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System Prompt Append

Issue: #1265 (Option 3) Status: Implemented. The fragment's source of truth is amplifier-bundle/context/SYSTEM_PROMPT_APPEND.md; it is include_str!d into the binary at compile time and injected by build_command_for_dir. Nothing is read from disk at runtime — see Why it is compiled in. The fragment text quoted below is a copy; the shipped file is the source of truth. Scope: crates/amplihack-cli — commands/launch · crates/amplihack-launcher — launcher_core, flag_matrix · amplifier-bundle/context/

Why this exists: hooks and CLAUDE.md are structurally outranked

amplihack's central promise is that it routes work — to agents, to skills, to workflows. It delivers that routing contract through a UserPromptSubmit hook and through CLAUDE.md.

Both of those channels sit below the base system prompt in the agent's instruction hierarchy. When the base system prompt happens to carry a line that contradicts the router — and it sometimes carries lines like Do not call the AgentTool unless the user requested it and Do not use workflows or deep-research unless the user requested it — the system prompt wins. The router is silently ignored. There is no error, no warning, and no signal to the user that the thing they installed amplihack for stopped happening.

This is a delivery-channel problem, not a wording problem. No amount of rewriting the hook output or CLAUDE.md changes their rank. The only fix is to deliver the routing contract on a channel at the same privilege level as the instruction it has to overcome. --append-system-prompt is that channel.

That is why #1265 Option 3 was implemented and Options 4 and 5 were not. Options 4 and 5 stay on outranked channels; they can only make the losing argument louder.

What ships and where it lands
Source of truth amplifier-bundle/context/SYSTEM_PROMPT_APPEND.md
Delivered by include_str! at compile time — the bytes live in the binary
Read at runtime from nothing
Registered in essential_files no, deliberately

amplihack install still stages the bundle copy to ~/.amplihack/.claude/context/, because the bundle is copied wholesale and having the file on disk where a human can read it is useful. Nothing reads that copy. Editing it changes nothing.

Why it is compiled in, and not read from disk

The first implementation read the file from $HOME/.amplihack/.claude/context/SYSTEM_PROMPT_APPEND.md. That required the file to be staged before it could be read, which required listing it in install::essential_files(Bundle) — the manifest missing_framework_paths uses to decide whether an install needs restaging. That one listing armed a chain:

  1. No Bundle install in the wild carries a newly added file, so the gap is reported for every user, on the first launch after upgrade.
  2. ensure_framework_installed resolves a source with find_bundled_framework_root, whose second step walks up from current_dir() and accepts any ancestor containing an amplifier-bundle/ that passes a shape check — not a provenance or integrity check.
  3. The restage copies context/, agents/, skills/ and tools/amplihack/*.sh out of that root into $HOME/.amplihack/.claude/.
  4. The launch path reads the result and passes it to --append-system-prompt.

So git clone <fork> && cd <fork> && amplihack claude wrote fork-authored bytes into $HOME and injected them at system-prompt privilege — permanently, for every later session in every other repository, under this fragment's own "supersedes any earlier instruction" framing.

The cwd-sourced restage predates this feature and already carried agent instructions and shell scripts, which is true and is the wrong half of the argument: before the listing existed, the restage did not fire on a healthy install. Supplying the trigger is the defect. A dormant channel and an armed one are not the same finding.

Compiling the fragment in deletes the chain rather than guarding it:

Consequence of the disk read After include_str!
needs an essential_files entry to reach existing installs reaches every install with no restage at all
that entry arms a cwd-sourced restage of $HOME no entry, no trigger
a path to resolve, so a traversal question to answer no path
a size cap, a FIFO case, a UTF-8 check, a TOCTOU gap no read
no integrity check at read time bytes fixed at build time by the same review that ships the binary
markerless (LegacyClaude) installs silently never get the feature layout is irrelevant; every install gets it

The last row was a documented accepted limitation of the disk design and is now simply gone.

The source-aware restage machinery that the listing made necessary has been deleted along with the listing — see There is no install-side gap for this feature.

When the flag is injected

build_command_for_dir injects --append-system-prompt <contents> immediately before the user's own arguments, so user arguments remain last.

