diff --git a/CHANGELOG.md b/CHANGELOG.md
index 643d4d900..c864a0aad 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -20,6 +20,7 @@ and adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 ### New Features
 
 - `codegraph files --format package` and `codegraph_files` with `format: "package"` now list a project's files by the package that owns them, the nearest `package.json` or `composer.json` above each file that names one, so a monorepo splits into its workspace members with no setup and files nothing claims get a section of their own (#1871). Thanks @LuisAlbertoMK.
+- `codegraph files --json` now marks each file as a test file or not, `--tests` and `--no-tests` filter the list in any format, and `codegraph node --file` says "test file" in its header, so your own tooling can ask CodeGraph instead of keeping a copy of its naming rules. Thanks @maxmilian. (#1877)
 - `codegraph install` now sets up IBM Bob, for Bob IDE and Bob Shell: it adds the CodeGraph MCP server to `~/.bob/settings/mcp.json`, or to `.bob/mcp.json` in a project. Thanks @QuentinDstl.
 - `codegraph install` now sets up Factory Droid: it adds the CodeGraph MCP server to `~/.factory/mcp.json` or a project's `.factory/mcp.json`, plus a short CodeGraph section in `AGENTS.md`, and Droid picks it up without a restart. Thanks @Rubyj and @KrystianMarek.
 - `codegraph install` now sets up Windsurf: it adds the CodeGraph MCP server to `~/.codeium/windsurf/mcp_config.json`, which Devin, Windsurf's new name, imports by default. Thanks @SEPURI-SAI-KRISHNA.
diff --git a/README.md b/README.md
index 27c5f5aa9..9511ef7ac 100644
--- a/README.md
+++ b/README.md
@@ -548,7 +548,7 @@ codegraph unlock [path]           # Remove a stale lock file that's blocking ind
 codegraph query <search>          # Search symbols (--kind, --limit, --json)
 codegraph explore <query>         # Relevant symbols' source + call paths in one shot (same output as the codegraph_explore MCP tool)
 codegraph node <symbol|file>      # One symbol's source + callers, or read a file with line numbers (same output as codegraph_node)
-codegraph files [path]            # Show file structure (--format, --filter, --max-depth, --json)
+codegraph files [path]            # Show file structure (--format, --filter, --max-depth, --tests, --json)
 codegraph callers <symbol>        # Find what calls a function/method (--limit, --json)
 codegraph callees <symbol>        # Find what a function/method calls (--limit, --json)
 codegraph impact <symbol>         # Analyze what code is affected by changing a symbol (--depth, --json)
diff --git a/__tests__/cli-files-test-classification.test.ts b/__tests__/cli-files-test-classification.test.ts
new file mode 100644
index 000000000..928460b1a
--- /dev/null
+++ b/__tests__/cli-files-test-classification.test.ts
@@ -0,0 +1,66 @@
+import { afterAll, beforeAll, describe, expect, it } from 'vitest';
+import { execFileSync } from 'child_process';
+import * as fs from 'fs';
+import * as os from 'os';
+import * as path from 'path';
+import { CodeGraph } from '../src';
+
+const BIN = path.resolve(__dirname, '../dist/bin/codegraph.js');
+
+describe('CLI test-file classification (#1877)', () => {
+  let dir: string;
+
+  function run(command: string, args: string[]): string {
+    return execFileSync(process.execPath, [BIN, command, ...args, '-p', dir], {
+      encoding: 'utf-8',
+      env: { ...process.env, CODEGRAPH_NO_DAEMON: '1', CODEGRAPH_WASM_RELAUNCHED: '1' },
+      stdio: ['ignore', 'pipe', 'pipe'],
+    });
+  }
+
+  beforeAll(async () => {
+    dir = fs.mkdtempSync(path.join(os.tmpdir(), 'codegraph-test-files-'));
+    fs.mkdirSync(path.join(dir, 'src'));
+    fs.writeFileSync(path.join(dir, 'src/app.ts'), 'export function app() { return 1; }\n');
+    fs.writeFileSync(path.join(dir, 'src/app.test.ts'), 'import { app } from "./app";\napp();\n');
+    const cg = CodeGraph.initSync(dir);
+    await cg.indexAll();
+    cg.close();
+  });
+
+  afterAll(() => fs.rmSync(dir, { recursive: true, force: true }));
+
+  it('reports the shared test-file predicate in files JSON and filters by it', () => {
+    const all = JSON.parse(run('files', ['--json']));
+    expect(all).toEqual(expect.arrayContaining([
+      expect.objectContaining({ path: 'src/app.ts', isTest: false }),
+      expect.objectContaining({ path: 'src/app.test.ts', isTest: true }),
+    ]));
+    expect(JSON.parse(run('files', ['--json', '--tests'])).map((file: { path: string }) => file.path))
+      .toEqual(['src/app.test.ts']);
+    expect(JSON.parse(run('files', ['--json', '--no-tests'])).map((file: { path: string }) => file.path))
+      .toEqual(['src/app.ts']);
+  });
+
+  it('keeps the filter in every output format', () => {
+    for (const format of ['tree', 'flat', 'grouped', 'package']) {
+      const tests = run('files', ['--tests', '--format', format]);
+      expect(tests).toContain('app.test.ts');
+      expect(tests).not.toMatch(/app\.ts/);
+      const source = run('files', ['--no-tests', '--format', format]);
+      expect(source).toMatch(/app\.ts/);
+      expect(source).not.toContain('app.test.ts');
+    }
+  });
+
+  it('says so when a filter leaves no files', () => {
+    expect(run('files', ['--tests', '--filter', 'src/app.ts'])).toContain('No files found');
+  });
+
+  it('marks test files in the node file header', () => {
+    expect(run('node', ['-f', 'src/app.test.ts', '--symbols-only']))
+      .toContain('**src/app.test.ts** · test file');
+    expect(run('node', ['-f', 'src/app.ts', '--symbols-only']))
+      .not.toContain('test file');
+  });
+});
diff --git a/__tests__/owning-package.test.ts b/__tests__/owning-package.test.ts
index 5a606a46b..f6df0ec67 100644
--- a/__tests__/owning-package.test.ts
+++ b/__tests__/owning-package.test.ts
@@ -329,7 +329,7 @@ describe('listing an indexed project by package', () => {
       const plain = JSON.parse(codegraph('files', '--json')) as Array<Record<string, unknown>>;
 
