feat: add test file preparation and path resolution utilities

coverage-service/lib/src/index.ts

@@ -14,6 +14,7 @@ export * from './providers/anthropic-provider';
 export * from './providers/local-llm-provider';
 export * from './prompts/test-generation';
 export * from './test-paths';
+export * from './prepare-test-generation';
 export * from './analysis/code-analyzer';
 
 import { AnthropicProvider } from './providers/anthropic-provider';

coverage-service/lib/src/prepare-test-generation.ts

@@ -0,0 +1,54 @@
+import type { RepositoryProvider } from './providers/repository-provider';
+import { resolveTestFileTarget } from './test-paths';
+
+export interface PreparedTestFileContext {
+  existingTestPaths: string[];
+  existingTests: string;
+  testOutputPath: string;
+  isUpdatingExistingTest: boolean;
+}
+
+/**
+ * Discover existing tests for a source file and decide whether to update
+ * an existing test file or create a new one at the inferred path.
+ */
+export async function prepareTestFileContext(
+  repoProvider: RepositoryProvider,
+  repoDir: string,
+  sourceFile: string,
+  framework: string,
+): Promise<PreparedTestFileContext> {
+  const existingTestPaths = await repoProvider.findExistingTests(
+    repoDir,
+    sourceFile,
+    framework,
+  );
+
+  const { testOutputPath, isUpdatingExistingTest } = resolveTestFileTarget(
+    existingTestPaths,
+    sourceFile,
+    framework,
+  );
+
+  const pathsToRead = isUpdatingExistingTest
+    ? [testOutputPath]
+    : existingTestPaths;
+
+  const parts = await Promise.all(
+    pathsToRead.map((p) => repoProvider.getFileContent(repoDir, p)),
+  );
+  const existingTests =
+    parts
+      .map((content, i) =>
+        content.length > 0 ? `// ${pathsToRead[i]}\n${content}` : '',
+      )
+      .filter(Boolean)
+      .join('\n\n---\n\n') || '(none found)';
+
+  return {
+    existingTestPaths,
+    existingTests,
+    testOutputPath,
+    isUpdatingExistingTest,
+  };
+}

coverage-service/lib/src/prompts/test-generation.ts

@@ -1,4 +1,4 @@
-import { inferJavaScriptTestFilePath, inferSourceImportPath, sourceFileExtension } from '../test-paths';
+import { inferJavaScriptTestFilePath, inferSourceImportPath, inferTestFilePath, sourceFileExtension } from '../test-paths';
 import type { TestGenerationContext } from '../types';
 
 function formatRepoPackages(packages: string[]): string {
@@ -28,12 +28,43 @@
   return ctx.exportedSymbols.map((s) => `- ${s}`).join('\n');
 }
 
+function outputTestPath(ctx: TestGenerationContext): string {
+  return ctx.testOutputPath ?? inferTestFilePath(ctx.file, ctx.framework);
+}
+
+function formatTestCountGuidance(ctx: TestGenerationContext): string {
+  const exports = ctx.exportedSymbols ?? [];
+  if (exports.length > 4) {
+    return `- Write focused tests covering **each exported symbol** (${exports.length} total: ${exports.join(', ')}). At least one test per symbol, especially for uncovered lines.`;
+  }
+  if (exports.length > 0) {
+    return `- Write 2–${Math.max(4, exports.length)} focused tests covering exported symbols (${exports.join(', ')}). At least one behavior per symbol when uncovered.`;
+  }
+  return '- Write 2–4 short, focused tests. One behavior per test.';
+}
+
+function formatTestOutputMode(ctx: TestGenerationContext): string {
+  const path = outputTestPath(ctx);
+  const countGuidance = formatTestCountGuidance(ctx);
+  if (ctx.isUpdatingExistingTest) {
+    return `## Mode: UPDATE existing test file
+- Output path: \`${path}\` (this file already exists — do NOT create a new file elsewhere).
+- Keep all existing tests that still apply; do not remove or rewrite passing tests unnecessarily.
+${countGuidance}
+- Do not duplicate test names or behaviors already present in Existing Tests.
+- Return the **complete updated test file** (existing + new tests).`;
+  }
+  return `## Mode: CREATE new test file
+- Output path: \`${path}\`
+${countGuidance}`;
+}
+
 function buildPythonPrompt(ctx: TestGenerationContext, symbols: string): string {
   return `You are writing Python unit tests for ONE production file. Tests must pass on first run with no manual fixes.
 
