Add preMcpToolCall hook support to all SDKs

[stephentoub]

Motivation

The copilot-agent-runtime 1.0.51 added a new preMcpToolCall hook that lets SDK consumers intercept MCP tool invocations before they execute -- specifically to inspect, set, replace, or remove the _meta field sent to the MCP server. This PR adds support for that hook across all five language SDKs.

Approach

Each SDK receives a new hook callback (OnPreMcpToolCall / on_pre_mcp_tool_call) that is invoked with contextual information about the pending MCP tool call (server name, tool name, arguments, working directory, timestamp, existing meta). The hook returns a tri-state output:

• Return null/None -- preserve the existing _meta unchanged
• Return output with metaToUse set to an object -- replace _meta with that object
• Return output with metaToUse set to null/None -- remove _meta entirely

Key design decisions

• Wire format uses "cwd" but the public SDK API exposes it as WorkingDirectory/working_directory for consistency with the rest of the SDK surface. All existing hook input types were renamed from Cwd to WorkingDirectory as well (with serialization attributes preserving the wire name).
• .NET uses a SerializeHookOutput helper to pre-serialize the typed output to JsonElement before boxing into the generic HooksInvokeResponse(object? Output) -- required because reflection-based serialization is disabled in the project.
• The hasHooks checks in .NET and Go were updated to include OnPreMcpToolCall so the runtime knows to invoke hooks.

Testing

• E2E tests added for all 5 SDKs (15 new tests total), covering all three meta manipulation scenarios (set, replace, remove)
• Tests use a shared MCP "meta-echo" server (test/harness/test-mcp-meta-echo-server.mjs) that echoes back the _meta it received in the tool result
• Three new snapshot YAML files in test/snapshots/pre_mcp_tool_call_hook/
• .NET serialization unit tests verifying correct JSON output for all hook return shapes
• All tests pass locally across .NET, Node.js, Go, Python, and Rust

Files of note

• test/harness/test-mcp-meta-echo-server.mjs -- shared MCP server for E2E tests
• dotnet/src/Types.cs -- PreMcpToolCallHookInput and PreMcpToolCallHookOutput types
• rust/src/hooks.rs -- Rust hook trait extension and dispatch logic
• test/harness/replayingCapiProxy.ts -- minor fix to forward hook RPC calls correctly

[Copilot]

Pull request overview

Adds cross-SDK support for the new preMcpToolCall runtime hook so consumers can inspect/modify/remove MCP request _meta before tool dispatch, and updates hook payload APIs to expose WorkingDirectory/workingDirectory while preserving the on-the-wire "cwd" key.

Changes:

• Introduces preMcpToolCall hook handlers and input/output types across Rust, .NET, Go, Node.js, and Python (including runtime dispatch/handler registration).
• Renames existing hook input cwd fields to WorkingDirectory/workingDirectory (with wire-format mapping back to "cwd").
• Adds shared MCP “meta-echo” test server + snapshot-driven E2E tests for set/replace/remove-meta scenarios; adds .NET serialization tests for hook output shapes.