       expect(plain.map((file) => Object.keys(file).sort())).toEqual(
-        plain.map(() => ['errors', 'language', 'nodeCount', 'path', 'size']),
+        plain.map(() => ['errors', 'isTest', 'language', 'nodeCount', 'path', 'size']),
       );
       expect(JSON.parse(codegraph('files', '--json', '--format', 'package'))).toEqual(plain);
     });
diff --git a/src/bin/codegraph.ts b/src/bin/codegraph.ts
index 001d35036..1cc271e11 100644
--- a/src/bin/codegraph.ts
+++ b/src/bin/codegraph.ts
@@ -1897,6 +1897,8 @@ program
   .option('-p, --path <path>', 'Project path')
   .option('--filter <dir>', 'Filter to files under this directory')
   .option('--pattern <glob>', 'Filter files matching this glob pattern')
+  .option('--tests', 'Show only test files')
+  .option('--no-tests', 'Exclude test files')
   .option('--format <format>', 'Output format (tree, flat, grouped, package)', 'tree')
   .option('--max-depth <number>', 'Maximum directory depth for tree format')
   .option('--no-metadata', 'Hide file metadata (language, symbol count)')
@@ -1905,6 +1907,7 @@ program
     path?: string;
     filter?: string;
     pattern?: string;
+    tests?: boolean;
     format?: string;
     maxDepth?: string;
     metadata?: boolean;
@@ -1939,6 +1942,10 @@ program
         files = files.filter(f => regex.test(f.path));
       }
 
