Let tool.execute.before short-circuit a tool call with a substitute result #32565

Problem

tool.execute.before can mutate a tool's args but can't stop the tool from running, and tool.execute.after only rewrites the result after it ran. There's no way for a plugin to answer a tool call itself and skip execution.

The runtime always runs the tool after the before-hook (packages/opencode/src/session/tools.ts, native path ~L88–93 and MCP path ~L129–139): it triggers tool.execute.before, then unconditionally calls item.execute(...).

Use case: a memory/cache plugin that already holds the answer for a read/grep (an indexed summary or cached result) and wants to return it instead of doing the read. Today it can only let the read happen and prepend a note in tool.execute.after, so the unnecessary work still runs. permission.ask can deny a call but returns an error, not a substitute result.

Proposal

Let tool.execute.before optionally supply a result and/or skip execution:

"tool.execute.before"?: (
  input: { tool: string; sessionID: string; callID: string },
  output: { args: any; result?: string; skip?: boolean },
) => Promise<void>

After the trigger, if skip is set (or result is provided), build the standard output from result and bypass item.execute(...). Backwards compatible — hooks that set neither behave exactly as today. The native and MCP paths return slightly different output shapes, so the substitute would need handling in both.

Alternatives

• tool.execute.after banner — the tool still runs.
• permission.ask deny — blocks, but yields an error rather than a clean result.

Happy to open a PR if this seems reasonable. Wanted to check the approach first per the contributing guide.