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fix(hooks): answer tool-hook denials to the model instead of aborting the run - #589
A tool hook Deny was mapped to TinyAgentsError::Tool, which admission does not answer, so the whole run aborted and the host saw "hosted agent invocation failed". This contradicted HookPermission::Deny's documented contract ("The agent is told why"). Now Deny returns ToolFailed(reason): the model gets a failed tool result and continues; the tool never runs. Deny with continue_: Some(false) keeps aborting, now as a new TinyAgentsError::ToolRefused / TerminalReason::ToolRefused / HostedErrorKind::ToolRefused with message "hosted agent invocation stopped: host policy refused a tool call". Ask unchanged. Found in a downstream host (Chottu) where a model emitting a blank tool name or calling a tool outside the host allowlist killed user turns and made a scheduled job fail every retry.
API Or Behavior Changes
Additive API (new variants on #[non_exhaustive] enums; TerminalReason may serialize "tool_refused"). Breaking behaviour for hosts that relied on Deny aborting: they must set continue_: Some(false). Side effects: a command hook that cannot execute and a ToolHook::before_tool returning Err both count as deny and are now answered to the model — the tool still never runs. Slot rollback on refusal kept (TOOL-12: denials do not spend tool budget); repeated denied calls are bounded by the model-call cap (tested). Suggest releasing as 3.2.0 with a release note (or 4.0.0 if strict semver for behaviour).
Tests
tests written first; reverting the hook mapping makes 4 of 5 new loop tests fail. Cover: deny answered + run reaches final answer; continue=false aborts with ToolRefused and no argument text in the error; Ask still fails; blank tool name denied → answered and model recovers; repeated denied calls stop at max_model_calls; tool body never runs. Gates: cargo fmt --check 0; clippy --workspace --all-targets (and --all-features) -D warnings 0; cargo doc with broken intra-doc links denied 0; cargo test --workspace: 5402 passed, 8 failed — the same 8 fail on clean upstream main 231d91c (3 providers::claude_code::driver tests needing a runnable fake claude script; 5 tinyagents-harness-tools skills::test::* "packed tool catalog is unavailable for this registration"). llvm-cov report total line coverage 88.51% (≥80).
A host tool-policy refusal that must end the run now has its own error
variant instead of a generic tool error. It classifies as
TerminalReason::ToolRefused and, at the hosted boundary, as
HostedErrorKind::ToolRefused with the fixed message "hosted agent
invocation stopped: host policy refused a tool call" rather than
"hosted agent invocation failed". The hosted stream's Failed item and
RunFailed event pick the same message from the run's terminal outcome.
The variant is non-retryable and carries no tool arguments.
A tool-hook Deny used to become TinyAgentsError::Tool, which the agent
loop does not answer, so the whole run aborted and hosted callers saw
"hosted agent invocation failed". That contradicted
HookPermission::Deny ("the agent is told why") and turned a blank or
invented tool name a host denies into a fatal error, since hooks run
before the unknown-tool policy.
A deny now returns ToolFailed: the call is answered with a failed tool
result carrying the hook, tool and reason, the tool never runs, and the
run continues. A deny with continue_ = Some(false) stops the run with
ToolRefused. Ask is unchanged.
An answered deny keeps releasing its tool-call slot like every other
before_tool refusal; a model that keeps calling a denied tool is bounded
by max_model_calls, covered by a loop test.
Behaviour change: hosts that relied on a deny ending the run must set
continue_ = Some(false).
No actionable comments were generated in the recent review.
Recent review info
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Reviewing files that changed from the base of the PR and between 8e6ef5c and ec3c5c6.
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crates/tinyagents-hooks/src/command/exec.rs
crates/tinyagents-hooks/src/command/exec_tests.rs
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Walkthrough
Tool-hook denials now return a failed tool result and continue the run by default. With continue_: Some(false), a denial stops the run with ToolRefused. The harness classifies this outcome as a failure and uses a fixed refusal message in hosted failures.
Changes: Tool-hook denial handling
Layer / File(s)
Summary
Select and test denial outcomes crates/tinyagents-harness/src/error.rs, crates/tinyagents-hooks/src/command/*, crates/tinyagents-hooks/src/tool/*, docs/modules/hooks/README.md, CHANGELOG.md
Tool-hook denials return ToolFailed by default and allow the run to continue. With continue_: Some(false), they return ToolRefused and stop the run. Tests cover denial and approval outcomes, tool execution, pending argument state, blank tool names, and model-call limits. Fail-closed denial messages no longer expose the underlying error text.
