x/tools/gopls: MCP: nil panic in summarizeDiagnostics (CodeAction without WorkspaceEdit)

[adonovan]

#!stacks
"sigpanic" && "mcp.summarizeDiagnostics:+6"

Issue created by stacks.

func summarizeDiagnostics(ctx context.Context, snapshot *cache.Snapshot, w io.Writer, diagnostics []*cache.Diagnostic, fixes map[*cache.Diagnostic]*protocol.CodeAction) error {
	for _, d := range diagnostics {
...		fix, ok := fixes[d]
		if ok {
			diff, err := toUnifiedDiff(ctx, snapshot, fix.Edit.DocumentChanges) <----- SEGV

This stack HZxbeA was reported by telemetry:

• crash/crash
• runtime.gopanic:+69,+0x153
• runtime.panicmem:=262,+0x2ff
• runtime.sigpanic:+19,+0x2cc
• golang.org/x/tools/gopls/internal/mcp.summarizeDiagnostics:+6,+0x184
• golang.org/x/tools/gopls/internal/mcp.(*handler).workspaceDiagnosticsHandler:+54,+0x5e3
• golang.org/x/tools/gopls/internal/mcp.(*handler).workspaceDiagnosticsHandler-fm:+0,+0x43
• golang.org/x/tools/internal/mcp.newServerToolErr[...].func1:+14,+0x12f
• golang.org/x/tools/internal/mcp.(*Server).callTool:+7,+0x1bf
• golang.org/x/tools/internal/mcp.init.serverMethod[...].func53:+1,+0x43
• golang.org/x/tools/internal/mcp.init.newMethodInfo[...].func55:+4,+0x4f
• golang.org/x/tools/internal/mcp.defaultReceivingMethodHandler[...]:+6,+0x9f
• golang.org/x/tools/internal/mcp.defaultReceivingMethodHandler[...]:+0,+0x5b
• golang.org/x/tools/internal/mcp.handleReceive[...]:+12,+0x15f
• golang.org/x/tools/internal/mcp.(*ServerSession).handle:+18,+0x1c7
• golang.org/x/tools/internal/mcp.connect[...].func1.1:+0,+0x47
• golang.org/x/tools/internal/jsonrpc2_v2.HandlerFunc.Handle:+1,+0x37
• golang.org/x/tools/internal/jsonrpc2_v2.(*Connection).handleAsync:+27,+0xdf

golang.org/x/tools/gopls@v0.20.0 go1.24.5 darwin/arm64 other,vscode (1)

Dups: p2z9Zg

[gabyhelp]

Related Issues

• x/tools/gopls: CodeAction: nil panic in findMatchingDiagnostics #71028 (closed)
• x/tools/gopls/internal/mcp: symbol references tool panics in ObjectLocation after nil result from types.LookupFieldOrMethod #74904 (closed)
• x/tools/gopls: CodeActions: "end < start" panic #74394 (closed)
• x/tools/gopls: sigpanic in methods of persistent.Map (corrupt g) #74870
• x/tools/gopls: SEGV in ParseDiagnostics (corrupt g or SP) #75464
• x/tools/gopls: nil deref panic in go.test code action #72907 (closed)
• x/tools/gopls: Implementation (by signatures): nil panic in dynamicFuncCallType #74305 (closed)
• x/tools/gopls: CodeActions: nil panic in refactorRewriteEliminateDotImport #74581
• x/tools/gopls: nil deref in x/telemetry/internal/counter.(*Counter).add #73890
• x/tools/gopls: analysisinternal.ZeroValue crash on (presumably) unsafe.Pointer #70585 (closed)

_{(Emoji vote if this was helpful or unhelpful; more detailed feedback welcome in this discussion.)}

[adonovan]

This stack p2z9Zg was reported by telemetry:

• crash/crash
• runtime.gopanic:+69,+0x14f
• runtime.panicmem:=262,+0x2e7
• runtime.sigpanic:+19,+0x2b8
• golang.org/x/tools/gopls/internal/mcp.summarizeDiagnostics:+6,+0x168
• golang.org/x/tools/gopls/internal/mcp.(*handler).workspaceDiagnosticsHandler:+54,+0x5c7
• golang.org/x/tools/gopls/internal/mcp.(*handler).workspaceDiagnosticsHandler-fm:+0,+0x43
• golang.org/x/tools/internal/mcp.newServerToolErr[...].func1:+14,+0x113
• golang.org/x/tools/internal/mcp.(*Server).callTool:+7,+0x1a3
• golang.org/x/tools/internal/mcp.init.serverMethod[...].func53:+1,+0x43
• golang.org/x/tools/internal/mcp.init.newMethodInfo[...].func55:+4,+0x4b
• golang.org/x/tools/internal/mcp.defaultReceivingMethodHandler[...]:+6,+0x9b
• golang.org/x/tools/internal/mcp.defaultReceivingMethodHandler[...]:+0,+0x5b
• golang.org/x/tools/internal/mcp.handleReceive[...]:+12,+0x147
• golang.org/x/tools/internal/mcp.(*ServerSession).handle:+18,+0x1bb
• golang.org/x/tools/internal/mcp.connect[...].func1.1:+0,+0x43
• golang.org/x/tools/internal/jsonrpc2_v2.HandlerFunc.Handle:+1,+0x37
• golang.org/x/tools/internal/jsonrpc2_v2.(*Connection).handleAsync:+27,+0xc3

golang.org/x/tools/gopls@v0.20.0 go1.25.1 darwin/arm64 neovim,other,vscode (1)