fix(mcp-stdio): propagate isError from CallToolResult to wire response [cherry-pick #3625 → release/2.0]#3635
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#3625) ## Summary Fixes a bug in the MCP stdio transport where `isError` was silently dropped from every `tools/call` response. ### Root Cause `HandleCallTool` in `McpStdioServer` used `CoerceToMcpContentBlocks` (reflection-based) to extract only the `Content` list from the `CallToolResult`, then constructed the JSON-RPC result with `new { content }`. The `IsError` property was never read, so all tool responses — successes and errors alike — looked identical on the wire. The HTTP transport was not affected; the MCP SDK serializes the full `CallToolResult` returned by `WithCallToolHandler`, correctly emitting `"isError": true` when set. ### Fix After `CoerceToMcpContentBlocks`, read `callResult.IsError` and include `isError` in the result object only when `== true`. The existing `WhenWritingNull` serializer option on `_jsonOptions` ensures a null `isError` (success path) is omitted from the wire — matching the MCP spec. ```csharp bool? isError = callResult.IsError; if (isError == true) { WriteResult(id, new { content, isError }); } else { WriteResult(id, new { content }); } ``` ### Tests Added (`McpStdioServerContentBlockTests`) - **`HandleCallTool_ErrorResult_EmitsIsErrorTrueOnWire`** — creates a real `CallToolResult { IsError = true }`, replicates the exact `HandleCallTool` conditional logic, and asserts `"isError": true` is present in the JSON-RPC wire output. - **`HandleCallTool_SuccessResult_OmitsIsErrorFromWire`** — verifies that a success result produces no `isError` field at all on the wire. --------- Co-authored-by: aaronburtle <93220300+aaronburtle@users.noreply.github.com> Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com> Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
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Pull request overview
Cherry-pick of #3625 to release/2.0. Fixes an MCP stdio bug where IsError from CallToolResult was dropped when constructing the JSON-RPC tools/call response, making tool errors indistinguishable from successes on the wire.
Changes:
- Extract a new
HandleCallToolAsync(JsonElement, CallToolResult)overload that includesisErrorin the result only whentrue, relying onWhenWritingNullto omit it on success. - Add regression tests for both error (
isError: trueemitted) and success (isErroromitted) wire output.
Reviewed changes
Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.
| File | Description |
|---|---|
| src/Azure.DataApiBuilder.Mcp/Core/McpStdioServer.cs | Adds an overload that propagates IsError into the JSON-RPC result. |
| src/Service.Tests/UnitTests/McpStdioServerContentBlockTests.cs | Adds regression tests verifying isError presence/absence on the wire. |
aaronburtle
approved these changes
May 23, 2026
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Cherry-pick of #3625 →
release/2.0This is a cherry-pick of PR #3625 (
fix(mcp-stdio): propagate isError from CallToolResult to wire response) frommaintorelease/2.0.Original PR Summary
Fixes a bug in the MCP stdio transport where
isErrorwas silently dropped from everytools/callresponse.Root Cause:
HandleCallToolusedCoerceToMcpContentBlocksto extract only theContentlist fromCallToolResult, then constructed the JSON-RPC result withnew { content }. TheIsErrorproperty was never read, so all tool responses looked identical on the wire.Fix: After
CoerceToMcpContentBlocks, readcallResult.IsErrorand includeisErrorin the result object only when== true. The existingWhenWritingNullserializer option ensuresisErroris omitted on the success path — matching the MCP spec.Tests Added (
McpStdioServerContentBlockTests):HandleCallTool_ErrorResult_EmitsIsErrorTrueOnWire— verifies"isError": trueis present for error resultsHandleCallTool_SuccessResult_OmitsIsErrorFromWire— verifiesisErroris absent for success resultsSquash commit cherry-picked:
2f824ffc2