antlr/antlr4 · critical · NotSupportedException

Couldn't identify final state of the precedence rule prefix

Error message

Couldn't identify final state of the precedence rule prefix section.

What it means

While preprocessing a left-recursive rule's ATN, the deserializer scans the star-loop states of the precedence-rule prefix section to find its end state (a LoopEndState whose first transition targets a RuleStopState). If no candidate matches, the serialized ATN does not have the expected shape and NotSupportedException is thrown — effectively a data/version corruption signal.

Source

Thrown at runtime/CSharp/src/Atn/ATNDeserializer.cs:135

						}
						if (!(state_3 is StarLoopEntryState))
						{
							continue;
						}
						ATNState maybeLoopEndState = state_3.Transition(state_3.NumberOfTransitions - 1).target;
						if (!(maybeLoopEndState is LoopEndState))
						{
							continue;
						}
						if (maybeLoopEndState.epsilonOnlyTransitions && maybeLoopEndState.Transition(0).target is RuleStopState)
						{
							endState = state_3;
							break;
						}
					}
					if (endState == null)
					{
						throw new NotSupportedException("Couldn't identify final state of the precedence rule prefix section.");
					}
					excludeTransition = ((StarLoopEntryState)endState).loopBackState.Transition(0);
				}
				else
				{
					endState = atn.ruleToStopState[i_13];
				}
				// all non-excluded transitions that currently target end state need to target blockEnd instead
				foreach (ATNState state_4 in atn.states)
				{
					foreach (Transition transition in state_4.transitions)
					{
						if (transition == excludeTransition)
						{
							continue;
						}
						if (transition.target == endState)
						{

View on GitHub (pinned to 7d5770395b)

Solutions

1. Regenerate all parsers/lexers with the tool version matching the Antlr4.Runtime package and rebuild
2. Remove stale generated files and bin/obj so no old serialized ATN survives
3. Verify only one consistent ANTLR version is referenced across the solution

try { new ATNDeserializer().Deserialize(data); } catch (NotSupportedException e) { /* ATN shape unrecognized: regenerate parsers with matching tool/runtime version */ }

When it happens

Trigger: Deserializing serialized ATN data produced by a different ANTLR version or tampered/truncated; a left-recursive rule's generated ATN not matching the C# runtime's expectations.

Common situations: Antlr4.Runtime NuGet version differing from the code generator version, stale generated .cs files after an upgrade, or hand-crafted ATN byte arrays.

Related errors

• Could not deserialize ATN with version {0} (expected {1}).
• The specified transition type is not valid.
• The specified state type {0} is not valid.
• The specified lexer action type {0} is not valid.
• Unrecognized ATN transition type.

AI-assisted analysis of antlr/antlr4@7d5770395b (2026-08-14). Data as JSON: /api/errors/706c081e7484bccc. Report an issue: GitHub.