fix(pi-stack): replace Type.Union/Literal with Type.String for JSON Schema 2020-12 compat

[chico-piaba]

Problem

When using claude-opus-4-6-thinking via the google-antigravity provider, the pi agent returns:

tools.N.custom.input_schema: JSON schema is invalid.
It must match JSON Schema draft 2020-12.

Root Cause

Three pi-stack extensions used Type.Union([Type.Literal(...)]) from @sinclair/typebox to define string enum parameters. This generates anyOf + const schemas.

The google-antigravity provider routes Claude models through Cloud Code Assist using the legacy parameters field (OpenAPI 3.03 Schema), which does not support anyOf + const patterns. The sanitizeSchemaForAntigravity() function converts const → enum, but the outer anyOf wrapper remains invalid.

Serialization chain

Type.Union([Type.Literal("a"), ...])
  → anyOf: [{ const: "a" }, ...]           ← Typebox generates this
    → sanitizeSchemaForAntigravity()
      → anyOf: [{ enum: ["a"] }, ...]      ← const becomes enum
        → OpenAPI 3.03 rejects anyOf        ← 💥 ERROR

Fix

Replaced Type.Union([Type.Literal(...)]) with Type.String({ description: "..." }) which is universally compatible across all providers and schema formats.

Files changed

Extension	Field	Before	After
session-analytics.ts	query_type	Type.Union([Type.Literal(...)])	Type.String()
governance-profiles.ts	action	Type.Union([Type.Literal(...)])	Type.String()
safe-boot.ts	action	Type.Union([Type.Literal(...)])	Type.String()

Testing

• Verified no remaining Type.Union/Type.Literal patterns in pi-stack extensions
• Changes only affect tool parameter schemas; runtime validation in execute() is unchanged
• Compatible with all providers: Anthropic, Google, OpenAI, GitHub Copilot

Prevention

For future tool schemas, prefer:

• ✅ Type.String({ description: "a | b | c" }) — universally safe
• ✅ StringEnum([...] as const) — generates { type: "string", enum: [...] }
• ❌ Type.Union([Type.Literal(...)]) — generates incompatible anyOf + const