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GH-4542: Collect all Flux elements in async MCP tool callbacks #6158

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GH-4542: Collect all Flux elements in async MCP tool callbacks #6158

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5e96423
feat: add REQUIRED constant to ToolChoiceBuilder in DeepSeek and Mini…
suryateja-g13 May 26, 2026
bbdcb42
fix: collect all Flux elements in async MCP tool callbacks
suryateja-g13 May 26, 2026
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31 changes: 24 additions & 7 deletions ...org/springframework/ai/mcp/annotation/method/tool/AbstractAsyncMcpToolMethodCallback.java
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Original file line number Diff line number Diff line change
Expand Up @@ -20,6 +20,7 @@

import io.modelcontextprotocol.spec.McpSchema.CallToolRequest;
import io.modelcontextprotocol.spec.McpSchema.CallToolResult;
import io.modelcontextprotocol.spec.McpSchema.Content;
import org.reactivestreams.Publisher;
import reactor.core.publisher.Flux;
import reactor.core.publisher.Mono;
Expand Down Expand Up @@ -80,24 +81,40 @@ protected Mono<CallToolResult> convertToCallToolResult(Object result) {
.build()));
}

// Handle Flux by taking the first element
// Handle Flux by collecting all elements
if (result instanceof Flux) {
Flux<?> fluxResult = (Flux<?>) result;

// Check if the Flux contains CallToolResult
// Check if the Flux contains CallToolResult — merge all content items
if (ReactiveUtils.isReactiveReturnTypeOfCallToolResult(this.toolMethod)) {
return ((Flux<CallToolResult>) fluxResult).next();
return ((Flux<CallToolResult>) fluxResult).collectList().map(results -> {
var builder = CallToolResult.builder();
for (CallToolResult r : results) {
for (Content c : r.content()) {
builder.addContent(c);
}
}
return builder.build();
});
}

// Handle Mono<Void> for VOID return type
// Handle Flux<Void> for VOID return type
if (ReactiveUtils.isReactiveReturnTypeOfVoid(this.toolMethod)) {
return fluxResult
.then(Mono.just(CallToolResult.builder().addTextContent(JsonParser.toJson("Done")).build()));
}

// Handle other Flux types by taking the first element and mapping
return fluxResult.next()
.map(this::mapValueToCallToolResult)
// Handle other Flux types by collecting all elements
return fluxResult.collectList().map(items -> {
var builder = CallToolResult.builder();
for (Object item : items) {
CallToolResult itemResult = this.mapValueToCallToolResult(item);
for (Content c : itemResult.content()) {
builder.addContent(c);
}
}
return builder.build();
})
.onErrorResume(e -> Mono.just(CallToolResult.builder()
.isError(true)
.addTextContent("Error invoking method: %s".formatted(e.getMessage()))
Expand Down
9 changes: 7 additions & 2 deletions ...va/org/springframework/ai/mcp/annotation/method/tool/AsyncMcpToolMethodCallbackTests.java
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Original file line number Diff line number Diff line change
Expand Up @@ -452,13 +452,18 @@ public void testMultipleFluxTool() throws Exception {
McpAsyncServerExchange exchange = mock(McpAsyncServerExchange.class);
CallToolRequest request = new CallToolRequest("multiple-flux-tool", Map.of("prefix", "item"));

// Flux tools should take the first element
// Flux tools should collect all elements and return them as separate content
// items
StepVerifier.create(callback.apply(exchange, request)).assertNext(result -> {
assertThat(result).isNotNull();
assertThat(result.isError()).isFalse();
assertThat(result.content()).hasSize(1);
assertThat(result.content()).hasSize(3);
assertThat(result.content().get(0)).isInstanceOf(TextContent.class);
assertThat(((TextContent) result.content().get(0)).text()).isEqualTo("item1");
assertThat(result.content().get(1)).isInstanceOf(TextContent.class);
assertThat(((TextContent) result.content().get(1)).text()).isEqualTo("item2");
assertThat(result.content().get(2)).isInstanceOf(TextContent.class);
assertThat(((TextContent) result.content().get(2)).text()).isEqualTo("item3");
}).verifyComplete();
}

Expand Down
9 changes: 7 additions & 2 deletions ...ringframework/ai/mcp/annotation/method/tool/AsyncStatelessMcpToolMethodCallbackTests.java
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Original file line number Diff line number Diff line change
Expand Up @@ -473,13 +473,18 @@ public void testMultipleFluxTool() throws Exception {
McpTransportContext context = mock(McpTransportContext.class);
CallToolRequest request = new CallToolRequest("multiple-flux-tool", Map.of("prefix", "item"));

// Flux tools should take the first element
// Flux tools should collect all elements and return them as separate content
// items
StepVerifier.create(callback.apply(context, request)).assertNext(result -> {
assertThat(result).isNotNull();
assertThat(result.isError()).isFalse();
assertThat(result.content()).hasSize(1);
assertThat(result.content()).hasSize(3);
assertThat(result.content().get(0)).isInstanceOf(TextContent.class);
assertThat(((TextContent) result.content().get(0)).text()).isEqualTo("item1");
assertThat(result.content().get(1)).isInstanceOf(TextContent.class);
assertThat(((TextContent) result.content().get(1)).text()).isEqualTo("item2");
assertThat(result.content().get(2)).isInstanceOf(TextContent.class);
assertThat(((TextContent) result.content().get(2)).text()).isEqualTo("item3");
}).verifyComplete();
}

