ACE Sidecar

Local coding agent cost optimization & observability — cut your Claude Code, Antigravity, and OpenAI Codex spend by up to 50% via real-time trajectory context compression and smart local model routing.

How It Cuts Costs · Quickstart · Trajectory Compression · Local Model Routing · Install · Configuration · Dashboard & Observability · Who Builds This · License

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How It Cuts Costs

Coding agents like Claude Code are token-heavy by design: every turn sends massive file reads, repetitive tool results, and bloated conversation histories.

ACE Sidecar runs a 100% local proxy on loopback (127.0.0.1) that actively reduces your API bill by 30% to 50%+ before requests ever hit the provider:

One Small Layer Between You and the Bill

┌────────────────────────────────────────────────────────┐
│                    Claude Code CLI                     │
│                (or VS Code Extension)                  │
└───────────────────────────┬────────────────────────────┘
                            │ ANTHROPIC_BASE_URL=http://127.0.0.1:8787
                            ▼
┌────────────────────────────────────────────────────────┐
│                   ACE Local Sidecar                    │
│                 (`ace up` on 127.0.0.1)                │
├────────────────────────────────────────────────────────┤
│ 1. Trajectory Compression                              │
│    • Tool Output Truncation: Head + tail (2KB cap)     │
│    • Read Deduplication: SHA-256 digest pointers       │
│    • Prose Compaction: Strips conversational fluff     │
│    • Idle Optimizer: 300s TTL-aware cache compaction   │
│                                                        │
│ 2. Local Model Routing (Claude Family)                 │
│    • Exploration / Grep / Search ──> Claude 3.5 Haiku  │
│    • Code Authoring / Editing    ──> Claude 3.7 Sonnet │
│    • Sticky Session Anchoring    ──> Retains 90% cache │
│    • Data Sensitivity Gate       ──> Quarantines data  │
└───────────────────────────┬────────────────────────────┘
                            │ Relays optimized payload & adapts params
                            ▼
┌────────────────────────────────────────────────────────┐
│                Anthropic Upstream API                  │
│               (https://api.anthropic.com)              │
└────────────────────────────────────────────────────────┘

100% locally hosted: Zero telemetry phone-home, zero remote data upload, zero external cloud accounts. Everything executes on your workstation.

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Quickstart

1. Install ACE Sidecar

# Recommended via uv
uv tool install ace-sidecar

# Or via pip
pip install ace-sidecar

2. Launch the Sidecar

ace up

Proxy listening on http://127.0.0.1:8787 with real-time compression and routing enabled.

3. Point Claude Code at the Sidecar

# In your terminal or ~/.bashrc / ~/.zshrc:
export ANTHROPIC_BASE_URL=http://127.0.0.1:8787

# Configure optimized MCP tools for Claude Code:
ace setup-claude

# Start coding as normal!
claude

Open the local dashboard at http://127.0.0.1:8787/dashboard to see live token savings, cache hit ratios, and dollar reductions.

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Core Optimization Engines

1. Trajectory Context Compression

• Tool Result Truncation: Automatically caps sprawling terminal outputs and file dumps at 2KB, preserving the diagnostic head lines and summary tail lines while eliding redundant middle output.
• Read Deduplication (SHA-256): When an agent re-reads an unchanged file across turns, the sidecar replaces the redundant content with a 120-byte digest pointer, saving tens of thousands of duplicate tokens.
• Fact-Preserving Prose Compression: Compresses assistant conversational chatter while preserving fenced code blocks, file paths, line numbers, and bash commands verbatim.
• Prompt Cache TTL Compactor: Anthropic prompt caches expire after 300 seconds of inactivity. When idle gap $> 300\text{s}$, the sidecar safely compacts older history into structured working state without breaking live turns.
• Fast Stdio MCP Tools (Search & Edit): Built-in Model Context Protocol server that replaces full-file dumps with atomic line-slice reads and hunk edits.

