mark3labs
bug: panic, nil pointer dereference in Stdio readResponses
Description
I need some help with a panic issue when using the stdio MCP client. I encountered the following panic
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x10 pc=0x5c27bd]
goroutine 27837 [running]:
bufio.(*Reader).ReadSlice(0x0, 0xa)
/usr/local/go/src/bufio/bufio.go:355 +0x3d
bufio.(*Reader).collectFragments(0x0, 0xa)
/usr/local/go/src/bufio/bufio.go:455 +0x70
bufio.(*Reader).ReadString(0x0?, 0x0?)
/usr/local/go/src/bufio/bufio.go:502 +0x1f
github.com/mark3labs/mcp-go/client/transport.(*Stdio).readResponses(0xc000f40240)
/Users/xanarry/go/pkg/mod/github.com/mark3labs/[email protected]/client/transport/stdio.go:170 +0x53
github.com/mark3labs/mcp-go/client/transport.(*Stdio).Start.func1()
/Users/xanarry/go/pkg/mod/github.com/mark3labs/[email protected]/client/transport/stdio.go:79 +0x2f
created by github.com/mark3labs/mcp-go/client/transport.(*Stdio).Start in goroutine 27955
/Users/xanarry/go/pkg/mod/github.com/mark3labs/[email protected]/client/transport/stdio.go:77 +0x8c
my mcp-go version is v0.29.0
I have checked my own code and the MCP client initialization logic, but I couldn't find any obvious reason for c.stdout to be nil at this point. My guess is that this happens when the MCP server subprocess exits unexpectedly, and the client tries to read from a closed or nil pipe. Should there be a panic recovery or more defensive check in the readResponses goroutine to avoid crashing the whole process? Or is there any recommended way to handle this scenario gracefully? Any advice or fix would be appreciated. Thank you!
I have tried to kill stdio mcp server process all the time, but I can not trigger this pannic, anybody help
@xanarry There has been some breaking changes recently but some bugs were also resolved which could be the issue. Could you try the latest release of the SDK? It should not have the issue.
@pottekkat thanks, I have updated it from v0.29.0 to v0.31.0, and continue to observe whether the problem will recur.
@pottekkat hello. pottekkat, I have updated sdk to v0.31.0, but it still quit without any warning signs today, the same panic error
I met the same problem.
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x685bf0]
goroutine 14 [running]:
github.com/mark3labs/mcp-go/server.(*MCPServer).HandleMessage(0xc0001bc000, {0x81b448?, 0xc000194460?}, {0xc00001e300, 0x74, 0x80})
github.com/mark3labs/[email protected]/server/request_handler.go:279 +0x17d0
github.com/mark3labs/mcp-go/server.(*SSEServer).handleMessage.func1({0x81b448?, 0xc000194460?})
github.com/mark3labs/[email protected]/server/sse.go:574 +0x89
created by github.com/mark3labs/mcp-go/server.(*SSEServer).handleMessage in goroutine 18
github.com/mark3labs/[email protected]/server/sse.go:570 +0x47c
My code:
sse := server.NewSSEServer(s)
sse.Start(":9090")
My mcp-go version:
github.com/mark3labs/mcp-go v0.43.0
Please let me know when this issue is fixed.
I met the same problem.
panic: runtime error: invalid memory address or nil pointer dereference [signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x685bf0] goroutine 14 [running]: github.com/mark3labs/mcp-go/server.(*MCPServer).HandleMessage(0xc0001bc000, {0x81b448?, 0xc000194460?}, {0xc00001e300, 0x74, 0x80}) github.com/mark3labs/[email protected]/server/request_handler.go:279 +0x17d0 github.com/mark3labs/mcp-go/server.(*SSEServer).handleMessage.func1({0x81b448?, 0xc000194460?}) github.com/mark3labs/[email protected]/server/sse.go:574 +0x89 created by github.com/mark3labs/mcp-go/server.(*SSEServer).handleMessage in goroutine 18 github.com/mark3labs/[email protected]/server/sse.go:570 +0x47cMy code:
sse := server.NewSSEServer(s) sse.Start(":9090")My mcp-go version:
github.com/mark3labs/mcp-go v0.43.0Please let me know when this issue is fixed.
To my disappointment, server.WithRecovery() didn't work because the program crashed and exited after the NPE error. If the program hadn't crashed and exited, this wouldn't be considered a bug.
I met the same problem.
panic: runtime error: invalid memory address or nil pointer dereference [signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x685bf0] goroutine 14 [running]: github.com/mark3labs/mcp-go/server.(*MCPServer).HandleMessage(0xc0001bc000, {0x81b448?, 0xc000194460?}, {0xc00001e300, 0x74, 0x80}) github.com/mark3labs/[email protected]/server/request_handler.go:279 +0x17d0 github.com/mark3labs/mcp-go/server.(*SSEServer).handleMessage.func1({0x81b448?, 0xc000194460?}) github.com/mark3labs/[email protected]/server/sse.go:574 +0x89 created by github.com/mark3labs/mcp-go/server.(*SSEServer).handleMessage in goroutine 18 github.com/mark3labs/[email protected]/server/sse.go:570 +0x47cMy code:
sse := server.NewSSEServer(s) sse.Start(":9090")My mcp-go version:
github.com/mark3labs/mcp-go v0.43.0Please let me know when this issue is fixed.To my disappointment,
server.WithRecovery()didn't work because the program crashed and exited after the NPE error. If the program hadn't crashed and exited, this wouldn't be considered a bug.
@pottekkat hello. pottekkat,The issue lies in the instantiation of this result - it's a nil value, and directly dereferencing it with * in the code caused this problem.
In github.com/mark3labs/[email protected]/server/request_handler.go:279
I met the same problem.
panic: runtime error: invalid memory address or nil pointer dereference [signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x685bf0] goroutine 14 [running]: github.com/mark3labs/mcp-go/server.(*MCPServer).HandleMessage(0xc0001bc000, {0x81b448?, 0xc000194460?}, {0xc00001e300, 0x74, 0x80}) github.com/mark3labs/[email protected]/server/request_handler.go:279 +0x17d0 github.com/mark3labs/mcp-go/server.(*SSEServer).handleMessage.func1({0x81b448?, 0xc000194460?}) github.com/mark3labs/[email protected]/server/sse.go:574 +0x89 created by github.com/mark3labs/mcp-go/server.(*SSEServer).handleMessage in goroutine 18 github.com/mark3labs/[email protected]/server/sse.go:570 +0x47cMy code:
sse := server.NewSSEServer(s) sse.Start(":9090")My mcp-go version:
github.com/mark3labs/mcp-go v0.43.0Please let me know when this issue is fixed.To my disappointment,
server.WithRecovery()didn't work because the program crashed and exited after the NPE error. If the program hadn't crashed and exited, this wouldn't be considered a bug.@pottekkat hello. pottekkat,The issue lies in the instantiation of this result - it's a nil value, and directly dereferencing it with * in the code caused this problem.
In
github.com/mark3labs/[email protected]/server/request_handler.go:279
Yes, I've encountered the same issue. Do you have any solutions in mind? Mcp-go is quite a useful framework, but its stability still needs improvement.
According to my tests, this issue still exists in the latest version.
