diff --git a/packages/compiler/src/frontend/lowering/lower-classes.ts b/packages/compiler/src/frontend/lowering/lower-classes.ts
index 06739166c..bfb7e06b8 100644
--- a/packages/compiler/src/frontend/lowering/lower-classes.ts
+++ b/packages/compiler/src/frontend/lowering/lower-classes.ts
@@ -6385,7 +6385,7 @@ export function collectInheritedJsFieldWrites(
  * hierarchy identifies the original declarations through aliases as well. */
 export function collectVirtualJsMethods(lowerer: Lowerer, files: readonly ts.SourceFile[]): void {
   const families: [ts.MethodDeclaration, ts.MethodDeclaration][] = [];
-  collectClassMethodMutations(lowerer, files);
+  lowerer.speculativeTypeQueries(() => collectClassMethodMutations(lowerer, files));
   const visit = (node: ts.Node): void => {
     if (ts.isClassDeclaration(node) || ts.isClassExpression(node)) {
       const accessors = node.members.filter(
diff --git a/packages/compiler/src/frontend/lowering/lowerer.ts b/packages/compiler/src/frontend/lowering/lowerer.ts
index aca306900..147a6232b 100644
--- a/packages/compiler/src/frontend/lowering/lowerer.ts
+++ b/packages/compiler/src/frontend/lowering/lowerer.ts
@@ -2825,7 +2825,8 @@ export class Lowerer {
             return null;
         }
         const className = this.classNamer(decl);
-        if (!this.collectingExprClasses.has(decl)) this.lowerClassExpressionInfo(decl);
+        if (!this.collectingExprClasses.has(decl) && !this.collectLocalClassForType(decl))
+          return null;
         return { kind: "object", className };
       },
       mixinIntersectionInstance: (widened) => mixinIntersectionInstanceType(this, widened),
@@ -6084,9 +6085,11 @@ export class Lowerer {
       for (const fp of parts) this.checker.prefetchSourceFile(fp.sf);
     }
     this.collectProgram(parts);
-    this.analyzeRuntimeOptionalArrayReads(parts);
-    this.analyzeDateJsonResults(parts);
-    this.analyzeGuardedRecordReadGlobals(parts);
+    this.speculativeTypeQueries(() => {
+      this.analyzeRuntimeOptionalArrayReads(parts);
+      this.analyzeDateJsonResults(parts);
+      this.analyzeGuardedRecordReadGlobals(parts);
+    });
     // Decorated classes analyze AFTER the whole collection pass: a
     // decorator's return type may name a subclass declared below the
     // class, and reference lowering needs each class's rebindability
@@ -6470,9 +6473,11 @@ export class Lowerer {
       collectionEdges.add(name);
     };
     this.collectProgram(parts);
-    this.analyzeRuntimeOptionalArrayReads(parts);
-    this.analyzeDateJsonResults(parts);
-    this.analyzeGuardedRecordReadGlobals(parts);
+    this.speculativeTypeQueries(() => {
+      this.analyzeRuntimeOptionalArrayReads(parts);
+      this.analyzeDateJsonResults(parts);
+      this.analyzeGuardedRecordReadGlobals(parts);
+    });
     // Decorated classes analyze post-collection here too: the %init seeds
     // lower the decoration calls, whose edges (decorator bodies, construct
     // thunks) reachable emit must see.
@@ -9615,6 +9620,49 @@ export class Lowerer {
     return lowerClassExpressionInfo(this, expr);
   }
 
+  /** Depth of the whole-program analyses that query types across every
+   * file before reachability is known (speculativeTypeQueries). */
+  private speculativeTypeQueryDepth = 0;
+
+  /** Runs a whole-program analysis whose type queries are speculative: they
+   * visit every file, reached or not, and no statement or declaration owns
+   * a refusal they trigger (collectLocalClassForType). */
+  speculativeTypeQueries(analysis: () => void): void {
+    this.speculativeTypeQueryDepth++;
+    try {
+      analysis();
+    } finally {
+      this.speculativeTypeQueryDepth--;
+    }
+  }
+
+  /** Collects a function-local class because a type query named its
+   * instance type. Collection and body lowering own a refusal of that
+   * class: the poison reaches the declaration or statement that asked, as
+   * before. A speculative whole-program analysis owns none, so there the
+   * class is unmappable for the query and its diagnostics are dropped;
+   * lowering the class's own declaration collects it again and reports
+   * them if the program reaches it. */
+  collectLocalClassForType(decl: ts.ClassLikeDeclaration): boolean {
+    if (this.speculativeTypeQueryDepth === 0 || this.fnStack.length > 0) {
+      this.lowerClassExpressionInfo(decl);
+      return true;
+    }
+    const sink: ScrDiagnostic[] = [];
+    const previousSink = this.diagSink;
+    this.diagSink = sink;
+    try {
+      this.lowerClassExpressionInfo(decl);
+    } catch (e) {
+      if (e instanceof PoisonError) return false;
+      throw e;
+    } finally {
+      this.diagSink = previousSink;
+    }
+    for (const d of sink) this.pushDiag(d);
+    return true;
+  }
+
   lowerClassExpression(expr: ts.ClassLikeDeclaration): IrExpr {
     return lowerClassExpression(this, expr);
   }
diff --git a/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/6102-local-class-type-query.js.json b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/6102-local-class-type-query.js.json
