 625 626 627 628 629 630 631 632 633 634 635 636 637 638 639 640 641 642 643 644 645 646 647 648 649 650 651 652 653 654 655 656 657 658 659 660 661 662 663 664 665 666 667 668 669 670 | |
listen
listen(
*,
tools_list_changed: bool = False,
prompts_list_changed: bool = False,
resources_list_changed: bool = False,
resource_subscriptions: Sequence[str] = ()
) -> AbstractAsyncContextManager[Subscription]
Open a subscriptions/listen stream of typed change events (2026-07-28 only).
Keyword args mirror the wire SubscriptionFilter; entering waits for the ack (honored subset: sub.honored):
async with client.listen(tools_list_changed=True) as sub:
async for event in sub:
tools = await client.list_tools() # refetch on change
A graceful close ends the loop; an abrupt drop raises SubscriptionLost. No replay: re-listen and refetch.
Raises:
| Type | Description |
|---|---|
ListenNotSupportedError
|
The negotiated protocol version predates 2026-07-28. |
MCPError
|
The server rejected the request or the connection failed first. |
SubscriptionLost
|
The stream ended before it was acknowledged. |
TimeoutError
|
The read timeout elapsed before the acknowledgment. |
Source code in src/mcp/client/client.py
672 673 674 675 676 677 678 679 680 681 682 683 684 685 686 687 688 689 690 691 692 693 694 695 696 697 698 699 700 701 702 703 | |
subscribe_resource
async
subscribe_resource(
uri: str, *, meta: RequestParamsMeta | None = None
) -> EmptyResult
Subscribe to resource updates (2025-era servers only).
Source code in src/mcp/client/client.py
719 720 721 722 723 724 725 | |
unsubscribe_resource
async
unsubscribe_resource(
uri: str, *, meta: RequestParamsMeta | None = None
) -> EmptyResult
Unsubscribe from resource updates (2025-era servers only).
Source code in src/mcp/client/client.py
727 728 729 730 731 732 733 | |
call_tool
async
call_tool(
name: str,
arguments: dict[str, Any] | None = None,
read_timeout_seconds: float | None = None,
progress_callback: ProgressFnT | None = None,
*,
input_responses: InputResponses | None = None,
request_state: str | None = None,
meta: RequestParamsMeta | None = None
) -> CallToolResult
Call a tool on the server.
If the server returns an InputRequiredResult, the embedded input
requests are dispatched to this client's sampling / elicitation / roots
callbacks and the call is retried automatically (up to
input_required_max_rounds). To drive the loop yourself — e.g. to
persist request_state across process restarts — use
client.session.call_tool(..., allow_input_required=True). Persisted
state is still subject to the server's TTL, request binding, and key
lifetime; a server on the default process-local key rejects it after a restart.
Result shapes claimed by this client's extensions are finished by the
owning claim's resolver, whose CallToolResult is returned; resolver
exceptions propagate as-is. To receive the claimed shape yourself, use
client.session.call_tool(..., allow_claimed=True).
Parameters:
| Name | Type | Description | Default |
|---|---|---|---|
name
|
str
|
The name of the tool to call. |
required |
arguments
|
dict[str, Any] | None
|
Arguments to pass to the tool. |
None
|
read_timeout_seconds
|
float | None
|
Timeout for each underlying |
None
|
progress_callback
|
ProgressFnT | None
|
Callback for progress updates. |
None
|
input_responses
|
InputResponses | None
|
Responses to seed the first call with (e.g. when
resuming from a persisted |
None
|
request_state
|
str | None
|
Opaque state to seed the first call with. |
None
|
meta
|
RequestParamsMeta | None
|
Additional metadata for the request. |
None
|
Returns:
| Type | Description |
|---|---|
CallToolResult
|
The tool result. |
Raises:
| Type | Description |
|---|---|
InputRequiredRoundsExceededError
|
|
MCPError
|
A callback returned |
ValidationError
|
The server returned a result that does not conform to the negotiated protocol version. |
Source code in src/mcp/client/client.py
735 736 737 738 739 740 741 742 743 744 745 746 747 748 749 750 751 752 753 754 755 756 757 758 759 760 761 762 763 764 765 766 767 768 769 770 771 772 773 774 775 776 777 778 779 780 781 782 783 784 785 786 787 788 789 790 791 792 793 794 795 796 797 798 799 800 801 802 803 804 805 806 807 808 | |
list_prompts
async
list_prompts(
*,
cursor: str | None = None,
meta: RequestParamsMeta | None = None,
cache_mode: CacheMode = "use"
) -> ListPromptsResult
List available prompts from the server.
Source code in src/mcp/client/client.py
810 811 812 813 814 815 816 817 818 819 820 821 822 823 824 | |
get_prompt
async
get_prompt(
name: str,
arguments: dict[str, str] | None = None,
*,
input_responses: InputResponses | None = None,
request_state: str | None = None,
meta: RequestParamsMeta | None = None
) -> GetPromptResult
Get a prompt from the server.
If the server returns an InputRequiredResult, the embedded input
requests are dispatched to this client's sampling / elicitation / roots
callbacks and the get is retried automatically (up to
input_required_max_rounds).
Parameters:
| Name | Type | Description | Default |
|---|---|---|---|
name
|
str
|
The name of the prompt. |
required |
arguments
|
dict[str, str] | None
|
Arguments to pass to the prompt. |
None
|
input_responses
|
InputResponses | None
|
Responses to seed the first call with (e.g. when
resuming from a persisted |
None
|
request_state
|
str | None
|
Opaque state to seed the first call with. |
None
|
meta
|
RequestParamsMeta | None
|
Additional metadata for the request. |
None
|
Returns:
| Type | Description |
|---|---|
GetPromptResult
|
The prompt content. |
Raises:
| Type | Description |
|---|---|
InputRequiredRoundsExceededError
|
|
MCPError
|
A callback returned |
ValidationError
|
The server returned a result that does not conform to the negotiated protocol version. |
Source code in src/mcp/client/client.py
826 827 828 829 830 831 832 833 834 835 836 837 838 839 840 841 842 843 844 845 846 847 848 849 850 851 852 853 854 855 856 857 858 859 860 861 862 863 864 865 | |
complete
async
complete(
ref: ResourceTemplateReference | PromptReference,
argument: dict[str, str],
context_arguments: dict[str, str] | None = None,
) -> CompleteResult
Get completions for a prompt or resource template argument.
Parameters:
| Name | Type | Description | Default |
|---|---|---|---|
ref
|
ResourceTemplateReference | PromptReference
|
Reference to the prompt or resource template |
required |
argument
|
dict[str, str]
|
The argument to complete |
required |
context_arguments
|
dict[str, str] | None
|
Additional context arguments |
None
|
Returns:
| Type | Description |
|---|---|
CompleteResult
|
Completion suggestions. |
Source code in src/mcp/client/client.py
890 891 892 893 894 895 896 897 898 899 900 901 902 903 904 905 906 | |
list_tools
async
list_tools(
*,
cursor: str | None = None,
meta: RequestParamsMeta | None = None,
cache_mode: CacheMode = "use"
) -> ListToolsResult
List available tools from the server.
Source code in src/mcp/client/client.py
908 909 910 911 912 913 914 915 916 917 918 919 920 921 922 923 924 925 926 927 928 | |
send_roots_list_changed
async
send_roots_list_changed() -> None
Send a notification that the roots list has changed.
Source code in src/mcp/client/client.py
930 931 932 933 934 | |