`textDocument/signatureHelp`'s response sometimes has `"activeParameter": -1` breaking LSP spec

[igor-ramazanov]

Describe the bug

Sometimes metals returns "activeParameter": -1 for the textDocument/signatureHelp request which breaks some clients1, since it's supposed2 to be a uinteger3.

kakoune-lsp debug log:

LSP: DEBG To server metals: {
  "id": 2277,
  "jsonrpc": "2.0",
  "method": "textDocument/signatureHelp",
  "params": {
    "position": {
      "character": 8,
      "line": 22
    },
    "textDocument": {
      "uri": "file:///<REDACTED>.scala"
    }
  }
}
...
LSP: DEBG From server metals: {
  "id": 2277,
  "jsonrpc": "2.0",
  "result": {
    "activeParameter": -1,
    "activeSignature": 0,
    "signatures": []
  }
}
LSP: ERRO kak-lsp crashed; disabling hooks for this buffer.
Please report an issue or send this crash report.
See the *debug* buffer for more info.

panicked at src/context.rs:257:61:
Failed to parse response: Error("invalid value: integer `-1`, expected u32", line: 0, column: 0)
disabled backtrace

• Scala: 3.7.4
• SBT: 1.12.0
• Scalac flags: -scalajs -Xsemanticdb -semanticdb-target <REDACTED>/target/scala-3.7.4/meta -Wall -Werror -deprecation -feature -no-indent -old-syntax
• JVM:

  $ java -version
  openjdk version "25" 2025-09-16
  OpenJDK Runtime Environment GraalVM CE 25+37.1 (build 25+37-jvmci-b01)
  OpenJDK 64-Bit Server VM GraalVM CE 25+37.1 (build 25+37-jvmci-b01, mixed mode, sharing)

• Metals config:

  {
    "autoImportBuild": "all",
    "bloopSbtAlreadyInstalled": false,
    "compilerOptions": {
      "overrideDefFormat": "unicode"
    },
    "defaultBspToBuildTool": false,
    "enableBestEffort": true,
    "enableIndentOnPaste": true,
    "enableSemanticHighlighting": false,
    "icons": "unicode",
    "isExitOnShutdown": false,
    "isHttpEnabled": true,
    "sbtScript": "/run/current-system/sw/bin/sbt",
    "statusBarProvider": "log-message",
    "superMethodLensesEnabled": true,
    "testUserInterface": "Code Lenses",
    "inlayHints": {
      "byNameParameters": {
        "enable": true
      },
      "closingLabels": {
        "enable": true
      },
      "hintsInPatternMatch": {
        "enable": true
      },
      "hintsXRayMode": {
        "enable": true
      },
      "implicitArguments": {
        "enable": true
      },
      "implicitConversions": {
        "enable": true
      },
      "inferredTypes": {
        "enable": true
      },
      "namedParameters": {
        "enable": true
      },
      "typeParameters": {
        "enable": true
      }
    }
  }

• Linux:

  $ uname --all
  Linux rocksteady 6.18.0 #1-NixOS SMP PREEMPT_DYNAMIC Sun Nov 30 22:42:10 UTC 2025 x86_64 GNU/Linux

• ScalaJS: 1.20.1

Unfortunately, I don't have a repro, but it consistently happens when:

object Foo {
  def bar(params: Int): Unit = ???
                  ^^^ - whole `Int` is selected
}

object Foo {
  def bar(params: [Int]): Unit = ???
                  ^^^^^ - surrounded with square brackets
}


object Foo {
  def bar(params: O[Int]): Unit = ???
                  ^ - start typing `Option`, and it crashes on `O`
}

Expected behavior

"activeParameter" should never be -1 as specified in the LSP specification.

Operating system

Linux

Editor/Extension

Other

Version of Metals

v1.6.4

Scala version/s

3.7.4

Extra context or search terms

No response

[tgodzik]

Thanks for reporting! This might happen in some cases, where the compiler is unable to recover from a parse or typer error. We should set 0 instead probably.

[tgodzik]

And for anyone looking at this, it would need to also be fixed for scala 3 in https://github.com/scala/scala3/blob/main/presentation-compiler/src/main/dotty/tools/pc/SignatureHelpProvider.scala

[igor-ramazanov]

Likely the offending line: https://github.com/scala/scala3/blob/f856285e0bf060a2dff8be372abf062703476e9b/compiler/src/dotty/tools/dotc/util/Signatures.scala#L252

[igor-ramazanov]

Poor man's temporary dirty fix:

diff --git a/metals/src/main/scala/scala/meta/internal/metals/Compilers.scala b/metals/src/main/scala/scala/meta/internal/metals/Compilers.scala
index 1f744c8f078..19a22add725 100644
--- a/metals/src/main/scala/scala/meta/internal/metals/Compilers.scala
+++ b/metals/src/main/scala/scala/meta/internal/metals/Compilers.scala
@@ -1174,6 +1174,12 @@ class Compilers(
         )
       ).asScala
     }.getOrElse(Future.successful(new SignatureHelp()))
+      .map {
+        case signatureHelp if signatureHelp.getActiveParameter == -1 =>
+          signatureHelp.setActiveParameter(0)
+          signatureHelp
+        case signatureHelp => signatureHelp
+      }
 
   def signatureHelp(
       id: BuildTargetIdentifier,
@@ -1184,6 +1190,12 @@ class Compilers(
         pc.signatureHelp(offsetParams).asScala
       }
       .getOrElse(Future.successful(new SignatureHelp()))
+      .map {
+        case signatureHelp if signatureHelp.getActiveParameter == -1 =>
+          signatureHelp.setActiveParameter(0)
+          signatureHelp
+        case signatureHelp => signatureHelp
+      }
 
   def selectionRange(
       params: SelectionRangeParams,

1. Clone this repo
2. Apply the above git patch
3. Do sbt +publishLocal
4. Do coursier bootstrap --java-opt -Dmetals.client=kakoune-lsp org.scalameta:metals_2.13:1.6.5-SNAPSHOT --output metals-kakoune-lsp --force --standalone
5. Update the kakoune-lsp config by setting the command = <a full path to the metal-kakoune-lsp binary>