diff --git a/README.md b/README.md
index b1719c4..ea10b3e 100644
--- a/README.md
+++ b/README.md
@@ -135,6 +135,7 @@ Current checks focus on patterns that often show up in unreviewed generated code
 - [error-obscuring catch blocks](src/rules/error-obscuring/README.md) (default-return or generic replacement error)
 - [empty catch blocks](src/rules/empty-catch/README.md)
 - [promise `.catch()` default fallbacks](src/rules/promise-default-fallbacks/README.md)
+- [stringified unknown errors](src/rules/stringified-unknown-errors/README.md)
 - [async wrapper / `return await` noise](src/rules/async-noise/README.md)
 - [pass-through wrappers](src/rules/pass-through-wrappers/README.md)
 - [barrel density](src/rules/barrel-density/README.md)
diff --git a/src/default-registry.ts b/src/default-registry.ts
index c740e45..a68ccd4 100644
--- a/src/default-registry.ts
+++ b/src/default-registry.ts
@@ -18,6 +18,7 @@ import { emptyCatchRule } from "./rules/empty-catch";
 import { errorObscuringRule } from "./rules/error-obscuring";
 import { errorSwallowingRule } from "./rules/error-swallowing";
 import { promiseDefaultFallbacksRule } from "./rules/promise-default-fallbacks";
+import { stringifiedUnknownErrorsRule } from "./rules/stringified-unknown-errors";
 import { barrelDensityRule } from "./rules/barrel-density";
 import { directoryFanoutHotspotRule } from "./rules/directory-fanout-hotspot";
 import { duplicateFunctionSignaturesRule } from "./rules/duplicate-function-signatures";
@@ -45,6 +46,7 @@ export function createDefaultRegistry(): Registry {
   registry.registerRule(errorObscuringRule);
   registry.registerRule(emptyCatchRule);
   registry.registerRule(promiseDefaultFallbacksRule);
+  registry.registerRule(stringifiedUnknownErrorsRule);
   registry.registerRule(barrelDensityRule);
   registry.registerRule(passThroughWrappersRule);
   registry.registerRule(duplicateFunctionSignaturesRule);
diff --git a/src/rules/stringified-unknown-errors/README.md b/src/rules/stringified-unknown-errors/README.md
new file mode 100644
index 0000000..a082b51
--- /dev/null
+++ b/src/rules/stringified-unknown-errors/README.md
@@ -0,0 +1,59 @@
+# defensive.stringified-unknown-errors
+
+Flags code that flattens unknown caught values into generic strings like `error.message` or `String(error)`.
+
+- **Family:** `defensive`
+- **Severity:** `strong`
+- **Scope:** `file`
+- **Requires:** `file.ast`
+
+## How it works
+
+The rule looks for conditional expressions like:
+
+- `error instanceof Error ? error.message : String(error)`
+- `err instanceof Error ? err.message : String(err)`
+- equivalent forms assigned into `message` / `error` properties or returned directly
+
+This pattern is common in generated wrapper code that wants a quick printable error value. It is sometimes reasonable, but repeated use often flattens richer failure objects into plain strings and makes downstream diagnostics more generic.
+
+To avoid obvious vendored noise, the rule skips very large bundled/generated files over `5000` logical lines.
+
+## Flagged examples
+
+```ts
+catch (error) {
+  return { success: false, error: error instanceof Error ? error.message : String(error) };
+}
+
+setError(err instanceof Error ? err.message : String(err));
+
+const message = error instanceof Error ? error.message : String(error);
+```
+
+## Usually ignored
+
+```ts
+catch (error) {
+  throw error;
+}
+
+catch (error) {
+  logger.error({ error });
+  return { success: false, error };
+}
+```
+
+## Scoring
+
+Each unknown-error stringification site adds `2` points.
+The file total is capped at `8`.
