diff --git a/README.md b/README.md
index 312d9e2..40386d9 100644
--- a/README.md
+++ b/README.md
@@ -146,3 +146,11 @@ Mocked unit tests are not evidence of a successful real sandbox run.
 
 Licensed under the Apache License, Version 2.0. See `LICENSE` for the full terms.
 Read `SECURITY.md` before enabling the workflow.
+
+### Optional system prompt
+
+Use `system-prompt` to pass a trusted absolute path to a UTF-8 file (up to
+128 KiB). Its contents are appended to the default Claude Code system prompt
+for planning and execution, not video editing. Omit it to retain the default.
+Keep repository-specific instructions in your own repository; the action does
+not interpret them or configure your environment. Do not include credentials.
diff --git a/action.yml b/action.yml
index 7a2d72f..a91fc1b 100644
--- a/action.yml
+++ b/action.yml
@@ -48,6 +48,9 @@ inputs:
   pool:
     description: Cua pool accessible to the supplied credentials.
     required: true
+  system-prompt:
+    description: Optional trusted absolute path to UTF-8 text appended to planning and execution system prompts (128 KiB maximum).
+    default: ''
   planning-prompt:
     description: Optional trusted absolute path overriding the planning prompt.
     default: ''
@@ -107,6 +110,7 @@ runs:
         CUA_BASE_URL: ${{ inputs.cua-base-url }}
         CUA_TOKEN_URL: ${{ inputs.cua-token-url }}
         E2E_POOL: ${{ inputs.pool }}
+        E2E_SYSTEM_PROMPT: ${{ inputs.system-prompt }}
         E2E_PLANNING_PROMPT: ${{ inputs.planning-prompt }}
         E2E_EXECUTION_PROMPT: ${{ inputs.execution-prompt }}
         E2E_VIDEO_EDITING_PROMPT: ${{ inputs.video-editing-prompt }}
diff --git a/agent/e2e_test_agent.py b/agent/e2e_test_agent.py
index 8874025..82e4bb6 100755
--- a/agent/e2e_test_agent.py
+++ b/agent/e2e_test_agent.py
@@ -178,6 +178,18 @@ async def guard(input_data, _tool_use_id, _context):
     return guard
 
 
+def agent_system_prompt() -> dict[str, str]:
+    prompt = {"type": "preset", "preset": "claude_code"}
+    extra = env("E2E_SYSTEM_PROMPT")
+    if extra:
+        with Path(extra).open("rb") as source:
+            content = source.read(128 * 1024 + 1)
+        if len(content) > 128 * 1024:
+            raise ValueError("System prompt exceeds the 128 KiB limit")
+        prompt["append"] = content.decode("utf-8")
+    return prompt
+
+
 async def run_agent(
     *,
     prompt: str,
@@ -233,7 +245,7 @@ async def run_agent(
         sandbox=sandbox,
         setting_sources=[],
         strict_mcp_config=bool(mcp_servers),
-        system_prompt={"type": "preset", "preset": "claude_code"},
+        system_prompt=agent_system_prompt(),
         tools=tools or ["Read", "Glob", "Grep", "Write"],
     )
     result: ResultMessage | None = None
diff --git a/scripts/run_action.py b/scripts/run_action.py
index 62ecd2f..03810cb 100644
--- a/scripts/run_action.py
+++ b/scripts/run_action.py
@@ -138,6 +138,8 @@ def command_env(self, phase):
             'MCP_PROXY_PORT': '3333',
         })
         environment.update(self.metadata)
+        if phase in ('plan', 'execute') and self.environment.get('E2E_SYSTEM_PROMPT'):
+            environment['E2E_SYSTEM_PROMPT'] = self.environment['E2E_SYSTEM_PROMPT']
         if phase in ('plan', 'execute', 'edit-video'):
             names = ('ANTHROPIC_API_KEY', 'ANTHROPIC_BASE_URL', 'ANTHROPIC_MODEL',
                      'E2E_PLANNING_PROMPT', 'E2E_EXECUTION_PROMPT', 'E2E_VIDEO_EDITING_PROMPT',
diff --git a/tests/test_action.py b/tests/test_action.py
index 04d6898..95af7a9 100644
--- a/tests/test_action.py
+++ b/tests/test_action.py
@@ -31,6 +31,15 @@ def setUp(self):
     def runner(self):
         return run_action.Runner(run_action.Config(self.env), self.env)
 
+    def test_system_prompt_is_only_forwarded_to_planning_and_execution(self):
+        self.env['E2E_SYSTEM_PROMPT'] = '/trusted/custom-system.md'
+        runner = self.runner()
+        for phase in ('plan', 'execute'):
+            self.assertEqual(runner.command_env(phase)['E2E_SYSTEM_PROMPT'],
+                             '/trusted/custom-system.md')
+        for phase in ('pr-context', 'claim-create', 'edit-video'):
+            self.assertNotIn('E2E_SYSTEM_PROMPT', runner.command_env(phase))
+
     def test_required_configuration_fails_before_running(self):
         for key in ('E2E_REPOSITORY', 'PR_NUMBER', 'HEAD_SHA', 'E2E_POOL',
                     'GH_TOKEN', 'CUA_CLIENT_ID', 'CUA_CLIENT_SECRET', 'ANTHROPIC_API_KEY',
diff --git a/tests/test_agent.py b/tests/test_agent.py
index 5230e6b..20e738a 100644
--- a/tests/test_agent.py
+++ b/tests/test_agent.py
@@ -46,6 +46,28 @@ class E2ETestAgentTests(unittest.TestCase):
     def setUp(self) -> None:
         self.agent = load_agent_module()
 
+    def test_custom_text_is_appended_to_system_prompt(self):
+        with tempfile.TemporaryDirectory() as directory:
+            context = Path(directory) / 'custom-system.md'
+            context.write_text('Three nspawn nodes and local Keycloak.')
+            with mock.patch.dict('os.environ', {'E2E_SYSTEM_PROMPT': str(context)}):
+                prompt = self.agent.agent_system_prompt()
+        self.assertEqual(prompt['preset'], 'claude_code')
+        self.assertIn('Three nspawn nodes and local Keycloak.', prompt['append'])
+
+    def test_default_system_prompt_is_unchanged(self):
+        with mock.patch.dict('os.environ', {}, clear=True):
+            self.assertEqual(self.agent.agent_system_prompt(),
+                             {'type': 'preset', 'preset': 'claude_code'})
+
+    def test_system_prompt_rejects_oversized_file(self):
+        with tempfile.TemporaryDirectory() as directory:
+            context = Path(directory) / 'large.md'
+            context.write_bytes(b'x' * (128 * 1024 + 1))
+            with mock.patch.dict('os.environ', {'E2E_SYSTEM_PROMPT': str(context)}):
+                with self.assertRaises(ValueError):
+                    self.agent.agent_system_prompt()
+
     def test_renders_all_prompt_tokens(self) -> None:
         with tempfile.TemporaryDirectory() as directory:
             prompt = Path(directory) / "prompt.md"