fix(coding-agent): chain tool_result extension patches

fixes #1280

packages/coding-agent/CHANGELOG.md

@@ -6,6 +6,7 @@
 
 - Fixed extra spacing between thinking-only assistant content and subsequent tool execution blocks when assistant messages contain no text
 - Fixed queued steering/follow-up/custom messages remaining stuck after threshold auto-compaction by resuming the agent loop when Agent-level queues still contain pending messages ([#1312](https://github.com/badlogic/pi-mono/pull/1312) by [@ferologics](https://github.com/ferologics))
+- Fixed `tool_result` extension handlers to chain result patches across handlers instead of last-handler-wins behavior ([#1280](https://github.com/badlogic/pi-mono/issues/1280))
 
 ## [0.52.6] - 2026-02-05
 

packages/coding-agent/docs/extensions.md

@@ -524,6 +524,11 @@ pi.on("tool_call", (event) => {
 
 Fired after tool executes. **Can modify result.**
 
+`tool_result` handlers chain like middleware:
+- Handlers run in extension load order
+- Each handler sees the latest result after previous handler changes
+- Handlers can return partial patches (`content`, `details`, or `isError`); omitted fields keep their current values
+
 ```typescript
 import { isBashToolResult } from "@mariozechner/pi-coding-agent";
 

packages/coding-agent/src/core/extensions/runner.ts

@@ -41,6 +41,7 @@ import type {
 	SessionBeforeTreeResult,
 	ToolCallEvent,
 	ToolCallEventResult,
+	ToolResultEvent,
 	ToolResultEventResult,
 	UserBashEvent,
 	UserBashEventResult,
@@ -98,7 +99,13 @@ interface BeforeAgentStartCombinedResult {
  */
 type RunnerEmitEvent = Exclude<
 	ExtensionEvent,
-	ToolCallEvent | UserBashEvent | ContextEvent | BeforeAgentStartEvent | ResourcesDiscoverEvent | InputEvent
+	| ToolCallEvent
+	| ToolResultEvent
+	| UserBashEvent
+	| ContextEvent
+	| BeforeAgentStartEvent
+	| ResourcesDiscoverEvent
+	| InputEvent
 >;
 
 export type ExtensionErrorListener = (error: ExtensionError) => void;
@@ -484,11 +491,9 @@ export class ExtensionRunner {
 		);
 	}
 
-	async emit(
-		event: RunnerEmitEvent,
-	): Promise<SessionBeforeCompactResult | SessionBeforeTreeResult | ToolResultEventResult | undefined> {
+	async emit(event: RunnerEmitEvent): Promise<SessionBeforeCompactResult | SessionBeforeTreeResult | undefined> {
 		const ctx = this.createContext();
-		let result: SessionBeforeCompactResult | SessionBeforeTreeResult | ToolResultEventResult | undefined;
+		let result: SessionBeforeCompactResult | SessionBeforeTreeResult | undefined;
 
 		for (const ext of this.extensions) {
 			const handlers = ext.handlers.get(event.type);
@@ -504,10 +509,6 @@ export class ExtensionRunner {
 							return result;
 						}
 					}
-
-					if (event.type === "tool_result" && handlerResult) {
-						result = handlerResult as ToolResultEventResult;
-					}
 				} catch (err) {
 					const message = err instanceof Error ? err.message : String(err);
 					const stack = err instanceof Error ? err.stack : undefined;
@@ -524,6 +525,56 @@ export class ExtensionRunner {
 		return result;
 	}
 
+	async emitToolResult(event: ToolResultEvent): Promise<ToolResultEventResult | undefined> {
+		const ctx = this.createContext();
+		const currentEvent: ToolResultEvent = { ...event };
+		let modified = false;
+
+		for (const ext of this.extensions) {
+			const handlers = ext.handlers.get("tool_result");
+			if (!handlers || handlers.length === 0) continue;
+
+			for (const handler of handlers) {
+				try {
+					const handlerResult = (await handler(currentEvent, ctx)) as ToolResultEventResult | undefined;
+					if (!handlerResult) continue;
+
+					if (handlerResult.content !== undefined) {
+						currentEvent.content = handlerResult.content;
+						modified = true;
+					}
+					if (handlerResult.details !== undefined) {
+						currentEvent.details = handlerResult.details;
+						modified = true;
+					}
+					if (handlerResult.isError !== undefined) {
+						currentEvent.isError = handlerResult.isError;
+						modified = true;
+					}
+				} catch (err) {
+					const message = err instanceof Error ? err.message : String(err);
+					const stack = err instanceof Error ? err.stack : undefined;
+					this.emitError({
+						extensionPath: ext.path,
+						event: "tool_result",
+						error: message,
+						stack,
+					});
+				}
+			}
+		}
+
+		if (!modified) {
+			return undefined;
+		}
+
+		return {
+			content: currentEvent.content,
+			details: currentEvent.details,
+			isError: currentEvent.isError,
+		};
+	}
+
 	async emitToolCall(event: ToolCallEvent): Promise<ToolCallEventResult | undefined> {
 		const ctx = this.createContext();
 		let result: ToolCallEventResult | undefined;