Expand Down
27 changes: 12 additions & 15 deletions ...t/java/org/springframework/ai/mcp/annotation/provider/tool/AsyncMcpToolProviderTests.java
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Original file line number Diff line number Diff line change
Expand Up @@ -533,11 +533,13 @@ public Flux<String> fluxHandlingTool(String input) {
StepVerifier.create(result).assertNext(callToolResult -> {
assertThat(callToolResult).isNotNull();
assertThat(callToolResult.isError()).isFalse();
assertThat(callToolResult.content()).hasSize(1);
assertThat(callToolResult.content()).hasSize(3);
assertThat(callToolResult.content().get(0)).isInstanceOf(TextContent.class);
// Flux results are typically concatenated or collected into a single response
String content = ((TextContent) callToolResult.content().get(0)).text();
assertThat(content).contains("test");
assertThat(((TextContent) callToolResult.content().get(0)).text()).isEqualTo("Item1: test");
assertThat(callToolResult.content().get(1)).isInstanceOf(TextContent.class);
assertThat(((TextContent) callToolResult.content().get(1)).text()).isEqualTo("Item2: test");
assertThat(callToolResult.content().get(2)).isInstanceOf(TextContent.class);
assertThat(((TextContent) callToolResult.content().get(2)).text()).isEqualTo("Item3: test");
}).verifyComplete();
}

Expand Down Expand Up @@ -956,18 +958,13 @@ public Flux<CustomResult> listResponseTool(String input) {

assertThat(result).isNotNull();
assertThat(result.isError()).isFalse();
assertThat(result.content()).hasSize(1);
assertThat(result.content()).hasSize(3);
assertThat(result.content().get(0)).isInstanceOf(McpSchema.TextContent.class);

String jsonText = ((TextContent) result.content().get(0)).text();
System.out.println("Actual JSON output: " + jsonText);

// The Flux might be serialized differently than expected, let's check what we
// actually get
// Based on the error, it seems like we're getting a single object instead of an
// array
// Let's adjust our assertion to match the actual behavior
assertThat(jsonText).contains("Processed: test - Item 1");
assertThat(((TextContent) result.content().get(0)).text()).contains("Processed: test - Item 1");
assertThat(result.content().get(1)).isInstanceOf(McpSchema.TextContent.class);
assertThat(((TextContent) result.content().get(1)).text()).contains("Processed: test - Item 2");
assertThat(result.content().get(2)).isInstanceOf(McpSchema.TextContent.class);
assertThat(((TextContent) result.content().get(2)).text()).contains("Processed: test - Item 3");
}

@Test
Expand Down
10 changes: 6 additions & 4 deletions ...g/springframework/ai/mcp/annotation/provider/tool/AsyncStatelessMcpToolProviderTests.java
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Original file line number Diff line number Diff line change
Expand Up @@ -529,11 +529,13 @@ public Flux<String> fluxHandlingTool(String input) {
StepVerifier.create(result).assertNext(callToolResult -> {
assertThat(callToolResult).isNotNull();
assertThat(callToolResult.isError()).isFalse();
assertThat(callToolResult.content()).hasSize(1);
assertThat(callToolResult.content()).hasSize(3);
assertThat(callToolResult.content().get(0)).isInstanceOf(TextContent.class);
// Flux results are typically concatenated or collected into a single response
String content = ((TextContent) callToolResult.content().get(0)).text();
assertThat(content).contains("test");
assertThat(((TextContent) callToolResult.content().get(0)).text()).isEqualTo("Item1: test");
assertThat(callToolResult.content().get(1)).isInstanceOf(TextContent.class);
assertThat(((TextContent) callToolResult.content().get(1)).text()).isEqualTo("Item2: test");
assertThat(callToolResult.content().get(2)).isInstanceOf(TextContent.class);
assertThat(((TextContent) callToolResult.content().get(2)).text()).isEqualTo("Item3: test");
}).verifyComplete();
}

Expand Down
4 changes: 4 additions & 0 deletions models/spring-ai-deepseek/src/main/java/org/springframework/ai/deepseek/api/DeepSeekApi.java
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Original file line number Diff line number Diff line change
Expand Up @@ -547,6 +547,10 @@ public static class ToolChoiceBuilder {
* Model will not call a function and instead generates a message
*/
public static final String NONE = "none";
/**
* Model must call at least one tool, but it can choose which one.
*/
public static final String REQUIRED = "required";

/**
* Specifying a particular function forces the model to call that function.
Expand Down
4 changes: 4 additions & 0 deletions models/spring-ai-minimax/src/main/java/org/springframework/ai/minimax/api/MiniMaxApi.java
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Original file line number Diff line number Diff line change
Expand Up @@ -625,6 +625,10 @@ public static class ToolChoiceBuilder {
* Model will not call a function and instead generates a message
*/
public static final String NONE = "none";
/**
* Model must call at least one tool, but it can choose which one.
*/
public static final String REQUIRED = "required";

/**
* Specifying a particular function forces the model to call that function.
Expand Down
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