docs: add ARCHITECTURE.md from Palace doc, cross-reference from README

ARCHITECTURE.md

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+# LiteLLM Dashboard Tool — Architecture & Design
+
+**Created:** 2026-06-04
+**Authors:** Kagamine Rin (Chief Engineer) & Ryan Neef (General / Co-Processor)
+
+## Product Definition
+
+A single Open WebUI **Tool** that provides LiteLLM proxy health status and cost breakdown. Designed for the Network but built for general use — any LiteLLM proxy instance.
+
+## OWI Integration Model
+
+**Type:** Open WebUI Tool (not a Pipeline, not a Filter)
+
+**How it works:**
+1. Tool is added to the OWI Workspace
+2. Tool is activated on a per-model or per-chat basis
+3. **Valves** pass the LiteLLM proxy address and master key
+4. No hardcoded connection details — the tool is portable
+
+**Why a Tool, not a Pipeline/Filter:**
+- Pipelines run on every request (wrong scope — this is on-demand, not per-request)
+- Filters modify request/response flow (wrong scope — this is informational)
+- Tools are invoked when needed, by the model, in context. This is the right pattern for "check cost" and "check health" — you ask, it answers.
+
+## Valve Configuration
+
+The tool needs two valves:
+
+| Valve | Type | Purpose | Default |
+|-------|------|---------|---------|
+| `litellm_url` | string | LiteLLM proxy base URL (e.g., `http://localhost:4000`) | `http://localhost:4000` |
+| `litellm_master_key` | string | LiteLLM master API key for auth | (empty — must be configured) |
+
+**Security consideration:** The master key will be stored in the OWI valve configuration. This is the same security model as other API keys in OWI (LiteLLM proxy key, etc.). The key is not exposed in chat context — OWI valves are injected at tool execution time, not displayed to the user.
+
+**Docker networking note:** If OWI runs inside Docker (typical), the `litellm_url` valve must use `host.docker.internal:4000` instead of `localhost:4000` to reach the LiteLLM proxy on the host.
+
+## Function 1: Health Check
+
+**LiteLLM Endpoint:** `GET {litellm_url}/health`
+**Auth:** `Authorization: Bearer {litellm_master_key}`
+
+### Response Processing
+
+The `/health` endpoint returns `healthy_endpoints` and `unhealthy_endpoints` arrays. Each endpoint object contains a massive `litellm_metadata` blob (~2KB per model) with full auth context. **This is stripped for token efficiency.**
+
+### Stripped Fields
+
+These fields are removed from the `/health` response because they bloat output without providing useful information:
+
+- `litellm_metadata` — ~2KB of auth/internal context per model
+- `safety_settings` — internal safety filter config
+- `cache_control_injection_points` — internal routing detail
+- `use_in_pass_through`, `use_litellm_proxy`, `merge_reasoning_content_in_choices` — internal flags
+
+### Retained Fields
+
+- `model` — the LiteLLM model identifier
+- `rpm` — rate limit
+- `timeout` — request timeout in seconds
+- `reasoning_effort` — if present and not "none" (e.g., "low")
+- `service_tier` — if present (e.g., "flex")
+
+### Output Format
+
+```
+LiteLLM Health Check — http://host.docker.internal:4000
+==================================================
+✅ Authenticated with master key
+
+Healthy: 8  |  Unhealthy: 0
+
+--- Healthy Models (8) ---
+  ✅ gemini-2.5-pro  — rpm: 60, timeout: 300.0s  (reasoning: low, tier: flex)
+  ✅ gemini-2.5-flash  — rpm: 60, timeout: 300.0s
+  ...
