gcf-go

Go implementation of GCF — the most token-efficient wire format for LLMs. A drop-in alternative to JSON and TOON for any structured data.

Built for the agentic loop, where the same structured context crosses the model boundary turn after turn. A single payload is 50-92% smaller than JSON, but GCF also deduplicates repeated structure across turns and sends only deltas when context changes, so by the 5th overlapping call each response costs 99% fewer tokens than JSON, and a 10-call session runs 94.4% cheaper than re-sending JSON every turn. Session dedup and delta both need local IDs and a multi-turn design that neither JSON nor TOON has.

• 100% comprehension on every frontier model, zero training. 29% fewer tokens than TOON and 56% fewer than JSON across 16 datasets; 91.2% on structurally complex code graphs (vs TOON 68.8%, JSON 54.1%).
• Proven lossless across 43,000,000,000+ round-trips in 5 formats and 6 languages. Zero runtime dependencies.
• One format, four properties no other single format holds at once: schema-free, lossless, token-compact (50-92% vs JSON), and model-readable with zero training. JSON is verbose, Protobuf needs a schema, MessagePack is binary, and TOON isn't reliably lossless.

2,500+ LLM evaluations. Full benchmarks.

Docs: gcformat.com · Playground · GCF vs TOON

Install

go get github.com/blackwell-systems/gcf-go

Zero dependencies. Single package. Don't want to change code? Use the MCP proxy for zero-code adoption.

CLI

Standalone binaries are attached to each release. The CLI is optional; it's for converting files from the command line without writing code.

# Install
go install github.com/blackwell-systems/gcf-go/cmd/gcf@latest

# Or download a binary from the latest release

# Usage
gcf encode < payload.json    # JSON to GCF
gcf decode < payload.gcf     # GCF to JSON
gcf stats  < payload.json    # token comparison

Quick Start

import gcf "github.com/blackwell-systems/gcf-go"

data := map[string]any{
    "employees": []map[string]any{
        {"id": 1, "name": "Alice", "department": "Engineering", "salary": 95000},
        {"id": 2, "name": "Bob", "department": "Sales", "salary": 72000},
    },
}
output := gcf.EncodeGeneric(data)

Output:

## employees [2]{department,id,name,salary}
Engineering|1|Alice|95000
Sales|2|Bob|72000

Works on any Go value: maps, slices, structs. One header declares field names, rows are positional values.

Graph Profile

For code graph data with symbols, edges, and distance groups:

p := &gcf.Payload{
    Tool: "context_for_task", TokenBudget: 5000, TokensUsed: 1847,
    Symbols: []gcf.Symbol{
        {QualifiedName: "pkg.Auth", Kind: "function", Score: 0.78, Provenance: "lsp", Distance: 0},
        {QualifiedName: "pkg.Server", Kind: "function", Score: 0.54, Provenance: "lsp", Distance: 1},
    },
    Edges: []gcf.Edge{{Source: "pkg.Server", Target: "pkg.Auth", EdgeType: "calls"}},
}
output := gcf.Encode(p)

Output:

GCF tool=context_for_task budget=5000 tokens=1847 symbols=2 edges=1
## targets
@0 fn pkg.Auth 0.78 lsp
## related
@1 fn pkg.Server 0.54 lsp
## edges [1]
@0<@1 calls

Decode

p, err := gcf.Decode(input)
if err != nil {
    log.Fatal(err)
}
fmt.Println(p.Tool, len(p.Symbols), "symbols", len(p.Edges), "edges")

Session Deduplication

Track transmitted symbols across multiple tool responses. Previously-sent symbols become bare references instead of full declarations:

sess := gcf.NewSession()

out1 := gcf.EncodeWithSession(payload1, sess) // full declarations
out2 := gcf.EncodeWithSession(payload2, sess) // reused symbols as "@N  # previously transmitted"

By the 5th call in a session: 86% fewer tokens than JSON from dedup alone, 99% stacked with delta encoding.

Streaming Encode

Write GCF output incrementally as symbols and edges arrive. Zero buffering, O(1) memory per row. Ideal for MCP servers that walk large graphs or paginate results:

enc := gcf.NewStreamEncoder(w, "context_for_task", gcf.StreamOptions{TokenBudget: 5000})

// Symbols emit immediately as they're discovered.
enc.WriteSymbol(gcf.Symbol{QualifiedName: "pkg.Auth", Kind: "function", Score: 0.95, Provenance: "lsp", Distance: 0})
enc.WriteSymbol(gcf.Symbol{QualifiedName: "pkg.Server", Kind: "function", Score: 0.60, Provenance: "lsp", Distance: 1})

// Edges emit immediately too.
enc.WriteEdge(gcf.Edge{Source: "pkg.Server", Target: "pkg.Auth", EdgeType: "calls"})

// Close emits the ##! summary trailer with final counts.
enc.Close()

Output:

GCF tool=context_for_task budget=5000
## targets
@0 fn pkg.Auth 0.95 lsp
## related
@1 fn pkg.Server 0.60 lsp
## edges [?]
@0<@1 calls
##! summary symbols=2 edges=1 counts=1,1,1

The [?] marker signals deferred count. The ##! summary trailer provides counts after the data. The LLM has both the data and the counts in context. Standard Decode() handles streaming output with no changes.

