Preserve Tool _meta when marshaling to JSON

[jaredly]

Description

Adds the _meta field to marshaled Tool json. Unmarshalling was already handled.

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• MCP spec compatibility implementation
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Code refactoring (no functional changes)
• Performance improvement
• Tests only (no functional changes)
• Other (please describe):

Checklist

• My code follows the code style of this project
• I have performed a self-review of my own code
• I have added tests that prove my fix is effective or that my feature works
• I have updated the documentation accordingly (didn't seem necessary)

MCP Spec Compliance

• This PR implements a feature defined in the MCP specification
• Link to relevant spec section: Link text
• Implementation follows the specification exactly (the current implementation doesn't do validation on _meta keys, and I didn't change that)

Additional Information

Summary by CodeRabbit

• New Features

  • JSON responses for tools now include an optional _meta field when metadata is present, providing additional contextual information while remaining backward compatible when metadata is absent.

• Tests

  • Added tests to ensure the _meta field is included when metadata is provided and omitted when metadata is nil.

[coderabbitai]

Walkthrough

Tool.MarshalJSON now conditionally includes a _meta JSON field when the Tool's Meta is non-nil. Two unit tests were added to verify _meta is present when Meta is set and omitted when Meta is nil.

Changes

Cohort / File(s)	Summary of Changes
Tool JSON marshaling mcp/tools.go	Update Tool.MarshalJSON to include a _meta key in the serialized JSON when Tool.Meta is non-nil.
Tests mcp/tools_test.go	Add TestToolMetaMarshaling and TestToolMetaMarshalingOmitsWhenNil to verify _meta is emitted when present and omitted when Meta is nil.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

Possibly related PRs

• task: add _meta field to relevant types as defined in MCP specification #429 — Updates Tool.MarshalJSON to conditionally emit _meta and/or adds Meta related changes affecting the same serialization logic.
• feat(protocol): allow additional fields in meta #293 — Changes representation/serialization of _meta/Meta in MCP Go, overlapping on JSON marshaling behavior.
• fix: tool annotation #165 — Modifies Tool.MarshalJSON and related annotations, touching the same marshaling implementation.

Suggested labels

type: enhancement, area: mcp spec

Suggested reviewers

• pottekkat
• robert-jackson-glean

Pre-merge checks and finishing touches

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 66.67% which is insufficient. The required threshold is 80.00%.	You can run @coderabbitai generate docstrings to improve docstring coverage.

✅ Passed checks (2 passed)

Check name	Status	Explanation
Title Check	✅ Passed	The title succinctly and accurately describes the primary change—adding preservation of the Tool’s _meta field during JSON marshaling—and uses clear, specific language without extraneous details.
Description Check	✅ Passed	The PR description follows the repository’s template by providing a concise Description, correctly checked Type of Change entries, a completed Checklist, and an MCP Spec Compliance section with a valid spec link; the optional Additional Information section remains empty but does not detract from completeness.

✨ Finishing touches

• 📝 Generate docstrings

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment

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📜 Recent review details

Configuration used: CodeRabbit UI

Review profile: CHILL

Plan: Pro

📥 Commits

Reviewing files that changed from the base of the PR and between e58af76 and cc17cf0.

📒 Files selected for processing (1)

• mcp/tools_test.go (1 hunks)

🚧 Files skipped from review as they are similar to previous changes (1)

• mcp/tools_test.go

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[coderabbitai]

Actionable comments posted: 0

🧹 Nitpick comments (1)

mcp/tools_test.go (1)

1400-1428: Test correctly verifies _meta marshaling. Consider adding a nil case.

The test appropriately validates that a non-nil Meta field is marshaled as _meta and matches the expected value.

To ensure complete coverage of the omitempty behavior, consider adding a test case that verifies _meta is omitted when Meta is nil:

+func TestToolMetaMarshalingOmitsWhenNil(t *testing.T) {
+	// Marshal a tool without Meta
+	data, err := json.Marshal(Tool{
+		Name:        "test-tool-no-meta",
+		Description: "A test tool without meta data",
+		InputSchema: ToolInputSchema{
+			Type:       "object",
+			Properties: map[string]any{},
+		},
+	})
+	assert.NoError(t, err)
+
+	// Unmarshal to map
+	var result map[string]any
+	err = json.Unmarshal(data, &result)
+	assert.NoError(t, err)
+
+	// Check that _meta field is not present
+	assert.NotContains(t, result, "_meta", "Tool without Meta should not include _meta field")
+}

📜 Review details

Configuration used: CodeRabbit UI

Review profile: CHILL

Plan: Pro

📥 Commits

Reviewing files that changed from the base of the PR and between 61b5d9e and e58af76.

📒 Files selected for processing (2)

• mcp/tools.go (1 hunks)
• mcp/tools_test.go (1 hunks)

🧰 Additional context used

🧬 Code graph analysis (2)

mcp/tools_test.go (2)

mcp/tools.go (4)

• Tool (557-574)
• Description (875-879)
• ToolInputSchema (632-632)
• Properties (1117-1121)

mcp/types.go (2)

• Meta (121-133)
• NewMetaFromMap (156-166)

mcp/tools.go (1)

mcp/types.go (1)

• Meta (121-133)

🔇 Additional comments (1)

mcp/tools.go (1)

616-619: LGTM!

The conditional marshaling of _meta when Meta is non-nil is correctly implemented and matches the existing pattern in CallToolResult.MarshalJSON (lines 478-480). This ensures spec compliance without breaking backward compatibility.