Feature request: let PostToolUse hooks replace tool output (updatedToolOutput)

Motivation

We build toonify-mcp, a context-compression tool: it converts large JSON/YAML tool output to a compact tabular format and collapses repetitive logs (60%+ token reduction on realistic payloads), while passing through anything it can't reproduce losslessly.

On Claude Code this works as a PostToolUse hook that returns hookSpecificOutput.updatedToolOutput, which replaces what the model sees — that replacement is what actually shrinks context.

Current Codex behavior

PostToolUseOutcome supports block / additionalContext / feedback, but no output replacement. The output parser already recognizes a Claude-style updatedMCPToolOutput field — and explicitly rejects it as unsupported (codex-rs/hooks/src/events/post_tool_use.rs, unsupported_updated_mcp_tool_output_fails_open), so the concept appears to be on your radar already.

additionalContext can't serve this use case: appending a compressed copy on top of the original output makes context larger, the exact opposite of the goal.

Request

Support an updatedToolOutput (and/or the already-parsed updatedMCPToolOutput) in PostToolUse hook output that replaces the tool result before it enters model context — schema-checked against the tool's output type, with fail-open on mismatch, like the current unsupported-field handling.

This would let context-optimization tools (compression, redaction, truncation policies) work across both ecosystems with the same hook contract.

Start in codex-rs/hooks/src/events/post_tool_use.rs, including unsupported_updated_mcp_tool_output_fails_open, to understand the existing parser and fail-open behavior. Trace how PostToolUseOutcome reaches model context; done means a supported updatedToolOutput or updatedMCPToolOutput can replace the result when schema-compatible, while mismatches fail open.