headroom/REALIGNMENT/02-architecture.md

02 — Realigned Target Architecture

The Rust-only proxy after Phase H. Each subsystem documented with its scope, invariants, file layout, and what it explicitly does NOT do.

---

2.1 Request lifecycle (Rust, post-Phase-C)

                                Client request
                                      │
                                      ▼
            ┌──────────────────────────────────────────────┐
            │ headroom-proxy (axum)                        │
            │                                              │
            │  1. classify_auth_mode(headers)              │  ← Phase F
            │     → "payg" | "oauth" | "subscription"      │
            │                                              │
            │  2. strip x-headroom-* from upstream-bound   │  ← Phase A (PR-A5)
            │                                              │
            │  3. byte-buffer body via RawValue            │  ← Phase A (PR-A4)
            │     (numeric precision preserved)            │
            │                                              │
            │  4. honor cache_control markers              │  ← Phase A (PR-A4)
            │     → frozen_message_count                   │
            │                                              │
            │  5. live_zone_compress(body, frozen_count,   │  ← Phase B
            │                       auth_mode)             │
            │     ├─ identify live-zone blocks             │
            │     ├─ per-block content-type detection      │
            │     ├─ dispatch to type-aware compressor     │
            │     ├─ token-validate; fallback to original  │
            │     ├─ CCR: hash-key, store, marker          │
            │     └─ replace block bytes in-place          │
            │                                              │
            │  6. tool_def_normalize(body)                 │  ← Phase E (PR-E1, E2)
            │     ├─ alpha-sort tools[]                    │
            │     └─ recursive-sort JSON Schema keys       │
            │                                              │
            │  7. cache_control_auto_place(body)           │  ← Phase E (PR-E3)
            │     (Anthropic; up to 4 ephemeral)           │
            │                                              │
            │  8. prompt_cache_key_inject(body)            │  ← Phase E (PR-E4)
            │     (OpenAI; only if not customer-set)       │
            │                                              │
            │  9. forward via reqwest with original bytes  │
            │     for unmodified envelope (RawValue diff)  │
            │                                              │
            │ 10. SSE response: byte-level state machine   │  ← Phase C (PR-C1)
            │     ├─ track blocks/items by id              │
            │     ├─ all delta types handled               │
            │     ├─ mid-stream error/ping/drop surfaced   │
            │     └─ pure passthrough to client            │
            │                                              │
            │ 11. usage telemetry (cache_read,             │  ← Phase G
            │     cache_creation, output_tokens, etc.)     │
            └──────────────────────────────────────────────┘
                                      │
                                      ▼
                                Upstream provider

---

2.2 The cache-safety invariants (every PR enforces)

Invariant I1 — Byte-faithful passthrough on unmutated bytes

For every request, the bytes sent to upstream are byte-equal (SHA-256) to the bytes received from the client, modulo only the byte ranges that a transform explicitly modified. No re-serialization through a Value type. No JSON-prettifier whitespace insertion. No \uXXXX ASCII escaping of UTF-8 user content.

Implementation: serde_json::value::RawValue for messages[*] entries; modified messages get fresh serialization, retained messages forward as exact byte copies. Workspace Cargo.toml adds arbitrary_precision + raw_value features.

Test gate: proxy_byte_faithful_anthropic_sha256 — record a real Anthropic /v1/messages payload, send it through the proxy with compression off, assert SHA-256 byte-equal at the upstream mock.

Invariant I2 — Cache hot zone never modified

The following are never mutated by Headroom:

• system (string or block list)
• tools[*] (other than alpha-sorting and JSON Schema key sorting in Phase E — both deterministic)
• Any message at index < frozen_message_count
• Reasoning items with encrypted_content
• Thinking blocks with signature
• redacted_thinking.data
• Compaction items ({"type": "compaction", "encrypted_content": ...})

Implementation: live_zone_compress walks messages from the tail, identifies live-zone blocks (latest user message, latest tool_result, latest function_call_output, latest local_shell_call_output, latest apply_patch_call_output), and ONLY modifies bytes within those blocks.

Test gate: cache_hot_zone_unchanged_under_compression — fixture with system + tools + 5 historical turns + new tool_result; assert system + tools + first 5 turns bytes equal at upstream.

Invariant I3 — Append-only

Once a message has appeared in any prior request to upstream, its bytes are frozen. Compression operates on the live zone (latest turn) only.

Implementation: frozen_message_count is the floor; any compressor that touches index < frozen_message_count is rejected at compile time (Rust trait constraint) or runtime (Python assertion).

Test gate: append_only_invariant_under_recompression — same input bytes through the compressor twice produces byte-equal output; retained messages are byte-equal across the two runs.

Invariant I4 — Determinism

For the same (input bytes, frozen_count, auth_mode), the compressor produces byte-equal output. No timestamps, no random seeds, no time-dependent decisions.

Implementation:

• TOIN is observation-only (Phase B PR-B5); it never alters request-time decisions.
• All hashing is BLAKE3 / SHA-256 with stable input ordering.
• Sort orders are explicit (BTreeMap for output, never HashMap).
• No Instant::now() in any compression code path.