Classify and report stopped runs crates/tinyagents-harness/src/terminal.rs, crates/tinyagents-harness/src/runtime/agent.rs, crates/tinyagents-harness/src/runtime/agent_tool_refusal_tests.rs, docs/modules/harness/*
ToolRefused maps to a failure terminal reason and hosted error kind. Hosted stream failures and RunFailed events use a fixed refusal message. Tests cover classification, conversion, sanitization, and retryability.
sequenceDiagram
participant Model
participant before_tool
participant ToolHook
participant AgentRuntime
participant TerminalOutcome
participant HostedStream
Model->>before_tool: Send tool call
before_tool->>ToolHook: Evaluate tool call
ToolHook-->>before_tool: Deny with continue_ set to false
before_tool->>AgentRuntime: Return ToolRefused
AgentRuntime->>TerminalOutcome: Classify as ToolRefused failure
AgentRuntime->>HostedStream: Emit sanitized refusal message
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Suggested reviewers:senamakel, tinysweeper
Merge Risk: Minimal · up to ec3c5
No actionable merge-blocking risk is identified in the reviewed change.
Pre-merge checks: 4 · 1
Failed checks: 1 warning
Check name
Status
Explanation
Resolution
Docstring Coverage
Docstring coverage is 60.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff with cyclomatic complexity of at least 2. Documenta…
Write docstrings for the functions missing them to satisfy the coverage threshold.
Passed checks: 4 passed
Check name
Status
Explanation
Description Check
Check skipped - CodeRabbit’s high-level summary is enabled.
Title check
The title clearly describes the primary change: tool-hook denials now return a response to the model instead of aborting the run.
Linked Issues check
Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check
Check skipped because no linked issues were found for this pull request.
Full details: Docstring Coverage
Explanation
Docstring coverage is 60.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff with cyclomatic complexity of at least 2. Documentation-only edits and deletions within surviving functions are included; unchanged diff context and fully deleted functions are excluded. Analyzed 15 functions across 10 files.
Resolution
Write docstrings for the functions missing them to satisfy the coverage threshold.
Autofix · Keep fixing CodeRabbit findings and required CI, and resolving merge conflicts
A rabbit checks each tool-call gate, A denied request can calmly wait. One flag can stop the running stream, A fixed reply keeps details clean. The burrow tests each path in sight, Then hops along beneath moonlight.
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Pull request purpose (title): fix(hooks): answer tool-hook denials to the model instead of aborting the run. Tiny Sweeper completed its review; deterministic results follow.
Review snapshot
Change surface
Files
Review signal
Count
Production
6
Active findings
0
Tests
4
Noted findings
0
Documentation
4
Resolved findings
0
Configuration
0
Pending checks/questions
0
Completeness: Complete Test assessment: No supported feature-to-test mapping was available; this does not mean tests are absent or passed.
What changed
A `tinyagents-hooks` tool-hook `Deny` no longer aborts the run. `ToolHooksMiddleware` returns `TinyAgentsError::ToolFailed` with the message `tool hook '<hook>' denied <tool>: <reason>`, so the agent loop answers the call with a failed tool result and the run continues; the tool never runs. A deny setting `continue_: Some(false)` instead stops the run with the new `TinyAgentsError::ToolRefused`, classified as `TerminalReason::ToolRefused` and, for hosted callers, `HostedErrorKind::ToolRefused` with the message `hosted agent invocation stopped: host policy refused a tool call`. The hosted stream `Failed` item and `RunFailed` event carry that refusal message when the run's typed outcome is `ToolRefused`. `Ask` is unchanged and still fails the run with `TinyAgentsError::Tool`. The refusal text names the hook, tool, and reason but never the call's arguments. A fail-closed command hook that cannot execute now denies with `hook '<label>' could not run` without embedding the underlying execution error, which is kept on `HookRun::error` for logging. The new enum variants are additive on the `#[non_exhaustive]` enums `TinyAgentsError`, `TerminalReason`, and `HostedErrorKind`. Documentation and the changelog were updated, including a behavior table for `Deny`, `Deny` with `continue_: Some(false)`, and `Ask`.
