diff --git a/dag/test/claim/match_arm_join_disjoint_coproduct_witness_test.dag b/dag/test/claim/match_arm_join_disjoint_coproduct_witness_test.dag
index aabf62b4bd6..789f840913f 100644
--- a/dag/test/claim/match_arm_join_disjoint_coproduct_witness_test.dag
+++ b/dag/test/claim/match_arm_join_disjoint_coproduct_witness_test.dag
@@ -1,5 +1,26 @@
 module test.claim.match_arm_join_disjoint_coproduct_witness_test
 
+import gunbc.compile_diagnostic_census {
+  CompileDiagnosticCensus,
+  CensusObserved,
+  CensusNotRunnable,
+  census_blocking_rows,
+  census_rows_of_class,
+  census_total_count
+}
+import std.types { String, Bool }
+import v2.std.live_tree { LiveTreeDisposition, ReadsLiveTree }
+
+data live_tree_disposition: LiveTreeDisposition = ReadsLiveTree
+
+fn maj_blocking_count(source: String, wanted: String) -> Int {
+  match compile_dag_diagnostic_census(source) {
+    CensusObserved { rows: rows } =>
+      census_total_count(rows: census_rows_of_class(rows: census_blocking_rows(rows: rows), wanted: wanted))
+    CensusNotRunnable { cause: _ } => 0 - 1
+  }
+}
+
 // A MATCH JOIN ANSWERED WITH ONE ARM AND NEVER JUDGED THE REST, SO THE VERDICT FLIPPED WITH ARM
 // ORDER. v1.compiler.infer builds a match expression's type by folding every arm's body type through
 // v1.compiler.infer_types prefer_specific_type, whose tie-break for two disjoint coproducts is the
@@ -178,3 +199,18 @@ test fn maj_matching_ground_element_collection_arms_still_compile() -> Bool {
     []
   )
 }
+
+// A DISJOINT ARM IS A SOURCE TYPE ERROR, so it is a typed TypeMismatch located at the offending arm,
+// never an InternalError: InternalError names a compiler defect (a state the compiler could not
+// have been handed), and reading it as one sent authors looking for a bug in a correct compiler.
+// Both the scalar and the collection ground relations are asserted by CLASS, and the InternalError
+// count is pinned to zero so the reclassification cannot silently revert.
+test fn maj_kernel_arm_mismatch_is_a_typed_mismatch() -> Bool {
+  maj_blocking_count(source: maj_kernel_arm_mismatch_fixture_source(), wanted: "TypeMismatch") == 1
+    && maj_blocking_count(source: maj_kernel_arm_mismatch_fixture_source(), wanted: "InternalError") == 0
+}
+
+test fn maj_collection_arm_mismatch_is_a_typed_mismatch() -> Bool {
+  maj_blocking_count(source: maj_ground_element_collection_mismatch_fixture_source(), wanted: "TypeMismatch") >= 1
+    && maj_blocking_count(source: maj_ground_element_collection_mismatch_fixture_source(), wanted: "InternalError") == 0
+}
diff --git a/src/v1/04_infer.dag b/src/v1/04_infer.dag
index 0fc07cbcaeb..ed4bfba9aa2 100644
--- a/src/v1/04_infer.dag
+++ b/src/v1/04_infer.dag
@@ -3081,13 +3081,9 @@ fn match_arm_join_diagnostics(unified_arm_type: Node, arm: ArmInferResult, scope
     arm_type: arm.body_type,
     scope: scope
   ) {
-    [inference_error(
-      message: concat(
-        "match arms produce proven-disjoint types: ",
-        node_type_shape(n: unified_arm_type, source_indices: scope.type_env.source_indices),
-        " vs ",
-        node_type_shape(n: arm.body_type, source_indices: scope.type_env.source_indices)
-      ),
+    [type_mismatch_error(
+      expected: node_type_shape(n: unified_arm_type, source_indices: scope.type_env.source_indices),
+      got: node_type_shape(n: arm.body_type, source_indices: scope.type_env.source_indices),
       span: arm_body(n: arm.typed_arm).span,
       module_name: scope.module_name
     )]
diff --git a/src/v1/stage0/src/v1_compiler_infer.rs b/src/v1/stage0/src/v1_compiler_infer.rs
index 1b84f5114e5..120321ba6bb 100644
--- a/src/v1/stage0/src/v1_compiler_infer.rs
+++ b/src/v1/stage0/src/v1_compiler_infer.rs
@@ -4312,22 +4312,14 @@ pub fn match_arm_join_diagnostics(
                 arm.body_type.clone(),
                 scope.clone(),
             ) {
-                Rc::new(vec![inference_error(
-                    v1_rt::concat(
-                        v1_rt::concat(
-                            v1_rt::concat(
-                                "match arms produce proven-disjoint types: ".to_string(),
-                                crate::v1_compiler_infer_types::node_type_shape(
-                                    unified_arm_type.clone(),
-                                    scope.type_env.clone().source_indices.clone(),
-                                ),
-                            ),
-                            " vs ".to_string(),
-                        ),
-                        crate::v1_compiler_infer_types::node_type_shape(
-                            arm.body_type.clone(),
-                            scope.type_env.clone().source_indices.clone(),
-                        ),
+                Rc::new(vec![type_mismatch_error(
+                    crate::v1_compiler_infer_types::node_type_shape(
+                        unified_arm_type.clone(),
+                        scope.type_env.clone().source_indices.clone(),
+                    ),
+                    crate::v1_compiler_infer_types::node_type_shape(
+                        arm.body_type.clone(),
+                        scope.type_env.clone().source_indices.clone(),
                     ),
                     crate::v1_std_core::arm_body(arm.typed_arm.clone())
                         .span