diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index d088b5f..03b7712 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -125,9 +125,10 @@ diffguardian/
 │   │       ├── index.ts        # Rule barrel file
 │   │       ├── R01_param_removed.ts
 │   │       ├── R02_param_reordered.ts
-│   │       ├── ...             # 28 rules total
+│   │       ├── ...             # 29 rules total
 │   │       ├── R29_type_alias_union_narrowed.ts
-│   │       └── R30_generator_toggle.ts
+│   │       ├── R30_generator_toggle.ts
+│   │       └── R31_abstract_modifier_added.ts
 │   │
 │   ├── reporter/
 │   │   ├── types.ts            # Reporter interface and config types
@@ -238,7 +239,7 @@ export const yourRule: FunctionRule = {
 
 Before submitting a new rule, ensure the following:
 
-1. Assign the next available rule ID (`R31`, `R32`, etc.)
+1. Assign the next available rule ID (`R32`, `R33`, etc.)
 2. Create the rule file in `src/classifier/rules/`
 3. Export it from `src/classifier/rules/index.ts`
 4. Write tests covering both positive and negative cases
diff --git a/README.md b/README.md
index 3f483e0..e96591a 100644
--- a/README.md
+++ b/README.md
@@ -33,7 +33,7 @@ The result? Silent regressions, broken CI/CD pipelines, and painful merge resolu
 ### The Solution
 Traditional diffs show **what changed**. Diff Guardian shows **what breaks**.
 
-Diff Guardian acts as an automated safety net. Using WASM-compiled Tree-Sitter grammars, it parses your code into abstract syntax tree (AST) signatures, compares the before and after states across any branch, and evaluates every diff against **28 strict production rules**. It then traces every call site across your ecosystem to show **exactly who is affected** before you merge or push.
+Diff Guardian acts as an automated safety net. Using WASM-compiled Tree-Sitter grammars, it parses your code into abstract syntax tree (AST) signatures, compares the before and after states across any branch, and evaluates every diff against **29 strict production rules**. It then traces every call site across your ecosystem to show **exactly who is affected** before you merge or push.
 
 ```
 $ npx dg compare main feature-branch
@@ -61,7 +61,7 @@ $ npx dg compare main feature-branch
 | Capability | Description |
 |---|---|
 | **AST-Powered Analysis** | Tree-Sitter WASM grammars parse your code into structural signatures — not regex |
-| **28 Classification Rules** | Covers parameter changes, return types, generics, visibility, enums, interfaces, type aliases, and more |
+| **29 Classification Rules** | Covers parameter changes, return types, generics, visibility, enums, interfaces, type aliases, and more |
 | **Blast Radius Tracing** | JIT import scanner and call-site tracer shows every consumer affected by a breaking change |
 | **CI/CD Native** | Auto-detects GitHub Actions and posts PR comments with full audit reports |
 | **Git Hook Enforcement** | Built-in Husky hooks block broken code at `pre-push`, `pre-merge-commit`, and `post-merge` |
@@ -185,7 +185,7 @@ Shows every file that imports the given symbol and where it is used:
 npx dg rules
 ```
 
-Prints all 28 classification rules with their IDs, names, targets, and descriptions.
+Prints all 29 classification rules with their IDs, names, targets, and descriptions.
 
 > For detailed examples and remediation guidance, see the [full rules documentation](https://diffguardian.vercel.app/docs/rules/all).
 
diff --git a/client/src/components/Terminal.tsx b/client/src/components/Terminal.tsx
index 4f0e317..d37aa5e 100644
--- a/client/src/components/Terminal.tsx
+++ b/client/src/components/Terminal.tsx
@@ -102,7 +102,7 @@ const COMMANDS: Record<string, TermLine[]> = {
     { text: "  R25 - Interface Prop Required [Target: interface]", type: "trace" },
     { text: "  R26 - Interface Prop Removed  [Target: interface]", type: "trace" },
     { text: "  R27 - Enum Member Changed    [Target: enum]", type: "trace" },
-    { text: "  ... and 19 more rules", type: "info" },
+    { text: "  ... and 20 more rules", type: "info" },
   ],
 
