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A v2 diff can't hold the snippet of an Edit's input (it needs line numbers and the operation), so on v2 an auto-approved Edit or Write showed its change only once the PostToolUse hook reported it, after the tool had already run. Only an asked one showed its approval patch first. ## What changes, v2 only The tool call shows the exact patch before the tool runs, auto-approved or not. ## How - **A PreToolUse hook for Edit and Write** (`fileChangePreviewHook`), registered only on v2 connections. Claude Code runs the tool only after the hook returns, so the file is still unchanged when the hook reads it. That's the same guarantee the approval patch in `canUseTool` relies on. The hook sends the same patch (`previewPatchContent`): - **Stream already showed the tool call:** a `tool_call_update` through the session's stream router, pinned like an approval patch. - **Hook came first:** the SDK dispatches hook callbacks as soon as it reads them, ahead of the message queue the agent consumes. The hook then emits the tool call with the patch via `ensureToolCallEmitted`, the permission flow's fix for the same race (#851). - **Subagent tools** are attributed exactly as in `canUseTool`. That rule moves into `toolUseOwner`, which both use, with no behavior change. - **Safe failure:** the hook makes no permission decision, and any failure only leaves the preview out. - **v1 and AIR** register no PreToolUse hook, so Claude Code waits on nothing new there and their output is unchanged. I first tried hanging the preview on the per-tool-use hook registry that PostToolUse uses. That fails when the hook comes before the stream, so the hook lives on the agent instead. ## Golden files The scenario harness now runs PreToolUse before each tool, as Claude Code does, and applies each hook's `matcher`. The v2 golden files of the four file-change scenarios change: - **`write-existing`** (auto-approved): the patch now arrives before the tool completes, and the final hook report no longer repeats it, because the tracker sees no change. - **`write-new`, `edit-with-permission`, `edit-rejected`:** the tool call gets the patch just before the permission request, which carries the same patch. No v1, AIR or plain golden file changes. ## Checks - `npm run check`, `npm run build` and all 2497 tests pass. - A new v2 test runs an auto-approved Edit with the hook both after and before the streamed tool use. It checks that the patch is computed from the file before the edit, arrives before the result, and isn't replaced. - A new test checks that only a v2 session registers the hook, with the `Edit|Write` matcher. - Mutations that fail tests: no hook on v2, the hook on v1 too, the hook-first path doing nothing, the already-shown path sending nothing, and Edit only. - Not pinned by any test: the pin itself. On v2 nothing later in the stream carries content for an Edit or Write, so it's only there for consistency with the approval patch. ## Follow-up AIR has the same gap for an auto-approved Write: it shows no diff until the hook. That's out of scope here, since it changes AIR's output.