From f99cdb649b0c61b07341bc31ecbf94b8f960bbef Mon Sep 17 00:00:00 2001 From: akashjainn <80935005+akashjainn@users.noreply.github.com> Date: Mon, 7 Sep 2026 04:16:16 -0400 Subject: [PATCH] mcp: base64-encode a non-header-safe Mcp-Name (#1246) MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Fixes: https://github.com/modelcontextprotocol/go-sdk/issues/1243 The 2026-07-28 transport spec only SHOULD-constrains names to header-safe characters, and lets a client Base64-wrap one that is not, using the `=?base64?...?=` sentinel. `setStandardHeaders` set `Mcp-Name` raw, while the sibling `Mcp-Param-*` path already ran its values through `encodeHeaderValue`, so a name outside the safe set went out unencoded. This routes `Mcp-Name` through `encodeHeaderValue` as well. #1242 landed the other direction — the server decoding the sentinel before comparing — so the two together close the round trip. Worth noting they interact: after #1242, a name that is literally shaped like the sentinel (`=?base64?abc?=`) is now rejected as invalid Base64, because the server tries to decode a value the client never encoded. The new test covers that case, and it fails on `main` today. `extractName` also covers `resources/read`, so this applies to resource URIs, which is where non-ASCII is most likely to show up in practice. ## Testing `TestMcpNameHeaderEncodingRoundTrip` drives `setStandardHeaders` and then feeds its output to `validateMcpHeaders`, so it pins the client and server halves against each other rather than asserting the encoding in isolation. Cases: an ASCII-safe name (must stay unencoded, no churn for the common path), non-ASCII, a leading space, a control character, and the sentinel-shaped literal above. Verified failing before the change and passing after. `go test ./...` passes. A note on process: [@remote-controlled-man](https://github.com/remote-controlled-man) asked about this issue earlier today and proposed much the same scope. I had the change written before I saw that comment and thought the #1242 interaction was worth surfacing either way, but I'm happy to close this if they'd rather carry it — please just say so. 🤖 Generated with [Claude Code](https://claude.com/claude-code) https://claude.ai/code/session_011Wf643Lx36V5wdwuuf2EdT --- mcp/streamable_headers.go | 7 ++++- mcp/streamable_headers_test.go | 54 ++++++++++++++++++++++++++++++++++ 2 files changed, 60 insertions(+), 1 deletion(-) diff --git a/mcp/streamable_headers.go b/mcp/streamable_headers.go index 77f2415..c2b8dfd 100644 --- a/mcp/streamable_headers.go +++ b/mcp/streamable_headers.go @@ -207,7 +207,12 @@ func setStandardHeaders(ctx context.Context, header http.Header, msg jsonrpc.Mes case *jsonrpc.Request: header.Set(methodHeader, msg.Method) if name, ok := extractName(msg.Method, msg.Params); ok { - header.Set(nameHeader, name) + // Names are only SHOULD-constrained to header-safe characters, so a + // name outside the safe set is Base64-wrapped with the =?base64?...?= + // sentinel, the same rule applied to Mcp-Param-* values. + if encoded, ok := encodeHeaderValue(name); ok { + header.Set(nameHeader, encoded) + } } if msg.Method == "tools/call" { if tool, ok := ctx.Value(toolContextKey).(*Tool); ok && tool != nil { diff --git a/mcp/streamable_headers_test.go b/mcp/streamable_headers_test.go index f0862f3..97af67c 100644 --- a/mcp/streamable_headers_test.go +++ b/mcp/streamable_headers_test.go @@ -1593,3 +1593,57 @@ func TestValidateParamHeaders_NestedArguments(t *testing.T) { t.Error("validateParamHeaders() = nil, want error for mismatched nested value") } } + +// TestMcpNameHeaderEncodingRoundTrip verifies that a name which is not +// header-safe is Base64-wrapped by the client per the 2026-07-28 transport +// spec, and that the server's validation accepts what the client produced. +func TestMcpNameHeaderEncodingRoundTrip(t *testing.T) { + tests := []struct { + name string + msg *jsonrpc.Request + wantHeader string + }{ + { + name: "ascii-safe tool name is sent as-is", + msg: &jsonrpc.Request{Method: "tools/call", Params: mustMarshal(&CallToolParams{Name: "my-tool"})}, + wantHeader: "my-tool", + }, + { + name: "non-ascii tool name is base64 encoded", + msg: &jsonrpc.Request{Method: "tools/call", Params: mustMarshal(&CallToolParams{Name: "café-tool"})}, + wantHeader: encodeBase64("café-tool"), + }, + { + name: "prompt name with a leading space is base64 encoded", + msg: &jsonrpc.Request{Method: "prompts/get", Params: mustMarshal(&GetPromptParams{Name: " leading-space"})}, + wantHeader: encodeBase64(" leading-space"), + }, + { + name: "resource uri with a control character is base64 encoded", + msg: &jsonrpc.Request{Method: "resources/read", Params: mustMarshal(&ReadResourceParams{URI: "file:///a\tb.txt"})}, + wantHeader: encodeBase64("file:///a\tb.txt"), + }, + { + name: "name that looks like the sentinel is base64 encoded", + msg: &jsonrpc.Request{Method: "tools/call", Params: mustMarshal(&CallToolParams{Name: "=?base64?abc?="})}, + wantHeader: encodeBase64("=?base64?abc?="), + }, + } + + for _, tt := range tests { + t.Run(tt.name, func(t *testing.T) { + header := http.Header{} + header.Set(protocolVersionHeader, minVersionForStandardHeaders) + setStandardHeaders(context.Background(), header, tt.msg) + + if got := header.Get(nameHeader); got != tt.wantHeader { + t.Errorf("Mcp-Name = %q, want %q", got, tt.wantHeader) + } + + // The server must accept the headers the client just generated. + if err := validateMcpHeaders(header, tt.msg, nil); err != nil { + t.Errorf("validateMcpHeaders() on client-generated headers = %v, want nil", err) + } + }) + } +}