   "npx dg init": [
diff --git a/client/src/components/docs/DocsSidebar.tsx b/client/src/components/docs/DocsSidebar.tsx
index 6d6c6ce..2880f81 100644
--- a/client/src/components/docs/DocsSidebar.tsx
+++ b/client/src/components/docs/DocsSidebar.tsx
@@ -77,6 +77,7 @@ const sections: SidebarSection[] = [
       { label: "R28 Exported", slug: "rules/r28" },
       { label: "R29 Union Narrowed", slug: "rules/r29" },
       { label: "R30 Generator Toggle", slug: "rules/r30" },
+      { label: "R31 Abstract Modifier Added", slug: "rules/r31" },
     ],
   },
   {
diff --git a/client/src/content/docs/cli-rules.tsx b/client/src/content/docs/cli-rules.tsx
index 4ffa20d..0eac142 100644
--- a/client/src/content/docs/cli-rules.tsx
+++ b/client/src/content/docs/cli-rules.tsx
@@ -39,6 +39,7 @@ const allRules: RuleEntry[] = [
   { id: "R28", name: "Visibility Widened", target: "function", severity: "safe", languages: "All", description: "A previously internal symbol becomes exported. The API surface expands, but existing callers are unaffected.", example: "function helper() -> export function helper()" },
   { id: "R29", name: "Type Alias Union Narrowed", target: "type_alias", severity: "breaking", languages: "TypeScript", description: "A string/number/boolean literal union type alias loses one of its members. Exhaustive switch statements and literal comparisons against the removed value will fail to compile or silently mishandle it.", example: "type Status = 'active' | 'inactive' -> type Status = 'active'" },
   { id: "R30", name: "Generator Function Toggle", target: "function", severity: "breaking", languages: "TS, Python", description: "A function adds or removes the generator modifier (function*, yield). The calling convention changes completely — callers receive an iterator instead of a direct value, or vice versa.", example: "function nextId() -> function* nextId()" },
+  { id: "R31", name: "Abstract Modifier Added", target: "function", severity: "breaking", languages: "All", description: "A concrete method is changed to abstract. Every existing subclass that does not already override this method will fail to compile, since a concrete implementation is no longer inherited.", example: "process(): void { ... } -> abstract process(): void;" },
 ];
 
 function SeverityBadge({ severity }: { severity: string }) {
diff --git a/client/src/content/docs/rules-data.ts b/client/src/content/docs/rules-data.ts
index 6d0e2e4..c7dd459 100644
--- a/client/src/content/docs/rules-data.ts
+++ b/client/src/content/docs/rules-data.ts
@@ -1072,6 +1072,45 @@ const id = nextId(); // id is now a Generator object, not a number`,
     ],
     relatedRules: ["R11", "R17", "R21"],
   },
+  "R31": {
+    id: "R31",
+    name: "Abstract Modifier Added",
+    severity: "breaking",
+    target: "function",
+    languages: ["All languages"],
+    summary: "Flags when a concrete method gains the `abstract` modifier. Every subclass that inherited the previous default implementation without overriding it will fail to compile, since that default no longer exists.",
+    whyItMatters: `An abstract method has no body — it's a contract every subclass must fulfill. When a previously concrete method becomes abstract, any subclass that relied on the inherited default implementation loses it instantly. The subclass compiles fine in isolation right up until this change; the break appears at every subclass definition, not at the method itself.`,
+    howItWorks: `The translator stamps an \`isAbstract: boolean\` flag on every \`FunctionSignature\` by detecting the \`abstract\` modifier keyword (TypeScript, Java) or the language's equivalent construct (Python's \`@abstractmethod\`, Go/Rust trait conventions) during AST parsing. The rule normalizes \`undefined\` to \`false\` and only fires when the modifier is newly added (\`false\` → \`true\`). Removing \`abstract\` (\`true\` → \`false\`) is intentionally not flagged — it only adds a usable default implementation, which breaks nothing.`,
+    beforeCode: `// service.ts
+export abstract class Service {
+  process(): void {
+    console.log('default behavior');
+  }
+}`,
+    afterCode: `// service.ts (BREAKING — method is now abstract)
+export abstract class Service {
+  abstract process(): void;
+}
+
+// subclass.ts — BREAKS
+export class MyService extends Service {}
+// Error: non-abstract class 'MyService' does not implement
+// inherited abstract member 'process'.`,
+    beforeLabel: "Before",
+    afterLabel: "After (converted to abstract)",
+    cliOutput: `$ npx dg check
+
+  [BREAKING] Service#process (modifier_changed)
+  src/service.ts:2
+  Method was made abstract. Every existing subclass that does not already override this method will fail to compile, since a concrete implementation is no longer inherited.`,
+    realWorldScenario: `A maintainer decides a base class method should no longer have a sensible default and marks it abstract to force every subclass to think about its own implementation. The change compiles cleanly in the file where it was made — the compiler errors only appear in every downstream subclass file, often in a different package or repository the maintainer never opened.`,
+    edgeCases: [
+      "isAbstract undefined on both sides (the vast majority of methods) is normalized to false — no false positive",
+      "Only the concrete → abstract direction fires; abstract → concrete is safe and silent by design",
+      "Applies only to methods (functions with a className) in practice, since free functions never receive isAbstract: true from any translator",
+    ],
+    relatedRules: ["R17", "R30"],
+  },
 };
 
