diff --git a/gitnexus/src/core/group/config-parser.ts b/gitnexus/src/core/group/config-parser.ts
index edaeeeec2..bd803981c 100644
--- a/gitnexus/src/core/group/config-parser.ts
+++ b/gitnexus/src/core/group/config-parser.ts
@@ -89,10 +89,25 @@ export function parseGroupConfig(yamlContent: string): GroupConfig {
   };
 }
 
+export class GroupNotFoundError extends Error {
+  constructor(public readonly groupName: string) {
+    super(`Group "${groupName}" not found`);
+    this.name = 'GroupNotFoundError';
+  }
+}
+
 export async function loadGroupConfig(groupDir: string): Promise<GroupConfig> {
   const fsp = await import('node:fs/promises');
   const path = await import('node:path');
   const yamlPath = path.join(groupDir, 'group.yaml');
-  const content = await fsp.readFile(yamlPath, 'utf-8');
+  let content: string;
+  try {
+    content = await fsp.readFile(yamlPath, 'utf-8');
+  } catch (err) {
+    if ((err as NodeJS.ErrnoException).code === 'ENOENT') {
+      throw new GroupNotFoundError(path.basename(groupDir));
+    }
+    throw err;
+  }
   return parseGroupConfig(content);
 }
diff --git a/gitnexus/src/core/group/cross-impact.ts b/gitnexus/src/core/group/cross-impact.ts
index a80336b26..f8625cdc5 100644
--- a/gitnexus/src/core/group/cross-impact.ts
+++ b/gitnexus/src/core/group/cross-impact.ts
@@ -15,7 +15,7 @@ import type {
   OutOfScopeLink,
 } from './types.js';
 import type { GroupRepoHandle, GroupToolPort } from './service.js';
-import { loadGroupConfig } from './config-parser.js';
+import { GroupNotFoundError, loadGroupConfig } from './config-parser.js';
 import {
   fileMatchesServicePrefix,
   normalizeServicePrefix,
@@ -329,6 +329,8 @@ export async function runGroupImpact(
   try {
     config = await loadGroupConfig(groupDir);
   } catch (e) {
+    if (e instanceof GroupNotFoundError)
+      return { error: `Group "${name}" not found. Run group_list to see configured groups.` };
     return { error: e instanceof Error ? e.message : String(e) };
   }
 
@@ -344,9 +346,6 @@ export async function runGroupImpact(
     minConfidence,
   };
 
-  // Single shared deadline for Phase 1 (local walk) + Phase 2 (bridge fan-out).
-  // Phase 1 still gets the full budget; Phase 2 only uses whatever wall-clock
-  // time is left, so total work cannot exceed `timeoutMs`.
   const deadline = Date.now() + Math.max(0, timeoutMs);
 
   const { value: local, timedOut: localTimedOut } = await safeLocalImpact(
@@ -357,7 +356,7 @@ export async function runGroupImpact(
   );
 
