Nvim-dap crashes on launch

[alt-romes]

The problem seems to be nvim-dap is confused about receiving a reverse-request for "runInTerminal" before receiving a response for "launch"

there are 3 types of messages: requests/responses/events

every request must have a response. events just happen

however, there are a few "special requests" because they go from the server to the client (the server requests something of the client). In this case, it requests to run something in the terminal.

But nvim-dap crashes with [ERROR] 2026-05-12 10:41:58 dap/session.lua:1090 "No callback found. Did the debug adapter send duplicate responses?" before we even send the launch response

This could be fixed in nvim-dap or in hdb. though we'd have to understand at what point does nvim-dap start allowing reverse-requests to do a workaround.

This likely regressed in the runInTerminal refactor, which made the runInTerminal reverse request get sent before se reply with the launch response. It may be easy to get back to sending the launch response eagerly even while the program is still loading.

[alt-romes]

Here's the server log and nvim-dap log:

$ hdb server --port 12345
Running DAP server on 12345...
TCP connection established from 127.0.0.1:60267
[127.0.0.1:60267][DEBUG][RECEIVED]
{
    "arguments": {
        "adapterID": "nvim-dap",
        "clientID": "neovim",
        "clientName": "neovim",
        "columnsStartAt1": true,
        "linesStartAt1": true,
        "locale": "en_US.UTF-8",
        "pathFormat": "path",
        "supportsProgressReporting": true,
        "supportsRunInTerminalRequest": true,
        "supportsStartDebuggingRequest": true,
        "supportsVariableType": true
    },
    "command": "initialize",
    "seq": 1,
    "type": "request"
}
[127.0.0.1:60267][DEBUG][SENT]
{
    "body": {
        "additionalModuleColumns": [
            {
                "attributeName": "Extra",
                "label": "Label"
            }
        ],
        "completionTriggerCharacters": [],
        "exceptionBreakpointFilters": [
            {
                "filter": "break-on-exception",
                "label": "All exceptions"
            },
            {
                "filter": "break-on-error",
                "label": "Uncaught exceptions"
            }
        ],
        "supportSuspendDebuggee": false,
        "supportTerminateDebuggee": false,
        "supportedChecksumAlgorithms": [],
        "supportsBreakpointLocationsRequest": true,
        "supportsCancelRequest": false,
        "supportsClipboardContext": false,
        "supportsCompletionsRequest": false,
        "supportsConditionalBreakpoints": true,
        "supportsConfigurationDoneRequest": true,
        "supportsDataBreakpoints": false,
        "supportsDelayedStackTraceLoading": false,
        "supportsDisassembleRequest": false,
        "supportsEvaluateForHovers": false,
        "supportsExceptionFilterOptions": false,
        "supportsExceptionInfoRequest": true,
        "supportsExceptionOptions": true,
        "supportsFunctionBreakpoints": true,
        "supportsGotoTargetsRequest": false,
        "supportsHitConditionalBreakpoints": true,
        "supportsInstructionBreakpoints": false,
        "supportsLoadedSourcesRequest": false,
        "supportsLogPoints": true,
        "supportsModulesRequest": false,
        "supportsReadMemoryRequest": false,
        "supportsRestartFrame": false,
        "supportsRestartRequest": false,
        "supportsSetExpression": false,
        "supportsSetVariable": false,
        "supportsSingleThreadExecutionRequests": false,
        "supportsStepBack": false,
        "supportsStepInTargetsRequest": false,
        "supportsSteppingGranularity": false,
        "supportsTerminateRequest": true,
        "supportsTerminateThreadsRequest": false,
        "supportsValueFormattingOptions": true,
        "supportsWriteMemoryRequest": false
    },
    "command": "initialize",
    "request_seq": 1,
    "seq": 2,
    "success": true,
    "type": "response"
}
[127.0.0.1:60267][DEBUG][RECEIVED]
{
    "arguments": {
        "debugServer": 12345,
        "entryArgs": [],
        "entryFile": "Main.hs",
        "entryPoint": "main",
        "extraGhcArgs": [],
        "name": "Haskell Debugger",
        "projectRoot": "/private/var/folders/_j/l1f15qm555z4qzvjjg6ftp_c0000gn/T/tmp.Mhob87dla7",
        "request": "launch",
        "type": "haskell-debugger"
    },
    "command": "launch",
    "seq": 2,
    "type": "request"
}
[2026-05-12T10:41:58.123670Z] [INFO] LogSetupMsg "Figuring out the right flags to compile the project using hie-bios..."
[2026-05-12T10:41:58.127569Z] [INFO] LogCradle Cradle{ cradleRootDir = "/private/var/folders/_j/l1f15qm555z4qzvjjg6ftp_c0000gn/T/tmp.Mhob87dla7", cradleOptsProg = CradleAction: Default}
[2026-05-12T10:41:58.127658Z] [INFO] LogSetupMsg "Checking GHC version against debugger version..."
...
[2026-05-12T10:41:58.223034Z] [INFO] LogSetupMsg "Session setup with hie-bios was successful."
[127.0.0.1:60267][DEBUG][SENT]
{
    "body": {
        "category": "console",
        "output": "libdir: /nix/store/zxnh5wx0w6n6rlj44r3kakkv3rx5fr3v-ghc-9.14.1/lib/ghc-9.14.1/lib\nunits: \nargs: -dynamic -fno-it\n"
    },
    "event": "output",
    "type": "event"
}
[127.0.0.1:60267][DEBUG][SENT]
{
    "body": {
Trying to connect to 60268
        "output": "Figuring out the right flags to compile the project using hie-bios...\n"
    },
    "event": "output",
    "type": "event"
}
[127.0.0.1:60267][INFO][Registered new debug session: debug-session:b7e659c5-9405-4c7f-b661-9c3bc08eab62]
[127.0.0.1:60267][DEBUG][SENT]
{
    "body": {
        "output": "Using cradle: Cradle{ cradleRootDir = \"/private/var/folders/_j/l1f15qm555z4qzvjjg6ftp_c0000gn/T/tmp.Mhob87dla7\", cradleOptsProg = CradleAction: Default}\n"
    },
    "event": "output",
    "type": "event"
}
[127.0.0.1:60267][DEBUG][SENT]
{
    "arguments": {
        "args": [
            "/Users/romes/.local/state/cabal/store/ghc-9.14.1-7f12/hskll-dbggr-0.13.1.0-50f13547/bin/hdb",
            "external-interpreter",
            "--port",
            "60268"
        ],
        "argsCanBeInterpretedByShell": false,
        "cwd": "",
        "kind": "integrated"
    },
    "command": "runInTerminal",
    "request_seq": 2,
    "seq": 3,
    "type": "request"
}
[127.0.0.1:60267][DEBUG][SENT]
{
    "body": {
        "output": "Checking GHC version against debugger version...\n"
    },
    "event": "output",
    "type": "event"
}
[127.0.0.1:60267][DEBUG][SENT]
{
    "command": "launch",
    "request_seq": 2,
    "seq": 3,
    "success": true,
    "type": "response"
}
[127.0.0.1:60267][DEBUG][SENT]
{
    "body": {
        "output": "Discovering session flags with hie-bios...\n"
    },
    "event": "output",
    "type": "event"
}
[127.0.0.1:60267][DEBUG][SENT]
{
    "event": "initialized",
    "type": "event"
}
[127.0.0.1:60267][DEBUG][SENT]
{
    "body": {
        "output": "invoking build tool to determine build flags (this may take some time depending on the cache)\n"
    },
    "event": "output",
    "type": "event"
}
[2026-05-12T10:41:58.226333Z] [INFO] Debugger launched successfully.
[127.0.0.1:60267][DEBUG][SENT]
{
    "body": {
        "output": "Session setup with hie-bios was successful.\n"
    },
    "event": "output",
    "type": "event"
}
[127.0.0.1:60267][INFO][SENT][Client has ended its connection]
[127.0.0.1:60267][INFO][SENT][Closing Connection]