+```
+
+### Error Paths
+
+| Condition | Response |
+|-----------|----------|
+| No master key configured | Warning shown, public health only |
+| 401 Unauthorized | Auth failed message |
+| 403 Forbidden | Permission denied message |
+| Connection refused | Connection failed message |
+| Timeout (>15s) | Timeout message |
+| Other non-200 | Status code + first 500 chars of response |
+
+## Function 2: Cost Breakdown
+
+**LiteLLM Endpoint:** `GET {litellm_url}/user/daily/activity`
+**Auth:** `Authorization: Bearer {litellm_master_key}`
+**Params:** `start_date` (YYYY-MM-DD), `end_date` (YYYY-MM-DD)
+
+### Response Processing
+
+The `/user/daily/activity` endpoint returns a rich breakdown structure:
+
+```
+results[] → per date:
+  metrics: { spend, prompt_tokens, completion_tokens, total_tokens, successful_requests, failed_requests, api_requests, cache_read_input_tokens, cache_creation_input_tokens }
+  breakdown:
+    models: { model_name → metrics }
+    model_groups: { group_name → metrics }
+    providers: { provider_name → metrics }
+    endpoints: { endpoint_path → metrics }
+    api_keys: { key_hash → metrics }
+    entities: { entity_id → metrics }
+```
+
+The tool aggregates per-model totals across all dates and formats a readable report.
+
+### Output Format
+
+```
+LiteLLM Cost Report: 2026-06-03 to 2026-06-04
+============================================================
+Total Spend:     $1.65
+Total Tokens:    2,011,201 (1,993,600 prompt + 17,601 completion)
+Cache Hit Rate:  69.5% (1,395,954 / 2,007,561 input tokens)
+Requests:        31 successful, 1 failed (32 total)
+
+By Model:
+  Model                      Spend |       Requests |    Tokens
+  gemini-3.5-flash          $1.20 |  26 ok, 0 fail | 2,003,116 tokens
+  gemini-3.1-flash-image    $0.45 |   5 ok, 0 fail |     8,085 tokens
+
+By Date:
+  2026-06-04: $1.28 | 27 ok, 0 fail | 2,004,642 tokens
+  2026-06-03: $0.37 |  4 ok, 0 fail |     6,559 tokens
+```
+
+### Key Metrics Surfaced
+
+- Total spend (USD)
+- Token counts (prompt, completion, total)
+- Cache hit rate (cache_read_input_tokens / total_input_tokens)
+- Request counts (successful + failed)
+- Per-model breakdown (sorted by spend, descending)
+- Per-date breakdown
+
+### Condensed/Stripped
+
+- `api_key_breakdown` sub-levels — too granular for general display
+- `metadata: {}` — empty in current responses
+- `cache_creation_input_tokens` — shown only when non-zero
+
+### Error Paths
+
+| Condition | Response |
+|-----------|----------|
+| No master key configured | Error — key required for cost data |
+| 401 Unauthorized | Auth failed message |
+| 403 Forbidden | Permission denied message |
+| Connection refused | Connection failed message |
+| Timeout (>30s) | Timeout message (large date ranges may be slow) |
+| No results | "No cost data found for [range]" |
+
+## Additional Endpoints (Future Consideration)
+
+| Endpoint | Purpose | Priority |
+|----------|----------|----------|
+| `GET /global/spend/keys` | Per-key lifetime spend | Nice-to-have |
+| `GET /spend/logs?start_date=&end_date=` | Individual transaction logs | Low (returned empty in testing) |
+| `GET /health/live` | Simple `{"status":"healthy"}` liveness check | Low (less useful than `/health`) |
+
+These are documented but not in scope for v1. The two-function design is the right scope.
+
+## Design Principles
+
+1. **Two functions, that's it.** No scope creep. Health check and cost breakdown.
+2. **Portable.** No hardcoded URLs or keys. Valves pass everything.
+3. **Token-efficient.** Strip litellm_metadata from health. Format cost data as readable text, not raw JSON dumps.
+4. **General-purpose.** Designed for the Network but usable by anyone with a LiteLLM proxy.
+5. **Read-only.** These endpoints are GET only. No mutations, no risk.