[Schema] SEP-2106: Allow non-object outputSchema and structuredContent (arrays/primitives)

[chr-hertel]

Implements the non-object output portion of SEP-2106 for the MCP Spec 2026-07-28 release.

Tracked by umbrella #337.

Spec summary

outputSchema fully drops the object-only restriction and accepts arrays/primitives/compositions. structuredContent widens from {[key:string]: unknown} to unknown, validated by outputSchema.

PHP SDK changes

• The outputSchema path (today the formatters in src/Capability/Formatter/) must accept array/scalar return types without object wrapping.
• Ensure structuredContent serialization handles non-object roots.
• Continue emitting the back-compat TextContent mirror when output is array/primitive (verify against spec).
• Client-side: Mcp\Client tool-listing types currently assume object-shaped structured content — relax type narrowing.

Related

• Umbrella [2026-07-28] Schema modernization & error code standardization #337