feat(supervise): typed RPC error taxonomy for dispatch

Introduce _RpcClientError and _RpcInternalError as distinct subclasses
of _RpcError so the dispatcher can handle bad requests and server-side
faults differently — returning client errors verbatim and logging
internal faults with their cause before replying ERR_INTERNAL.

Wrap write_proposal and archive_proposal IO with _RpcInternalError
so OS failures surface through the typed path instead of the bare
Exception fallback. All existing raise _RpcError(...) call sites
converted to _RpcClientError.

Closes #253

bot_bottle/git_http_backend.py

@@ -148,7 +148,13 @@ class GitHttpHandler(BaseHTTPRequestHandler):
             key, _, value = line.decode("latin1").partition(":")
             value = value.strip()
             if key.lower() == "status":
-                status = int(value.split()[0])
+                try:
+                    status = int(value.split()[0])
+                except (ValueError, IndexError):
+                    self.log_message(
+                        "malformed CGI Status header %r; using 500", value,
+                    )
+                    status = 500
             else:
                 headers.append((key, value))
         self.send_response(status)

bot_bottle/supervise_server.py

@@ -90,19 +90,19 @@ def parse_jsonrpc(body: bytes) -> JsonRpcRequest:
     try:
         raw = json.loads(body)
     except json.JSONDecodeError as e:
-        raise _RpcError(ERR_PARSE, f"parse error: {e}") from e
+        raise _RpcClientError(ERR_PARSE, f"parse error: {e}") from e
     if not isinstance(raw, dict):
-        raise _RpcError(ERR_INVALID_REQUEST, "request must be a JSON object")
+        raise _RpcClientError(ERR_INVALID_REQUEST, "request must be a JSON object")
     if raw.get("jsonrpc") != JSONRPC_VERSION:
-        raise _RpcError(ERR_INVALID_REQUEST, "jsonrpc field must be '2.0'")
+        raise _RpcClientError(ERR_INVALID_REQUEST, "jsonrpc field must be '2.0'")
     method = raw.get("method")
     if not isinstance(method, str):
-        raise _RpcError(ERR_INVALID_REQUEST, "method must be a string")
+        raise _RpcClientError(ERR_INVALID_REQUEST, "method must be a string")
     params = raw.get("params", {})
     if params is None:
         params = {}
     if not isinstance(params, dict):
-        raise _RpcError(ERR_INVALID_PARAMS, "params must be an object")
+        raise _RpcClientError(ERR_INVALID_PARAMS, "params must be an object")
     rpc_id = raw.get("id", _NO_ID)
     is_notification = rpc_id is _NO_ID
     return JsonRpcRequest(
@@ -117,12 +117,23 @@ _NO_ID = object()
 
 
 class _RpcError(Exception):
+    """Base class for all typed RPC errors that surface as JSON-RPC error responses."""
     def __init__(self, code: int, message: str):
         super().__init__(message)
         self.code = code
         self.message = message
 
 
+class _RpcClientError(_RpcError):
+    """Caller sent a bad request; returned verbatim, no server-side logging."""
+
+
+class _RpcInternalError(_RpcError):
+    """Server-side fault; logged at ERROR with cause, always returns ERR_INTERNAL."""
+    def __init__(self, message: str) -> None:
+        super().__init__(ERR_INTERNAL, message)
+
+
 def jsonrpc_result(request_id: object, result: object) -> bytes:
     payload = {"jsonrpc": JSONRPC_VERSION, "id": request_id, "result": result}
     return (json.dumps(payload) + "\n").encode("utf-8")
@@ -290,7 +301,7 @@ def validate_proposed_file(tool: str, content: str) -> None:
     catches obvious paste-errors / wrong-tool selections before they
     enter the queue."""
     if not content.strip():
-        raise _RpcError(ERR_INVALID_PARAMS, f"{tool}: proposed file is empty")
+        raise _RpcClientError(ERR_INVALID_PARAMS, f"{tool}: proposed file is empty")
     if tool == _sv.TOOL_CAPABILITY_BLOCK:
         # Dockerfiles are too varied to validate syntactically beyond
         # non-empty. The operator reads the diff in the TUI.
@@ -299,17 +310,17 @@ def validate_proposed_file(tool: str, content: str) -> None:
         try:
             config = load_config(content)
         except ValueError as e:
-            raise _RpcError(
+            raise _RpcClientError(
                 ERR_INVALID_PARAMS,
                 f"{tool}: proposed routes.yaml is not valid: {e}",
             ) from e
         if config.log != LOG_OFF:
-            raise _RpcError(
+            raise _RpcClientError(
                 ERR_INVALID_PARAMS,
                 f"{tool}: proposed routes.yaml must not change egress logging",
             )
     else:
-        raise _RpcError(ERR_INVALID_PARAMS, f"unknown tool {tool!r}")
+        raise _RpcClientError(ERR_INVALID_PARAMS, f"unknown tool {tool!r}")
 
