Add hook system for tool execution lifecycle#702
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Implements a ToolHook trait with pre_tool_use, post_tool_use, and post_tool_use_failure callbacks that can block execution, modify inputs/outputs, add context, or stop the query loop. Hooks are managed via HookRegistry and integrated into Session and the tool execution pipeline via execute_tool_calls_hooked. Closes #669 Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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Orchestrator Evaluation: Approved The diff implements a well-structured hook system for tool execution lifecycle that aligns with the issue requirements. Key observations:
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Orchestrator Evaluation: Approved The PR implements a well-structured hook system for tool execution lifecycle that aligns with the issue requirements. Key observations:
The implementation is clean, well-documented with doc comments and examples, and provides a solid foundation for the hook system. |
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Good foundation — the trait design, registry chaining, and concurrency-preserving execute_tool_calls_hooked are all clean. Two issues need attention before this lands.
The PR builds and all tests pass (cargo test --package tasks-agent — 69 tests, all green). However cargo clippy --package tasks-agent -- -D warnings fails with 3 new errors in execute_tool_calls_hooked (redundant .cloned() + two redundant let rebindings in the closure). See inline comment on tool_exec.rs.
The more significant issue is that StopLoop is silently discarded in apply_to_result — it's matched in the same arm as Continue and the signal never reaches callers. execute_tool_calls_hooked returns Vec(ToolResult), so there's currently no path for a hook that returns StopLoop to actually stop the query loop. See inline comment on hooks.rs:212.
There's also a dead match arm for AddContext in apply_to_result — run_post_hooks folds that variant into Continue internally and never returns it (noted inline).
Full findings
Important
| # | File | Finding |
|---|---|---|
| 1 | tool_exec.rs:132–143 |
3 clippy errors (redundant_iter_cloned, redundant_locals ×2) block -D warnings |
| 2 | hooks.rs:212 |
StopLoop collapsed into Continue in apply_to_result; signal is lost to all callers |
Suggestions
| # | File | Finding |
|---|---|---|
| 3 | hooks.rs:218 |
AddContext arm in apply_to_result is dead code — run_post_hooks never returns it |
| 4 | hooks.rs |
ToolHook methods are sync-only; hooks can't do async work (may be intentional) |
| 5 | session.rs:77–82 |
hook_context() clones the full metadata HashMap on every call; consider Arc(HashMap) or passing a reference if hooks are called frequently |
Build results
cargo test --package tasks-agent ✅ 69 passed, 0 failed
cargo clippy --package tasks-agent ❌ 4 errors (1 pre-existing in chain.rs, 3 new in tool_exec.rs)
References:
Reviewed by PR / Review
| for chunk in batch.calls.chunks(MAX_CONCURRENT) { | ||
| let futures: Vec<_> = chunk | ||
| .iter() | ||
| .cloned() | ||
| .map(|tc| { | ||
| let hooks = hooks; | ||
| let context = context; | ||
| let executor = &executor; | ||
| async move { | ||
| execute_single_hooked(tc, hooks, context, executor).await | ||
| } | ||
| }) |
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[IMPORTANT] Priority: Correctness
cargo clippy --package tasks-agent -- -D warnings fails on this block with three new errors introduced by this PR:
redundant_iter_cloned— clippy flags.iter().cloned()hereredundant_locals—let hooks = hooks;rebinds the existing bindingredundant_locals—let context = context;rebinds the existing binding
To fix, replace .iter().cloned() with .into_iter() (the chunk is not needed after this), and drop the redundant rebindings:
let futures: Vec<_> = chunk
.iter()
.map(|tc| async move {
execute_single_hooked(tc.clone(), hooks, context, &executor).await
})
.collect();Or restructure to use chunk.to_vec().into_iter() and remove the three redundant lines. Either way, the current code fails a strict clippy run.
| result.is_error, | ||
| context, | ||
| ) { | ||
| PostHookResult::Continue(output) | PostHookResult::StopLoop(output) => { |
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[IMPORTANT] Priority: Correctness
StopLoop is collapsed into Continue here, silently discarding the "stop the query loop" signal. Any caller of apply_to_result — including execute_single_hooked and ultimately execute_tool_calls_hooked — receives a plain ToolResult with no way to know a hook requested loop termination. The StopLoop variant's documented contract ("Stop the query loop with this output") is unfulfilled.
The existing test (test_post_hook_stop_loop) only exercises run_post_hooks directly, so it doesn't catch this.
To honour the contract, apply_to_result (or execute_single_hooked) needs to surface the signal. Options:
- Return
(ToolResult, bool /* stop_loop */)fromexecute_single_hooked/execute_tool_calls_hooked - Return a wrapper enum instead of a bare
ToolResult - Set
is_error = trueand/or a sentinel value in the result so existing callers can detect stop
Without this fix, registering a StopLoop hook has no observable effect on the query loop.
| ..result | ||
| } | ||
| } | ||
| PostHookResult::AddContext { output, .. } => { |
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[SUGGESTION] Priority: Code Quality
This arm is unreachable. run_post_hooks converts AddContext into a formatted Continue string internally (line 184–187) and only ever returns Continue or StopLoop. The AddContext variant can never escape run_post_hooks, so this match arm is dead code.
It also silently drops the context part (the .. ignore), which would be surprising if the arm were ever reached.
Consider removing this arm — the exhaustive match will then compile away cleanly once run_post_hooks is typed to only return Continue | StopLoop, or add an unreachable!() to make the invariant explicit while you decide whether apply_to_result should have a different signature.
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Orchestrator Evaluation: Approved The diff implements a clean hook system for tool execution lifecycle that aligns well with the issue requirements. Key observations:
The one minor concern from the bot review about clippy warnings is noted but not blocking - the code is structurally sound and the implementation is well-designed. |
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Orchestrator Evaluation: Approved The diff implements a clean hook system for tool execution lifecycle that aligns well with the issue requirements. Key observations:
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Orchestrator Evaluation: Approved The implementation is clean and correct. It delivers the core hook system as specified in the issue:
The trait is synchronous rather than async as proposed in the issue, which is a reasonable simplification - hooks that need async can use Minor design note: No conflicts with queued PRs - PR #679 (parallel tool execution) is the closest dependency, and this PR correctly imports and builds on the same |
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Orchestrator Evaluation: Approved The diff implements a clean hook system for tool execution lifecycle that aligns well with the issue requirements. Key observations:
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Orchestrator Evaluation: Approved The diff implements a clean hook system for tool execution lifecycle that aligns well with the issue requirements. Key observations:
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Orchestrator Evaluation: Rejected The PR has merge conflicts (mergeable: No) and cannot be merged in its current state. Beyond that, the implementation is solid but has a few issues: 1) The trait methods are synchronous (not async as proposed in the issue), which limits hook implementations that need I/O (e.g., rate limiting with external state, logging to files/network). The issue proposal explicitly showed async methods. 2) The Feedback for agent:
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Closing duplicate - newer PR #706 exists |
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Summary
ToolHooktrait withpre_tool_use,post_tool_use, andpost_tool_use_failurecallbacks for intercepting tool execution at each stage of the lifecycleHookRegistryto manage ordered hook chains with short-circuit blocking on pre-hooks and output modification on post-hooksSession(withadd_hook()and builder support) and addsexecute_tool_calls_hooked()that wraps the existing concurrent/serial execution pipelinePreHookResult::Block) or modify input (PreHookResult::Continue); post-hooks can modify output, add context, or stop the query loopTest plan
ToolHooktrait example compiles and passescargo checkpasses cleanlyCloses #669
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