feat: add configurable default prompt file for org-level agent instructions [close OPE-36] (#1187)

* feat: add configurable default prompt file for org-level agent instructions

* linting

* Update CUSTOMIZATION.md

Co-authored-by: Brace Sproul <braceasproul@gmail.com>

* fix: address PR review feedback on default prompt

---------

Co-authored-by: Brace Sproul <braceasproul@gmail.com>

CUSTOMIZATION.md

@@ -385,6 +385,45 @@ The system prompt is assembled in `agent/prompt.py` from modular sections. You c
 | `CODE_REVIEW_GUIDELINES_SECTION` | How the agent reviews code changes |
 | `COMMUNICATION_SECTION` | Formatting and messaging guidelines |
 
+### Default prompt file
+
+Open SWE supports a `default_prompt.md` file for org-level instructions that apply to **every** agent run, regardless of which repository is being worked on. This is the recommended way to set default repository preferences, org conventions, and shared guidelines.
+
+The file is loaded at agent startup and injected into the system prompt between the task overview and repository setup sections.
+
+**Location:** [`default_prompt.md`](./default_prompt.md) in the project root.
+
+**Override:** Set the `DEFAULT_PROMPT_PATH` environment variable to use a different file:
+
+```bash
+DEFAULT_PROMPT_PATH="/path/to/my-org-prompt.md"
+```
+
+**Format:** Write plain markdown. The content is injected as-is under a `### Custom Instructions` heading in the system prompt. Example:
+
+```markdown
+# Default Prompt
+
+## Default Repository
+
+When no repository is specified, work on the **my-app** repository under **my-org**.
+
+## Organization Conventions
+
+- Use conventional commits: feat:, fix:, chore:
+- Always tag the requesting user when work is complete
+```
+
+**Loading order:** Default prompt → System prompt sections → AGENTS.md (per-repo). If the file is missing or empty, it is silently skipped — no error is raised.
+
+**When to use `default_prompt.md` vs `AGENTS.md`:**
+
+| | `default_prompt.md` | `AGENTS.md` |
+|---|---|---|
+| Scope | All tasks, all repos | Single repository |
+| Location | Open SWE project root | Target repo root |
+| Use for | Default repo, org conventions | Repo-specific coding standards |
+
 ### Using AGENTS.md
 
 Drop an `AGENTS.md` file in the root of any repository to add repo-specific instructions. The agent reads it from the sandbox at startup and appends it to the system prompt. This is the easiest way to encode conventions per-repo without modifying Open SWE's code.

agent/prompt.py

@@ -1,5 +1,39 @@
+import logging
+import os
+from pathlib import Path
+
 from .utils.github_comments import UNTRUSTED_GITHUB_COMMENT_OPEN_TAG
 
+logger = logging.getLogger(__name__)
+
+DEFAULT_PROMPT_PATH = os.environ.get(
+    "DEFAULT_PROMPT_PATH",
+    str(Path(__file__).resolve().parent.parent / "default_prompt.md"),
+)
+
+
+def _load_default_prompt() -> str:
+    """Load custom prompt from the default prompt file.
+
+    Returns empty string if the file doesn't exist or can't be read.
+    """
+    try:
+        path = Path(DEFAULT_PROMPT_PATH)
+        if path.is_file():
+            content = path.read_text().strip()
+            if content:
+                # Escape curly braces so .format() doesn't choke on them
+                escaped = content.replace("{", "{{").replace("}", "}}")
+                return f"""---
+
+### Custom Instructions
+
+{escaped}"""
+    except Exception:
+        logger.warning("Failed to read default prompt file at %s", DEFAULT_PROMPT_PATH)
+    return ""
+
+
 WORKING_ENV_SECTION = """---
 
 ### Working Environment
@@ -282,9 +316,10 @@ When you have completed your implementation, follow these steps in order:
 Always call `commit_and_open_pr` followed by the appropriate reply tool once implementation is complete and code quality checks pass."""
 
 
-SYSTEM_PROMPT = (
+SYSTEM_PROMPT_TEMPLATE = (
     WORKING_ENV_SECTION
     + TASK_OVERVIEW_SECTION
+    + "{default_prompt_section}"
     + REPO_SETUP_SECTION
     + FILE_MANAGEMENT_SECTION
     + TASK_EXECUTION_SECTION
@@ -305,8 +340,10 @@ def construct_system_prompt(
     linear_project_id: str = "",
     linear_issue_number: str = "",
 ) -> str:
-    return SYSTEM_PROMPT.format(
+    default_prompt_section = _load_default_prompt()
+    return SYSTEM_PROMPT_TEMPLATE.format(
         working_dir=working_dir,
         linear_project_id=linear_project_id or "<PROJECT_ID>",
         linear_issue_number=linear_issue_number or "<ISSUE_NUMBER>",
+        default_prompt_section=default_prompt_section,
     )

default_prompt.md

@@ -0,0 +1,3 @@
+# Default Prompt
+
+When a repository is not explicitly mentioned, use `langchain-ai/langchainplus`. Always assume the organization is `langchain-ai`.