diff --git a/gitnexus/src/mcp/local/local-backend.ts b/gitnexus/src/mcp/local/local-backend.ts
index 2cf60e64b..72741bd2d 100644
--- a/gitnexus/src/mcp/local/local-backend.ts
+++ b/gitnexus/src/mcp/local/local-backend.ts
@@ -163,10 +163,15 @@ function normalizeToolParams(
   const normalized = { ...input };
   for (const { canonical, aliases } of definitions) {
     const keys = [canonical, ...aliases];
-    const supplied = keys.flatMap((key) => {
+    const supplied: Array<{ key: string; value: string }> = [];
+    for (const key of keys) {
+      if (!Object.prototype.hasOwnProperty.call(input, key)) continue;
       const value = input[key];
-      return typeof value === 'string' && value.trim() ? [{ key, value: value.trim() }] : [];
-    });
+      if (typeof value !== 'string' || !value.trim()) {
+        return { error: `MCP parameter ${method}.${key} must be a non-empty string.` };
+      }
+      supplied.push({ key, value: value.trim() });
+    }
     const distinctValues = new Set(supplied.map(({ value }) => value));
     if (distinctValues.size > 1) {
       return {
@@ -179,6 +184,14 @@ function normalizeToolParams(
     for (const alias of aliases) delete normalized[alias];
     if (supplied.length > 0) normalized[canonical] = supplied[0].value;
   }
+
+  if (
+    method === 'impact' &&
+    typeof normalized.target !== 'string' &&
+    (typeof normalized.target_uid !== 'string' || !normalized.target_uid.trim())
+  ) {
+    return { error: 'MCP impact requires target, name, symbol, or target_uid.' };
+  }
   return { params: normalized };
 }
 
diff --git a/gitnexus/src/mcp/tools.ts b/gitnexus/src/mcp/tools.ts
index 57f71f675..792f62b3b 100644
--- a/gitnexus/src/mcp/tools.ts
+++ b/gitnexus/src/mcp/tools.ts
@@ -591,7 +591,6 @@ SERVICE: optional monorepo path prefix (case-sensitive path segments). When "rep
         },
       },
       required: ['direction'],
-      anyOf: [{ required: ['target'] }, { required: ['name'] }, { required: ['symbol'] }],
     },
   },
   {
diff --git a/gitnexus/test/unit/calltool-dispatch.test.ts b/gitnexus/test/unit/calltool-dispatch.test.ts
index cbd2f3cef..7439f0239 100644
--- a/gitnexus/test/unit/calltool-dispatch.test.ts
+++ b/gitnexus/test/unit/calltool-dispatch.test.ts
@@ -391,6 +391,43 @@ describe('LocalBackend.callTool', () => {
     expect(resolveSpy).not.toHaveBeenCalled();
   });
 
+  it.each([
+    ['impact', { target: '', direction: 'upstream' }],
+    ['impact', { name: 42, direction: 'upstream' }],
+    ['context', { name: 'validate', file: '   ' }],
+  ])('rejects invalid %s aliases before repository resolution', async (method, params) => {
+    const resolveSpy = vi.spyOn(backend, 'resolveRepo');
+
+    const result = await backend.callTool(method, params);
+
+    expect(result.error).toMatch(/non-empty string/i);
+    expect(resolveSpy).not.toHaveBeenCalled();
+  });
+
+  it('rejects a missing impact target before repository resolution', async () => {
+    const resolveSpy = vi.spyOn(backend, 'resolveRepo');
+
+    const result = await backend.callTool('impact', { direction: 'upstream' });
+
+    expect(result.error).toMatch(/requires target, name, symbol, or target_uid/i);
+    expect(resolveSpy).not.toHaveBeenCalled();
+  });
+
+  it('preserves target_uid-only impact dispatch', async () => {
+    const impactSpy = vi
+      .spyOn(backend as any, 'impact')
+      .mockResolvedValue({ status: 'normalized' });
+
+    await backend.callTool('impact', {
+      target_uid: 'Function:src/auth.ts:validate',
+      direction: 'upstream',
+    });
+
+    expect(impactSpy.mock.calls[0][1]).toMatchObject({
+      target_uid: 'Function:src/auth.ts:validate',
+    });
+  });
+
   it('normalizes impact aliases before @group forwarding', async () => {
     resolveAtMemberMock.mockResolvedValue({ ok: true, repoPath: '/tmp/test-project' });
     const groupImpactSpy = vi
diff --git a/gitnexus/test/unit/tools.test.ts b/gitnexus/test/unit/tools.test.ts
index 4757fd9f7..82cd718fc 100644
--- a/gitnexus/test/unit/tools.test.ts
+++ b/gitnexus/test/unit/tools.test.ts
@@ -144,17 +144,15 @@ describe('GITNEXUS_TOOLS', () => {
     expect(apiImpactTool.inputSchema.required).toEqual([]);
   });
 
-  it('impact tool requires direction and accepts target, name, or symbol', () => {
+  it('impact tool requires direction and advertises target, name, or symbol without combinators', () => {
     const impactTool = GITNEXUS_TOOLS.find((t) => t.name === 'impact')!;
     expect(impactTool.inputSchema.required).toContain('direction');
     expect(impactTool.inputSchema.required).not.toContain('target');
     expect(impactTool.inputSchema.properties.name).toMatchObject({ type: 'string' });
     expect(impactTool.inputSchema.properties.symbol).toMatchObject({ type: 'string' });
-    expect(impactTool.inputSchema.anyOf).toEqual([
-      { required: ['target'] },
-      { required: ['name'] },
-      { required: ['symbol'] },
-    ]);
+    expect(impactTool.inputSchema).not.toHaveProperty('anyOf');
+    expect(impactTool.inputSchema).not.toHaveProperty('oneOf');
+    expect(impactTool.inputSchema).not.toHaveProperty('allOf');
   });
 
   it('impact tool advertises the PDG-only `line` statement anchor (integer, min 0, not required)', () => {