Merge pull request #713 from code-yeongyu/fix/mcp-null-tool-schema

fix(mcp): strip null-valued schema types

Author: code-yeongyu

packages/coding-agent/CHANGELOG.md

@@ -14,6 +14,7 @@
 
 - Fixed dynamic project rules being re-injected for every distinct matching tool target: unchanged rule content now stays deduplicated while it remains in the live agent context, then becomes eligible again after accepted compaction or a rule-content change ([#712](https://github.com/code-yeongyu/senpi/pull/712)).
 - Fixed MCP prompt slash commands going missing after startup-raced server connections: the MCP service now emits a catalog-registration signal after publishing each snapshot, and prompt command registration waits for the server's prompt metadata instead of inferring readiness from tool registration ([#706](https://github.com/code-yeongyu/senpi/pull/706)).
+- Fixed HTTP 400 failures from MCP servers that emit invalid JSON-null `type` keywords by stripping only those malformed values at the shared MCP schema-conversion boundary while preserving valid JSON Schema null types ([#713](https://github.com/code-yeongyu/senpi/pull/713)).
 
 ### Removed
 

packages/coding-agent/src/core/extensions/builtin/mcp/changes.md

@@ -1,5 +1,31 @@
 # mcp Extension Changes
 
+## Strip invalid null-valued MCP schema types (2026-08-04)
+
+### What changed
+- `expose/schema-compat.ts` now omits JSON-null `type` keywords while
+  recursively resolving MCP tool input schemas into TypeBox definitions.
+- Valid JSON Schema null types remain unchanged, including `type: "null"` and
+  union arrays such as `type: ["string", "null"]`.
+- `test/mcp/schema-compat.test.ts` covers root, nested property, and combiner
+  branch null values plus both valid null-type forms.
+
+### Why
+- Some MCP servers emit `type: null`. JSON Schema permits the string `"null"`
+  but not the JSON null value; strict OpenAI-compatible providers reject the
+  malformed tool definition with HTTP 400 before the model can answer.
+- Sanitizing at MCP conversion protects every provider adapter that receives
+  the registered tool, rather than patching one provider-specific wire path.
+
+### Why extension system couldn't handle this alone
+- The MCP builtin owns conversion from external `tools/list` schemas to the
+  registered `ToolDefinition`. Other extensions cannot rewrite that private
+  schema conversion before the tool enters the shared provider pipeline.
+
+### Expected merge conflict zones
+- LOW: `expose/schema-compat.ts` recursive `$ref` copy loop.
+- LOW: `test/mcp/schema-compat.test.ts` schema-conversion cases.
+
 ## Session-expiry retry uses the full service reconnect (2026-08-03)
 
 ### What changed

packages/coding-agent/src/core/extensions/builtin/mcp/expose/schema-compat.ts

@@ -152,6 +152,7 @@ function resolveRefs(value: unknown, root: unknown, seenRefs: Set<string>, warni
 
 	const result: Record<string, unknown> = {};
 	for (const [key, child] of Object.entries(value)) {
+		if (key === "type" && child === null) continue;
 		result[key] = resolveRefs(child, root, seenRefs, warnings);
 	}
 	return result;

packages/coding-agent/test/mcp/schema-compat.test.ts

@@ -27,6 +27,32 @@ describe("mcp schema compatibility", () => {
 		expect(Object.getOwnPropertyDescriptor(result.schema, "additionalProperties")).toBeUndefined();
 	});
 
+	it("strips null-valued type keywords recursively", () => {
+		const result = convertJsonSchemaToTypeBox({
+			type: null,
+			properties: {
+				pattern: { type: null, description: "ast-grep pattern" },
+				jsonNull: { type: "null" },
+				maybeText: { type: ["string", "null"] },
+				choice: {
+					anyOf: [{ type: null, const: "all" }, { type: "string" }],
+				},
+			},
+		});
+
+		expect(result.warnings).toEqual([]);
+		expect(JSON.parse(JSON.stringify(result.schema))).toEqual({
+			properties: {
+				pattern: { description: "ast-grep pattern" },
+				jsonNull: { type: "null" },
+				maybeText: { type: ["string", "null"] },
+				choice: {
+					anyOf: [{ const: "all" }, { type: "string" }],
+				},
+			},
+		});
+	});
+
 	it("falls back to a permissive object and warning for unresolvable refs", () => {
 		const result = convertJsonSchemaToTypeBox({
 			type: "object",