packages/coding-agent/src/core/extensions/wrapper.ts

@@ -4,7 +4,7 @@
 
 import type { AgentTool, AgentToolUpdateCallback } from "@mariozechner/pi-agent-core";
 import type { ExtensionRunner } from "./runner.js";
-import type { RegisteredTool, ToolCallEventResult, ToolResultEventResult } from "./types.js";
+import type { RegisteredTool, ToolCallEventResult } from "./types.js";
 
 /**
  * Wrap a RegisteredTool into an AgentTool.
@@ -72,7 +72,7 @@ export function wrapToolWithExtensions<T>(tool: AgentTool<any, T>, runner: Exten
 
 				// Emit tool_result event - extensions can modify the result
 				if (runner.hasHandlers("tool_result")) {
-					const resultResult = (await runner.emit({
+					const resultResult = await runner.emitToolResult({
 						type: "tool_result",
 						toolName: tool.name,
 						toolCallId,
@@ -80,7 +80,7 @@ export function wrapToolWithExtensions<T>(tool: AgentTool<any, T>, runner: Exten
 						content: result.content,
 						details: result.details,
 						isError: false,
-					})) as ToolResultEventResult | undefined;
+					});
 
 					if (resultResult) {
 						return {
@@ -94,7 +94,7 @@ export function wrapToolWithExtensions<T>(tool: AgentTool<any, T>, runner: Exten
 			} catch (err) {
 				// Emit tool_result event for errors
 				if (runner.hasHandlers("tool_result")) {
-					await runner.emit({
+					await runner.emitToolResult({
 						type: "tool_result",
 						toolName: tool.name,
 						toolCallId,

packages/coding-agent/test/extensions-runner.test.ts

@@ -399,6 +399,98 @@ describe("ExtensionRunner", () => {
 		});
 	});
 
+	describe("tool_result chaining", () => {
+		it("chains content modifications across handlers", async () => {
+			const extCode1 = `
+				export default function(pi) {
+					pi.on("tool_result", async (event) => {
+						return {
+							content: [...event.content, { type: "text", text: "ext1" }],
+						};
+					});
+				}
+			`;
+			const extCode2 = `
+				export default function(pi) {
+					pi.on("tool_result", async (event) => {
+						return {
+							content: [...event.content, { type: "text", text: "ext2" }],
+						};
+					});
+				}
+			`;
+			fs.writeFileSync(path.join(extensionsDir, "tool-result-1.ts"), extCode1);
+			fs.writeFileSync(path.join(extensionsDir, "tool-result-2.ts"), extCode2);
+
+			const result = await discoverAndLoadExtensions([], tempDir, tempDir);
+			const runner = new ExtensionRunner(result.extensions, result.runtime, tempDir, sessionManager, modelRegistry);
+
+			const chained = await runner.emitToolResult({
+				type: "tool_result",
+				toolName: "my_tool",
+				toolCallId: "call-1",
+				input: {},
+				content: [{ type: "text", text: "base" }],
+				details: { initial: true },
+				isError: false,
+			});
+
+			expect(chained).toBeDefined();
+			const chainedContent = chained?.content;
+			expect(chainedContent).toBeDefined();
+			expect(chainedContent![0]).toEqual({ type: "text", text: "base" });
+			expect(chainedContent).toHaveLength(3);
+			const appendedText = chainedContent!
+				.slice(1)
+				.filter((item): item is { type: "text"; text: string } => item.type === "text")
+				.map((item) => item.text);
+			expect(appendedText.sort()).toEqual(["ext1", "ext2"]);
+		});
+
+		it("preserves previous modifications when later handlers return partial patches", async () => {
+			const extCode1 = `
+				export default function(pi) {
+					pi.on("tool_result", async () => {
+						return {
+							content: [{ type: "text", text: "first" }],
+							details: { source: "ext1" },
+						};
+					});
+				}
+			`;
+			const extCode2 = `
+				export default function(pi) {
+					pi.on("tool_result", async () => {
+						return {
+							isError: true,
+						};
+					});
+				}
+			`;
+			fs.writeFileSync(path.join(extensionsDir, "tool-result-partial-1.ts"), extCode1);
+			fs.writeFileSync(path.join(extensionsDir, "tool-result-partial-2.ts"), extCode2);
+
+			const result = await discoverAndLoadExtensions([], tempDir, tempDir);
+			const runner = new ExtensionRunner(result.extensions, result.runtime, tempDir, sessionManager, modelRegistry);
+
+			const chained = await runner.emitToolResult({
+				type: "tool_result",
+				toolName: "my_tool",
+				toolCallId: "call-2",
+				input: {},
+				content: [{ type: "text", text: "base" }],
+				details: { initial: true },
+				isError: false,
+			});
+
+			expect(chained).toEqual({
+				content: [{ type: "text", text: "first" }],
+				details: { source: "ext1" },
+				isError: true,
+			});
+		});
+	});
+
 	describe("hasHandlers", () => {
 		it("returns true when handlers exist for event type", async () => {
 			const extCode = `