Test gate: Property test — for arbitrary valid input, compress(input) == compress(compress(input).original) (idempotence on already-compressed); compress(input) == compress(input) (run-to-run determinism).

Invariant I5 — Token-aware, not byte-aware

Every compression is validated post-compression with a tokenizer. If compressed.tokens >= original.tokens, the original is forwarded.

Implementation: Phase B PR-B4. Per-content-type byte thresholds: code>2KB, JSON>1KB, logs>500B, plain text>5KB. Below threshold = no compression attempted (overhead exceeds savings).

Test gate: proptest_compression_token_count_non_increasing — for arbitrary valid inputs from a strategy, tokens(output) ≤ tokens(input).

Invariant I6 — Position-preserving

Compression never reorders blocks within a content array, never splits one block into multiple, never adds inline metadata fields to existing blocks.

Implementation: Compressor signature is fn(block: &mut Block) -> Result<()> — operates in place. Block type, tool_use_id / call_id, is_error, all sibling fields preserved.

Side-channel metadata: A separate marker block (text-type, sibling) carries CCR retrieval directives. Never an extra field on the original block.

Invariant I7 — Tool definitions normalized, not compressed

Tools are sorted alphabetically by name; JSON Schema keys are sorted recursively; description whitespace is normalized. The bytes of each tool definition's input_schema.properties[*].description are otherwise preserved.

Implementation: Phase E PR-E1, PR-E2.

Invariant I8 — signature, encrypted_content, redacted_thinking.data are sacrosanct

These are passthrough only. Never inspected, never decoded, never transformed.

Implementation: Compressor block-type dispatch has explicit no-op arms for these types. The Bedrock/Vertex native paths (Phase D) preserve them unlike the LiteLLM converter.

Invariant I9 — TOIN observes, never mutates request bytes

TOIN's pattern stats grow across requests. Recommendations are published to disk between deploys. The compressor reads recommendations at startup, not per-request.

Implementation: Phase B PR-B5. TOIN's in-memory state writes are append-only; reads never block compression.

Invariant I10 — Auth mode gates compression policy

PAYG: aggressive (full live-zone compression, CCR, tool injection, Phase 3 stabilization). OAuth: passthrough-prefer (live-zone lossless only, no auto-cache_control, no auto-prompt_cache_key, no X-Forwarded-*). Subscription: stealth-prefer (everything OAuth does PLUS preserve accept-encoding, never inject X-Headroom-* upstream, never mutate User-Agent).

Implementation: Phase F PR-F1, PR-F2.

---

2.3 The compressor module layout (post-Phase-B)

crates/headroom-core/src/
├── lib.rs                     # public surface
├── tokenizer/                 # KEEP (HF + tiktoken impls)
│   ├── mod.rs
│   ├── hf_impl.rs
│   ├── tiktoken_impl.rs
│   ├── estimator.rs
│   └── registry.rs
├── ccr.rs                     # KEEP, hardened (persistent backend)
├── signals/                   # KEEP — drives live-zone consumers
│   ├── mod.rs
│   ├── line_importance.rs
│   ├── keyword_detector.rs
│   └── tiered.rs
├── transforms/                # the compressors
│   ├── mod.rs
│   ├── safety.rs              # MOVED from context/safety.rs (Phase B)
│   ├── live_zone.rs           # NEW — live-zone block dispatcher (Phase B)
│   ├── content_detector.rs    # KEEP
│   ├── detection.rs           # KEEP
│   ├── magika_detector.rs     # KEEP
│   ├── unidiff_detector.rs    # KEEP
│   ├── adaptive_sizer.rs      # KEEP
│   ├── anchor_selector.rs     # KEEP
│   ├── tag_protector.rs       # KEEP
│   ├── log_compressor.rs      # KEEP
│   ├── search_compressor.rs   # KEEP
│   ├── diff_compressor.rs     # KEEP
│   ├── kompress_compressor.rs # NEW — Phase H Rust port via `ort` crate
│   ├── smart_crusher/         # KEEP (25 files, correctly scoped)
│   └── pipeline/              # SHRUNK — only the live-zone orchestrator
│       ├── mod.rs
│       ├── orchestrator.rs    # rewrite to live-zone-only
│       ├── traits.rs          # LosslessTransform / LossyTransform
│       └── offloads/          # KEEP — JSON, log, search, diff offloads
└── auth_mode.rs               # NEW — Phase F (classify_auth_mode helper)