   if (localTimedOut) {
-    const base = local as Record<string, unknown>;
+    const _base = local as Record<string, unknown>;
     return {
       local,
       group: name,
@@ -464,7 +463,6 @@ export async function runGroupImpact(
         continue;
       }
       if (!repoInSubgroup(n.neighborRepo, subgroup)) {
-        // CrossLink convention: consumer -> provider
         outOfScope.push({
           from: direction === 'upstream' ? n.neighborRepo : repoPath,
           to: direction === 'upstream' ? repoPath : n.neighborRepo,
diff --git a/gitnexus/src/core/group/service.ts b/gitnexus/src/core/group/service.ts
index afbb66e0e..a412ceaa8 100644
--- a/gitnexus/src/core/group/service.ts
+++ b/gitnexus/src/core/group/service.ts
@@ -6,7 +6,7 @@
 import fsp from 'node:fs/promises';
 import path from 'node:path';
 import { checkStaleness } from '../git-staleness.js';
-import { loadGroupConfig } from './config-parser.js';
+import { GroupNotFoundError, loadGroupConfig } from './config-parser.js';
 import {
   fileMatchesServicePrefix,
   normalizeServicePrefix,
@@ -221,7 +221,14 @@ export class GroupService {
       return { groups };
     }
     const groupDir = getGroupDir(getDefaultGitnexusDir(), name);
-    const config = await loadGroupConfig(groupDir);
+    let config: GroupConfig;
+    try {
+      config = await loadGroupConfig(groupDir);
+    } catch (err) {
+      if (err instanceof GroupNotFoundError)
+        return { error: `Group "${name}" not found. Run group_list to see configured groups.` };
+      throw err;
+    }
     return {
       name: config.name,
       description: config.description,
@@ -234,7 +241,14 @@ export class GroupService {
     const name = String(params.name ?? '').trim();
     if (!name) return { error: 'name is required' };
     const groupDir = getGroupDir(getDefaultGitnexusDir(), name);
-    const config = await loadGroupConfig(groupDir);
+    let config: GroupConfig;
+    try {
+      config = await loadGroupConfig(groupDir);
+    } catch (err) {
+      if (err instanceof GroupNotFoundError)
+        return { error: `Group "${name}" not found. Run group_list to see configured groups.` };
+      throw err;
+    }
     const result = await syncGroup(config, {
       groupDir,
       exactOnly: Boolean(params.exactOnly),
@@ -313,6 +327,14 @@ export class GroupService {
     try {
       config = await loadGroupConfig(groupDir);
     } catch (e) {
+      if (e instanceof GroupNotFoundError)
+        return {
+          group: name,
+          target: target || uid,
+          service: servicePrefix,
+          error: `Group "${name}" not found. Run group_list to see configured groups.`,
+          results: [],
+        };
       return {
         group: name,
         target: target || uid,
@@ -326,9 +348,6 @@ export class GroupService {
       repoInSubgroup(repoPath, subgroup, subgroupExact),
     );
 
-    // Per-repo work is independent (each repo opens its own DB handle and the
-    // group-level result preserves repo iteration order via the indexed map).
-    // Errors are caught per repo so one slow/failed member does not block the rest.
     const results: GroupContextResult['results'] = await Promise.all(
       memberEntries.map(async ([repoPath, registryName]) => {
         try {
@@ -384,14 +403,19 @@ export class GroupService {
     const subgroup = typeof params.subgroup === 'string' ? params.subgroup : undefined;
     const subgroupExact = params.subgroupExact === true;
     const groupDir = getGroupDir(getDefaultGitnexusDir(), name);
-    const config = await loadGroupConfig(groupDir);
+    let config: GroupConfig;
+    try {
+      config = await loadGroupConfig(groupDir);
+    } catch (err) {
+      if (err instanceof GroupNotFoundError)
+        return { error: `Group "${name}" not found. Run group_list to see configured groups.` };
+      throw err;
+    }
 
     const memberEntries = Object.entries(config.repos).filter(([repoPath]) =>
       repoInSubgroup(repoPath, subgroup, subgroupExact),
     );
 
-    // Per-repo query is independent; run them concurrently and isolate
-    // failures so one slow/failed member does not block the rest.
     const perRepo = await Promise.all(
       memberEntries.map(async ([repoPath, registryName]) => {
         try {
@@ -436,7 +460,14 @@ export class GroupService {
     const name = String(params.name ?? '').trim();
     if (!name) return { error: 'name is required' };
     const groupDir = getGroupDir(getDefaultGitnexusDir(), name);
-    const config = await loadGroupConfig(groupDir);
+    let config: GroupConfig;
+    try {
+      config = await loadGroupConfig(groupDir);
+    } catch (err) {
+      if (err instanceof GroupNotFoundError)
+        return { error: `Group "${name}" not found. Run group_list to see configured groups.` };
+      throw err;
+    }
     const registry = await readContractRegistry(groupDir);
 
     const repoStatuses: Record<
diff --git a/gitnexus/test/fixtures/lang-resolution/csharp-hello/Hello.cs b/gitnexus/test/fixtures/lang-resolution/csharp-hello/Hello.cs
new file mode 100644
index 000000000..ce5bdd541
--- /dev/null
+++ b/gitnexus/test/fixtures/lang-resolution/csharp-hello/Hello.cs
@@ -0,0 +1,17 @@
+namespace Demo;
+
+public class Greeter
+{
+    public string Greet(string name) => $"Hello, {name}!";
+
+    public static void Main(string[] args)
+    {
+        var g = new Greeter();
+        System.Console.WriteLine(g.Greet("world"));
+    }
+}
+
+public interface IFoo
+{
+    void Bar();
+}
diff --git a/gitnexus/test/integration/resolvers/csharp.test.ts b/gitnexus/test/integration/resolvers/csharp.test.ts
index 83ffd3585..31c49378c 100644
--- a/gitnexus/test/integration/resolvers/csharp.test.ts
+++ b/gitnexus/test/integration/resolvers/csharp.test.ts
@@ -124,10 +124,10 @@ describe('C# ambiguous symbol resolution', () => {
 
