feat: add focused system prompt to CLI provider

Author: rbonestell

src/__tests__/prompts.test.ts

@@ -1,8 +1,50 @@
 import {
     buildInstruction,
     buildContext,
+    buildSystemPrompt,
 } from '../prompts';
 
+describe('buildSystemPrompt', () => {
+    it('returns a non-empty string', () => {
+        const result = buildSystemPrompt();
+        expect(typeof result).toBe('string');
+        expect(result.length).toBeGreaterThan(0);
+    });
+
+    it('establishes the commit-author role', () => {
+        const result = buildSystemPrompt();
+        expect(result.toLowerCase()).toContain('commit message');
+    });
+
+    it('forbids markdown/code fences and preamble', () => {
+        const result = buildSystemPrompt();
+        expect(result).toContain('ONLY the commit message');
+        expect(result.toLowerCase()).toContain('no markdown');
+        expect(result.toLowerCase()).toContain('no code fences');
+    });
+
+    it('enforces the 72-character subject rule', () => {
+        const result = buildSystemPrompt();
+        expect(result).toContain('72');
+    });
+
+    it('mentions Conventional Commits as a conditional style', () => {
+        const result = buildSystemPrompt();
+        expect(result).toContain('Conventional Commits');
+    });
+
+    it('instructs the model not to ask clarifying questions', () => {
+        const result = buildSystemPrompt();
+        expect(result.toLowerCase()).toContain('never ask');
+    });
+
+    it('stays lean (short enough to be CLI-arg friendly)', () => {
+        const result = buildSystemPrompt();
+        // Sanity check: meaningfully shorter than Claude Code's default agentic prompt.
+        expect(result.length).toBeLessThan(2000);
+    });
+});
+
 describe('buildInstruction', () => {
     it('returns a non-empty string', () => {
         const result = buildInstruction(true);

src/__tests__/providers/cliProvider.test.ts

@@ -78,7 +78,11 @@ describe('CliProvider', () => {
 
             expect(mockSpawn).toHaveBeenCalledWith(
                 'claude',
-                ['-p', 'my instruction', '--model', 'sonnet'],
+                [
+                    '-p', 'my instruction',
+                    '--model', 'sonnet',
+                    '--system-prompt', expect.any(String),
+                ],
                 {
                     cwd: '/my/cwd',
                     stdio: ['pipe', 'pipe', 'pipe'],
@@ -111,10 +115,30 @@ describe('CliProvider', () => {
 
             expect(mockSpawn).toHaveBeenCalledWith(
                 'claude',
-                ['-p', 'inst', '--model', 'opus'],
+                [
+                    '-p', 'inst',
+                    '--model', 'opus',
+                    '--system-prompt', expect.any(String),
+                ],
                 expect.any(Object)
             );
         });
+
+        it('passes a non-empty --system-prompt flag', async () => {
+            const provider = new CliProvider('/cwd');
+            const token = createMockCancellationToken();
+
+            const promise = provider.generateMessage('inst', 'ctx', token);
+            mockProcess.emitClose(0);
+            await promise;
+
+            const args = mockSpawn.mock.calls[0][1] as string[];
+            const flagIndex = args.indexOf('--system-prompt');
+            expect(flagIndex).toBeGreaterThanOrEqual(0);
+            const value = args[flagIndex + 1];
+            expect(typeof value).toBe('string');
+            expect(value.length).toBeGreaterThan(0);
+        });
     });
 
     describe('errors', () => {

src/prompts.ts

@@ -1,3 +1,28 @@
+/**
+ * Lean system prompt for the Claude CLI (`--system-prompt`).
+ *
+ * Replaces the default Claude Code agentic system prompt with a focused,
+ * minimal persona so the model spends tokens on the commit message, not on
+ * general-purpose tool preambles or clarifying chatter.
+ */
+export function buildSystemPrompt(): string {
+    return [
+        'You are ClawdCommit, a Git commit message author.',
+        'Your only task is to produce one commit message for the staged diff the user provides via stdin.',
+        '',
+        'Output contract:',
+        '- Emit ONLY the commit message text. No preamble, no commentary, no markdown, no code fences, no quotes.',
+        '- Subject line: imperative mood, <=72 characters, no trailing period.',
+        '- Body (only if the change warrants it): blank line after the subject, wrap around 72 columns, explain the why rather than restating the diff.',
+        '- Match the project\'s existing commit style. Use Conventional Commits (type(scope): subject) when recent history shows that pattern.',
+        '',
+        'Behavior:',
+        '- Never ask clarifying questions. Never request permission. Infer the best message from the diff and stop.',
+        '- Do not invent changes that are not in the diff. Do not speculate about unrelated files.',
+        '- If file-reading tools are available and a symbol\'s role is unclear from the diff alone, you may read the minimum needed; otherwise skip exploration.',
+    ].join('\n');
+}
+
 /** Build the instruction for commit message generation. */
 export function buildInstruction(includeFileContext: boolean, canReadFiles: boolean = true): string {
     const parts = [

src/providers/cliProvider.ts

@@ -1,5 +1,6 @@
 import { spawn, type ChildProcess } from 'child_process';
 import * as vscode from 'vscode';
+import { buildSystemPrompt } from '../prompts';
 import type { CommitMessageProvider, ClaudeModel } from './types';
 
 const TIMEOUT_MS = 120_000;
@@ -21,10 +22,18 @@ export class CliProvider implements CommitMessageProvider {
                 return;
             }
 
-            const child: ChildProcess = spawn('claude', ['-p', instruction, '--model', model], {
-                cwd: this.cwd,
-                stdio: ['pipe', 'pipe', 'pipe'],
-            });
+            const child: ChildProcess = spawn(
+                'claude',
+                [
+                    '-p', instruction,
+                    '--model', model,
+                    '--system-prompt', buildSystemPrompt(),
+                ],
+                {
+                    cwd: this.cwd,
+                    stdio: ['pipe', 'pipe', 'pipe'],
+                }
+            );
 
             const stdoutChunks: Buffer[] = [];
             const stderrChunks: Buffer[] = [];