 
 # --- MCP handlers ----------------------------------------------------------
@@ -382,17 +393,17 @@ def handle_tools_call(
     doesn't need operator approval."""
     name = params.get("name")
     if not isinstance(name, str):
-        raise _RpcError(ERR_INVALID_PARAMS, "tools/call missing 'name'")
+        raise _RpcClientError(ERR_INVALID_PARAMS, "tools/call missing 'name'")
     if name == _sv.TOOL_LIST_EGRESS_ROUTES:
         return handle_list_egress_routes(typing.cast(dict[str, object], params.get("arguments", {})), config)
 
     args_raw = params.get("arguments", {})
     if not isinstance(args_raw, dict):
-        raise _RpcError(ERR_INVALID_PARAMS, "tools/call 'arguments' must be an object")
+        raise _RpcClientError(ERR_INVALID_PARAMS, "tools/call 'arguments' must be an object")
 
     justification = args_raw.get("justification")
     if not isinstance(justification, str) or not justification.strip():
-        raise _RpcError(
+        raise _RpcClientError(
             ERR_INVALID_PARAMS,
             f"{name}: 'justification' is required and must be a non-empty string",
         )
@@ -401,13 +412,13 @@ def handle_tools_call(
         file_field = PROPOSED_FILE_FIELD[name]
         proposed_file = args_raw.get(file_field)
         if not isinstance(proposed_file, str):
-            raise _RpcError(
+            raise _RpcClientError(
                 ERR_INVALID_PARAMS,
                 f"{name}: '{file_field}' is required and must be a string",
             )
         validate_proposed_file(name, proposed_file)
     else:
-        raise _RpcError(ERR_INVALID_PARAMS, f"unknown tool {name!r}")
+        raise _RpcClientError(ERR_INVALID_PARAMS, f"unknown tool {name!r}")
 
     proposal = _sv.Proposal.new(
         bottle_slug=config.bottle_slug,
@@ -416,7 +427,10 @@ def handle_tools_call(
         justification=justification,
         current_file_hash=_sv.sha256_hex(proposed_file),
     )
-    _sv.write_proposal(config.queue_dir, proposal)
+    try:
+        _sv.write_proposal(config.queue_dir, proposal)
+    except OSError as e:
+        raise _RpcInternalError(f"failed to write proposal to queue: {e}") from e
     sys.stderr.write(
         f"supervise: queued proposal {proposal.id} ({name}) "
         f"for bottle {config.bottle_slug}; waiting for operator...\n"
@@ -436,7 +450,10 @@ def handle_tools_call(
             "content": [{"type": "text", "text": text}],
             "isError": False,
         }
-    _sv.archive_proposal(config.queue_dir, proposal.id)
+    try:
+        _sv.archive_proposal(config.queue_dir, proposal.id)
+    except OSError as e:
+        raise _RpcInternalError(f"failed to archive proposal: {e}") from e
 
     text = format_response_text(response)
     return {
@@ -512,7 +529,7 @@ class MCPHandler(http.server.BaseHTTPRequestHandler):
 
         try:
             req = parse_jsonrpc(body)
-        except _RpcError as e:
+        except _RpcClientError as e:
             self._write_jsonrpc(jsonrpc_error(None, e.code, e.message))
             return
 