 ## Scope
 - Generate ONLY the complete test file content. No markdown, no explanations, no extra files unless required for imports to work.
-- Write 2–4 short, focused tests. One behavior per test.
+${formatTestOutputMode(ctx)}
 - Test the file: ${ctx.file}
 
 ${formatFullSourceHeader(ctx)}## Context
@@ -134,7 +165,7 @@
 }
 
 function buildJavaScriptPrompt(ctx: TestGenerationContext, symbols: string): string {
-  const testFile = inferJavaScriptTestFilePath(ctx.file);
+  const testFile = outputTestPath(ctx);
   const importPath = inferSourceImportPath(testFile, ctx.file);
   const importExt = sourceFileExtension(ctx.file);
   const repoHasJest = ctx.repoPackages.some((p) => normalizePackageName(p) === 'jest');
@@ -156,9 +187,8 @@
 
 ## Scope
 - Generate ONLY the complete test file content. No markdown, no explanations.
-- Write 2–4 short, focused tests. One behavior per test.
+${formatTestOutputMode(ctx)}
 - Production file: ${ctx.file}
-- Save tests to: ${testFile}
 
 ${formatFullSourceHeader(ctx)}## Context
 
@@ -290,6 +320,7 @@
     return (
       'You write simple, correct JavaScript unit tests that pass on first run. ' +
       'Prefer node:test and node:assert unless the repo already uses Jest/Vitest. ' +
+      'When updating an existing test file, preserve working tests and append new cases for uncovered lines. ' +
       'Use correct relative ESM import paths with the source file extension (.js, .jsx, .ts, .tsx). ' +
       'Only import symbols listed as exported; never import private functions. ' +
       'For CLI files with no exports, test via child_process.spawnSync instead of importing. ' +
@@ -300,6 +331,7 @@
 
   return (
     'You write simple, correct unit tests that pass on first run. Mock all external I/O. ' +
+    'When updating an existing test file, preserve working tests and append new cases for uncovered lines. ' +
     'Use exact types and defaults from source models. Return only runnable test file code. ' +
     'Declare minimal extra test-only packages on the first line as # test-deps: pkg1, pkg2'
   );

coverage-service/lib/src/providers/anthropic-provider.ts

@@ -4,6 +4,10 @@ import type { GeneratedTest, TestGenerationContext } from '../types';
 
 import type { TestGenerationProvider } from './test-generation-provider';
 
+function outputPathFor(context: TestGenerationContext): string {
+  return context.testOutputPath ?? inferTestFilePath(context.file, context.framework);
+}
+
 export interface AnthropicProviderConfig {
   apiKey: string;
   model?: string;
@@ -43,7 +47,7 @@ export class AnthropicProvider implements TestGenerationProvider {
     const content = raw.replace(/^```[\w]*\n?/gm, '').replace(/```$/gm, '').trim();
 
     return {
-      filePath: inferTestFilePath(context.file, context.framework),
+      filePath: outputPathFor(context),
       content,
       targetFile: context.file,
     };

coverage-service/lib/src/providers/github-provider.ts

@@ -7,6 +7,7 @@
 import { Octokit } from '@octokit/rest';
 
 import type { ChangedFile } from '../types';
+import { orderedTestFileCandidates } from '../test-paths';
 
 import type {
   CheckoutPROptions,
@@ -261,59 +262,55 @@
   async findExistingTests(
     repoDir: string,
     sourceFile: string,
+    framework = 'pytest',
   ): Promise<string[]> {
     const { readdir } = await import('fs/promises');
-    const baseName = sourceFile.replace(/\.[^.]+$/, '');
-    const dir = join(repoDir, sourceFile.split('/').slice(0, -1).join('/'));
-    const patterns = [
-      `${baseName}.test.ts`,
-      `${baseName}.spec.ts`,
-      `${baseName}.test.js`,
-      `${baseName}.spec.js`,
-      `test_${baseName.split('/').pop()}.py`,
-    ];
 
-    const results: string[] = [];
-    for (const pattern of patterns) {
-      const full = join(dir, pattern.split('/').pop()!);
-      if (existsSync(full)) results.push(full);
-    }
+    const found = new Set<string>();
+    const add = (rel: string) => found.add(rel.replace(/\\/g, '/'));
 