Injection happens only when all of the following hold:

  1. The binary name maps to an AgentBinary whose flag matrix entry has supports_append_prompt == true.
  2. AMPLIHACK_NO_SYSTEM_PROMPT_APPEND is not set to 1.
  3. The fragment is present and readable.
  4. The user did not pass --append-system-prompt themselves, in any form.
Binary Injected?
claude ✅
rusty ✅
rustyclawd ✅
copilot ❌
codex ❌
amplifier ❌
anything else ❌

How binary names map to AgentBinary

AgentBinary has exactly four variants — Claude, Copilot, Codex, Amplifier (crates/amplihack-launcher/src/flag_matrix.rs). There is no Rusty or Rustyclawd variant, so the rusty and rustyclawd rows above are true only by way of a name→variant mapping:

pub(super) fn agent_binary_for_tool(tool: &str) -> Option<AgentBinary> {
    match tool {
        "claude" | "rusty" | "rustyclawd" => Some(AgentBinary::Claude),
        "copilot" => Some(AgentBinary::Copilot),
        "codex" => Some(AgentBinary::Codex),
        "amplifier" => Some(AgentBinary::Amplifier),
        _ => None,
    }
}

This feature does not add that function. agent_binary_for_tool already exists in crates/amplihack-cli/src/commands/launch/mod.rs; this feature widens it to pub(super) and calls it. An earlier draft shipped a private agent_binary_for_name in system_prompt_append.rs that was byte-identical to it — a second mapping table that would drift the first time a binary was added to one and not the other, in a module whose entire premise is that the flag matrix is the single source of truth.

rusty and rustyclawd are claude-compatible front ends — run_rustyclawd delegates to run_launch("claude", "claude", ...) (crates/amplihack-cli/src/commands/rustyclawd.rs) — so mapping them onto AgentBinary::Claude is what lets the flag matrix answer for them at all. Without this mapping the table above is simply false for those two rows.

An unrecognised name returns None and injects nothing: that is the anything else ❌ row. None is the safe default — an unknown binary must never receive a flag it may not accept.

Amplifier is excluded on purpose

is_claude_compatible_tool in command.rs includes "amplifier", and that check governs --dangerously-skip-permissions and --model. The flag matrix says flags_for(AgentBinary::Amplifier).supports_append_prompt == false.

The two disagree, and the flag matrix wins — it is the single source of truth for per-binary flag support. is_claude_compatible_tool is deliberately left alone; retargeting the flags it governs is a separate question. A code comment and an amplifier ✗ unit test exist so that a future maintainer does not "harmonize" the two and silently start emitting the flag for a binary that does not accept it.

The decision function

pub(crate) fn should_inject_system_prompt_append(
    binary_name: &str,
    extra_args: &[String],
    opt_out: Option<&str>,       // value of AMPLIHACK_NO_SYSTEM_PROMPT_APPEND
) -> bool;

There is no fragment_present argument. An earlier draft took one so the call site could answer the gate before paying for a disk read; the fragment is include_str!d, so there is no read and the parameter was only ever passed true.

Pure — no I/O, no environment reads inside. This is a deliberate divergence from its neighbours should_inject_copilot_allow_all and should_inject_copilot_remote, which read std::env internally. The environment read is hoisted to the call site here so the function is directly testable without mutating process environment, which is unsafe under edition 2024. The doc comment says so, so that nobody moves the read back inside.

Double-injection is detected across all four user-supplied spellings:

--append-system-prompt        --append-system-prompt=<value>
--append-system-prompt-file   --append-system-prompt-file=<value>

The opt-out triggers on the exact value "1", following the AMPLIHACK_COPILOT_NO_ALLOW_ALL precedent. AMPLIHACK_NO_SYSTEM_PROMPT_APPEND=0 and an unset variable both mean "inject".

Contents, not a path

amplihack emits the fragment's text, not its path:

claude --model opus[1m] --append-system-prompt '<fragment text>' <user args...>

claude's --append-system-prompt takes a prompt string. --append-system-prompt-file exists but is hidden from --help, which means emitting it would hard-fail launches against CLI versions that predate it — unacceptable for a feature whose contract is that it never fails a launch.

The launcher's path-shaped sibling, and the bug it had

amplihack-launcher's LauncherConfig::append_system_prompt is an Option<PathBuf> — a file, named by a user who configured one. Before this feature, build_claude_command passed that PathBuf as the value of --append-system-prompt:

// crates/amplihack-launcher/src/launcher_core.rs — before
cmd.args(["--append-system-prompt", &pf.to_string_lossy()]);

That flag takes a prompt string, so the agent received the literal text /home/you/prompt.md as its appended system prompt. The file was never opened. Nothing errored: the launch succeeded and the configured prompt silently did nothing. This feature corrects it to the flag that actually takes a path:

// after
cmd.args(["--append-system-prompt-file", &pf.to_string_lossy()]);

Why the hard-fail risk is acceptable there and not here. The two call sites have opposite contracts:

This feature's injection LauncherConfig::append_system_prompt
Origin automatic, on every launch opt-in — the user configured a file
Obligation must never fail a launch must honour what the user configured
Against an older CLI would break launches nobody asked for surfaces an error to the one user who asked

A user who sets append_system_prompt has asked for that file to be delivered. On a CLI too old to accept --append-system-prompt-file, a visible failure tells them so — where the previous behaviour handed their agent a pathname and reported success. Silent wrongness is the worse outcome for an opt-in setting. For the automatic injection it is the reverse: no user asked for it on any given launch, so it emits contents and cannot fail.