@@ -520,11 +537,19 @@ class MCPHandler(http.server.BaseHTTPRequestHandler):
 
         try:
             result = self._dispatch(req, config)
-        except _RpcError as e:
+        except _RpcClientError as e:
             self._write_jsonrpc(jsonrpc_error(req.id, e.code, e.message))
             return
-        except Exception as e:  # noqa: W0718 — catch-all for RPC dispatch errors
+        except _RpcInternalError as e:
-            sys.stderr.write(f"supervise: internal error: {e}\n")
+            cause = e.__cause__
+            detail = f": {cause}" if cause else ""
+            sys.stderr.write(f"supervise: internal error: {e.message}{detail}\n")
+            sys.stderr.flush()
+            self._write_jsonrpc(jsonrpc_error(req.id, ERR_INTERNAL, "internal error"))
+            return
+        except Exception as e:  # noqa: W0718 — unexpected errors
+            sys.stderr.write(f"supervise: unexpected error: {type(e).__name__}: {e}\n")
+            sys.stderr.flush()
             self._write_jsonrpc(jsonrpc_error(req.id, ERR_INTERNAL, "internal error"))
             return
 
@@ -543,7 +568,7 @@ class MCPHandler(http.server.BaseHTTPRequestHandler):
             return handle_tools_list(req.params)
         if method == "tools/call":
             return handle_tools_call(req.params, config)
-        raise _RpcError(ERR_METHOD_NOT_FOUND, f"method not found: {method}")
+        raise _RpcClientError(ERR_METHOD_NOT_FOUND, f"method not found: {method}")
 
     def _write_jsonrpc(self, body: bytes) -> None:
         self.send_response(200)

tests/unit/test_git_http_backend.py

@@ -256,6 +256,57 @@ class TestGitHttpBackend(unittest.TestCase):
             os.environ["GIT_GATE_ACCESS_HOOK"] = value
 
 
+class TestMalformedStatusHeader(unittest.TestCase):
+    """Malformed CGI Status: headers must not propagate as unhandled exceptions;
+    the handler should fall back to HTTP 500."""
+
+    def setUp(self):
+        from http.server import ThreadingHTTPServer
+        import tempfile
+        self._tmp = tempfile.mkdtemp()
+        os.environ["GIT_PROJECT_ROOT"] = self._tmp
+        self._server = ThreadingHTTPServer(("127.0.0.1", 0), GitHttpHandler)
+        self._thread = threading.Thread(
+            target=self._server.serve_forever, daemon=True,
+        )
+        self._thread.start()
+        self._port = self._server.server_port
+
+    def tearDown(self):
+        self._server.shutdown()
+        self._server.server_close()
+        os.environ.pop("GIT_PROJECT_ROOT", None)
+        import shutil
+        shutil.rmtree(self._tmp, ignore_errors=True)
+
+    def _get_with_backend_response(self, cgi_response: bytes) -> int:
+        with mock.patch(
+            "bot_bottle.git_http_backend.subprocess.run",
+            return_value=mock.Mock(returncode=0, stdout=cgi_response),
+        ):
+            req = urllib.request.Request(
+                f"http://127.0.0.1:{self._port}/repo.git/info/refs",
+                method="GET",
+            )
+            try:
+                with urllib.request.urlopen(req, timeout=3) as resp:
+                    return resp.status
+            except urllib.error.HTTPError as e:  # type: ignore
+                return e.code
+
+    def test_empty_status_value_returns_500(self):
+        status = self._get_with_backend_response(
+            b"Status: \r\nContent-Type: text/plain\r\n\r\n"
+        )
+        self.assertEqual(500, status)
+
+    def test_non_numeric_status_returns_500(self):
+        status = self._get_with_backend_response(
+            b"Status: bad\r\nContent-Type: text/plain\r\n\r\n"
+        )
+        self.assertEqual(500, status)
+
+
 class TestContentLengthBounds(unittest.TestCase):
     """PRD 0041: malformed or oversized Content-Length is rejected before
     git http-backend is invoked."""

