diff --git a/packages/coding-agent/CHANGELOG.md b/packages/coding-agent/CHANGELOG.md
index e14adb45b..06e400d1e 100644
--- a/packages/coding-agent/CHANGELOG.md
+++ b/packages/coding-agent/CHANGELOG.md
@@ -17,6 +17,7 @@
 - Fixed direct RPC `steer` and `follow_up` commands bypassing extension `input` handlers ([#8718](https://github.com/earendil-works/pi/issues/8718)).
 - Fixed premature missing-model errors after login by waiting for catalog discovery. Radius now defaults to `balanced`, falling back to the first available Radius model when needed.
 - Fixed fullscreen mode reserving a blank row for custom footers that render zero rows ([#8919](https://github.com/earendil-works/pi/issues/8919)).
+- Fixed extension tools without parameter schemas to be rejected during registration instead of breaking provider requests ([#9300](https://github.com/earendil-works/pi/issues/9300)).
 
 ## [0.85.1] - 2026-09-05
 
diff --git a/packages/coding-agent/src/core/extensions/loader.ts b/packages/coding-agent/src/core/extensions/loader.ts
index 32ca6c952..d06f801a4 100644
--- a/packages/coding-agent/src/core/extensions/loader.ts
+++ b/packages/coding-agent/src/core/extensions/loader.ts
@@ -286,6 +286,11 @@ function createExtensionAPI(
 
 		registerTool(tool: ToolDefinition): void {
 			assertActive();
+			if (typeof tool.parameters !== "object" || tool.parameters === null || Array.isArray(tool.parameters)) {
+				throw new Error(
+					`Tool "${tool.name}" registered by extension "${extension.path}" must define an object parameter schema.`,
+				);
+			}
 			extension.tools.set(tool.name, {
 				definition: tool,
 				sourceInfo: extension.sourceInfo,
diff --git a/packages/coding-agent/test/extensions-runner.test.ts b/packages/coding-agent/test/extensions-runner.test.ts
index 70f06a3cf..d33b2668f 100644
--- a/packages/coding-agent/test/extensions-runner.test.ts
+++ b/packages/coding-agent/test/extensions-runner.test.ts
@@ -390,6 +390,32 @@ describe("ExtensionRunner", () => {
 			expect(tools.map((t) => t.definition.name).sort()).toEqual(["tool_a", "tool_b"]);
 		});
 
+		// Regression test for #9300.
+		it("rejects extension tools without a parameter schema", async () => {
+			const extensionPath = path.join(extensionsDir, "missing-parameters.js");
+			fs.writeFileSync(
+				extensionPath,
+				`export default function(pi) {
+	pi.registerTool({
+		name: "noop",
+		label: "No-op",
+		description: "Do nothing",
+		execute: async () => ({ content: [{ type: "text", text: "ok" }] }),
+	});
+}`,
+			);
+
+			const result = await loadExtensions([extensionPath], tempDir);
+
+			expect(result.extensions).toHaveLength(0);
+			expect(result.errors).toEqual([
+				{
+					path: extensionPath,
+					error: `Failed to load extension: Tool "noop" registered by extension "${extensionPath}" must define an object parameter schema.`,
+				},
+			]);
+		});
+
 		it("keeps first tool when two extensions register the same name", async () => {
 			const first = `
 				import { Type } from "typebox";