 /** Return ordered list of rule IDs */
diff --git a/src/classifier/rules/R31_abstract_modifier_added.ts b/src/classifier/rules/R31_abstract_modifier_added.ts
new file mode 100644
index 0000000..73343b3
--- /dev/null
+++ b/src/classifier/rules/R31_abstract_modifier_added.ts
@@ -0,0 +1,48 @@
+/**
+ * RULE 31: Abstract Modifier Added
+ * Flags when a concrete method gains the `abstract` modifier.
+ * This is a breaking change because every existing subclass that does not
+ * already override this method was previously inheriting a working default
+ * implementation. Once the method becomes abstract, that default is gone —
+ * those subclasses will fail to compile until they supply their own
+ * implementation.
+ *
+ * The reverse direction (abstract → concrete) is safe and non-breaking, but
+ * still reported as an advisory `safe` result: subclasses now have an
+ * optional default implementation available, and downstream developers may
+ * want to know they can delete their now-unnecessary override boilerplate.
+ */
+
+import { FunctionRule, RuleResult } from '../types';
+
+export const abstractModifierAddedRule: FunctionRule = {
+  id: 'R31',
+  name: 'Abstract Modifier Added',
+  description: 'Flags when a concrete method is changed to abstract, forcing every subclass to supply its own implementation.',
+  languages: 'all',
+  target: 'function',
+
+  check(oldSig, newSig): RuleResult | null {
+    // Normalize undefined to false for safe boolean comparison
+    const wasAbstract = oldSig.isAbstract === true;
+    const isAbstractNow = newSig.isAbstract === true;
+
+    if (!wasAbstract && isAbstractNow) {
+      return {
+        severity: 'breaking',
+        changeType: 'modifier_changed',
+        message: `Method '${newSig.name}' was made abstract. Every existing subclass that does not already override this method will fail to compile, since a concrete implementation is no longer inherited.`,
+      };
+    }
+
+    if (wasAbstract && !isAbstractNow) {
+      return {
+        severity: 'safe',
+        changeType: 'modifier_changed',
+        message: `Method '${newSig.name}' is no longer abstract and now has a default implementation. Existing subclass overrides remain valid and are now optional — safe to remove if the default is sufficient.`,
+      };
+    }
+
+    return null;
+  },
+};
diff --git a/src/classifier/rules/index.ts b/src/classifier/rules/index.ts
index efed75a..f4b3ee9 100644
--- a/src/classifier/rules/index.ts
+++ b/src/classifier/rules/index.ts
@@ -26,3 +26,4 @@ export * from './R27_enum_changed';
 export * from './R28_exported';
 export * from './R29_type_alias_union_narrowed';
 export * from './R30_generator_toggle';
+export * from './R31_abstract_modifier_added';
diff --git a/src/parsers/translators/typescript.ts b/src/parsers/translators/typescript.ts
index 0200f0a..e9b7764 100644
--- a/src/parsers/translators/typescript.ts
+++ b/src/parsers/translators/typescript.ts
@@ -8,6 +8,7 @@
  * Covers every TypeScript/JavaScript construct that produces a public API:
  *  - Top-level functions            (function_declaration, generator_function_declaration)
  *  - Class methods                  (method_definition)
+ *  - Abstract class methods         (abstract_method_signature)
  *  - Interface method signatures    (method_signature)
  *  - Exported arrow functions       (lexical_declaration → arrow_function)
  *  - Class constructors             (constructor)
@@ -84,6 +85,12 @@ const FN_QUERY_SRC_TS = `
     parameters: (formal_parameters) @params
     return_type: (type_annotation)? @return
   ) @fn
+
+  (abstract_method_signature
+    name: (property_identifier) @name
+    parameters: (formal_parameters) @params
+    return_type: (type_annotation)? @return
+  ) @fn
 `;
 
