handleCallToolVariant: panic recovery returns (nil, nil) crashing mcp-go; triggered by tiktoken cache corruption

[Melodeiro]

Bug Description

call_tool_read, call_tool_write, and call_tool_destructive panic with nil pointer dereference. The client receives an empty HTTP response and the connection is dropped.

Two independent bugs combine to cause the crash:

Bug 1: Corrupted tiktoken cache (trigger)

tiktoken-go v0.1.8 (load.go:31) does http.Get(blobpath) and caches the response body without checking resp.StatusCode. If Azure Blob Storage returns a 404 (or any HTTP error), the XML error response is cached as a valid BPE file:

/tmp/data-gym-cache/9b5ad71b2ce5302211f9c61530b329a4922fc6a4

Content (223 bytes instead of ~1.6MB):

<?xml version="1.0" encoding="utf-8"?>
<Error><Code>ResourceNotFound</Code><Message>The specified resource does not exist.</Message></Error>

This causes tokens.NewTokenizer() to fail. The fallback in runtime.go:145 also calls GetEncoding() which reads the same corrupted cache → returns (nil, error). The error is discarded with _:

tokenizer, _ = tokens.NewTokenizer(tokenizerEncoding, logger.Sugar(), false)

The nil *DefaultTokenizer is then assigned to the tokens.Tokenizer interface field — creating a non-nil interface with nil underlying value (Go spec: an interface holding a nil concrete value is itself non-nil).

Bug 2: Panic recovery returns (nil, nil) (amplifier)

handleCallToolVariant (mcp.go:1012) has a defer with recover(), but uses unnamed return values:

func (p *MCPProxyServer) handleCallToolVariant(...) (*mcp.CallToolResult, error) {
    defer func() {
        if r := recover(); r != nil {
            p.logger.Error("Recovered from panic in handleCallToolVariant", ...)
            // BUG: no return value set — function returns (nil, nil)
        }
    }()

When the tokenizer nil dereference panics at mcp.go:1245:

1. recover() catches the panic and logs it
2. Function returns zero values: (*mcp.CallToolResult)(nil), (error)(nil)
3. mcp-go HandleMessage (request_handler.go:331) does return createResponse(baseMessage.ID, *result) — second panic on nil dereference
4. This second panic is not recovered → HTTP goroutine crashes → client gets empty response

Additional: unsafe type assertion (mcp.go:~1253)

Encoding: tokenizer.(*tokens.DefaultTokenizer).GetDefaultEncoding(),

This is a non-checked type assertion that panics if the underlying value is nil, even when the interface is non-nil.

Reproduction

1. Delete tiktoken cache: rm /tmp/data-gym-cache/9b5ad71b2ce5302211f9c61530b329a4922fc6a4
2. Block the Azure URL (simulate 404) and restart mcpproxy
3. Any call_tool_read/write/destructive call will panic

Or simply corrupt the cache file:

echo "corrupted" > /tmp/data-gym-cache/9b5ad71b2ce5302211f9c61530b329a4922fc6a4

Suggested Fixes

1. Named return values in panic recovery (critical)

func (p *MCPProxyServer) handleCallToolVariant(...) (callResult *mcp.CallToolResult, callErr error) {
    defer func() {
        if r := recover(); r != nil {
            p.logger.Error("Recovered from panic in handleCallToolVariant", ...)
            callResult = mcp.NewToolResultError(fmt.Sprintf("Internal proxy error: %v", r))
            callErr = nil
        }
    }()

2. Safe tokenizer nil check before use

isTokenizerValid := false
if tokenizer != nil {
    if dt, ok := tokenizer.(*tokens.DefaultTokenizer); ok && dt != nil {
        isTokenizerValid = true
    }
}
if isTokenizerValid {
    // ... count tokens
}

3. Upstream: tiktoken-go should check HTTP status before caching

load.go:31 should verify resp.StatusCode == 200 before writing to cache.

Versions Tested

All versions from v0.9.0+ (when tokenizer was added in commit 4ad01d4) are affected:

• v0.14.0 (mcp-go@v0.43.1) — same panic
• v0.17.8, v0.18.1, v0.19.1 (mcp-go@v0.44.0-beta.2) — same panic
• v0.8.6 and below — not affected (no tokenizer code)

Workaround

Delete the corrupted tiktoken cache file and restart mcpproxy:

rm /tmp/data-gym-cache/9b5ad71b2ce5302211f9c61530b329a4922fc6a4
systemctl --user restart mcpproxy

Environment

• OS: Linux amd64 (Ubuntu)
• mcpproxy: v0.19.1
• mcp-go: v0.44.0-beta.2
• tiktoken-go: v0.1.8
• Go: 1.25.7