What causes "void warn(String message)" in spring-projects/spring-ai?

Use ctx.warn("...") within an async tool and subscribe to (or chain) the returned Mono

void warn(String message)

Error message

void warn(String message)

What it means

This is not an error but the interface method declaration warn(String) on McpAsyncRequestContext: the reactive context API's method to send a WARN-level logging notification to the connected MCP client, returning Mono<Void>. It appears in error listings only because it is an API surface member.

Solutions

Use ctx.warn("...") within an async tool and subscribe to (or chain) the returned Mono<Void>
For imperative code, use the sync context's warn(String) instead

Example fix

// before (async context, imperative style)
ctx.warn("disk nearly full"); // does nothing alone
// after
return ctx.warn("disk nearly full").then(Mono.just(result));

Defensive patterns

Strategy: try-catch

Try / catch

try {
  ctx.warn("disk nearly full").block();
} catch (Exception e) {
  logger.warn("Failed to deliver warn log notification to MCP client", e);
}

Prevention

Remember async context logging methods return Mono<Void> and must be subscribed
Chain logging Monos into the tool's reactive pipeline
Use the sync context for imperative logging

When it happens

Trigger: Invoked when a tool implementation calls ctx.warn("message") on an async (reactive) MCP request context to emit a client-side warning log notification.

Common situations: Tool code that wants the MCP client (e.g. an LLM host) to display warning logs during tool execution uses this method as part of the logging family log/debug/info/warn/error.

Related errors

AI-assisted analysis of spring-projects/spring-ai@98a7beda4f (2026-09-11). Data as JSON: /api/errors/94db03ae11e56ae4. Report an issue: GitHub.

Appendix: source

Thrown at mcp/mcp-annotations/src/main/java/org/springframework/ai/mcp/annotation/context/McpAsyncRequestContext.java:97

	Mono<Void> progress(Consumer<ProgressSpec> progressSpec);

	Mono<Void> progress(ProgressNotification progressNotification);

	// --------------------------------------
	// Ping
	// --------------------------------------
	Mono<Object> ping();

	// --------------------------------------
	// Logging
	// --------------------------------------
	Mono<Void> log(Consumer<LoggingSpec> logSpec);

	Mono<Void> debug(String message);

	Mono<Void> info(String message);

	Mono<Void> warn(String message);

	Mono<Void> error(String message);

}

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