# DELETED in Phase B:
# context/                     ← except safety.rs which moved
# scoring/
# relevance/

crates/headroom-proxy/src/
├── lib.rs
├── main.rs
├── config.rs
├── error.rs
├── proxy.rs                   # Phase A: pure passthrough on /v1/messages
                               # Phase C: + /v1/chat/completions, /v1/responses
├── headers.rs                 # Phase F: conditional X-Forwarded-*
├── websocket.rs               # Phase C: WS Codex flow
├── sse/                       # NEW — Phase C
│   ├── mod.rs
│   ├── parser.rs              # byte-level state machine
│   ├── anthropic.rs           # 4-event dance + delta types
│   ├── openai_chat.rs         # tool_call accumulation
│   └── openai_responses.rs    # output items + reasoning summary
├── compression/
│   ├── mod.rs                 # routing by path × auth_mode
│   ├── live_zone_anthropic.rs # NEW (Phase B)
│   ├── live_zone_openai.rs    # NEW (Phase C)
│   ├── tool_def_normalize.rs  # NEW (Phase E)
│   ├── cache_control.rs       # NEW (Phase E)
│   └── model_limits.rs        # KEEP
├── bedrock/                   # NEW — Phase D
│   ├── mod.rs
│   ├── sigv4.rs
│   ├── invoke.rs
│   └── eventstream.rs
├── vertex/                    # NEW — Phase D
│   ├── mod.rs
│   ├── adc.rs
│   └── stream_raw_predict.rs
└── observability/             # NEW — Phase G
    ├── mod.rs
    ├── prometheus.rs
    ├── cache_hit_rate.rs
    └── compression_ratio.rs

# DELETED:
# compression/icm.rs           ← Phase A PR-A1
# compression/anthropic.rs     ← Phase A PR-A1 (replaced with live_zone_anthropic.rs in Phase B)

---

2.4 The auth-mode policy matrix (Phase F)

Policy aspect	PAYG	OAuth	Subscription
Live-zone compression	aggressive	lossless-only	lossless-only
CCR enabled	yes	yes	yes (long-session)
Tool def alpha-sort	yes	yes	yes
JSON Schema key sort	yes	yes	yes
Auto cache_control placement	yes	NO (could void scope)	NO
Auto prompt_cache_key injection	yes (OpenAI)	NO	NO
anthropic-beta mutation	NO	NO	NO
X-Headroom-* upstream	NO	NO	NO
X-Forwarded-* upstream	yes	yes	NO
User-Agent rewrite	NO	NO	NO
accept-encoding strip	OK	OK	NO (preserve)
Lossy compressors (LLMLingua)	OK	NO	NO
Memory injection	live-zone tail	live-zone tail (gated)	live-zone tail (gated)
TOIN aggregation key	(mode, model)	(mode, model)	(mode, model)
Authorization log redaction	first 12 chars	first 12 chars	first 12 chars

---

2.5 Preserved primitives detail

TOIN (post-Phase-B-PR-B5)

// Strict observation-only.
pub trait Telemetry {
    fn record_compression(
        &self,
        auth_mode: AuthMode,
        model: ModelFamily,
        structure_hash: StructureHash,
        outcome: CompressionOutcome,
    );
    // No request-time hint API. Period.
}

// Recommendations published between deploys via:
//   $ cargo run -p headroom-toin-publish -- --auth-mode payg --model claude-3-7-sonnet
// Output: recommendations.toml committed to repo, loaded by compressor at startup.

CCR (post-Phase-B-PR-B7)

pub trait CcrStore: Send + Sync {
    fn put(&self, hash: ContentHash, original: Bytes, ttl: Duration) -> Result<()>;
    fn get(&self, hash: ContentHash) -> Result<Option<Bytes>>;
    fn purge_expired(&self) -> usize;
}

pub struct SqliteCcrStore { ... }   // primary backend
pub struct RedisCcrStore { ... }    // optional, for multi-worker

// `ccr_retrieve` tool registered on every request for sessions that ever did CCR.
// Marker injection format: `<<ccr:HASH>>` appended to compressed block content.
// Markers are deterministic (hash is content-addressed); replay-safe.

Kompress-base (post-Phase-H-PR-H4 Rust port)

// Plain-text §8.6 compressor. Used only as a last resort, only on live-zone
// user-message text exceeding 5KB.
pub struct KompressCompressor {
    // ONNX runtime via `ort` crate. Model deterministic for fixed weights.
    session: ort::Session,
    threshold_bytes: usize,
}

impl LossyTransform for KompressCompressor { ... }

---

2.6 What this architecture explicitly does NOT do

• Does NOT drop messages from history. Ever. ICM is gone.
• Does NOT modify system, tools, or any old turn.
• Does NOT inject Headroom's own tools into customer prompts unless CCR has already fired in this session (and then always, never toggling).
• Does NOT consult TOIN at request time. Recommendations are loaded at startup only.
• Does NOT shell out to RTK from the proxy. RTK lives on the wrap-CLI side (project-decided 2026-05-01).
• Does NOT translate Anthropic ↔ OpenAI shapes. Each provider has its own native handler. Bedrock and Vertex have native envelopes (Phase D).
• Does NOT compress on /v1/responses/compact or /v1/conversations (different shapes; passthrough only).
• Does NOT rewrite request headers except to strip x-headroom-* from upstream-bound headers and add conditional X-Forwarded-* (PAYG/OAuth only).
• Does NOT add User-Agent headers. The customer's UA passes through verbatim.
• Does NOT compress images, base64 blobs, or audio (out of scope for this realignment).
• Does NOT modify tool_use.input JSON key order, tool_calls.function.arguments string contents, phase field, V4A patches, local_shell_call.action.command argv arrays, or any encrypted/redacted/compaction content.