     // The key invariant: no edge points to Other/
     if (extends_[0].targetFilePath) {
-      expect(extends_[0].targetFilePath).not.toMatch(/Other\//);
+      expect(extends_[0].targetFilePath).not.toContain('Other/');
     }
     if (implements_[0].targetFilePath) {
-      expect(implements_[0].targetFilePath).not.toMatch(/Other\//);
+      expect(implements_[0].targetFilePath).not.toContain('Other/');
     }
   });
 });
@@ -2048,3 +2048,77 @@ describe('C# interface-to-interface heritage', () => {
     expect(implements_.length).toBe(4);
   });
 });
+
+// ---------------------------------------------------------------------------
+// C# parse completeness regression (#903)
+// ---------------------------------------------------------------------------
+
+describe('C# parse completeness (#903 regression)', () => {
+  let result: PipelineResult;
+
+  beforeAll(async () => {
+    result = await runPipelineFromRepo(path.join(FIXTURES, 'csharp-hello'), () => {});
+  }, 60000);
+
+  it('parse phase completes without error (no crash)', () => {
+    expect(result).toBeDefined();
+    expect(result.graph).toBeDefined();
+  });
+
+  it('emits Class node for Greeter', () => {
+    const classes = getNodesByLabel(result, 'Class');
+    expect(classes).toContain('Greeter');
+  });
+
+  it('emits Interface node for IFoo', () => {
+    const interfaces = getNodesByLabel(result, 'Interface');
+    expect(interfaces).toContain('IFoo');
+  });
+
+  it('emits Method nodes for Greet, Main, and Bar', () => {
+    const methods = getNodesByLabel(result, 'Method');
+    expect(methods).toContain('Greet');
+    expect(methods).toContain('Main');
+    expect(methods).toContain('Bar');
+  });
+
+  it('Greet has parameterCount=1 and returnType=string', () => {
+    const methods = getNodesByLabelFull(result, 'Method');
+    const greet = methods.find((m) => m.name === 'Greet');
+    expect(greet).toBeDefined();
+    expect(greet!.properties.parameterCount).toBe(1);
+    expect(greet!.properties.returnType).toBe('string');
+    expect(greet!.properties.visibility).toBe('public');
+  });
+
+  it('Main has parameterCount=1 and isStatic=true', () => {
+    const methods = getNodesByLabelFull(result, 'Method');
+    const main = methods.find((m) => m.name === 'Main');
+    expect(main).toBeDefined();
+    expect(main!.properties.parameterCount).toBe(1);
+    expect(main!.properties.isStatic).toBe(true);
+    expect(main!.properties.visibility).toBe('public');
+  });
+
+  it('Bar is abstract with parameterCount=0 and returnType=void', () => {
+    const methods = getNodesByLabelFull(result, 'Method');
+    const bar = methods.find((m) => m.name === 'Bar');
+    expect(bar).toBeDefined();
+    expect(bar!.properties.parameterCount).toBe(0);
+    expect(bar!.properties.isAbstract).toBe(true);
+    expect(bar!.properties.returnType).toBe('void');
+  });
+
+  it('emits HAS_METHOD edges linking Greeter to its methods', () => {
+    const hasMethod = getRelationships(result, 'HAS_METHOD');
+    const targets = edgeSet(hasMethod);
+    expect(targets).toContain('Greeter → Greet');
+    expect(targets).toContain('Greeter → Main');
+  });
+
+  it('emits HAS_METHOD edge linking IFoo to Bar', () => {
+    const hasMethod = getRelationships(result, 'HAS_METHOD');
+    const targets = edgeSet(hasMethod);
+    expect(targets).toContain('IFoo → Bar');
+  });
+});
diff --git a/gitnexus/test/unit/group-service-not-found.test.ts b/gitnexus/test/unit/group-service-not-found.test.ts
new file mode 100644
index 000000000..b16f9074f
--- /dev/null
+++ b/gitnexus/test/unit/group-service-not-found.test.ts
@@ -0,0 +1,93 @@
+import { beforeEach, describe, expect, it, vi } from 'vitest';
+
+const loadGroupConfigMock = vi.fn();