-    try {
-      const entries = await readdir(join(repoDir, 'test'), { recursive: true });
-      for (const entry of entries) {
-        const name = String(entry);
-        if (
-          name.includes(baseName.split('/').pop()!) &&
-          (name.endsWith('.test.ts') ||
-            name.endsWith('.spec.ts') ||
-            name.startsWith('test_'))
-        ) {
-          results.push(join(repoDir, 'test', name));
-        }
+    for (const candidate of orderedTestFileCandidates(sourceFile, framework)) {
+      const full = join(repoDir, candidate);
+      if (existsSync(full)) {
+        add(candidate);
       }
-    } catch {
-      // no test directory
     }
 
-    try {
-      const entries = await readdir(join(repoDir, 'tests'), { recursive: true });
-      for (const entry of entries) {
-        const name = String(entry);
-        if (
-          name.includes(baseName.split('/').pop()!) &&
-          (name.endsWith('.test.ts') ||
-            name.endsWith('.spec.ts') ||
-            name.startsWith('test_'))
-        ) {
-          results.push(join(repoDir, 'tests', name));
+    const baseName = sourceFile.split('/').pop()!.replace(/\.[^.]+$/, '');
+
+    for (const testRoot of ['tests', 'test', 'src'] as const) {
+      try {
+        const entries = await readdir(join(repoDir, testRoot), {
+          recursive: true,
+        });
+        for (const entry of entries) {
+          const name = String(entry);
+          if (
+            !name.includes(baseName) ||
+            !(
+              name.endsWith('.test.ts') ||
+              name.endsWith('.test.tsx') ||
+              name.endsWith('.test.js') ||
+              name.endsWith('.test.jsx') ||
+              name.endsWith('.spec.ts') ||
+              name.endsWith('.spec.js') ||
+              name.startsWith('test_')
+            )
+          ) {
+            continue;
+          }
+          const rel = `${testRoot}/${name}`.replace(/\\/g, '/');
+          const full = join(repoDir, rel);
+          if (existsSync(full)) {
+            add(rel);
+          }
         }
+      } catch {
+        // no directory
       }
-    } catch {
-      // no tests directory
     }
 
-    return results;
+    return [...found];
   }
 
   private async runGit(args: string[], cwd?: string): Promise<void> {

coverage-service/lib/src/providers/local-llm-provider.ts

@@ -4,6 +4,10 @@ import type { GeneratedTest, TestGenerationContext } from '../types';
 
 import type { TestGenerationProvider } from './test-generation-provider';
 
+function outputPathFor(context: TestGenerationContext): string {
+  return context.testOutputPath ?? inferTestFilePath(context.file, context.framework);
+}
+
 export interface LocalLLMProviderConfig {
   baseUrl?: string;
   model?: string;
@@ -41,7 +45,7 @@ export class LocalLLMProvider implements TestGenerationProvider {
       .trim();
 
     return {
-      filePath: inferTestFilePath(context.file, context.framework),
+      filePath: outputPathFor(context),
       content,
       targetFile: context.file,
     };

coverage-service/lib/src/providers/openai-provider.ts

@@ -7,6 +7,10 @@ import type { GeneratedTest, TestGenerationContext } from '../types';
 
 import type { TestGenerationProvider } from './test-generation-provider';
 
+function outputPathFor(context: TestGenerationContext): string {
+  return context.testOutputPath ?? inferTestFilePath(context.file, context.framework);
+}
+
 export interface OpenAIProviderConfig {
   apiKey: string;
   model?: string;
@@ -50,7 +54,7 @@ export class OpenAIProvider implements TestGenerationProvider {
     const content = this.stripMarkdown(data.choices[0]?.message?.content ?? '');
 
     return {
-      filePath: inferTestFilePath(context.file, context.framework),
+      filePath: outputPathFor(context),
       content,
       targetFile: context.file,
     };

coverage-service/lib/src/providers/repository-provider.ts

@@ -25,5 +25,9 @@ export interface RepositoryProvider {
     headBranch: string,
   ): Promise<ChangedFile[]>;
   getFileContent(repoDir: string, filePath: string): Promise<string>;
-  findExistingTests(repoDir: string, sourceFile: string): Promise<string[]>;
+  findExistingTests(
+    repoDir: string,
+    sourceFile: string,
+    framework?: string,
+  ): Promise<string[]>;
 }