diff --git a/AGENTS.md b/AGENTS.md
index 4256a3c75..4ca7d7e8b 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -250,6 +250,15 @@ wait
 
 This does not apply to lightweight LLM-only targets (azure, openai, gemini, openrouter) which can run with higher concurrency.
 
+### Writing Tests
+
+Tests should be lean and focused on what matters. Follow these principles:
+
+- **Only test new or changed behavior.** Don't write tests for existing behavior that's already covered by the 1600+ core tests. If you fix a bug, test the fix and its edge cases — not the surrounding module.
+- **One test per distinct behavior.** Don't write separate tests for trivially different inputs that exercise the same code path.
+- **No tests for obvious code.** If a function returns `undefined` for missing input and that's a one-line null check, you don't need a test for it unless it's a regression risk.
+- **Regression tests > comprehensive tests.** A test that would have caught the bug is worth more than five tests that exercise happy paths.
+
 ### Verifying Evaluator Changes
 
 Unit tests alone are insufficient for evaluator changes. After implementing or modifying evaluators:
diff --git a/packages/core/src/evaluation/formatting/segment-formatter.ts b/packages/core/src/evaluation/formatting/segment-formatter.ts
index 8596a947f..666ac857a 100644
--- a/packages/core/src/evaluation/formatting/segment-formatter.ts
+++ b/packages/core/src/evaluation/formatting/segment-formatter.ts
@@ -54,9 +54,13 @@ export function formatSegment(
       return undefined;
     }
 
-    // Agent mode: return file reference only
+    // Agent mode: return file reference with absolute path so agents can locate
+    // the file regardless of their working directory. resolvedPath is set by
+    // message-processor.ts during file resolution; fall back to the display
+    // path when it is absent (e.g. in unit tests with synthetic segments).
     if (mode === 'agent') {
-      return `<file: path="${filePath}">`;
+      const absolutePath = asString(segment.resolvedPath) ?? filePath;
+      return `<file: path="${absolutePath}">`;
     }
 
     // LM mode: return embedded content with XML tags
diff --git a/packages/core/test/evaluation/formatting/segment-formatter.test.ts b/packages/core/test/evaluation/formatting/segment-formatter.test.ts
new file mode 100644
index 000000000..d44642b02
--- /dev/null
+++ b/packages/core/test/evaluation/formatting/segment-formatter.test.ts
@@ -0,0 +1,20 @@
+import { describe, expect, it } from 'bun:test';
+
+import { formatSegment } from '../../../src/evaluation/formatting/segment-formatter.js';
+
+describe('formatSegment agent mode file paths', () => {
+  it('uses resolvedPath (absolute) when available', () => {
+    const segment = {
+      type: 'file',
+      path: 'snippets/data.csv',
+      text: 'content',
+      resolvedPath: '/abs/path/to/snippets/data.csv',
+    };
+    expect(formatSegment(segment, 'agent')).toBe('<file: path="/abs/path/to/snippets/data.csv">');
+  });
+
+  it('falls back to display path when resolvedPath is absent', () => {
+    const segment = { type: 'file', path: 'snippets/data.csv', text: 'content' };
+    expect(formatSegment(segment, 'agent')).toBe('<file: path="snippets/data.csv">');
+  });
+});