Delta Encoding

When the consumer already has a prior context pack, send only what changed:

delta := &gcf.DeltaPayload{
    Tool:     "context_for_task",
    BaseRoot: "aaa111",
    NewRoot:  "bbb222",
    Removed:  []gcf.Symbol{{QualifiedName: "pkg.OldFunc", Kind: "function"}},
    Added:    []gcf.Symbol{{QualifiedName: "pkg.NewFunc", Kind: "function", Score: 0.85, Provenance: "rwr"}},
    DeltaTokens: 30,
    FullTokens:  200,
}

output := gcf.EncodeDelta(delta)

81.2% savings on re-queries where the pack changed slightly.

Generic Encoding

Encode any Go value (not just graph payloads) into GCF tabular format:

data := map[string]any{
    "employees": []map[string]any{
        {"id": 1, "name": "Alice", "department": "Engineering", "salary": 95000},
        {"id": 2, "name": "Bob", "department": "Sales", "salary": 72000},
    },
}
output := gcf.EncodeGeneric(data)

Output:

## employees [2]{id,name,department,salary}
1|Alice|Engineering|95000
2|Bob|Sales|72000

Works on maps, slices, structs, and primitives. Arrays of uniform objects get tabular rows. Nested objects use ## key section headers.

API

Function	Description
Encode(p *Payload) string	Encode a graph payload to GCF text
EncodeGeneric(data any) string	Encode any value to GCF tabular format
Decode(input string) (*Payload, error)	Parse GCF text back to a Payload
EncodeWithSession(p *Payload, s *Session) string	Encode with session deduplication
EncodeDelta(d *DeltaPayload) string	Encode a delta (added/removed only)
NewStreamEncoder(w, tool, opts) *StreamEncoder	Create a streaming encoder (zero-buffering)
NewSession() *Session	Create a new session tracker (thread-safe)

Types

Type	Purpose
Payload	Full GCF payload: tool, budget, symbols, edges, pack root
Symbol	Graph node: qualified name, kind, score, provenance, distance
Edge	Directed relationship: source, target, edge type
DeltaPayload	Diff between two packs: added/removed symbols and edges
Session	Thread-safe tracker for multi-call deduplication
StreamEncoder	Streaming encoder: WriteSymbol, WriteEdge, WriteBareRef, Close
StreamOptions	Config for streaming: TokenBudget, TokensUsed, PackRoot, Session
KindAbbrev / KindExpand	Bidirectional kind abbreviation maps

Benchmarks

2,500+ LLM evaluations across 11 models, 4 providers, and 50+ independent test runs.

	GCF	TOON	JSON
Comprehension (23 runs, 10 models)	91.2%	68.8%	54.1%
Generation (28 runs, 9 models)	5/5	1.0/5	5.0/5
Input tokens (500 symbols)	11,090	16,378	53,341
Output tokens (100 symbols)	5,976	8,937	16,121

GCF wins 15/16 datasets on the expanded token efficiency benchmark. Full results: gcformat.com/guide/benchmarks

Implementations

Language	Package	Repository
Go	go get github.com/blackwell-systems/gcf-go	gcf-go
TypeScript	npm install @blackwell-systems/gcf	gcf-typescript
Python	pip install gcf-python	gcf-python
Rust	cargo add gcf	gcf-rust
Swift	Swift Package Manager	gcf-swift
Kotlin	JitPack	gcf-kotlin
MCP Proxy	pip install gcf-proxy	gcf-proxy (bidirectional, session dedup, HTTP frontend)
Claude Code Plugin	/plugin install	gcf-claude-plugin (one-command install, session stats hook)
Codex Plugin	codex plugin add	gcf-codex-plugin (one-command install, session stats hook)
VS Code	ext install blackwell-systems.gcf-vscode	gcf-vscode (syntax highlighting)
n8n	npm install n8n-nodes-gcf	gcf-n8n-nodes (workflow encode/decode)
Tree-sitter	npm install tree-sitter-gcf	tree-sitter-gcf

Zero runtime dependencies. Permanently. All six implementations depend only on their language's standard library. No transitive dependencies. No supply chain risk. This is a permanent commitment: GCF will never take on external runtime dependencies. MIT licensed. All implementations support both generic profile (encodeGeneric) and graph profile (encode). CLI included in all 6 languages.

Specification: SPEC v3.4.1 Stable with 204 conformance fixtures, 43,000,000,000+ lossless round-trips verified across 5 formats and 6 languages. All implementations at v2.4.0+ (Go v1.5.0). Cross-language 6x6 matrix verified.

Adopted by

Chrome DevTools MCP (46K stars, Google Chrome DevTools team) · Speakeasy (API tooling, customers include Google, Verizon, Mistral AI, DocuSign, Vercel) · OmniRoute (6.1K stars) · NetClaw (556 stars) · ctx (510 stars) · NeuroNest · Open Data Products SDK (Linux Foundation) · Raycast · and more

License

MIT - Dayna Blackwell