fix: return isError for argument validation failures#2511
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- Rename NewServerToolWithRawContextHandler -> NewServerTool. This is the preferred constructor for raw mcp.ToolHandler tools because it avoids creating closures at registration time, which matters for per-request servers that re-register all tools on every request. - Rename deprecated generic NewServerTool[In, Out] -> NewServerToolWithDeps to free up the simpler name and make its closure-based nature explicit. The dynamic tools package is the only legitimate user of this constructor because DynamicToolDependencies differs from the standard ToolDependencies. - Remove deprecated NewServerToolFromHandler. Its only callers can use the new NewServerTool directly via context-injected deps. - Update all call sites in dependencies.go, dynamic_tools.go, and registry_test.go. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
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Pull request overview
Updates the pkg/inventory ServerTool constructors so JSON argument unmarshalling failures are surfaced to MCP clients as tool execution errors (CallToolResult.IsError: true) instead of JSON-RPC protocol errors, enabling agents to see the validation message and retry with corrected arguments.
Changes:
- Return a
*mcp.CallToolResultwithIsError: truewhenjson.Unmarshalof tool arguments fails inNewServerToolWithDepsandNewServerToolWithContextHandler. - Add unit tests covering invalid-argument behavior for both constructors plus a valid-argument success path.
Show a summary per file
| File | Description |
|---|---|
| pkg/inventory/server_tool.go | Converts argument-unmarshal failures from returned Go errors into MCP-visible CallToolResult errors (IsError: true). |
| pkg/inventory/server_tool_test.go | Adds tests ensuring invalid arguments yield IsError: true (and handlers are not invoked), plus a success-case test. |
Copilot's findings
- Files reviewed: 5/5 changed files
- Comments generated: 0
When tool argument unmarshalling fails (wrong types, malformed JSON), return a CallToolResult with IsError: true instead of a Go error. Returning a Go error is converted by the SDK into a JSON-RPC protocol error (-32603), which is invisible to agents and prevents self-correction. Returning IsError: true with the validation message lets agents see the problem and retry with corrected arguments. Affects: - NewServerToolWithDeps (was NewServerTool prior to the rename in #2510) - NewServerToolWithContextHandler Fixes #1952. Re-applies #2488 by @blackwell-systems on top of the NewServerTool rename. Co-authored-by: blackwell-systems <236632453+blackwell-systems@users.noreply.github.com> Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
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May 20, 2026 08:14
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Re-applies #2488 by @blackwell-systems on top of the
NewServerToolrename in #2510.What
When tool argument unmarshalling fails (wrong types, malformed JSON), the constructors in
pkg/inventory/server_tool.gopreviously returned(nil, err). The Go SDK converts a returned error into a JSON-RPC protocol error (-32603), which is invisible to agents and prevents them from self-correcting.This change returns a
CallToolResultwithIsError: trueand the validation message instead, so the agent sees the problem and can retry with corrected arguments.Affects:
NewServerToolWithDeps(wasNewServerToolprior to the rename in refactor: simplify NewServerTool naming #2510)NewServerToolWithContextHandlerTests
Adds
pkg/inventory/server_tool_test.gocovering both constructors (invalid args →IsError: true) plus a success path.Stacking & credit
NewServerTool→NewServerToolWithDeps). Targets that branch as base.Co-authored-byon the commit.Fixes #1952.
Closes #2488 (superseded).