TinyCode

A simplified MVP that shows how Claude Code works under the hood — agentic loop, tool use, streaming, all in ~300 lines of TypeScript.

pnpm tsx src/cli.ts claude-sonnet-4-6 5 tools

How It Works

The agentic loop — a simple while loop that calls Claude, executes tools, and repeats.

User types prompt │ ▼ ┌─────────────────────────────────────────────────────┐ │ client.messages.stream() │ │ Send: system prompt + conversation + tool defs │ │ Stream: text deltas appear in terminal in realtime │ └─────────────────────┬───────────────────────────────┘ │ stop_reason? ╱ ╲ "end_turn" "tool_use" │ │ Return to Execute tool locally REPL ┌──────────────────────┐ │ read_file → fs.readFile │ │ write_file → fs.writeFile │ │ edit_file → string replace │ │ list_files → glob │ │ run_command→ child_process │ └──────────┬───────────┘ │ Append tool_result to conversation history │ ▼ Loop back to stream() max 20 iterations

Five Core Tools

The minimum set needed for an agentic coding assistant.

read_file

Read file contents with line numbers. Truncates at 50KB to stay within context budget.

write_file

Create or overwrite files. Auto-creates parent directories if they don't exist.

edit_file

Precise string replacement. Finds an exact match and replaces it — fails if not unique.

list_files

Glob-based file discovery. Ignores node_modules and .git. Returns up to 200 matches.

run_command

Execute shell commands with 30s timeout. Returns stdout, stderr, and exit code.

API Calls Per Session

Each agentic turn = 1 API call. Complexity determines how many turns a task needs.

Session Type	API Calls	Input Tokens	Output Tokens	Est. Cost
Simple Q&A	1	200 – 500	300 – 1.5K	$0.001 – $0.005
Read & Explain File	2 – 3	2K – 8K	1K – 4K	$0.01 – $0.05
Single File Edit	3 – 5	5K – 15K	2K – 8K	$0.03 – $0.10
Multi-File Refactor	10 – 30	20K – 80K	10K – 40K	$0.15 – $0.60
Build New Feature	15 – 50+	50K – 200K	20K – 100K	$0.50 – $3.00

Context grows with each turn because the full message history is sent every time. By turn 20, input tokens can be 5–10x the initial prompt.

Latency Profile

Streaming makes the experience feel responsive even when total generation time is several seconds.

300-800ms

First token (streaming)

10-500ms

Tool execution (local)

1-5s

Full agentic turn

<1s

User-perceived latency

Tool	Typical Latency	Notes
`read_file`	5 – 50ms	Filesystem read, limited to 50KB
`write_file`	5 – 50ms	Filesystem write + mkdir
`edit_file`	5 – 100ms	Read + string replace + write
`list_files`	10 – 200ms	Glob traversal, varies with directory size
`run_command`	50ms – 30s	Depends on command; timeout at 30s

Source Architecture

~300 lines of TypeScript, 6 files, zero abstractions.

File	Purpose	Key Function
`src/cli.ts`	REPL entry point	readline loop, command dispatch
`src/agent.ts`	Agentic loop engine	`runAgentTurn()` — stream + tool dispatch
`src/render.ts`	Terminal output	ANSI colors, streaming text, tool call display
`src/tools/index.ts`	Tool registry	Definitions + `executeTool()` dispatch
`src/tools/*.ts`	Tool implementations	5 files, one per tool