+      if (options.tests !== undefined) {
+        files = files.filter(f => isTestPath(f.path) === options.tests);
+      }
+
       if (files.length === 0) {
         info('No files found matching the criteria.');
         cg.destroy();
@@ -1949,6 +1956,7 @@ program
       if (options.json) {
         const output = files.map(f => ({
           path: f.path,
+          isTest: isTestPath(f.path),
           language: f.language,
           nodeCount: f.nodeCount,
           size: f.size,
diff --git a/src/mcp/tools.ts b/src/mcp/tools.ts
index 9de05825f..b8e7dcbf1 100644
--- a/src/mcp/tools.ts
+++ b/src/mcp/tools.ts
@@ -36,7 +36,7 @@ import {
 } from '../sync/worktree';
 import type { PendingFile } from '../sync';
 import type { Node, Edge, SearchResult, Subgraph, NodeKind, Language } from '../types';
-import { isTestFile, normalizeNameToken, splitAtCjkSwitches, isQueryNameToken } from '../search/query-utils';
+import { isTestFile, isTestPath, normalizeNameToken, splitAtCjkSwitches, isQueryNameToken } from '../search/query-utils';
 import { groupDefinitions, isQualifiedSymbol, lastQualifierPart, matchesSymbol, qualifiedSymbolMatches } from '../graph/symbol-lookup';
 import { goPackageReaderFor } from '../graph/go-package';
 import {
@@ -8277,6 +8277,7 @@ export class ToolHandler {
     }
 
     const filePath = resolved.path;
+    const testLabel = isTestPath(filePath) ? ' · test file' : '';
     const nodes = cg.getNodesInFile(filePath)
       .filter((n) => n.kind !== 'file' && n.kind !== 'import' && n.kind !== 'export')
       .sort((a, b) => a.startLine - b.startLine);
@@ -8300,7 +8301,7 @@ export class ToolHandler {
 
     // symbolsOnly → the cheap structural overview, no source.
     if (opts.symbolsOnly) {
-      const out = [`**${filePath}** — ${nodes.length} symbol${nodes.length === 1 ? '' : 's'}, ${depSummary}`, ''];
+      const out = [`**${filePath}**${testLabel} — ${nodes.length} symbol${nodes.length === 1 ? '' : 's'}, ${depSummary}`, ''];
       if (nodes.length) out.push(...symbolMap('**Symbols**'));
       else out.push('_No indexed symbols in this file._');
       out.push('', '> Drop `symbolsOnly` (or pass `offset`/`limit`) to read the source, like Read.');
@@ -8310,7 +8311,7 @@ export class ToolHandler {
     // SECURITY (#383): never dump a raw config/data file — a yaml/properties
     // line is `key: <secret>`. Summarize by key and point to a real Read.
     if (CONFIG_LEAF_LANGUAGES.has(resolved.language)) {
-      const out = [`**${filePath}** — configuration/data file, ${depSummary}`, ''];
+      const out = [`**${filePath}**${testLabel} — configuration/data file, ${depSummary}`, ''];
       if (nodes.length) out.push(...symbolMap('**Keys (values withheld for safety)**'));
       out.push('', '> Values may be secrets, so codegraph indexes keys only. Read the file directly if you need a value.');
       return this.textResult(this.truncateOutput(out.join('\n')));
@@ -8324,7 +8325,7 @@ export class ToolHandler {
       try { content = readFileSync(abs, 'utf-8'); } catch { content = null; }
     }
     if (content === null) {
-      const out = [`**${filePath}** — could not read from disk (it may have moved since indexing). ${depSummary}`, ''];
+      const out = [`**${filePath}**${testLabel} — could not read from disk (it may have moved since indexing). ${depSummary}`, ''];
       if (nodes.length) out.push(...symbolMap('**Symbols**'));
       out.push('', `> Read \`${filePath}\` directly for its current content.`);
       return this.textResult(this.truncateOutput(out.join('\n')));
@@ -8344,11 +8345,11 @@ export class ToolHandler {
     const DEFAULT_LIMIT = 2000;
     const offset = Math.max(1, opts.offset ?? 1);
     if (offset > total) {
-      return this.textResult(`**${filePath}** has ${total} line${total === 1 ? '' : 's'} — offset ${offset} is past the end. ${depSummary}`);
+      return this.textResult(`**${filePath}**${testLabel} has ${total} line${total === 1 ? '' : 's'} — offset ${offset} is past the end. ${depSummary}`);
     }
     const maxLines = Math.max(1, opts.limit ?? DEFAULT_LIMIT);
     const start = offset - 1; // 0-based
-    const header = `**${filePath}** — ${total} lines, ${nodes.length} symbol${nodes.length === 1 ? '' : 's'} · ${depSummary}`;
+    const header = `**${filePath}**${testLabel} — ${total} lines, ${nodes.length} symbol${nodes.length === 1 ? '' : 's'} · ${depSummary}`;
 
     // Numbered lines, byte-for-byte Read's shape: `<n>\t<line>`, no left-pad.
     const numbered: string[] = [];