MCP Server Part 9: Background callbacks by KoolADE85 · Pull Request #3766 · plotly/dash

KoolADE85 · 2026-05-07T17:36:27Z

Summary

Adds support for MCP Tasks — when an LLM calls a tool backed by a Dash background callback, the tool returns a taskId immediately and the LLM polls for results.

New dash/mcp/tasks/ module that lists, starts, and cancels background callbacks per the MCP spec SEP-1686
A new tool to trigger background callbacks (for clients that don't yet implement MCP Tasks natively)
Background callback tools' descriptions auto-include the polling instructions so the LLM knows how/when to retrieve results
Thorough integration tests

camdecoster · 2026-05-13T20:00:29Z

+if TYPE_CHECKING:
+ from dash.mcp.primitives.tools.callback_adapter import CallbackAdapter


Why was this necessary?

CallbackAdapter is only used for its type:
def format_callback_response( response: CallbackExecutionResponse, callback: CallbackAdapter, ) -> CallToolResult:
and the TYPE_CHECKING constant just prevents circular import errors.

camdecoster · 2026-05-13T20:01:16Z

+
+
+def task_result_to_tool_result(create_task_result: CreateTaskResult) -> CallToolResult:
+ """Wrap a CreateTaskResult as a CallToolResult with polling instructions.


Ahem...
Suggested change
"""WrapaCreateTaskResultasaCallToolResultwithpollinginstructions.
"""
WrapaCreateTaskResultasaCallToolResultwithpollinginstructions.

camdecoster · 2026-05-13T20:06:53Z

+ "This is a long-running background operation. "
+ "It returns a taskId immediately. "


These get joined with newlines, so you could probably skip the ending spaces.
Suggested change
"This is a long-running background operation."
"It returns a taskId immediately."
"This is a long-running background operation."
"It returns a taskId immediately."

This is actually implicit string concatenation (no commas), so they form a single string (without newlines).

camdecoster · 2026-05-13T20:12:16Z

 )
+
+
+def task_result_to_tool_result(create_task_result: CreateTaskResult) -> CallToolResult:


This is minor, but CreateTaskResult makes me think that you're creating something. That's not the case though. Is there another name option?

Well, we are creating a task in MCP parlance, which is just some long-running operation that agents need to wait for.
The name CreateTaskResult comes from the MCP package itself so it's not really ours to rename.

camdecoster · 2026-05-13T20:24:48Z

- def call_tool(cls, tool_name: str, arguments: dict[str, Any]) -> CallToolResult:
+ def call_tool(
+ cls, tool_name: str, arguments: dict[str, Any], task: dict | None = None
+ ) -> CallToolResult | CreateTaskResult:


Would CreateTaskResult get returned? In task_result_to_tool_result the result is wrapped in CallToolResult.

camdecoster · 2026-05-13T21:47:58Z

+ tool_name: str,
+ arguments: dict[str, Any],
+ task: dict | None = None,
+ ) -> CallToolResult:


Could the result here be CreateTaskResult?

camdecoster · 2026-05-14T23:49:37Z

+ task_status = get_task(task_id)
+ if task_status.status == "completed":
+ return get_task_result(task_id)


Is the task guaranteed to still be available between these two calls? For example, if you have two clients polling for the same task, could one of them grab the result before the other and cause an error?

…move Flask dependency in MCP implementation

- Reorder status checks in get_task: result_ready takes priority over job_running. A background process can write its result to the cache before the process fully exits, causing a false "working" status even after the result is available (fixes test_mcpbg016). - Annotate response as CallbackExecutionResponse so mypy accepts it as the argument to _prepare_response and format_callback_response (fixes Typing Tests CI failure). Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

KoolADE85 requested review from T4rk1n, camdecoster and ndrezn as code owners May 7, 2026 17:36

KoolADE85 force-pushed the feature/mcp-server-integration branch 2 times, most recently from 3c39eac to a1ca057Compare May 8, 2026 22:10

KoolADE85 force-pushed the feature/mcp-background-callbacks branch from 3e146f8 to 0a82ca0Compare May 8, 2026 22:11

KoolADE85 force-pushed the feature/mcp-server-integration branch from a1ca057 to 409be55Compare May 11, 2026 23:19

KoolADE85 force-pushed the feature/mcp-background-callbacks branch from fd13290 to 5c45bafCompare May 11, 2026 23:19

KoolADE85 force-pushed the feature/mcp-server-integration branch from 409be55 to b788678Compare May 13, 2026 18:04

KoolADE85 force-pushed the feature/mcp-background-callbacks branch 2 times, most recently from 1366bd4 to f4254e2Compare May 14, 2026 17:13

camdecoster reviewed May 14, 2026
View reviewed changes

KoolADE85 added 7 commits May 19, 2026 09:17

Implement background callback support
493a9dc

Ensure that background callback expiry is communicated in MCP tool
62bcde8

lint
806d4cd

Install diskcache extra in lint-unit and typing CI jobs
311a593

Fix mypy errors in CI
77d3d66

Fix bug storing metadata on background callbacks
b54c7ce

Enable programmatic querying of background callback status/result; re…
3ef3409
…move Flask dependency in MCP implementation

KoolADE85 force-pushed the feature/mcp-background-callbacks branch from f4254e2 to 3ef3409Compare May 19, 2026 15:18

KoolADE85 changed the base branch from feature/mcp-server-integration to mcpMay 19, 2026 15:18

robertclausand others added 2 commits May 19, 2026 09:48

code review feedback
eae9504

camdecoster approved these changes May 19, 2026
View reviewed changes

camdecoster assigned KoolADE85 May 20, 2026

KoolADE85 added 2 commits May 20, 2026 09:36

Revert match/case refactor due to compatibility problems
7460799

automatically load env vars with direnv
57e9a2c

KoolADE85 merged commit 2cf8f91 into mcpMay 20, 2026
52 of 55 checks passed

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MCP Server Part 9: Background callbacks - #3766

MCP Server Part 9: Background callbacks#3766
KoolADE85 merged 11 commits into
mcpfrom
feature/mcp-background-callbacks

KoolADE85 commented May 7, 2026

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		if TYPE_CHECKING:
		from dash.mcp.primitives.tools.callback_adapter import CallbackAdapter



		def task_result_to_tool_result(create_task_result: CreateTaskResult) -> CallToolResult:
		"""Wrap a CreateTaskResult as a CallToolResult with polling instructions.

	"""WrapaCreateTaskResultasaCallToolResultwithpollinginstructions.
	"""
	WrapaCreateTaskResultasaCallToolResultwithpollinginstructions.

		"This is a long-running background operation. "
		"It returns a taskId immediately. "

-"This is a long-running background operation."
-"It returns a taskId immediately."
+"This is a long-running background operation."
+"It returns a taskId immediately."

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KoolADE85 commented May 7, 2026

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