 /**
diff --git a/tests/e2e/ast-e2e.test.ts b/tests/e2e/ast-e2e.test.ts
index 01e9c3c..ac208c2 100644
--- a/tests/e2e/ast-e2e.test.ts
+++ b/tests/e2e/ast-e2e.test.ts
@@ -178,4 +178,29 @@ describe('E2E AST Pipeline — WASM + Engine integration', () => {
     expect(changes[0].changeType).toBe('symbol_added');
   });
 
+  it('✅ Identifies a concrete method converted to abstract (R31)', async () => {
+    const oldCode = `
+      export abstract class Service {
+        process(): void {
+          console.log('default behavior');
+        }
+      }
+    `;
+    const newCode = `
+      export abstract class Service {
+        abstract process(): void;
+      }
+    `;
+
+    const diff = createMockDiff(oldCode, newCode);
+    const parsedDiffs = await mapper.buildSignatureCache([diff]);
+    const changes = engine.compare(parsedDiffs[0]);
+
+    expect(changes).toHaveLength(1);
+    expect(changes[0].severity).toBe('breaking');
+    expect(changes[0].changeType).toBe('modifier_changed');
+    expect(changes[0].message).toContain('abstract');
+    expect(changes[0].symbolType).toBe('function');
+  });
+
 });
diff --git a/tests/rules/function-rules.test.ts b/tests/rules/function-rules.test.ts
index ee0d4d0..751d914 100644
--- a/tests/rules/function-rules.test.ts
+++ b/tests/rules/function-rules.test.ts
@@ -38,6 +38,7 @@ import { defaultValueChangedRule }     from '../../src/classifier/rules/R23_defa
 import { constructorChangedRule }      from '../../src/classifier/rules/R24_constructor_changed';
 import { exportedRule }                from '../../src/classifier/rules/R28_exported';
 import { generatorToggledRule }        from '../../src/classifier/rules/R30_generator_toggle';
+import { abstractModifierAddedRule }   from '../../src/classifier/rules/R31_abstract_modifier_added';
 
 // ═══════════════════════════════════════════════════════════════════════════════
 // R01: Parameter Removed
@@ -1436,3 +1437,50 @@ describe('R30 — generatorToggledRule', () => {
     expect(result).toBeNull();
   });
 });
+
+// ─────────────────────────────────────────────────────────────────────────────
+// R31: Abstract Modifier Added
+// ─────────────────────────────────────────────────────────────────────────────
+
+describe('R31 — abstractModifierAddedRule', () => {
+
+  it('✅ True Positive: flags concrete method → abstract', () => {
+    const oldSig = mockFnSig({ isAbstract: false });
+    const newSig = mockFnSig({ isAbstract: true });
+
+    const result = abstractModifierAddedRule.check(oldSig, newSig);
+
+    expect(result).not.toBeNull();
+    expect(asSingle(result).severity).toBe('breaking');
+    expect(asSingle(result).changeType).toBe('modifier_changed');
+    expect(asSingle(result).message).toContain('abstract');
+  });
+
+  it('✅ True Positive (Safe): flags abstract → concrete as advisory', () => {
+    const oldSig = mockFnSig({ isAbstract: true });
+    const newSig = mockFnSig({ isAbstract: false });
+
+    const result = abstractModifierAddedRule.check(oldSig, newSig);
+
+    expect(result).not.toBeNull();
+    expect(asSingle(result).severity).toBe('safe');
+    expect(asSingle(result).changeType).toBe('modifier_changed');
+  });
+
+  it('🚫 False Positive: both concrete must return null', () => {
+    const sig = mockFnSig({ isAbstract: false });
+
+    const result = abstractModifierAddedRule.check(sig, sig);
+
+    expect(result).toBeNull();
+  });
+
+  it('🔲 Edge Case: isAbstract undefined on both sides must not throw and must return null', () => {
+    const oldSig = mockFnSig({});
+    const newSig = mockFnSig({});
+
+    const result = abstractModifierAddedRule.check(oldSig, newSig);
+
+    expect(result).toBeNull();
+  });
+});