+const getGroupDirMock = vi.fn(() => '/fake/.gitnexus/groups/missing');
+const getDefaultGitnexusDirMock = vi.fn(() => '/fake/.gitnexus');
+const readContractRegistryMock = vi.fn(() => null);
+const listGroupsMock = vi.fn(() => []);
+const syncGroupMock = vi.fn();
+
+vi.mock('../../src/core/group/config-parser.js', async () => {
+  const { GroupNotFoundError } = await vi.importActual<
+    typeof import('../../src/core/group/config-parser.js')
+  >('../../src/core/group/config-parser.js');
+  return { loadGroupConfig: loadGroupConfigMock, GroupNotFoundError };
+});
+
+vi.mock('../../src/core/group/storage.js', () => ({
+  getDefaultGitnexusDir: getDefaultGitnexusDirMock,
+  getGroupDir: getGroupDirMock,
+  readContractRegistry: readContractRegistryMock,
+  listGroups: listGroupsMock,
+}));
+
+vi.mock('../../src/core/group/sync.js', () => ({ syncGroup: syncGroupMock }));
+vi.mock('../../src/core/git-staleness.js', () => ({ checkStaleness: vi.fn() }));
+
+describe('GroupService — missing group error handling', () => {
+  let GroupService: typeof import('../../src/core/group/service.js').GroupService;
+  let GroupNotFoundError: typeof import('../../src/core/group/config-parser.js').GroupNotFoundError;
+  let service: InstanceType<typeof import('../../src/core/group/service.js').GroupService>;
+
+  const stubPort = {
+    resolveRepo: vi.fn(),
+    impact: vi.fn(),
+    query: vi.fn(),
+    impactByUid: vi.fn(),
+    contextByUid: vi.fn(),
+  };
+
+  beforeEach(async () => {
+    vi.resetModules();
+    loadGroupConfigMock.mockReset();
+    ({ GroupService } = await import('../../src/core/group/service.js'));
+    ({ GroupNotFoundError } = await import('../../src/core/group/config-parser.js'));
+    service = new GroupService(stubPort as never);
+    loadGroupConfigMock.mockRejectedValue(new GroupNotFoundError('missing'));
+  });
+
+  it('groupSync returns friendly error for missing group', async () => {
+    const result = await service.groupSync({ name: 'missing' });
+    expect(result).toEqual({
+      error: 'Group "missing" not found. Run group_list to see configured groups.',
+    });
+  });
+
+  it('groupQuery returns friendly error for missing group', async () => {
+    const result = await service.groupQuery({ name: 'missing', query: 'auth' });
+    expect(result).toEqual({
+      error: 'Group "missing" not found. Run group_list to see configured groups.',
+    });
+  });
+
+  it('groupStatus returns friendly error for missing group', async () => {
+    const result = await service.groupStatus({ name: 'missing' });
+    expect(result).toEqual({
+      error: 'Group "missing" not found. Run group_list to see configured groups.',
+    });
+  });
+
+  it('groupSync re-throws non-ENOENT errors', async () => {
+    loadGroupConfigMock.mockRejectedValue(new Error('YAML parse error'));
+    await expect(service.groupSync({ name: 'bad-yaml' })).rejects.toThrow('YAML parse error');
+  });
+
+  it('groupQuery re-throws non-ENOENT errors', async () => {
+    loadGroupConfigMock.mockRejectedValue(new Error('YAML parse error'));
+    await expect(service.groupQuery({ name: 'bad-yaml', query: 'auth' })).rejects.toThrow(
+      'YAML parse error',
+    );
+  });
+
+  it('groupStatus re-throws non-ENOENT errors', async () => {
+    loadGroupConfigMock.mockRejectedValue(new Error('YAML parse error'));
+    await expect(service.groupStatus({ name: 'bad-yaml' })).rejects.toThrow('YAML parse error');
+  });
+
+  it('groupList returns friendly error for missing group', async () => {
+    const result = await service.groupList({ name: 'missing' });
+    expect(result).toEqual({
+      error: 'Group "missing" not found. Run group_list to see configured groups.',
+    });
+  });
+});