diff --git a/README.md b/README.md
index f1f28a1..44b07d3 100644
--- a/README.md
+++ b/README.md
@@ -56,23 +56,23 @@ const tenantSafeText = tenantFilter.censor("message text");
 ```
 
 The default shared instance is exported as `filter` and uses the built-in strict
-and loose term lists. It is mutable through `setStrict`, `setLoose`,
-`addStrict`, and `addLoose`, so changes affect later calls that use the same
-shared instance.
+and loose term lists. It is read-only for shared use: `check`, `censor`, and
+`analyze` are available, while `setStrict`, `setLoose`, `addStrict`, and
+`addLoose` throw instead of mutating process-wide state.
 
 Use `createProfanityFilter(...)` when per-request, per-tenant, or test-local
-dictionaries must be isolated from the shared mutable `filter`.
+dictionaries must be mutable and isolated from the shared `filter`.
 
 ### Shared Default Vs Isolated Filters
 
 Use the exported `filter` for the package's shared built-in Russian behavior
 when your code only calls `check`, `censor`, or `analyze`. It is a process-local
-object: every module that imports `filter` receives the same mutable instance.
+read-only object: every module that imports `filter` receives the same instance,
+and mutation methods throw to protect shared dictionary state.
 
-Do not call `setStrict`, `setLoose`, `addStrict`, or `addLoose` on the shared
-`filter` for application-specific, tenant-specific, request-specific, or
-test-local terms. Those calls intentionally mutate that shared instance and
-therefore affect future checks performed through the same import.
+Do not use the shared `filter` for application-specific, tenant-specific,
+request-specific, or test-local terms. Use a factory-created filter for those
+runtime dictionaries.
 
 Use `createProfanityFilter(...)`,
 `createProfanityFilterFromDictionary(dictionary)`, or
@@ -194,11 +194,12 @@ const looseOnly = createProfanityFilter([], ["banned"]);
 const builtIn = createProfanityFilter();
 ```
 
-All filter instances expose stable `name: "profanity"` plus `check`, `censor`,
-`analyze`, `setStrict`, `setLoose`, `addStrict`, and `addLoose`.
-Runtime mutation methods affect only the instance they are called on. Calling
-them on the shared `filter` mutates shared package state; calling them on a
-factory-created filter mutates only that isolated instance.
+All factory-created filter instances expose stable `name: "profanity"` plus
+`check`, `censor`, `analyze`, `setStrict`, `setLoose`, `addStrict`, and
+`addLoose`. Runtime mutation methods affect only the instance they are called
+on. The shared `filter` is typed as `ReadonlyProfanityFilter`, keeps the same
+read methods, and rejects mutation methods at runtime for JavaScript consumers;
+factory-created filters remain the mutable boundary.
 
 ### Language Dictionaries
 
@@ -405,9 +406,9 @@ into different compiled matcher views.
 - Ranges are UTF-16 offsets into the original source string.
 - Runtime dictionaries do not support caller-provided regular expressions.
 - Runtime string terms do not receive taxonomy metadata.
-- The shared `filter` instance is mutable and process-local; reserve its
-  mutation methods for deliberate global changes.
-- Use factory-created filters for application-specific, tenant-specific,
+- The shared `filter` instance is process-local and read-only; use its
+  `check`, `censor`, and `analyze` methods for built-in Russian behavior.
+- Use factory-created mutable filters for application-specific, tenant-specific,
   request-specific, or test-local runtime dictionary changes.
 - Built-in corpus behavior is intentionally locked by compatibility tests.
 
@@ -427,9 +428,8 @@ Intentional public-package changes:
 - The filter exposes `check(text): boolean` for boolean-only detection.
 - `createProfanityFilter()` without arguments creates an instance with compiled
   views of the built-in Russian dictionary.
-- The exported `filter` remains a shared mutable default for backward
-  compatibility; factory-created filters are the recommended boundary for
-  isolated runtime mutation.
+- The exported `filter` is a shared read-only default; factory-created filters
+  are the mutable boundary for isolated runtime mutation.
 - Masking preserves JavaScript string length for astral code points.
 
