[FEATURE]: schema-constrained structured outputs (JSON Schema), similar to Codex #10456

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

OpenCode currently supports JSON-formatted output, but there is no way to enforce conformance to a user-supplied schema (e.g. JSON Schema).

For CI and automation use cases (code review bots, PR comments, policy checks), best-effort JSON is not sufficient. Outputs need to be schema-constrained and deterministic, or the run should fail clearly.

OpenAI Codex provides this via Structured Outputs (JSON Schema), as shown in the Codex CI/code-review guide:

• https://cookbook.openai.com/examples/codex/build_code_review_with_codex_sdk

That guide demonstrates passing an output schema file and getting guaranteed schema-compliant output, which makes CI integration reliable.

Proposed functionality (illustrative)

opencode run review \
  --output-schema review.schema.json \
  --output-format json

Where:

• review.schema.json is a JSON Schema
• OpenCode guarantees the final output conforms to the schema
• Failure to conform results in a clear error (not best-effort JSON)
• This feature might need to be limited to subset of the available models.

Important distinction

This is not about:

• prompting the model to “output JSON”
• validating JSON after the fact

The request is for schema enforcement at generation time, similar to Codex’s structured output support.

Why this matters

• Enables reliable CI/CD and PR review automation
• Avoids validation + retry loops
• Complements OpenCode’s LSP support by making outputs machine-actionable

Happy to help test if this is something you’d consider adding.