Features
Modified — Tool-hook Deny is answered to the model: `ToolHooksMiddleware` maps a default `Deny` to `TinyAgentsError::ToolFailed` instead of `TinyAgentsError::Tool`, so the call receives a failed tool result carrying `tool hook '<hook>' denied <tool>: <reason>` and the run continues rather than aborting. (crates/tinyagents-hooks/src/tool/mod.rswhere, CHANGELOG.mdduring the 2.x migration are no longer part of the TinyAgents workspace.)
Added — Fatal refusal via continue_: Some(false): A deny with `continue_: Some(false)` stops the run with the new `TinyAgentsError::ToolRefused`, which `TerminalOutcome` classifies as `TerminalReason::ToolRefused` with class `Failure`, and `HostedErrorKind::ToolRefused` maps the hosted message to `hosted agent invocation stopped: host policy refused a tool call`; it is never retryable. (crates/tinyagents-harness/src/error.rspub enum TinyAgentsError {, crates/tinyagents-harness/src/terminal.rspub enum TerminalReason {, crates/tinyagents-harness/src/runtime/agent.rspub enum HostedErrorKind {)
Added — Hosted stream and event refusal message selection: `sanitize_hosted_stream_item` and `sanitize_hosted_event` choose the fixed failure text from the run's typed terminal outcome, so a `ToolRefused` outcome yields the refusal-specific message while other errors keep the generic `hosted agent invocation failed`. (crates/tinyagents-harness/src/runtime/agent.rsfn sanitize_hosted_event(record: &mut EventRecord) {, crates/tinyagents-harness/src/runtime/agent.rsfn hosted_error_message(kind: HostedErrorKind) -> &'static str {)
Modified — Fail-closed hook deny no longer embeds the execution error: When a fail-closed command hook fails to execute, the deny reason set on `agent_message` is now the static `hook '<label>' could not run` rather than including the spawner error text, so execution details cannot reach the model transcript; the raw error stays on `HookRun::error` for logging. (crates/tinyagents-hooks/src/command/exec.rsfn finish(, crates/tinyagents-hooks/src/command/exec_tests.rsmod spawner {)
Modified — Continue-on-deny documentation and changelog: Docs describe the deny/continue/ask behavior table, the argument-free refusal text, and slot release bounded by `RunLimits::max_model_calls`; the changelog marks the deny behavior change as Breaking (behaviour). (docs/modules/hooks/README.mdthe model. `HookIdentity` lets a host provide session, agent, and working-path, CHANGELOG.mdduring the 2.x migration are no longer part of the TinyAgents workspace.)
Tests
No supported feature-to-test mapping was produced. Test execution is not inferred.
Positive: The critique lane answered after reviewing 14 of 14 files with 0 findings, reporting that the fail-closed deny message now omits spawner error details and the edited assertions match the production path; it notes the code index is behind this pull request so retrieved context may be out of date.
Lane summary: The change replaces the fail-closed denial's agent-facing message with a generic "hook '<label>' could not run" so spawner error details no longer reach the model transcript, while still keeping the raw error on `HookRun::error` for logging; the accompanying test asserts both halves. The edited assertions match the production code path (`finish` sets `error: Some(error)` and a deny message without the detail), so nothing here looks broken. _The code index is behind this pull request (indexed at `8e6ef5c0a760`), so retrieved context may be out of date._
security
Conclusion: Success
Scope reviewed: all assigned evidence
Positive: The security lane answered after reviewing 10 of 10 applicable files with 0 findings, finding that the change stops raw exec error text from reaching the model-facing denial message while keeping it on the error field, narrowing what reaches the transcript.
Lane summary: The change stops an exec failure's raw error text from being echoed into the model-facing denial message (`HookOutput::deny` sets `agent_message`, which the doc comment says is 'Shown to the model'), keeping it only on the `error` field. The accompanying test pins that the detail is absent from the model message but present on the error. No attacker-controlled path to a dangerous sink changes; the edit narrows what reaches the transcript. The change looks sound. _The code index is behind this pull request (indexed at `8e6ef5c0a760`), so retrieved context may be out of date._
tests
Conclusion: Success
Scope reviewed: all assigned evidence
Lane summary: The incremental change drops the raw runner error from the fail-closed deny reason so tool-hook refusals echoed to the model cannot carry subprocess internals, and `exec_tests.rs` now asserts both that the reason omits the detail and that it stays in the run's `error` field — an assertion that fails if the redaction is reverted. That behaviour change is covered; no defect found. _The code index is behind this pull request (indexed at `8e6ef5c0a760`), so retrieved context may be out of date._
commits
Conclusion: Success
Scope reviewed: all assigned evidence
Positive: The commits lane answered that no sensitive content was found in committed files.