tests/unit/test_supervise_server.py

@@ -20,6 +20,7 @@ import supervise as _sv  # noqa: E402  # type: ignore
 
 from bot_bottle import supervise_server  # noqa: E402
 from bot_bottle.supervise_server import (
+    ERR_INTERNAL,
     ERR_INVALID_PARAMS,
     ERR_INVALID_REQUEST,
     ERR_METHOD_NOT_FOUND,
@@ -29,7 +30,9 @@ from bot_bottle.supervise_server import (
     PROPOSED_FILE_FIELD,
     ServerConfig,
     TOOL_DEFINITIONS,
+    _RpcClientError,
     _RpcError,
+    _RpcInternalError,
     _response_timeout_from_env,
     format_response_text,
     handle_initialize,
@@ -77,6 +80,65 @@ class TestValidation(unittest.TestCase):
         self.assertIn("must not change egress logging", cm.exception.message)
 
 
+# --- Error taxonomy --------------------------------------------------------
+
+
+class TestRpcErrorTaxonomy(unittest.TestCase):
+    def test_rpc_client_error_is_rpc_error(self):
+        e = _RpcClientError(ERR_INVALID_PARAMS, "bad param")
+        self.assertIsInstance(e, _RpcError)
+        self.assertEqual(ERR_INVALID_PARAMS, e.code)
+        self.assertEqual("bad param", e.message)
+
+    def test_rpc_internal_error_is_rpc_error(self):
+        e = _RpcInternalError("disk full")
+        self.assertIsInstance(e, _RpcError)
+        self.assertEqual(ERR_INTERNAL, e.code)
+        self.assertEqual("disk full", e.message)
+
+    def test_rpc_internal_error_preserves_cause(self):
+        cause = OSError("no space left on device")
+        try:
+            raise _RpcInternalError("failed to write") from cause
+        except _RpcInternalError as e:
+            self.assertIs(cause, e.__cause__)
+
+    def test_parse_error_is_client_error(self):
+        with self.assertRaises(_RpcClientError):
+            parse_jsonrpc(b"{bad json")
+
+    def test_validation_error_is_client_error(self):
+        with self.assertRaises(_RpcClientError):
+            validate_proposed_file(_sv.TOOL_EGRESS_ALLOW, "routes: nope\n")
+
+    def test_unknown_tool_in_tools_call_is_client_error(self):
+        config = ServerConfig(bottle_slug="dev", queue_dir=Path("/unused"))
+        with self.assertRaises(_RpcClientError) as cm:
+            handle_tools_call({"name": "no-such-tool", "arguments": {}}, config)
+        self.assertEqual(ERR_INVALID_PARAMS, cm.exception.code)
+
+
+class TestRpcInternalErrorOnIoFailure(unittest.TestCase):
+    def test_write_proposal_os_error_raises_internal(self):
+        config = ServerConfig(
+            bottle_slug="dev",
+            queue_dir=Path("/dev/null/cannot-exist"),
+        )
+        with self.assertRaises(_RpcInternalError) as cm:
+            handle_tools_call(
+                {
+                    "name": _sv.TOOL_CAPABILITY_BLOCK,
+                    "arguments": {
+                        "dockerfile": "FROM python:3.13\n",
+                        "justification": "x",
+                    },
+                },
+                config,
+            )
+        self.assertEqual(ERR_INTERNAL, cm.exception.code)
+        self.assertIsNotNone(cm.exception.__cause__)
+
+
 # --- JSON-RPC parsing ------------------------------------------------------
 
 
@@ -469,6 +531,26 @@ class TestHttpEndToEnd(unittest.TestCase):
         )
         self.assertEqual(ERR_METHOD_NOT_FOUND, result["error"]["code"])  # type: ignore[index]
 
+    def test_internal_error_returns_err_internal_over_http(self):
+        with patch.object(
+            supervise_server._sv, "write_proposal",
+            side_effect=OSError("disk full"),
+        ):
+            result = self._post_jsonrpc({
+                "jsonrpc": "2.0",
+                "id": 99,
+                "method": "tools/call",
+                "params": {
+                    "name": _sv.TOOL_CAPABILITY_BLOCK,
+                    "arguments": {
+                        "dockerfile": "FROM python:3.13\n",
+                        "justification": "x",
+                    },
+                },
+            })
+        self.assertIn("error", result)
+        self.assertEqual(ERR_INTERNAL, result["error"]["code"])  # type: ignore[index]
+
     def test_health_endpoint(self):
         conn = http.client.HTTPConnection("127.0.0.1", self.port, timeout=5)
         try: