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StructuredOutputCallToolHandler doesn't handle error based CallToolResult #538

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StructuredOutputCallToolHandler doesn't handle error based CallToolResult#538
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tzolov

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@jc-hcl
jc-hcl
opened on Sep 4, 2025

java-sdk/mcp/src/main/java/io/modelcontextprotocol/server/McpAsyncServer.java

Lines 377 to 379 in 681fe7f

return this.delegateCallToolResult.apply(exchange, request).map(result -> {
if (outputSchema == null) {

The method is checking for various combinations of content compared to the declared output schema, but if the result is an error then this should be just returned irrespective of the schema and structuredContent values i.e.

if (Boolean.TRUE.equals(result.isError())) {
    return result;
}

Activity

  1. tzolov
    self-assigned this
    on Sep 6, 2025
  2. tzolov
    added a commit that references this issue on Sep 7, 2025

    fix: align tool error handling with MCP specification

    7e3f76e
  3. tzolov
    closed this as completedin c7736b6on Sep 8, 2025
  4. tzolov
    added a commit that references this issue on Sep 8, 2025

    fix: Skip structured output validation for error tool results

    210a813
  5. He-Pin
    added a commit that references this issue on Sep 8, 2025

    fix: Skip structured output validation for error tool results

    c7736b6
  6. rhishirajnema
    added a commit that references this issue on Jun 25, 2026

    link #115926 Upgrade MCP SDK fork to upstream v0.18.2 for streamable …

    7f007e3
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