fix: stop waiting on the silent stdin an agent's shell hands vf

[Bradenream]

Summary

Claude Code runs any Bash command that contains a heredoc with stdin connected to a Unix socket. It never writes to that socket and never closes it; commands without a heredoc get /dev/null instead. vf took the socket for a piped body. Every body command in such a call waited the full 10s stdin deadline and then failed, even with every field given as a flag. An agent then had to retry with </dev/null and remember to add it to every later write.

In agent mode, a socket on stdin that delivers nothing within 250ms is now taken to carry no body. Pipes and files keep the full 10s in every mode. Shells hand vf a pipe for producer | vf and a file for redirects and heredocs, never a socket. So slow producers (curl … | vf) and the README's "pipe a body, override fields with flags" still work. A socket comes from a program that spawned vf, and a program that means to send a body writes it as it starts vf: a Node parent's first byte arrives within 0.1ms, even with 100 spawns at once.

The output package owns agent detection and imports flagutil, so flagutil cannot import it to ask. Instead output installs IsAgentMode in flagutil when the program starts (internal/output/agentmodecheck.go), and flagutil asks it each time rather than keeping a copy. So the stdin rule follows however agent mode is finally resolved, including an explicit --agent-mode once the separate fix for that flag lands.

Options compared

Each option was measured against the existing stdin tests plus the four new cases:

Option	Silent socket no longer stalls	Slow piped body still read
Skip stdin when flags satisfy the body	only when a body flag is given	no: dropped when flags are also given (a README case)
Short window for any stdin, agent mode	yes	no: slow pipelines dropped in agent mode
Skip stdin that is not yet readable	yes	no: python3, node and slow producers dropped (5 tests fail)
This PR: short window for a socket, agent mode	yes	yes (13/13)

Deliberate limit: an idle pipe in agent mode still waits 10s, because a pipe is also how a slow producer | vf arrives. Claude Code does not hand vf one.

Before and after

vf agent update --prompt x, run in an agent's Bash call that contains a heredoc:

	master	this PR
Time	10.0s	0.8s
Result	failed to read stdin: timed out…, exit 1	prompt sent, exit 0

Test plan

• gofmt, go vet ./... and go test ./... pass; go.mod is unchanged.
• internal/flagutil/stdin_test.go covers the decision rule and the bounded read:
  • a silent reader means no body;
  • a body that starts in time is read;
  • the first bytes, then the rest, are both read;
  • EOF returns at once;
  • a slow body with no limit is read.
• test/stdin-handling.test.ts has 4 new cases, and the file passes 13/13:
  • a silent socket returns at once, with flags and without (both take 10s and fail on master);
  • a socket that writes at spawn is still read;
  • a slow shell producer merged with flags is still read.
• Reproduced and verified inside a real Claude Code Bash call.
• CI

The 4 vitest cases that already fail on master (docs-command ×2, flag-errors, flag-raw-text) are fixed by #37. With both PRs applied to current master, the suite passes 82/82.

[Copilot]

Copilot review overview

🟡 Changes recommended

Explicit --agent-mode overrides are bypassed by early cached detection, producing incorrect stdin behavior.

Review effort: Balanced
Findings: 1

Open (1)

• Honor explicit --agent-mode flag overrides before cached agent detection · New

What changed in this PR

Prevents agent-mode commands from waiting on silent stdin sockets while preserving normal pipe behavior.

Changes:

• Adds socket-specific 250ms silence handling.
• Synchronizes agent mode with stdin handling.
• Adds Go and integration regression tests.

File	Description
internal/​flagutil/​stdin.go	Implements bounded socket handling.
internal/​flagutil/​metadata.go	Routes body reads through new logic.
internal/​output/​agentmode.go	Propagates agent-mode state.
internal/​flagutil/​stdin_test.go	Tests bounded reads and classification.
test/​stdin-handling.test.ts	Tests real socket and pipe behavior.

Files not reviewed (2)

• internal/flagutil/metadata.go: Generated file
• internal/output/agentmode.go: Generated file

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[effervescentia]

Merge activity

• Oct 2, 6:35 PM UTC: The merge label 'merge' was detected. This PR will be added to the Graphite merge queue once it meets the requirements.
• Oct 2, 6:35 PM UTC: effervescentia added this pull request to the Graphite merge queue.
• Oct 2, 6:35 PM UTC: CI is running for this pull request on a draft pull request (#47) due to your merge queue CI optimization settings.
• Oct 2, 6:36 PM UTC: Merged by the Graphite merge queue via draft PR: #47.