Send OpenCode source-control prompts over stdin (#4859)

Co-authored-by: Orca <help@stably.ai>

src/shared/commit-message-agent-spec.test.ts

@@ -331,6 +331,60 @@ describe('buildArgs (Codex)', () => {
   })
 })
 
+describe('buildArgs (OpenCode)', () => {
+  const spec = getCommitMessageAgentSpec('opencode')!
+
+  it('runs `opencode run` without passing the prompt via argv', () => {
+    const prompt = `PROMPT ${'x'.repeat(1024)}`
+    const args = spec.buildArgs({
+      prompt,
+      model: 'opencode/deepseek-v4-flash-free'
+    })
+
+    expect(args).toEqual([
+      'run',
+      '--model',
+      'opencode/deepseek-v4-flash-free',
+      '--agent',
+      'build',
+      '--format',
+      'default'
+    ])
+    expect(args).not.toContain(prompt)
+    expect(args).not.toContain('')
+    expect(spec.promptDelivery).toBe('stdin')
+  })
+
+  it('emits --variant <level> when thinking level is supplied', () => {
+    const args = spec.buildArgs({
+      prompt: 'PROMPT',
+      model: 'opencode/gpt-5.4-mini',
+      thinkingLevel: 'high'
+    })
+
+    expect(args).toEqual([
+      'run',
+      '--model',
+      'opencode/gpt-5.4-mini',
+      '--agent',
+      'build',
+      '--format',
+      'default',
+      '--variant',
+      'high'
+    ])
+  })
+
+  it('omits --variant when no thinking level is supplied', () => {
+    const args = spec.buildArgs({
+      prompt: 'PROMPT',
+      model: 'opencode/gpt-5.4-mini'
+    })
+
+    expect(args).not.toContain('--variant')
+  })
+})
+
 describe('buildArgs (Antigravity)', () => {
   const spec = getCommitMessageAgentSpec('antigravity')!
 

src/shared/commit-message-agent-spec.ts

@@ -336,8 +336,10 @@ export const COMMIT_MESSAGE_AGENT_SPECS: Partial<Record<TuiAgent, CommitMessageA
     id: 'opencode',
     label: 'OpenCode',
     binary: 'opencode',
-    promptDelivery: 'argv',
-    buildArgs: ({ prompt, model, thinkingLevel }) => [
+    // Why: Source Control AI prompts can include large staged diffs; OpenCode
+    // accepts the prompt on stdin, which avoids cross-platform argv limits.
+    promptDelivery: 'stdin',
+    buildArgs: ({ model, thinkingLevel }) => [
       'run',
       '--model',
       model,
@@ -345,8 +347,7 @@ export const COMMIT_MESSAGE_AGENT_SPECS: Partial<Record<TuiAgent, CommitMessageA
       'build',
       '--format',
       'default',
-      ...(thinkingLevel ? ['--variant', thinkingLevel] : []),
-      prompt
+      ...(thinkingLevel ? ['--variant', thinkingLevel] : [])
     ],
     modelSource: 'dynamic',
     modelDiscovery: { binary: 'opencode', args: ['models'], parse: parseLineModels },

src/shared/commit-message-plan.test.ts

@@ -33,7 +33,7 @@ describe('planCommitMessageGeneration', () => {
     })
   })
 
-  it('plans OpenCode run with prompt in argv and model variant', () => {
+  it('plans OpenCode run with prompt on stdin and model variant', () => {
     const result = planCommitMessageGeneration(
       {
         agentId: 'opencode',
@@ -56,10 +56,39 @@ describe('planCommitMessageGeneration', () => {
           '--format',
           'default',
           '--variant',
-          'high',
-          'PROMPT'
+          'high'
         ],
-        stdinPayload: null,
+        stdinPayload: 'PROMPT',
+        label: 'OpenCode'
+      }
+    })
+  })
+
+  it('keeps OpenCode preset command overrides while sending the prompt on stdin', () => {
+    const result = planCommitMessageGeneration(
+      {
+        agentId: 'opencode',
+        model: 'opencode/gpt-5.4-mini',
+        agentCommandOverride: 'npx opencode'
+      },
+      'PROMPT'
+    )
+
+    expect(result).toEqual({
+      ok: true,
+      plan: {
+        binary: 'npx',
+        args: [
+          'opencode',
+          'run',
+          '--model',
+          'opencode/gpt-5.4-mini',
+          '--agent',
+          'build',
+          '--format',
+          'default'
+        ],
+        stdinPayload: 'PROMPT',
         label: 'OpenCode'
       }
     })