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aggregate_records should default fields to * #3529

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aggregate_records should default fields to *
souvikghosh04 May 7, 2026
68066b1
Copilot review fixes
souvikghosh04 May 7, 2026
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42 changes: 36 additions & 6 deletions src/Azure.DataApiBuilder.Mcp/BuiltInTools/AggregateRecordsTool.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -78,7 +78,8 @@ public class AggregateRecordsTool : IMcpTool
},
""field"": {
""type"": ""string"",
""description"": ""Field name to aggregate, or * with count to count all rows.""
""description"": ""Field name to aggregate, or * with count to count all rows. Defaults to '*' when omitted (only valid with function 'count')."",
""default"": ""*""
},
""distinct"": {
""type"": ""boolean"",
Expand Down Expand Up @@ -128,7 +129,7 @@ public class AggregateRecordsTool : IMcpTool
""description"": ""Opaque cursor from a previous endCursor for next-page retrieval. Requires groupby and first. Do not construct manually.""
}
},
""required"": [""entity"", ""function"", ""field""]
""required"": [""entity"", ""function""]
}"
)
};
Expand Down Expand Up @@ -367,13 +368,42 @@ public async Task<CallToolResult> ExecuteAsync(
$"Invalid function '{function}'. Must be one of: count, avg, sum, min, max.", logger);
}

// Parse field
if (!root.TryGetProperty("field", out JsonElement fieldElement) || string.IsNullOrWhiteSpace(fieldElement.GetString()))
// Parse field. When omitted, default to '*' (only valid with function 'count').
// For non-count functions, an explicit numeric field name is required.
string field;
if (root.TryGetProperty("field", out JsonElement fieldElement))
{
return McpResponseBuilder.BuildErrorResult(toolName, "InvalidArguments", "Missing required argument 'field'.", logger);
if (fieldElement.ValueKind != JsonValueKind.String)
{
return McpResponseBuilder.BuildErrorResult(toolName, "InvalidArguments",
$"Argument 'field' must be a string. Got: '{fieldElement.ValueKind}'.", logger);
}

if (!string.IsNullOrWhiteSpace(fieldElement.GetString()))
{
field = fieldElement.GetString()!;
}
else if (function == "count")
{
field = "*";
}
else
{
return McpResponseBuilder.BuildErrorResult(toolName, "InvalidArguments",
$"Missing required argument 'field'. The default value '*' is only valid with function 'count'; for function '{function}', provide a specific numeric field name.", logger);
}
}
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else
{
if (function != "count")
{
return McpResponseBuilder.BuildErrorResult(toolName, "InvalidArguments",
$"Missing required argument 'field'. The default value '*' is only valid with function 'count'; for function '{function}', provide a specific numeric field name.", logger);
}

field = "*";
}

string field = fieldElement.GetString()!;
bool isCountStar = function == "count" && field == "*";

if (field == "*" && function != "count")
Expand Down
42 changes: 40 additions & 2 deletions src/Service.Tests/Mcp/AggregateRecordsToolTests.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -62,7 +62,15 @@ public void GetToolMetadata_HasRequiredSchemaProperties()

CollectionAssert.Contains(requiredFields, "entity");
CollectionAssert.Contains(requiredFields, "function");
CollectionAssert.Contains(requiredFields, "field");
CollectionAssert.DoesNotContain(requiredFields, "field",
"'field' must not be required; it defaults to '*'.");

// Verify the field property declares '*' as its default value.
JsonElement fieldProp = properties.GetProperty("field");
Assert.IsTrue(fieldProp.TryGetProperty("default", out JsonElement fieldDefault),
"'field' property must declare a default value.");
Assert.AreEqual("*", fieldDefault.GetString(),
"'field' default must be '*'.");

// Verify all schema properties exist with correct types
AssertSchemaProperty(properties, "entity", "string");
Expand Down Expand Up @@ -119,7 +127,8 @@ public async Task AggregateRecords_Disabled_ReturnsToolDisabledError(bool runtim
[DataTestMethod]
[DataRow("{\"function\": \"count\", \"field\": \"*\"}", null, DisplayName = "Missing entity")]
[DataRow("{\"entity\": \"Book\", \"field\": \"*\"}", null, DisplayName = "Missing function")]
[DataRow("{\"entity\": \"Book\", \"function\": \"count\"}", null, DisplayName = "Missing field")]
[DataRow("{\"entity\": \"Book\", \"function\": \"avg\"}", "field", DisplayName = "Missing field for non-count function")]
[DataRow("{\"entity\": \"Book\", \"function\": \"sum\"}", "field", DisplayName = "Missing field for sum function")]
[DataRow("{\"entity\": \"Book\", \"function\": \"median\", \"field\": \"price\"}", "median", DisplayName = "Invalid function 'median'")]
public async Task AggregateRecords_MissingOrInvalidRequiredArgs_ReturnsInvalidArguments(string json, string expectedInMessage)
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{
Expand Down Expand Up @@ -155,6 +164,12 @@ public async Task AggregateRecords_NullArguments_ReturnsInvalidArguments()
DisplayName = "Star field with avg (must mention count)")]
[DataRow("{\"entity\": \"Book\", \"function\": \"count\", \"field\": \"*\", \"distinct\": true}", "DISTINCT",
DisplayName = "Distinct with count(*)")]
[DataRow("{\"entity\": \"Book\", \"function\": \"count\", \"distinct\": true}", "DISTINCT",
DisplayName = "Distinct with implicit count(*) default (issue #3280)")]
[DataRow("{\"entity\": \"Book\", \"function\": \"avg\"}", "field",
DisplayName = "Non-count function without field returns InvalidArguments (issue #3280)")]
[DataRow("{\"entity\": \"Book\", \"function\": \"count\", \"field\": 123}", "field",
DisplayName = "Non-string field value returns InvalidArguments")]
public async Task AggregateRecords_InvalidFieldFunctionCombination_ReturnsInvalidArguments(string json, string expectedInMessage)
{
IServiceProvider sp = CreateDefaultServiceProvider();
Expand Down Expand Up @@ -246,6 +261,29 @@ public async Task AggregateRecords_SimpleCountStar_PassesValidation()
"Simple count(*) must pass input validation.");
}

/// <summary>
/// Verifies that count without an explicit field passes validation by defaulting to '*' (issue #3280).
/// </summary>
[TestMethod]
public async Task AggregateRecords_CountWithoutField_PassesValidation()
{
IServiceProvider sp = CreateDefaultServiceProvider();

CallToolResult result = await ExecuteToolAsync(sp,
"{\"entity\": \"Book\", \"function\": \"count\"}");

// May fail at metadata resolution (no real DB), but must NOT fail with InvalidArguments.
if (result.IsError != true)
{
return;
}

JsonElement content = ParseContent(result);
string errorType = content.GetProperty("error").GetProperty("type").GetString()!;
Assert.AreNotEqual("InvalidArguments", errorType,
"count without field must pass input validation by defaulting to '*' (issue #3280).");
}

/// <summary>
/// Verifies that the orderby schema property has no default value (fix for #3279).
/// </summary>
Expand Down
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