diff --git a/JuliaSyntax/AGENTS.md b/JuliaSyntax/AGENTS.md
new file mode 100644
index 0000000000000..b5953f2154c32
--- /dev/null
+++ b/JuliaSyntax/AGENTS.md
@@ -0,0 +1,7 @@
+# JuliaSyntax Agent Instructions
+
+See [../AGENTS.md](../AGENTS.md) for general instructions.
+
+## Parser changes
+
+When modifying the parser, always verify that AST production comments (e.g., `# f.[x]  ==>  (error f x)`) accurately reflect the actual parser output.
diff --git a/JuliaSyntax/CLAUDE.md b/JuliaSyntax/CLAUDE.md
new file mode 120000
index 0000000000000..47dc3e3d863cf
--- /dev/null
+++ b/JuliaSyntax/CLAUDE.md
@@ -0,0 +1 @@
+AGENTS.md
\ No newline at end of file
diff --git a/JuliaSyntax/src/julia/parser.jl b/JuliaSyntax/src/julia/parser.jl
index 75c806caaa60a..eedfe03653c41 100644
--- a/JuliaSyntax/src/julia/parser.jl
+++ b/JuliaSyntax/src/julia/parser.jl
@@ -1738,6 +1738,14 @@ function parse_call_chain(ps::ParseState, mark, is_macrocall=false)
                 emit(ps, emark, K"error",
                      error="the .' operator for transpose is discontinued")
                 emit(ps, mark, K"dotcall", POSTFIX_OP_FLAG)
+            elseif k == K"[" || k == K"{"
+                # f.[x]  ==>  (error f x)
+                # f.{x}  ==>  (error f x)
+                # Parse as broadcasted brackets, then wrap in error
+                close = k == K"[" ? K"]" : K"}"
+                bump(ps, TRIVIA_FLAG)
+                parse_cat(ParseState(ps, end_symbol=true), close, ps.end_symbol)
+                emit(ps, mark, K"error", error="brackets are not allowed after `.`")
             else
                 if saw_misplaced_atsym
                     # If we saw a misplaced `@` earlier, this might be the place
diff --git a/JuliaSyntax/test/diagnostics.jl b/JuliaSyntax/test/diagnostics.jl
index 151aad919c0ed..e258386c1d3d2 100644
--- a/JuliaSyntax/test/diagnostics.jl
+++ b/JuliaSyntax/test/diagnostics.jl
@@ -48,6 +48,10 @@ end
         Diagnostic(3, 4, :error, "`@` must appear on first or last macro name component")
     @test diagnostic("@M.(x)") ==
         Diagnostic(1, 3, :error, "dot call syntax not supported for macros")
+    @test diagnostic("a.[x]") ==
+        Diagnostic(1, 5, :error, "brackets are not allowed after `.`")
+    @test diagnostic("a.{x}") ==
+        Diagnostic(1, 5, :error, "brackets are not allowed after `.`")
 
     @test diagnostic("try x end") ==
         Diagnostic(1, 9, :error, "try without catch or finally")
diff --git a/test/syntax.jl b/test/syntax.jl
index 5691ca7135993..851ceec87da49 100644
--- a/test/syntax.jl
+++ b/test/syntax.jl
@@ -934,8 +934,8 @@ g21054(>:) = >:2
 @test g21054(-) == -2
 
 # issue #21168
-@test_broken Meta.lower(Main, :(a.[1])) == Expr(:error, "invalid syntax \"a.[1]\"")
-@test_broken Meta.lower(Main, :(a.{1})) == Expr(:error, "invalid syntax \"a.{1}\"")
+@test_parseerror "a.[1]"
+@test_parseerror "a.{1}"
 
 # Issue #21225
 let abstr = Meta.parse("abstract type X end")