Show a summary per file

File	Description
test/snapshots/pre_mcp_tool_call_hook/should_set_meta_via_premcptoolcall_hook.yaml	New replay snapshot for “set meta” scenario.
test/snapshots/pre_mcp_tool_call_hook/should_replace_meta_via_premcptoolcall_hook.yaml	New replay snapshot for “replace meta” scenario.
test/snapshots/pre_mcp_tool_call_hook/should_remove_meta_via_premcptoolcall_hook.yaml	New replay snapshot for “remove meta” scenario.
test/harness/test-mcp-meta-echo-server.mjs	Adds MCP stdio server that echoes received _meta for E2E validation.
test/harness/replayingCapiProxy.ts	Adds optional debug logging to help diagnose snapshot prefix mismatches.
rust/tests/e2e/support.rs	Removes now-unneeded dead_code suppression for repo_root().
rust/tests/e2e/pre_mcp_tool_call_hook.rs	Adds Rust E2E tests covering set/replace/remove meta via hook.
rust/tests/e2e/hooks_extended.rs	Updates tests to assert working_directory instead of cwd.
rust/tests/e2e.rs	Registers new Rust E2E module.
rust/src/hooks.rs	Adds PreMcpToolCall hook types + dispatch; renames hook cwd fields to working_directory (wire "cwd").
rust/examples/hooks.rs	Updates example to use working_directory.
python/e2e/test_pre_mcp_tool_call_hook_e2e.py	Adds Python E2E tests for preMcpToolCall meta manipulation.
python/e2e/test_hooks_extended_e2e.py	Updates extended hook test to expect workingDirectory.
python/copilot/session.py	Adds new hook TypedDicts/handler type and dispatch wiring; remaps "cwd" to workingDirectory for hook inputs.
nodejs/test/e2e/pre_mcp_tool_call_hook.e2e.test.ts	Adds Node E2E tests for preMcpToolCall meta manipulation.
nodejs/test/e2e/hooks_extended.e2e.test.ts	Updates extended hook test assertions to use workingDirectory.
nodejs/src/types.ts	Adds new PreMcpToolCall* types and hook registration; renames hook base input cwd to workingDirectory.
nodejs/src/session.ts	Remaps raw hook input { cwd } to { workingDirectory } during deserialization.
go/types.go	Adds Go PreMcpToolCall hook types + meta tri-state helpers; renames several public Cwd-named fields to WorkingDirectory (wire "cwd").
go/session.go	Wires preMcpToolCall into hook invoke dispatcher.
go/internal/e2e/session_e2e_test.go	Updates assertions to use Context.WorkingDirectory.
go/internal/e2e/pre_mcp_tool_call_hook_e2e_test.go	Adds Go E2E tests for meta set/replace/remove via hook.
go/internal/e2e/mcp_and_agents_e2e_test.go	Updates MCP stdio config field to WorkingDirectory.
go/internal/e2e/hooks_extended_e2e_test.go	Updates hook extended test to use WorkingDirectory.
go/internal/e2e/client_options_e2e_test.go	Updates captured CLI JSON struct to WorkingDirectory field (wire "cwd").
go/client.go	Updates hasHooks detection to include OnPreMcpToolCall.
dotnet/test/Unit/SerializationTests.cs	Adds unit tests validating serialization shapes for PreMcpToolCallHookOutput and boxed JsonElement outputs.
dotnet/test/E2E/PreMcpToolCallHookE2ETests.cs	Adds .NET E2E tests for meta set/replace/remove via hook.
dotnet/test/E2E/HookLifecycleAndOutputE2ETests.cs	Updates hook lifecycle assertions to use WorkingDirectory.
dotnet/src/Types.cs	Adds PreMcpToolCallHookInput/Output; renames hook input Cwd properties to WorkingDirectory (wire "cwd").
dotnet/src/Session.cs	Adds hook dispatch for preMcpToolCall and pre-serialization helper for output.
dotnet/src/Client.cs	Updates hasHooks detection to include OnPreMcpToolCall.

Copilot's findings

• Files reviewed: 32/32 changed files
• Comments generated: 4

[github-actions]

Commit pushed: bd6ef76

Generated by Java Codegen Agentic Fix · ● 9.2M

[github-actions]

Generated by SDK Consistency Review Agent for issue #1366 · ● 13.3M

[github-actions]

Cross-SDK Consistency Review ✅

This PR maintains excellent cross-SDK consistency. Here's a summary of the review:

Coverage

The preMcpToolCall hook is added to all 6 SDK implementations: Node.js/TypeScript, Python, Go, .NET, Java, and Rust.

API naming consistency (language idioms respected)

SDK	Hook property	Input type	Output type
.NET	OnPreMcpToolCall	PreMcpToolCallHookInput	PreMcpToolCallHookOutput?
Go	OnPreMcpToolCall	PreMcpToolCallHookInput	*PreMcpToolCallHookOutput
Java	setOnPreMcpToolCall / getOnPreMcpToolCall	PreMcpToolCallHookInput	PreMcpToolCallHookOutput
Node.js	onPreMcpToolCall	PreMcpToolCallHookInput	PreMcpToolCallHookOutput | null | undefined
Python	on_pre_mcp_tool_call	PreMcpToolCallHookInput	PreMcpToolCallHookOutput | None
Rust	(trait-based)	PreMcpToolCallHookInput	PreMcpToolCallHookOutput

cwd → WorkingDirectory rename

The wire-format rename is applied consistently across all SDKs — public API uses WorkingDirectory/working_directory while JSON serialization preserves the "cwd" wire name via attributes/tags.

Test coverage

E2E tests for all three meta-manipulation scenarios (set, replace, remove) are present for all SDKs.

No inconsistencies found.

Generated by SDK Consistency Review Agent for issue #1366 · ● 7.2M · ◷