new file mode 100644
index 000000000..86c6ba9e8
--- /dev/null
+++ b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/6102-local-class-type-query.js.json
@@ -0,0 +1,6 @@
+{
+ "order": [
+  "<repo>/tests/corpus/6102-local-class-type-query.js"
+ ],
+ "diags": []
+}
diff --git a/packages/compiler/test/ts7/baselines/order-parity/tests/diagnostics/local-class-type-query-brand.ts.json b/packages/compiler/test/ts7/baselines/order-parity/tests/diagnostics/local-class-type-query-brand.ts.json
new file mode 100644
index 000000000..e17ea3bf5
--- /dev/null
+++ b/packages/compiler/test/ts7/baselines/order-parity/tests/diagnostics/local-class-type-query-brand.ts.json
@@ -0,0 +1,6 @@
+{
+ "order": [
+  "<repo>/tests/diagnostics/local-class-type-query-brand.ts"
+ ],
+ "diags": []
+}
diff --git a/packages/compiler/test/ts7/baselines/order-parity/tests/diagnostics/local-class-type-query.ts.json b/packages/compiler/test/ts7/baselines/order-parity/tests/diagnostics/local-class-type-query.ts.json
new file mode 100644
index 000000000..3c89fba9f
--- /dev/null
+++ b/packages/compiler/test/ts7/baselines/order-parity/tests/diagnostics/local-class-type-query.ts.json
@@ -0,0 +1,6 @@
+{
+ "order": [
+  "<repo>/tests/diagnostics/local-class-type-query.ts"
+ ],
+ "diags": []
+}
diff --git a/tests/corpus/6102-local-class-type-query.js b/tests/corpus/6102-local-class-type-query.js
new file mode 100644
index 000000000..92fe35725
--- /dev/null
+++ b/tests/corpus/6102-local-class-type-query.js
@@ -0,0 +1,42 @@
+// A whole-program analysis that queries types before reachability is known
+// (the computed-key write census here) can name a function-local class whose
+// collection is refused. That query has no owner for the refusal: the class
+// stays unmapped for it, and the refusal belongs to the class's own
+// declaration, which this program never executes (it used to crash the
+// compiler with an uncaught PoisonError).
+function main(run) {
+  if (!run) return "skipped";
+  const Project = class {
+    constructor() {
+      this.name = "p";
+    }
+    close() {
+      if (this.missingFilesMap) {
+        this.missingFilesMap.clear();
+        this.missingFilesMap = void 0;
+      }
+    }
+    add(k) {
+      (this.missingFilesMap || (this.missingFilesMap = new Map())).set(k, 1);
+    }
+  };
+  const holder = { p: new Project() };
+  const key = String(run);
+  holder.p[key] = "q";
+  return holder.p.name;
+}
+
+class Counter {
+  constructor() {
+    this.counts = {};
+  }
+  bump(key) {
+    this.counts[key] = (this.counts[key] || 0) + 1;
+    return this.counts[key];
+  }
+}
+
+const counter = new Counter();
+const word = process.argv.length > 99 ? "unused" : "word";
+counter.bump(word);
+console.log(main(0), counter.bump(word));
diff --git a/tests/diagnostics/local-class-type-query-brand.ts b/tests/diagnostics/local-class-type-query-brand.ts
new file mode 100644
index 000000000..566304627
--- /dev/null
+++ b/tests/diagnostics/local-class-type-query-brand.ts
@@ -0,0 +1,14 @@
+// The same speculative element-type query, reaching a function-local class
+// with a private brand through the function's returned array.
+function make() {
+  class Box {
+    #v = 1;
+    read(): number {
+      return this.#v;
+    }
+  }
+  return [new Box()];
+}
+const xs = make();
+const x = xs[0];
+console.log(x === undefined ? -1 : x.read());
diff --git a/tests/diagnostics/local-class-type-query.ts b/tests/diagnostics/local-class-type-query.ts
new file mode 100644
index 000000000..217653ff2
--- /dev/null
+++ b/tests/diagnostics/local-class-type-query.ts
@@ -0,0 +1,14 @@
+// A whole-program analysis queries the element type of `items` before any
+// body lowers. The function-local class it names is refused: the refusal is
+// reported once, at the class (it used to crash the compiler with an uncaught
+// PoisonError).
+function make() {
+  class Box {
+    static count = 0;
+    v = 1;
+  }
+  return new Box();
+}
+const items = [make()];
+const first = items[0];
+console.log(first ? first.v : 0);
diff --git a/tests/harness/__snapshots__/local-class-type-query-brand.ts.txt b/tests/harness/__snapshots__/local-class-type-query-brand.ts.txt
new file mode 100644
index 000000000..6db3418a8
--- /dev/null
+++ b/tests/harness/__snapshots__/local-class-type-query-brand.ts.txt
@@ -0,0 +1,6 @@
+local-class-type-query-brand.ts:4:3 - error SC1090: function-local classes with private brands are not supported yet
+
+  3 | function make() {
+  4 |   class Box {
+    |   ^~~~~~~~~~~
+  5 |     #v = 1;
\ No newline at end of file
diff --git a/tests/harness/__snapshots__/local-class-type-query.ts.txt b/tests/harness/__snapshots__/local-class-type-query.ts.txt
new file mode 100644
index 000000000..87489fcd5
--- /dev/null
+++ b/tests/harness/__snapshots__/local-class-type-query.ts.txt
@@ -0,0 +1,6 @@
+local-class-type-query.ts:6:3 - error SC1090: function-local classes with statics, decorators, or generic members are not supported yet
+
+  5 | function make() {
+  6 |   class Box {
+    |   ^~~~~~~~~~~
+  7 |     static count = 0;
\ No newline at end of file