fix(mcp-clients): support JSON Schema union type arrays in the validation hint

[pedrofrxncx]

Source: bug found auditing apps/api/src/mcp-clients/validation-hint.ts after this week's coerce/retry hardening (#6210, #6216, #6242).

Why a maintainer wants it: JSON Schema lets a property's type be a union array (e.g. a nullable field as ["string", "null"]"), and the MCP SDK's Tool.inputSchema.properties values are untyped custom objects, so nothing stops an upstream server from advertising one. matchesDeclaredType and coerceArgsToSchema only ever compared against a single type string, so any correctly-typed value against a union-typed property was always reported as "Wrong type" in the repair hint sent back to the agent — actively misleading it about what's actually wrong — and coerceArgsToSchema could never recognize a string value as already valid for such a property.

Failure scenario: a tool declares note: { type: ["string", "null"] }. The agent sends a correct string value but the call still fails validation for an unrelated reason (e.g. a missing required field). The generated hint falsely says note (expected string,null, got string) even though note was fine, confusing the retry. Regression test: validation-hint.test.ts — "does not flag a nullable union type's matching member as wrong-typed" / "flags a value matching none of a union type's members" / "coerces against a union type, skipping when string is already a member".

Verify: bun test apps/api/src/mcp-clients/validation-hint.test.ts (19 pass).

Checks run locally: bun run fmt, cd apps/api && bunx tsc --noEmit (clean), bunx oxlint apps/api/src/mcp-clients/validation-hint.ts apps/api/src/mcp-clients/validation-hint.test.ts (0 warnings/errors), targeted test file above. Full CI validates the rest.

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Summary by cubic

Fixes validation and repair hints for tools that declare JSON Schema union types (e.g., ["string", "null"]). Previously, union-typed properties were always flagged as wrong-typed; now we correctly match and coerce across union members.

• Accepts type as string | string[] and normalizes to an array.
• matchesDeclaredType matches any union member (keeps special handling for "integer").
• coerceArgsToSchema tries coercion across union members and skips coercion when "string" is already allowed.
• buildValidationHint formats expected union types as a|b and no longer flags values that match a union member.
• Adds targeted tests; no behavior change for single-type schemas.

^{Written for commit 1c2adfb. Summary will update on new commits.}