fix(coding-agent): merge piped stdin into initial prompt closes #2315

packages/coding-agent/README.md

@@ -451,6 +451,12 @@ pi config                    # Enable/disable package resources
 | `--mode rpc` | RPC mode for process integration (see [docs/rpc.md](docs/rpc.md)) |
 | `--export <in> [out]` | Export session to HTML |
 
+In print mode, pi also reads piped stdin and merges it into the initial prompt:
+
+```bash
+cat README.md | pi -p "Summarize this text"
+```
+
 ### Model Options
 
 | Option | Description |
@@ -525,6 +531,9 @@ pi "List all .ts files in src/"
 # Non-interactive
 pi -p "Summarize this codebase"
 
+# Non-interactive with piped stdin
+cat README.md | pi -p "Summarize this text"
+
 # Different model
 pi --provider openai --model gpt-4o "Help me refactor"
 

packages/coding-agent/src/cli/initial-message.ts

@@ -0,0 +1,43 @@
+import type { ImageContent } from "@mariozechner/pi-ai";
+import type { Args } from "./args.js";
+
+export interface InitialMessageInput {
+	parsed: Args;
+	fileText?: string;
+	fileImages?: ImageContent[];
+	stdinContent?: string;
+}
+
+export interface InitialMessageResult {
+	initialMessage?: string;
+	initialImages?: ImageContent[];
+}
+
+/**
+ * Combine stdin content, @file text, and the first CLI message into a single
+ * initial prompt for non-interactive mode.
+ */
+export function buildInitialMessage({
+	parsed,
+	fileText,
+	fileImages,
+	stdinContent,
+}: InitialMessageInput): InitialMessageResult {
+	const parts: string[] = [];
+	if (stdinContent !== undefined) {
+		parts.push(stdinContent);
+	}
+	if (fileText) {
+		parts.push(fileText);
+	}
+
+	if (parsed.messages.length > 0) {
+		parts.push(parsed.messages[0]);
+		parsed.messages.shift();
+	}
+
+	return {
+		initialMessage: parts.length > 0 ? parts.join("") : undefined,
+		initialImages: fileImages && fileImages.length > 0 ? fileImages : undefined,
+	};
+}

packages/coding-agent/src/main.ts

@@ -11,6 +11,7 @@ import { createInterface } from "readline";
 import { type Args, parseArgs, printHelp } from "./cli/args.js";
 import { selectConfig } from "./cli/config-selector.js";
 import { processFileArguments } from "./cli/file-processor.js";
+import { buildInitialMessage } from "./cli/initial-message.js";
 import { listModels } from "./cli/list-models.js";
 import { selectSession } from "./cli/session-picker.js";
 import { APP_NAME, getAgentDir, getModelsPath, VERSION } from "./config.js";
@@ -311,28 +312,22 @@ async function handlePackageCommand(args: string[]): Promise<boolean> {
 async function prepareInitialMessage(
 	parsed: Args,
 	autoResizeImages: boolean,
+	stdinContent?: string,
 ): Promise<{
 	initialMessage?: string;
 	initialImages?: ImageContent[];
 }> {
 	if (parsed.fileArgs.length === 0) {
-		return {};
+		return buildInitialMessage({ parsed, stdinContent });
 	}
 
 	const { text, images } = await processFileArguments(parsed.fileArgs, { autoResizeImages });
-
+	return buildInitialMessage({
-	let initialMessage: string;
+		parsed,
-	if (parsed.messages.length > 0) {
+		fileText: text,
-		initialMessage = text + parsed.messages[0];
+		fileImages: images,
-		parsed.messages.shift();
+		stdinContent,
-	} else {
+	});
-		initialMessage = text;
-	}
-
-	return {
-		initialMessage,
-		initialImages: images.length > 0 ? images : undefined,
-	};
 }
 
 /** Result from resolving a session argument */
@@ -675,13 +670,12 @@ export async function main(args: string[]) {
 	}
 
 	// Read piped stdin content (if any) - skip for RPC mode which uses stdin for JSON-RPC
+	let stdinContent: string | undefined;
 	if (parsed.mode !== "rpc") {
-		const stdinContent = await readPipedStdin();
+		stdinContent = await readPipedStdin();
 		if (stdinContent !== undefined) {
 			// Force print mode since interactive mode requires a TTY for keyboard input
 			parsed.print = true;
-			// Prepend stdin content to messages
-			parsed.messages.unshift(stdinContent);
 		}
 	}
 
@@ -704,7 +698,11 @@ export async function main(args: string[]) {
 		process.exit(1);
 	}
 
-	const { initialMessage, initialImages } = await prepareInitialMessage(parsed, settingsManager.getImageAutoResize());
+	const { initialMessage, initialImages } = await prepareInitialMessage(
+		parsed,
+		settingsManager.getImageAutoResize(),
+		stdinContent,
+	);
 	const isInteractive = !parsed.print && parsed.mode === undefined;
 	const mode = parsed.mode || "text";
 	initTheme(settingsManager.getTheme(), isInteractive);

packages/coding-agent/test/initial-message.test.ts

@@ -0,0 +1,47 @@
+import { describe, expect, test } from "vitest";
+import type { Args } from "../src/cli/args.js";
+import { buildInitialMessage } from "../src/cli/initial-message.js";
+
+function createArgs(messages: string[] = []): Args {
+	return {
+		messages: [...messages],
+		fileArgs: [],
+		unknownFlags: new Map(),
+	};
+}
+
+describe("buildInitialMessage", () => {
+	test("merges piped stdin with the first CLI message into one prompt", () => {
+		const parsed = createArgs(["Summarize the text given"]);
+		const result = buildInitialMessage({
+			parsed,
+			stdinContent: "README contents\n",
+		});
+
+		expect(result.initialMessage).toBe("README contents\nSummarize the text given");
+		expect(parsed.messages).toEqual([]);
+	});
+
+	test("uses stdin as the initial prompt when no CLI message is present", () => {
+		const parsed = createArgs();
+		const result = buildInitialMessage({
+			parsed,
+			stdinContent: "README contents",
+		});
+
+		expect(result.initialMessage).toBe("README contents");
+		expect(parsed.messages).toEqual([]);
+	});
+
+	test("combines stdin, file text, and first CLI message in one prompt", () => {
+		const parsed = createArgs(["Explain it", "Second message"]);
+		const result = buildInitialMessage({
+			parsed,
+			stdinContent: "stdin\n",
+			fileText: "file\n",
+		});
+
+		expect(result.initialMessage).toBe("stdin\nfile\nExplain it");
+		expect(parsed.messages).toEqual(["Second message"]);
+	});
+});