2. Local Model Routing

• Cost Arbitrage Across Task Phases: High-volume, low-complexity agent turns (searching repository structure, running greps, reading file heads) are routed to Claude 3.5 Haiku ($0.80/1M vs $3.00/1M), saving ~73% on input costs for exploration turns.
• Session Stickiness (Cache Protection): Anthropic prompt caching offers a 90% discount ($0.30/1M reads vs $3.00/1M). The router anchors to the destination model on Turn 1 and sticks to it, preventing mid-session model thrashing from invalidating the prompt cache.
• Pluggable Escalation Triggers:
  • Explicit User Override: Type "use opus" or "switch to sonnet" in your prompt to dynamically re-anchor.
  • Complexity Jump: Escalates to Claude 3.7 Sonnet with extended thinking when task difficulty jumps ($\Delta C \ge 0.40$).
  • Consecutive Error Escalation: Automatically escalates to flagship reasoning if the agent encounters $\ge 2$ consecutive test or tool failures.
  • Data Sensitivity Gate: Detects secrets (.env, credentials, AWS keys, JWTs) and routes them to strict compliance profiles.
• Claude Family Wire Protocol Parity: Parameter adaptation automatically strips thinking blocks and clamps max_tokens when down-routing to Haiku so Claude Code never encounters a 400 Bad Request.

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Configuration (~/.ace/routing.yaml)

Routing and compression are fully customizable without touching Python code. The configuration defaults strictly to the tested Claude model family for 100% CLI stability:

# ~/.ace/routing.yaml
version: "1.0"
active_profile: "balanced"

# Allowed candidate models. Add custom ARNs (e.g. AWS Bedrock / Vertex) or remove models.
allowed_models:
  - "claude-3-5-haiku-20241022"
  - "claude-3-5-sonnet-20241022"
  - "claude-3-7-sonnet-20250219"
  - "claude-3-opus-20240229"

profiles:
  balanced:
    exploration_model: "claude-3-5-haiku-20241022"
    authoring_model: "claude-3-7-sonnet-20250219"
    reasoning_model: "claude-3-7-sonnet-20250219"
    enable_thinking_on_reasoning: true
    thinking_budget: 4096

  economy:
    exploration_model: "claude-3-5-haiku-20241022"
    authoring_model: "claude-3-5-haiku-20241022"
    reasoning_model: "claude-3-7-sonnet-20250219"

  flagship:
    exploration_model: "claude-3-7-sonnet-20250219"
    authoring_model: "claude-3-7-sonnet-20250219"
    reasoning_model: "claude-3-7-sonnet-20250219"

Routing CLI Commands

# View active routing profile and allowed models
ace routing list

# Dry-run test a routing decision on a prompt
ace routing test "where is main.py defined?"
# Result: Routes to claude-3-5-haiku-20241022 (exploration)

ace routing test "use opus to review distributed consensus design"
# Result: Routes to claude-3-opus-20240229 (explicit override)

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Dashboard & Observability

Even with optimization active, ACE Sidecar provides complete, transparent observability into your coding sessions:

• Unified History: Auto-discovers transcripts from ~/.claude/projects, ~/.gemini/antigravity/brain, and ~/.codex/sessions.
• Real Spend Against Provider Rates: Tracks input, output, cache-read, and cache-write rates based on versioned provider catalogs.
• Counterfactual Savings Rail: Shows exact token and dollar savings recovered by context compression and model routing.
• Where the Time Goes: Wall-clock breakdown across model generation, tool execution, human think-time, and approval prompts.
• Prometheus Metrics: Live metrics exposed at GET /metrics for Grafana, Datadog, or OpenTelemetry.

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Endpoints

Endpoint	Purpose
POST /v1/messages	Loopback proxy interceptor for Claude Code and coding agents
GET /dashboard	Visual web dashboard showing costs, savings, and session timelines
GET /healthz	Health check, active compression state, and routing status
GET /api/stats	JSON breakdown of saved bytes, saved tokens, and routed models
GET /metrics	Prometheus text exposition metrics

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Who Builds This

ACE Sidecar is built by ACE Fleet.

ACE Fleet builds inference efficiency middleware for enterprises running production AI. While Fleet manages multi-tenant cloud traffic, ACE Sidecar is the open-source, local vertical designed specifically to optimize and compress individual developer coding agent workloads on their own machines.

	ACE Sidecar (this repo)	ACE Fleet
Scope	Single developer workstation	Organization-wide inference fabric
Workload	Coding agents (Claude Code, Antigravity, Codex)	Production APIs, services, multi-agent systems
Action	Acts locally: Compresses context & routes models on loopback	Acts centrally: Dynamic routing, global cache, cloud fleets
Privacy	100% local, zero data egress	Multi-tenant VPC / Private Cloud
License	Open source (AGPL-3.0)	Enterprise

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License

GNU Affero General Public License v3.0.