mcp: send empty arrays for completions and prompts with nothing to return - #1290
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guglielmo-san merged 2 commits intoSep 24, 2026
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A completion handler with no suggestions naturally returns
&CompleteResult{}, and a prompt handler can return a GetPromptResult
with no messages. Both went out with JSON null ("values": null,
"messages": null), although the schema requires arrays, so clients that
validate results reject them: the TypeScript SDK's CompleteResult and
GetPromptResult schemas fail with "expected array, received null".
Replace nil with an empty slice in the handler's result before sending,
as callTool already does for tool content. An input-required prompt
result is left as is, like a tool result.
guglielmo-san
approved these changes
Sep 24, 2026
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A completion handler with nothing to suggest naturally returns
&CompleteResult{}, and a prompt handler can return aGetPromptResultwith no messages. Both went out with JSON null,{"completion":{"values":null}}and{"messages":null}, although the schema requires arrays. So a client that validates results rejects them. With the TypeScript SDK (1.29), bothCompleteResultSchemaandGetPromptResultSchemafail withexpected array, received null, while[]parses fine. The Go client does not notice, because it decodes null as a nil slice.This replaces nil with an empty slice in the handler's result before it is sent, the same way
callToolalready handles tool content (// avoid "null"). An input-required prompt result is left as is, like a tool result. Also likecallTool, this covers what the handler returns: a receiving middleware that returns its own result is not normalized.The test is an extension of
TestNoJSONNull: its server gets a completion handler and a prompt that return empty results, and the client calls both. It fails without the change, on each of the two separately.