Unexpected hook event: postToolUseFailure

[galiacheng]

I’m using Sonnet 4.6 with SDK 0.2.2 DotNet, but I’m receiving a postToolUseFailure event, which doesn’t appear to be supported.
It seems to have replaced the original postToolUse event, causing my script not to execute.
Could you help confirm whether this behavior is coming from the model?

[github-actions]

Investigation Findings

Thanks for the detailed report! I've investigated this across the full SDK codebase.

What I found

The postToolUseFailure hook type is a new event introduced by newer versions of the Copilot CLI (surfacing with Sonnet 4.6). The SDK (v0.2.2) does not yet support it.

How the SDK currently handles it: All four SDK implementations (Node.js, Python, Go, .NET) are designed to gracefully ignore any unrecognised hook types — they return null/undefined/nil without error. This is an intentional forward-compatibility design. Relevant code paths:

• Node.js: nodejs/src/session.ts L905-908 — unknown hook type returns undefined
• Python: python/copilot/session.py L1831-1833 — unknown hook type returns None
• Go: go/session.go L454-456 — default: return nil, nil
• .NET: dotnet/src/Session.cs L967 — _ => null

The Go test suite even uses postToolUseFailure explicitly as the example of a future/unknown hook type (see go/session_test.go L548-550).

Root cause of your issue

When a tool call fails, the CLI now sends a postToolUseFailure event instead of (or in addition to) postToolUse. Because the SDK doesn't expose an onPostToolUseFailure handler, there's no way to subscribe to it — and your existing onPostToolUse handler won't fire for those failure cases.

Classification

This is working as designed from the SDK's perspective (the SDK correctly ignores unknown hooks without crashing), but the SDK needs to be updated to expose the new postToolUseFailure hook type to users. I've labelled this as an enhancement.

The fix would involve:

1. Adding onPostToolUseFailure (or similar) to SessionHooks in all four SDKs
2. Mapping it in each SDK's hook-dispatch switch/map
3. Defining input/output types for the failure case (which may differ from PostToolUseHookInput)

In the meantime, there is no workaround through the SDK's typed hook API — you'd need to wait for SDK support to be added.

Generated by Bug Handler for issue #1126 · ● 468.8K · ◷

[stephentoub]

This is resolved. The postToolUseFailure hook is now a first-class, supported hook in the .NET SDK (added in #1421, 2026-05-26). SessionHooks exposes OnPostToolUseFailure (with PostToolUseFailureHookInput / PostToolUseFailureHookOutput types), and Session dispatches the postToolUseFailure event to it — so it is no longer an "unexpected"/unsupported event.

To answer your question: the event does not come from the model. The CLI emits postToolUseFailure (instead of postToolUse) when a tool call ends in a non-success result, so to react to failed tool calls register an OnPostToolUseFailure handler alongside OnPostToolUse. Update to the latest @github/copilot-sdk to pick this up. Closing as fixed.

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