[Bug] Piped stdin and the CLI instruction are concatenated without a separator

[zfaustk]

Affected area

Coding agent and CLI

What happened?

When print mode receives both piped stdin and a CLI instruction, the two inputs can be concatenated without a separator.

readPipedStdin() trims the stdin value. buildInitialMessage() then combines stdin and the first CLI message with parts.join(""). In the documented command below, the initial prompt becomes README contentsSummarize this text:

cat README.md | prime-agent -p "Summarize this text"

The existing initial-message test does not catch this because it calls buildInitialMessage() directly with a stdin value that still ends in \n.

Steps to reproduce

On current main, run this from packages/coding-agent:

npx tsx -e 'import { buildInitialMessage } from "./src/cli/initial-message.ts"; const rawStdin = "README contents\n"; const stdinContent = rawStdin.trim() || undefined; const parsed = { messages: ["Summarize this text"], fileArgs: [], unknownFlags: new Map(), diagnostics: [] }; console.log(JSON.stringify(buildInitialMessage({ parsed, stdinContent }).initialMessage));'

Output:

"README contentsSummarize this text"

Expected behavior

Piped stdin and the explicit instruction should remain distinguishable in the assembled prompt. A newline between them would produce README contents\nSummarize this text, whether or not the original stdin ended with a newline.

Prime Agent version

main at 9f9501146e869466acaca66dac49cff857b7b4f9

Environment

macOS 26.3.2 arm64, Node.js 24.14.1

Additional context

I can add a focused regression test and a narrow fix if maintainers want this behavior changed and invite the implementation.

[RelaxJonh]

Confirmed on Windows 11 (native, not WSL) with Prime Agent 0.7.2 and Node.js 24.x.

The reproduction from the OP works identically on Windows:

npx tsx -e 'import { buildInitialMessage } from "./src/cli/initial-message.ts"; const rawStdin = "README contents\n"; const stdinContent = rawStdin.trim() || undefined; const parsed = { messages: ["Summarize this text"], fileArgs: [], unknownFlags: new Map(), diagnostics: [] }; console.log(JSON.stringify(buildInitialMessage({ parsed, stdinContent }).initialMessage));'

Output: "README contentsSummarize this text"

The root cause is parts.join("") in buildInitialMessage() at packages/coding-agent/src/cli/initial-message.ts:33. When stdin is piped and trimmed, and a CLI message follows, the two are concatenated without any separator.

This also affects Windows pipe input (e.g. type README.md | prime-agent -p "Summarize") — the behavior is platform-independent since the bug is in the string join logic, not in I/O handling.

I can confirm the fix is straightforward: either parts.join("\n") or adding a newline before appending the CLI message. Happy to test a patch on Windows if maintainers want this addressed.