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Fix McpHeaderEncoder.DecodeValue throwing on the degenerate base64 wrapper #1805

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Fix McpHeaderEncoder.DecodeValue throwing on the degenerate base64 wrapper #1805

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3 changes: 2 additions & 1 deletion src/ModelContextProtocol.Core/Protocol/McpHeaderEncoder.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -126,7 +126,8 @@ public static class McpHeaderEncoder

// Check for Base64 wrapper. The spec requires the sentinel markers to be
// case-sensitive and exactly lowercase per SEP-2243.
if (headerValue.StartsWith(Base64Prefix, StringComparison.Ordinal) &&
if (headerValue.Length >= Base64Prefix.Length + Base64Suffix.Length &&
headerValue.StartsWith(Base64Prefix, StringComparison.Ordinal) &&
headerValue.EndsWith(Base64Suffix, StringComparison.Ordinal))
{
var base64Content = headerValue.Substring(
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10 changes: 10 additions & 0 deletions tests/ModelContextProtocol.Tests/Client/McpHeaderEncoderTests.cs
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Expand Up @@ -104,6 +104,16 @@ public void DecodeValue_ValidBase64_Decodes()
Assert.Equal("Hello", result);
}

[Fact]
public void DecodeValue_DegenerateWrapper_ReturnsLiteralValue()
{
// "=?base64?=" matches both the prefix "=?base64?" and the suffix "?=" because they
// overlap on the shared '?', but it is too short to contain any base64 content. It must be
// returned as-is rather than throwing when the wrapper is stripped.
var result = McpHeaderEncoder.DecodeValue("=?base64?=");
Assert.Equal("=?base64?=", result);
}

[Fact]
public void DecodeValue_CaseSensitivePrefix_ReturnsLiteralValue()
{
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