fix(tools): add the native function schema for generate_image

generate_image existed in the prompt sections, the executor and the image_gen
MCP server, but not in FUNCTION_TOOL_SCHEMAS. API models are only sent schemas
from that list, so the tool selector could pick it and the schema filter would
drop it again — the model then improvised text-shaped calls it had no way to
make work.

The existing JSON fallback in function_call_to_tool_block already produces
content the executor parses, so no converter branch is needed. A required-args
entry is, though: without it a prompt-less call falls through to the line
parser and the whole JSON blob becomes the image prompt.

Fixes #5520

Co-Authored-By: Claude <noreply@anthropic.com>

src/tool_schemas.py

@@ -26,6 +26,9 @@ _REQUIRED_NATIVE_TOOL_ARGS = {
     "write_file": ("path",),
     "edit_file": ("path",),
     "apply_patch": ("patch_text", "patchText", "patch"),
+    # Without this, a prompt-less call falls through to the line parser in
+    # tool_execution and the whole JSON blob becomes the image prompt.
+    "generate_image": ("prompt",),
 }
 
 # ---------------------------------------------------------------------------
@@ -1030,6 +1033,23 @@ FUNCTION_TOOL_SCHEMAS = [
             }
         }
     },
+    {
+        "type": "function",
+        "function": {
+            "name": "generate_image",
+            "description": "Generate an image from a text prompt. Art, illustrations, photos.",
+            "parameters": {
+                "type": "object",
+                "properties": {
+                    "prompt": {"type": "string", "description": "Image description prompt"},
+                    "model": {"type": "string", "description": "Model name (auto-detects if omitted)"},
+                    "size": {"type": "string", "description": "Image size (default 1024x1024)"},
+                    "quality": {"type": "string", "description": "Quality: low, medium, high, auto (default medium)"},
+                },
+                "required": ["prompt"]
+            }
+        }
+    },
     {
         "type": "function",
         "function": {

tests/test_generate_image_native_schema.py

@@ -0,0 +1,61 @@
+"""Issue #5520 — generate_image had no native function schema.
+
+The tool exists in the fenced-block prompt, the executor, and the image_gen MCP
+server, but not in FUNCTION_TOOL_SCHEMAS. API models (native function calling)
+are only sent schemas from that list, so they could never call it and improvised
+malformed text calls instead.
+"""
+
+import json
+
+
+def _schemas():
+    import src.agent_tools  # noqa: F401  (tool_schemas <-> agent_tools import cycle)
+    from src.tool_schemas import FUNCTION_TOOL_SCHEMAS
+
+    return {s["function"]["name"]: s["function"] for s in FUNCTION_TOOL_SCHEMAS}
+
+
+def _convert(arguments):
+    import src.agent_tools  # noqa: F401
+    from src.tool_schemas import function_call_to_tool_block
+
+    return function_call_to_tool_block("generate_image", json.dumps(arguments))
+
+
+def test_generate_image_is_offered_to_native_function_calling_models():
+    schemas = _schemas()
+    assert "generate_image" in schemas, (
+        "generate_image has no native schema, so it is filtered out of the tools "
+        "sent to API models even when the selector picks it"
+    )
+    assert schemas["generate_image"]["parameters"]["required"] == ["prompt"]
+
+
+def test_native_call_reaches_the_executor_with_its_arguments_intact():
+    from src.tool_execution import _build_mcp_args
+
+    args = {"prompt": "a cat riding a bicycle", "model": "gpt-image-1",
+            "size": "1024x1024", "quality": "high"}
+    block = _convert(args)
+    assert block is not None and block.tool_type == "generate_image"
+    assert _build_mcp_args("generate_image", block.content) == args
+
+
+def test_call_without_a_prompt_is_rejected_instead_of_drawing_its_own_arguments():
+    """No prompt key means the line parser takes the whole JSON blob as the prompt,
+    so the model gets an image of its own arguments."""
+    assert _convert({"size": "512x512"}) is None
+    assert _convert({}) is None
+    assert _convert({"prompt": "   "}) is None
+
+
+def test_advertised_parameters_are_the_ones_the_image_server_accepts():
+    """Anything the schema advertises but the executor drops is a silent no-op."""
+    from mcp_servers.image_gen_server import list_tools
+    import asyncio
+
+    server_schema = asyncio.run(list_tools())[0].inputSchema
+    assert set(_schemas()["generate_image"]["parameters"]["properties"]) == set(
+        server_schema["properties"]
+    )