Implement SEP-2200: Clarify tool result content visibility #2193

Activity

1. felixweinberger

   added this to 2026-07-28 Spec Implementationon May 29, 2026
2. github-project-automation

   moved this to Todo in 2026-07-28 Spec Implementationon May 29, 2026
3. mattzcarey commented on May 29, 2026

   

   mattzcarey

   on May 29, 2026

   Member

   More actions

   Status — SEP-2200 (Clarify tool result content visibility)

   Track: C — independent, starts now.
   Spec: SEP-2200 PR was closed, but the normative content still landed in docs/specification/draft/server/tools.mdx via other commits. The spec page treats this as a real release-1 SEP.

   What's done

   • Nothing in main enforcing the new rules. packages/core/src/types/spec.types.ts currently has the old prose (isError: true → "any errors that originate from the tool SHOULD be reported inside the result object").

   Outstanding (acceptance criteria, summary)

   • R-2200-4 (wire): Tool that returns structuredContent SHOULD also return a content representation of the same information. Remove the SDK's auto-JSON-stringify of structuredContent into content (the old inspector behavior from change step name #1089).
   • R-2200-5 (wire): When outputSchema is defined AND result is not an error, server MUST provide structuredContent conforming to the schema. The "not an error" qualifier is new — if isError=true with no/non-conforming structuredContent, SDK MUST NOT reject.
   • R-2200-6 (sdk): Clients SHOULD NOT validate structuredContent against outputSchema when isError=true. (Today the client validator likely throws on outputSchema mismatch regardless of isError.)
   • R-2200-1 (guidance): Clients SHOULD NOT forward both content and structuredContent verbatim to the model.
   • R-2200-7 (guidance): When both present they SHOULD be semantically equivalent.
   • R-2200-8 (guidance): Conversational/agentic clients SHOULD use content, fall back to structuredContent if empty.
   • R-2200-9 (guidance): Programmatic/typed clients SHOULD prefer structuredContent.
   • R-2200-10 (guidance): isError=true → error message in content; structuredContent MAY be omitted.

   Suggested first PR

   1. Lift the "auto-stringify structuredContent into content" branch in the SDK helper and replace it with explicit caller behavior.
   2. In the client validator, gate outputSchema validation on !result.isError.
   3. Update JSDoc on CallToolResult.structuredContent to reference the new prose.

   Coordination

   • Touches the same file as Implement SEP-2106: Tools inputSchema & outputSchema Conform to JSON Schema 2020-12 #2192 (spec.types.ts). Bundling the structuredContent widening (SEP-2106 R-2106-6) with the visibility rules (SEP-2200) avoids duplicate review of the same lines.
   • No cross-track dependencies.
4. mattzcarey

   self-assigned this

   on May 29, 2026
5. norika1207-lab commented on Jun 2, 2026

   

   norika1207-lab

   on Jun 2, 2026 · Hidden as spam

   show commentMore actions
6. mattzcarey commented on Jun 9, 2026

   

   mattzcarey

   on Jun 9, 2026

   Member

   More actions

   Closing: SEP-2200 was declined — the spec PR (modelcontextprotocol/modelcontextprotocol#2200) was closed unmerged on 2026-05-25 after the Core Maintainer vote was re-run, with the stated preference to instead fix the underlying tools/call interface with polymorphic return types in a future spec revision. The draft spec contains no visibility language to implement (the adjacent structured-content guidance is covered by SEP-2106 / #2249). If a successor SEP lands, it should get its own tracking issue.
7. mattzcarey

   closed this as completedon Jun 9, 2026
8. github-project-automation

   moved this from Todo to Done in 2026-07-28 Spec Implementationon Jun 9, 2026