+
+## Benchmark signal
+
+Full pinned benchmark against the exact `known-ai-vs-solid-oss` cohort:
+
+- Signal score: **0.88 / 1.00**
+- Best separating metric: **findings / file (0.88)**
+- Hit rate: **7/9 AI repos** vs **1/9 mature OSS repos**
+- Full results: [experimental rule report](../../../reports/autoresearch-candidate-rule.md#defensivestringified-unknown-errors)
diff --git a/src/rules/stringified-unknown-errors/index.ts b/src/rules/stringified-unknown-errors/index.ts
new file mode 100644
index 0000000..41fe048
--- /dev/null
+++ b/src/rules/stringified-unknown-errors/index.ts
@@ -0,0 +1,169 @@
+import * as ts from "typescript";
+import type { RulePlugin } from "../../core/types";
+import { getLineNumber, unwrapExpression, walk } from "../../facts/ts-helpers";
+import { delta } from "../../rule-delta";
+
+const MAX_LOGICAL_LINES = 5000;
+
+type MatchKind =
+  | "stringified-unknown-error"
+  | "returned-stringified-unknown-error"
+  | "property-stringified-unknown-error";
+
+type StringifiedUnknownErrorMatch = {
+  line: number;
+  kind: MatchKind;
+};
+
+function isErrorIdentifier(expression: ts.Expression, allowedNames: Set<string>): boolean {
+  const unwrapped = unwrapExpression(expression);
+  return ts.isIdentifier(unwrapped) && allowedNames.has(unwrapped.text);
+}
+
+function isStringCallOfError(expression: ts.Expression, allowedNames: Set<string>): boolean {
+  const unwrapped = unwrapExpression(expression);
+  return (
+    ts.isCallExpression(unwrapped) &&
+    ts.isIdentifier(unwrapped.expression) &&
+    unwrapped.expression.text === "String" &&
+    unwrapped.arguments.length === 1 &&
+    isErrorIdentifier(unwrapped.arguments[0]!, allowedNames)
+  );
+}
+
+function isErrorMessageAccess(expression: ts.Expression, allowedNames: Set<string>): boolean {
+  const unwrapped = unwrapExpression(expression);
+  return (
+    ts.isPropertyAccessExpression(unwrapped) &&
+    unwrapped.name.text === "message" &&
+    isErrorIdentifier(unwrapped.expression, allowedNames)
+  );
+}
+
+function isErrorInstanceofCheck(expression: ts.Expression, allowedNames: Set<string>): boolean {
+  const unwrapped = unwrapExpression(expression);
+  return (
+    ts.isBinaryExpression(unwrapped) &&
+    unwrapped.operatorToken.kind === ts.SyntaxKind.InstanceOfKeyword &&
+    isErrorIdentifier(unwrapped.left, allowedNames) &&
+    ts.isIdentifier(unwrapExpression(unwrapped.right)) &&
+    unwrapExpression(unwrapped.right).text === "Error"
+  );
+}
+
+function summarizeConditional(
+  node: ts.ConditionalExpression,
+  sourceFile: ts.SourceFile,
+): StringifiedUnknownErrorMatch | null {
+  const allowedNames = new Set<string>();
+
+  const condition = unwrapExpression(node.condition);
+  if (
+    ts.isBinaryExpression(condition) &&
+    condition.operatorToken.kind === ts.SyntaxKind.BarBarToken
+  ) {
+    for (const side of [condition.left, condition.right]) {
+      const unwrapped = unwrapExpression(side);
+      if (
+        ts.isBinaryExpression(unwrapped) &&
+        unwrapped.operatorToken.kind === ts.SyntaxKind.InstanceOfKeyword
+      ) {
+        const left = unwrapExpression(unwrapped.left);
+        if (ts.isIdentifier(left)) {
+          allowedNames.add(left.text);
+        }
+      }
+    }
+  } else if (isErrorInstanceofCheck(condition, new Set(["error", "err", "e", "cause"]))) {
+    const left = unwrapExpression((condition as ts.BinaryExpression).left) as ts.Identifier;
+    allowedNames.add(left.text);
+  }
+
+  if (allowedNames.size === 0) {
+    return null;
+  }
+
+  const whenTrue = unwrapExpression(node.whenTrue);
+  const whenFalse = unwrapExpression(node.whenFalse);
+  const isEitherDirection =
+    (isErrorMessageAccess(whenTrue, allowedNames) &&