 ## Architecture
diff --git a/src/filter.ts b/src/filter.ts
index 3694635..47ac6bd 100644
--- a/src/filter.ts
+++ b/src/filter.ts
@@ -33,6 +33,7 @@ import {
   type ProfanityMatchRange,
   type ProfanitySeverity,
   type ProfanityTermList,
+  type ReadonlyProfanityFilter,
 } from "./types.js";
 import {
   compileLooseLiteralPatterns,
@@ -156,6 +157,29 @@ function createFilter(state: FilterState): ProfanityFilter {
   };
 }
 
+function createReadOnlyFilter(
+  filter: ProfanityFilter,
+): ReadonlyProfanityFilter {
+  const readOnlyFilter = Object.freeze({
+    name: filter.name,
+    analyze: filter.analyze,
+    check: filter.check,
+    censor: filter.censor,
+    setStrict: rejectReadOnlyFilterMutation,
+    addStrict: rejectReadOnlyFilterMutation,
+    setLoose: rejectReadOnlyFilterMutation,
+    addLoose: rejectReadOnlyFilterMutation,
+  });
+
+  return readOnlyFilter;
+}
+
+function rejectReadOnlyFilterMutation(): never {
+  throw new TypeError(
+    "The shared profanity filter is read-only. Use createProfanityFilter() or a dictionary factory to create a mutable filter.",
+  );
+}
+
 function createState(
   strictTerms: ProfanityTermList,
   looseTerms: ProfanityTermList,
@@ -536,4 +560,6 @@ const isAtLeastSeverity = (
   PROFANITY_SEVERITY_RANK[severity] >= PROFANITY_SEVERITY_RANK[minSeverity];
 
 export const profanityFilter = createProfanityFilter;
-export const filter = createProfanityFilter(STRICT_BASE, LOOSE_BASE);
+export const filter: ReadonlyProfanityFilter = createReadOnlyFilter(
+  createProfanityFilter(STRICT_BASE, LOOSE_BASE),
+);
diff --git a/src/index.ts b/src/index.ts
index ce8d52b..dc81219 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -8,6 +8,7 @@ export type {
   ProfanitySeverity,
   ProfanityTaxonomyMetadata,
   ProfanityTermList,
+  ReadonlyProfanityFilter,
 } from "./types.js";
 export type {
   ProfanityLanguageDictionary,
diff --git a/src/types.ts b/src/types.ts
index 8d77859..51cd2f0 100644
--- a/src/types.ts
+++ b/src/types.ts
@@ -32,11 +32,14 @@ export interface ProfanityMatchRange extends Readonly<
 
 export const PROFANITY_FILTER_NAME = "profanity";
 
-export interface ProfanityFilter {
+export interface ReadonlyProfanityFilter {
   readonly name: typeof PROFANITY_FILTER_NAME;
   analyze(text: string, options?: ProfanityMatchOptions): ProfanityMatchRange[];
   check(text: string, options?: ProfanityMatchOptions): boolean;
   censor(text: string, options?: ProfanityMatchOptions): string;
+}
+
+export interface ProfanityFilter extends ReadonlyProfanityFilter {
   setStrict(list: ProfanityTermList): void;
   setLoose(list: ProfanityTermList): void;
   addStrict(term: unknown): void;
diff --git a/tests/api.spec.ts b/tests/api.spec.ts
index 1f34242..348f14d 100644
--- a/tests/api.spec.ts
+++ b/tests/api.spec.ts
@@ -19,6 +19,7 @@ import {
   type ProfanityMatchRange,
   type ProfanitySeverity,
   type ProfanityTaxonomyMetadata,
+  type ReadonlyProfanityFilter,
   russianProfanityDictionary,
   validateProfanityLanguageDictionary,
 } from "../src";
@@ -141,11 +142,38 @@ describe("public API", () => {
     >();
   });
 