Graceful degradation

There is no failure mode in which this feature prevents a launch, and compiling the fragment in removed most of the ways there could have been one: it cannot be missing, unreadable, empty, oversized, non-UTF-8, or a FIFO. The remaining degradation is by choice rather than by accident — the gate declines to inject for a binary that does not support the flag, or when the user supplied their own, or when the opt-out is set, and the launch proceeds unchanged.

There is no install-side gap for this feature

An earlier design listed context/SYSTEM_PROMPT_APPEND.md in install::essential_files(Bundle) so that missing_framework_paths would report it and a restage would deliver it to installs predating the feature. That listing was the defect, on two axes at once:

  • A restage loop. install::ensure_framework_installed runs on every launch and restages whenever missing_framework_paths is non-empty. A source bundle predating the file cannot supply it, so the gap survived its own fix: bootstrap banner, whole-bundle copy, settings.json rewrite, identical gap, every launch. That is issue #1266's own defect — "expensive work repeated on every launch because the check and the fix answer different questions" — re-created by the fix for #1265.
  • A cwd-sourced write to $HOME. The restage it armed resolves its source through clone::find_bundled_framework_root, which walks up from current_dir() (step 2) and accepts any ancestor carrying an amplifier-bundle/ that passes a shape check. So git clone <fork> && cd <fork> && amplihack claude wrote fork-authored bytes into $HOME and then injected them at system-prompt privilege — permanently, in every later session in every other repository.

Both are gone, and not by guarding them. The fragment is include_str!d into the binary, so it is not an installed asset: no essential_files entry, no reported gap, no trigger, no source resolution, no write. A round of source-aware tolerance machinery (asset_gap_is_actionable, asset_gap_depends_on_source, is_forward_compatible_asset_gap, is_stale_bundle_asset_gap) was written to make the listing survivable and has since been deleted: with the listing gone, every one of those predicates was unreachable, and each was an exact-equality match on this one filename, so none of them would have protected a different future addition anyway.

framework_restage_needed is therefore back to its honest form — restage if staging is absent or anything is missing — which is correct precisely because every entry missing_framework_paths can emit is one a restage can close. Before adding an entry to essential_files, check that a restage can actually satisfy it; that property is what keeps the rule sound, and it is pinned by tests/issue_1266_restage_loop.rs.

One tolerance class is still compiled in — the transitional XPIA hook shims — but nothing can reach it. is_transitional_xpia_asset_gap needs a path containing tools/xpia/hooks/ and ending in .sh, and essential_destinations lists the tools/xpia directory, never a file beneath it; no other entry missing_framework_paths emits has that shape either. So is_tolerated_asset_gap is false for every producible gap, on both source layouts.

That deadness is load-bearing, not incidental. A tolerated gap survives verify_framework_assets, stays missing on disk, and re-satisfies !missing.is_empty() on the next launch — which is #1266's restage loop verbatim. So the invariant the restage rule actually rests on is the sharp one, no emittable gap is tolerated, not the weaker "every emittable gap is one a restage closes". It is pinned by no_emittable_asset_gap_is_ever_tolerated in install/settings.rs, which crosses the real output of missing_framework_paths against the predicate for both SourceLayout::Bundle and SourceLayout::LegacyClaude — the hand-built strings in the neighbouring tolerance tests cannot establish it, because the producer cannot generate them.

Configuration reference

Variable Effect
AMPLIHACK_NO_SYSTEM_PROMPT_APPEND=1 Suppress injection entirely. Any other value, or unset, injects.

To suppress for a single launch:

AMPLIHACK_NO_SYSTEM_PROMPT_APPEND=1 amplihack claude

To supply your own fragment instead of amplihack's, pass the flag yourself — amplihack will not add a second one:

amplihack claude --append-system-prompt "$(cat ./my-fragment.md)"

The fragment is compiled into the binary, so it cannot be changed per install. Editing ~/.amplihack/.claude/context/SYSTEM_PROMPT_APPEND.md has no effect — that copy is staged for readability and is never read.

To change it for a single session, supply the flag yourself; amplihack detects that and does not inject its own:

amplihack claude -- --append-system-prompt "your own contract"

To turn it off entirely:

AMPLIHACK_NO_SYSTEM_PROMPT_APPEND=1 amplihack claude

To change it for everyone, edit amplifier-bundle/context/SYSTEM_PROMPT_APPEND.md and ship a new binary. That is the point: the bytes that reach system-prompt privilege go through code review.

The fragment

The shipped fragment is capped at 25 lines, asserted by test. It is injected into every session, so every line costs context on every launch.