+      isStringCallOfError(whenFalse, allowedNames)) ||
+    (isErrorMessageAccess(whenFalse, allowedNames) && isStringCallOfError(whenTrue, allowedNames));
+
+  if (!isEitherDirection) {
+    return null;
+  }
+
+  let kind: MatchKind = "stringified-unknown-error";
+  if (ts.isReturnStatement(node.parent)) {
+    kind = "returned-stringified-unknown-error";
+  } else if (ts.isPropertyAssignment(node.parent)) {
+    kind = "property-stringified-unknown-error";
+  }
+
+  return {
+    line: getLineNumber(sourceFile, node.getStart(sourceFile)),
+    kind,
+  };
+}
+
+function findStringifiedUnknownErrors(sourceFile: ts.SourceFile): StringifiedUnknownErrorMatch[] {
+  const matches: StringifiedUnknownErrorMatch[] = [];
+
+  walk(sourceFile, (node) => {
+    if (!ts.isConditionalExpression(node)) {
+      return;
+    }
+
+    const match = summarizeConditional(node, sourceFile);
+    if (match) {
+      matches.push(match);
+    }
+  });
+
+  return matches;
+}
+
+export const stringifiedUnknownErrorsRule: RulePlugin = {
+  id: "defensive.stringified-unknown-errors",
+  family: "defensive",
+  severity: "strong",
+  scope: "file",
+  requires: ["file.ast"],
+  delta: delta.byLocations(),
+  supports(context) {
+    return context.scope === "file" && Boolean(context.file);
+  },
+  evaluate(context) {
+    if (context.file!.logicalLineCount > MAX_LOGICAL_LINES) {
+      return [];
+    }
+
+    const sourceFile = context.runtime.store.getFileFact<ts.SourceFile>(
+      context.file!.path,
+      "file.ast",
+    );
+    if (!sourceFile) {
+      return [];
+    }
+
+    const matches = findStringifiedUnknownErrors(sourceFile);
+    if (matches.length === 0) {
+      return [];
+    }
+
+    return [
+      {
+        ruleId: "defensive.stringified-unknown-errors",
+        family: "defensive",
+        severity: "strong",
+        scope: "file",
+        path: context.file!.path,
+        message: `Found ${matches.length} unknown-error normalization${matches.length === 1 ? "" : "s"} that collapse caught values into generic strings`,
+        evidence: matches.map((match) => `line ${match.line}: ${match.kind}`),
+        score: Math.min(8, matches.length * 2),
+        locations: matches.map((match) => ({ path: context.file!.path, line: match.line })),
+      },
+    ];
+  },
+};
diff --git a/tests/heuristics.test.ts b/tests/heuristics.test.ts
index 7bf4e0c..df126b7 100644
--- a/tests/heuristics.test.ts
+++ b/tests/heuristics.test.ts
@@ -44,6 +44,11 @@ describe("heuristic rule pack", () => {
         "  }",
         "}",
         "",
+        "export function getErrorMessage(error: unknown) {",
+        "  const message = error instanceof Error ? error.message : String(error);",
+        "  return { message };",
+        "}",
+        "",
       ].join("\n"),
       "src/service.ts": [
         "function getData(id: string) {",
@@ -80,6 +85,7 @@ describe("heuristic rule pack", () => {
     expect(ruleIds.has("comments.placeholder-comments")).toBe(true);
     expect(ruleIds.has("defensive.error-obscuring")).toBe(true);
     expect(ruleIds.has("defensive.promise-default-fallbacks")).toBe(true);
+    expect(ruleIds.has("defensive.stringified-unknown-errors")).toBe(true);
     expect(ruleIds.has("defensive.async-noise")).toBe(true);
     expect(ruleIds.has("structure.pass-through-wrappers")).toBe(true);
     expect(ruleIds.has("structure.barrel-density")).toBe(true);
diff --git a/tests/stringified-unknown-errors.test.ts b/tests/stringified-unknown-errors.test.ts
new file mode 100644
index 0000000..a45bad3
--- /dev/null
+++ b/tests/stringified-unknown-errors.test.ts
@@ -0,0 +1,106 @@
+import { afterEach, describe, expect, test } from "bun:test";
+import { mkdtemp, mkdir, rm, writeFile } from "node:fs/promises";