+  it("exports the shared default filter as a read-only type", () => {
+    expectTypeOf(filter).toEqualTypeOf<ReadonlyProfanityFilter>();
+    expectTypeOf(createProfanityFilter()).toMatchTypeOf<{
+      setStrict(list: readonly unknown[]): void;
+      setLoose(list: readonly unknown[]): void;
+      addStrict(term: unknown): void;
+      addLoose(term: unknown): void;
+    }>();
+  });
+
   it("exposes the default instance and the compatible factory alias", () => {
     expect(filter.name).toBe(PROFANITY_FILTER_NAME);
     expect(profanityFilter(["fff"], []).censor("fff ggg")).toBe("*** ggg");
   });
 
+  it("keeps the shared default filter read-only", () => {
+    const errorMessage =
+      "The shared profanity filter is read-only. Use createProfanityFilter() or a dictionary factory to create a mutable filter.";
+
+    expect(Object.isFrozen(filter)).toBe(true);
+    expect(() => filter.addStrict("shared-only")).toThrow(errorMessage);
+    expect(() => filter.addLoose("sharedloose")).toThrow(errorMessage);
+    expect(() => filter.setStrict(["replacement-only"])).toThrow(errorMessage);
+    expect(() => filter.setLoose(["replacementloose"])).toThrow(errorMessage);
+
+    expect(filter.check("блядь")).toBe(true);
+    expect(filter.check("shared-only")).toBe(false);
+    expect(filter.check("s-h-a-r-e-d-l-o-o-s-e")).toBe(false);
+    expect(filter.check("replacement-only")).toBe(false);
+    expect(filter.check("r-e-p-l-a-c-e-m-e-n-t-l-o-o-s-e")).toBe(false);
+  });
+
   it("exposes a stable filter name and check helper", () => {
     expect(filter.name).toBe(PROFANITY_FILTER_NAME);
     expect(filter.check("привет блядь")).toBe(true);
@@ -735,6 +763,18 @@ describe("public API", () => {
     );
   });
 
+  it("keeps factory-created filters mutable after the default export is read-only", () => {
+    const mutable = createProfanityFilter([], []);
+
+    mutable.addStrict("strict-only");
+    mutable.addLoose("looseonly");
+
+    expect(mutable.check("strict-only")).toBe(true);
+    expect(mutable.check("l-o-o-s-e-o-n-l-y")).toBe(true);
+    expect(filter.check("strict-only")).toBe(false);
+    expect(filter.check("l-o-o-s-e-o-n-l-y")).toBe(false);
+  });
+
   it("keeps built-in rules active when appending runtime literals", () => {
     const strict = createProfanityFilter(undefined, []);
     strict.addStrict("custom");
diff --git a/tests/dist-public-api-smoke.ts b/tests/dist-public-api-smoke.ts
index fd17366..5059358 100644
--- a/tests/dist-public-api-smoke.ts
+++ b/tests/dist-public-api-smoke.ts
@@ -17,6 +17,7 @@ import {
   type ProfanityMatchRange,
   type ProfanitySeverity,
   type ProfanityTaxonomyMetadata,
+  type ReadonlyProfanityFilter,
 } from "../dist/index.js";
 
 const category: ProfanityCategory = "OBSCENE_MAT";
@@ -50,15 +51,20 @@ const dictionaryFilter: ProfanityFilter =
   createProfanityFilterFromDictionary(dictionary);
 const compiledDictionaryFilter: ProfanityFilter =
   createProfanityFilterFromCompiledDictionary(compiledDictionary);
+const sharedFilter: ReadonlyProfanityFilter = filter;
 const match: ProfanityMatchRange | undefined = strict.analyze(
   "alpha beta gamma delta",
   options,
 )[0];
 
 filter.check("plain text");
+sharedFilter.censor("plain text");
 dictionaryFilter.check("plain text");
 compiledDictionaryFilter.check("plain text");
 
+// @ts-expect-error The shared default filter is read-only.
+filter.addStrict("not-allowed");
+
 if (PROFANITY_FILTER_NAME !== "profanity") {
   throw new Error("Unexpected filter name declaration.");
 }