Lane summary: No sensitive content in committed files. _The code index is behind this pull request (indexed at `8e6ef5c0a760`), so retrieved context may be out of date._
description
Conclusion: Success
Scope reviewed: all assigned evidence
Positive: The description lane answered with no reportable defect, finding the redaction consistent with the pull request's stated goal of keeping hook details out of model-visible denial text and noting the body already documents that a command hook that cannot execute counts as a deny answered to the model.
Lane summary: This revision's changes are narrow: a fail-closed hook's deny reason no longer embeds the underlying execution error (only a static "hook '<label>' could not run"), with a test asserting the error text stays out of agent_message while remaining in `error`. That matches the pull request's stated goal of keeping hook details out of the model-visible denial text, and the PR body already documents that a command hook that cannot execute counts as a deny and is answered to the model. No defect found. _The code index is behind this pull request (indexed at `8e6ef5c0a760`), so retrieved context may be out of date._
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Actionable comments posted: 1
🪄 Fix comments: Fix CodeRabbit comments on this PR
Prompt to fix review comments
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Inline comments:
Review comments at @crates/tinyagents-hooks/src/tool/mod.rs:
- Around line 204-208: Update the hook error handling that builds the message
using hook.name() and event.tool_name so execution failures sent through the
ToolFailed path use a fixed, non-sensitive reason and log detailed errors with
tracing; preserve explicit exit-code-2 denial reasons, including their
documented stderr text.
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priority: p3Whenever. Cosmetic, a nicety, or a cleanup with no user visible effect.
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Summary
A tool hook Deny was mapped to TinyAgentsError::Tool, which admission does not answer, so the whole run aborted and the host saw "hosted agent invocation failed". This contradicted HookPermission::Deny's documented contract ("The agent is told why"). Now Deny returns ToolFailed(reason): the model gets a failed tool result and continues; the tool never runs. Deny with continue_: Some(false) keeps aborting, now as a new TinyAgentsError::ToolRefused / TerminalReason::ToolRefused / HostedErrorKind::ToolRefused with message "hosted agent invocation stopped: host policy refused a tool call". Ask unchanged. Found in a downstream host (Chottu) where a model emitting a blank tool name or calling a tool outside the host allowlist killed user turns and made a scheduled job fail every retry.
API Or Behavior Changes
Additive API (new variants on #[non_exhaustive] enums; TerminalReason may serialize "tool_refused"). Breaking behaviour for hosts that relied on Deny aborting: they must set continue_: Some(false). Side effects: a command hook that cannot execute and a ToolHook::before_tool returning Err both count as deny and are now answered to the model — the tool still never runs. Slot rollback on refusal kept (TOOL-12: denials do not spend tool budget); repeated denied calls are bounded by the model-call cap (tested). Suggest releasing as 3.2.0 with a release note (or 4.0.0 if strict semver for behaviour).
Tests
tests written first; reverting the hook mapping makes 4 of 5 new loop tests fail. Cover: deny answered + run reaches final answer; continue=false aborts with ToolRefused and no argument text in the error; Ask still fails; blank tool name denied → answered and model recovers; repeated denied calls stop at max_model_calls; tool body never runs. Gates: cargo fmt --check 0; clippy --workspace --all-targets (and --all-features) -D warnings 0; cargo doc with broken intra-doc links denied 0; cargo test --workspace: 5402 passed, 8 failed — the same 8 fail on clean upstream main 231d91c (3 providers::claude_code::driver tests needing a runnable fake claude script; 5 tinyagents-harness-tools skills::test::* "packed tool catalog is unavailable for this registration"). llvm-cov report total line coverage 88.51% (≥80).
Documentation
docs/modules/harness/hosting.md, terminal-outcome.md, docs/modules/hooks/README.md behaviour table, command/types.rs continue_ doc, CHANGELOG.md entry marked Breaking (behaviour).
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