fix(call-actor): synthesize structuredContent on MCP server pass-through#859
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MQ37 merged 1 commit intoMay 18, 2026
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handleMcpToolCall returned { content: result.content } only — no structuredContent,
no isError. Since #415 added outputSchema to call-actor, MCP SDK >= 1.11.4 rejects
the response with -32600 'has an output schema but did not return structured content'
once the client has called listTools() to cache validators (i.e. every real client).
Synthesize a sentinel RunResponse matching getActorRunOutputSchema.required keys —
runId/actorId/status/storages/summary/nextStep — and forward result.isError. The
remote tool's payload still flows through content; no behavior change for callers
that don't validate against outputSchema.
Reproduces with mcpc:
mcpc --json @apify tools-call call-actor \
actor:='"apify/actors-mcp-server:search-apify-docs"' input:='{"query":"weather"}'
This was referenced May 18, 2026
MQ37
merged commit 9cf3a39
into
feat/migrate-direct-actor-tools-runresponse
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Context
call-actorwith MCP-server pass-through syntax (actor: "apify/actors-mcp-server:search-apify-docs") fails on master withMCP error -32600: Tool call-actor has an output schema but did not return structured content. Repro withmcpc --json @apify tools-call call-actor actor:='"apify/actors-mcp-server:search-apify-docs"' input:='{"query":"weather"}'. Regression introduced in #415 (f9512c4, 2026-01-29) whenoutputSchemawas added tocall-actor; the MCP-pass-through code inhandleMcpToolCallhas been returning{ content }only since #274 (2025-09-18). Both ingredients sat dormant until the SDK on^1.25.2(already enforcing since 1.11.4) saw them combined.Solution
In
handleMcpToolCall, synthesize a sentinelRunResponsematchinggetActorRunOutputSchema.required(runId: 'mcp-passthrough',actorId: baseActorName,status,storages: {},summary,nextStep) and forwardresult.isErrorfrom the remote tool. The remote tool's payload still flows throughcontent. Stacks on #853.Worth your attention
'mcp-passthrough'is a deliberate non-Apify literal so logs / dashboards never mistake it for a real run id. Open tomcp-passthrough:${mcpToolName}for greppability per tool if preferred.client.listTools(), so the SDK never builds the validator cache. The new test attests/integration/suite.ts:813callslistTools()first to mirror real clients (mcpc, Claude Desktop), which is the only way to surface this class of bug at the integration layer.call-actordoing two unrelated things (Apify run vs. MCP tool pass-through) under one tool name is the root cause; this PR keeps that shape and just makes the response spec-compliant. Follow-up tracked in call-actor mixes Apify-run and MCP-tool pass-through under one schema #860 (see comment).Follow-up
call-actorMCP pass-through into a dedicated tool, or move to code-mode as the default. Issue: call-actor mixes Apify-run and MCP-tool pass-through under one schema #860