+import os from "node:os";
+import path from "node:path";
+import { DEFAULT_CONFIG } from "../src/config";
+import { analyzeRepository } from "../src/core/engine";
+import { Registry } from "../src/core/registry";
+import { createDefaultRegistry } from "../src/default-registry";
+import { stringifiedUnknownErrorsRule } from "../src/rules/stringified-unknown-errors";
+
+const tempDirs: string[] = [];
+
+afterEach(async () => {
+  await Promise.all(tempDirs.splice(0).map((dir) => rm(dir, { recursive: true, force: true })));
+});
+
+async function writeRepoFiles(rootDir: string, files: Record<string, string>): Promise<void> {
+  for (const [relativePath, content] of Object.entries(files)) {
+    const absolutePath = path.join(rootDir, relativePath);
+    await mkdir(path.dirname(absolutePath), { recursive: true });
+    await writeFile(absolutePath, content);
+  }
+}
+
+async function createTempRepo(files: Record<string, string>): Promise<string> {
+  const rootDir = await mkdtemp(path.join(os.tmpdir(), "slop-scan-stringified-errors-"));
+  tempDirs.push(rootDir);
+  await writeRepoFiles(rootDir, files);
+  return rootDir;
+}
+
+function createCandidateRegistry(): Registry {
+  const baseRegistry = createDefaultRegistry();
+  const registry = new Registry();
+
+  for (const language of baseRegistry.getLanguages()) {
+    registry.registerLanguage(language);
+  }
+
+  for (const provider of baseRegistry.getFactProviders()) {
+    registry.registerFactProvider(provider);
+  }
+
+  registry.registerRule(stringifiedUnknownErrorsRule);
+  return registry;
+}
+
+describe("stringified-unknown-errors rule", () => {
+  test("flags unknown errors collapsed into generic strings", async () => {
+    const rootDir = await createTempRepo({
+      "src/slop.ts": [
+        "export function getMessage(error: unknown) {",
+        "  return error instanceof Error ? error.message : String(error);",
+        "}",
+        "",
+        "export function toResult(err: unknown) {",
+        "  return { ok: false, error: err instanceof Error ? err.message : String(err) };",
+        "}",
+        "",
+        "export function updateState(error: unknown) {",
+        "  const next = error instanceof Error ? error.message : String(error);",
+        "  return { message: next };",
+        "}",
+      ].join("\n"),
+      "src/legit.ts": [
+        "export function rethrow(error: unknown) {",
+        "  throw error;",
+        "}",
+        "",
+        "export function preserve(error: unknown) {",
+        "  return { error };",
+        "}",
+      ].join("\n"),
+    });
+
+    const result = await analyzeRepository(rootDir, DEFAULT_CONFIG, createCandidateRegistry());
+    const finding = result.findings.find(
+      (nextFinding) => nextFinding.ruleId === "defensive.stringified-unknown-errors",
+    );
+
+    expect(finding).toBeDefined();
+    expect(finding?.path).toBe("src/slop.ts");
+    expect(finding?.evidence).toEqual([
+      "line 2: returned-stringified-unknown-error",
+      "line 6: property-stringified-unknown-error",
+      "line 10: stringified-unknown-error",
+    ]);
+    expect(result.findings).toHaveLength(1);
+  });
+
+  test("ignores giant bundled files that would otherwise create vendor noise", async () => {
+    const hugeFile = [
+      ...Array.from({ length: 5001 }, (_, index) => `export const filler${index} = ${index};`),
+      "export const message = error instanceof Error ? error.message : String(error);",
+      "",
+    ].join("\n");
+
+    const rootDir = await createTempRepo({
+      "src/bundle.ts": hugeFile,
+    });
+
+    const result = await analyzeRepository(rootDir, DEFAULT_CONFIG, createCandidateRegistry());
+
+    expect(result.findings).toHaveLength(0);
+  });
+});