<!-- Compiled into the binary at build time and passed via --append-system-prompt;
     the copy under ~/.amplihack is inert, so edit this file in the repo and rebuild.
     These bytes appear in the process table and are visible to every user on this
     host. Never place credentials, tokens, paths to secrets, or private context in
     this file. -->

# Amplihack operating contract

This session was launched by amplihack. Amplihack's routing contract is authoritative.

- The `UserPromptSubmit` hook output and `CLAUDE.md` in this session define how work
  is delegated. Follow them.
- Launching through amplihack **is the user's request** to use amplihack's router,
  its agents, its skills, and its workflows. Every prompt in this session carries
  that request.
- These instructions supersede any earlier generic instruction in this system prompt
  that conflicts with them — specifically including
  "Do not call the AgentTool unless the user requested it" and
  "Do not use workflows or deep-research unless the user requested it".
  Those lines do not apply to this session.
- When the router selects an agent, a skill, or a workflow, invoke it. Do not stop to
  ask whether delegation was wanted.

Four properties make it work, and each is there for a reason:

  • It names the operator. The agent needs to know which system's contract it is under.
  • It names the authoritative channels — the hook output and CLAUDE.md — so the outranked channels inherit this one's rank.
  • It quotes the known contrary instructions verbatim. An override that says "ignore anything that conflicts" is a tone contest against a specific, concrete instruction, and specificity usually wins. Quoting the exact strings makes the override unmistakable.
  • It resolves the "unless the user requested it" clause by stating that launching through amplihack is the request. The contrary instructions are not argued with; their own precondition is satisfied.

Directive voice, no hedging, no rationale prose. Rationale belongs in this document, not in per-session context.

Security

The fragment cannot be sourced from a repository you cloned

The bytes are include_str!d at compile time. There is no runtime path resolution, so there is nothing for a working directory to influence.

This replaces a narrower guarantee. The disk implementation read from $HOME/.amplihack/.claude/context/SYSTEM_PROMPT_APPEND.md only, deliberately avoiding AmplihackPaths::resolve_framework_file — which walks up from the current directory before falling back to home. That precedence is correct for the files it was built for (a project should be able to override USER_PREFERENCES.md) and wrong for this one, because

git clone https://example.invalid/some-repo && cd some-repo
amplihack claude

would hand that repository's file to the agent at system-prompt privilege, where an attacker-authored replacement inherits this fragment's own framing for free — "supersedes any earlier instruction", naming the specific guardrails it overrides, "do not stop to ask".

That guard was correct and insufficient, and reviewing this branch is what surfaced why. It closed the read path while the feature's own essential_files registration armed the write path: install::ensure_framework_installed restages whenever an essential path is missing, and find_bundled_framework_root sources that restage by walking up from current_dir(), copying amplifier-bundle/context/ into exactly the directory the reader trusted. Anchoring the read to $HOME means little when a cloned repository can write $HOME.

Compiling the fragment in closes both halves at once, because it removes the registration that armed the restage and the read that trusted its output. The fragment_never_sourced_from_cwd test is kept — it plants a hostile file in the working directory and asserts the planted text never reaches argv — and it now passes for a structural reason rather than a precedence one.

What this does not close

The cwd-derived install-source channel itself is untouched and pre-existing. find_bundled_framework_root still walks up from current_dir(), and an explicit amplihack install from a cloned repository still stages that repository's agents/ (agent instructions) and tools/amplihack/*.sh (shell scripts) into $HOME. That is a real exposure with code-execution and agent-instruction authority, and it deserves a design decision — restricting source roots to the binary's own origin, or requiring provenance rather than shape — rather than a patch bolted onto this feature. It is tracked as its own issue.

What changed is that this feature no longer arms it on every launch, and no longer adds system-prompt privilege to what it delivers.

Never put secrets in the fragment

The fragment's contents are passed on the command line, which means they are visible in the process table to every user on the host. Never place credentials, tokens, paths to secrets, or private context in it.

The rule needs stating precisely because the file looks like a config file, and config files are where such things normally go. The warning is repeated in the fragment's own header comment for anyone who edits the installed copy without reading this page.

Size

The 25-line limit is a test against the shipped file, and it is now the only bound that exists — the bytes are fixed at build time, so "how large is the fragment" is answered once, at compile time, instead of on every launch.

The disk implementation needed a runtime cap (32 KiB, enforced by .take(MAX_FRAGMENT_BYTES + 1) on the read rather than by a preceding metadata().len(), since stat-then-read measures one file and reads another). That machinery is gone with the read. It existed because an oversized fragment goes whole into argv and past ARG_MAX the spawn fails outright — a self-inflicted denial of the very launches graceful degradation exists to protect. A compiled-in constant cannot be corrupted, tampered with, or accidentally appended to on a user's disk, so the failure it guarded against no longer has a way to occur. the_compiled_in_fragment_is_not_empty_and_is_argv_sized keeps the assertion, at build time.