Nvim:

[ERROR] 2026-05-12 10:41:58 dap/session.lua:1090	"No callback found. Did the debug adapter send duplicate responses?"	{
  command = "launch",
  request_seq = 2,
  seq = 3,
  success = true,
  type = "response"
}

[alt-romes]

Sending the launch ack right after receiving the launch request might be the right way to do this. Because what really signals readiness is the initialized event.

[alt-romes]

diff --git a/hdb-dap/Development/Debug/Adapter/Server.hs b/hdb-dap/Development/Debug/Adapter/Server.hs
index 5260c01..b985a8a 100644
--- a/hdb-dap/Development/Debug/Adapter/Server.hs
+++ b/hdb-dap/Development/Debug/Adapter/Server.hs
@@ -139,11 +139,12 @@ talk l servConf prefer_internal_interpreter = \ case
     let runInTerminal = fromMaybe False $ supportsRunInTerminalRequest =<< clientCaps
 #endif
 
+    sendLaunchResponse   -- ack
+
     initDebugger (contramap DAPSessionLog l) servConf
       runInTerminal prefer_internal_interpreter
       launch_args
 
-    sendLaunchResponse   -- ack
     sendInitializedEvent -- our debugger is only ready to be configured after it has launched the session
 
     liftLogIO l <& DAPLaunchLog (WithSeverity (T.pack "Debugger launched successfully.") Info)

[alt-romes]

Ah! After taking a look at nvim's source I've realized the bug is actually that our Reverse request does not conform to the protocol:

[127.0.0.1:60267][DEBUG][SENT]
{
    "arguments": {
        "args": [
            "/Users/romes/.local/state/cabal/store/ghc-9.14.1-7f12/hskll-dbggr-0.13.1.0-50f13547/bin/hdb",
            "external-interpreter",
            "--port",
            "60268"
        ],
        "argsCanBeInterpretedByShell": false,
        "cwd": "",
        "kind": "integrated"
    },
    "command": "runInTerminal",
    "request_seq": 2,
    "seq": 3,
    "type": "request"
}

nvim-dap uses the request_seq field as a proxy to determine if something is a response. Thus, it considers our reverse request to be a response!

The root fix is to make our reverse request not have a request_seq field. It's probably picking that up from the context because we leverage the existing request/response mechanisms in the dap library.

[alt-romes]

I've opened haskell-debugger/dap#34

[alt-romes]

Fixed by haskell-debugger/dap#35

[alt-romes]

This ticket can be closed once dap >= 0.6.1 is released and we bump the min version of dap to 0.6.1