Capstone 01 — Terminal-Native Coding Agent

Problem

Coding agents became the dominant AI application category in 2026. Claude Code (Anthropic), Cursor 3 with Composer 2 and Agent Tabs (Cursor), Amp (Sourcegraph), OpenCode (112k stars), Factory Droids, and Google Jules all ship variations of the same architecture: a terminal harness, a permissioned tool surface, a sandbox, and a plan-act-observe loop built around a frontier model. The frontier is narrow — Live-SWE-agent reached 79.2% on SWE-bench Verified with Opus 4.5 — but the engineering craft is wide. Most failure modes are not model mistakes. They are tool-loop instability, context poisoning, runaway token cost, and destructive filesystem operations.

You cannot reason about these agents from the outside. You have to build one, watch the loop crash on turn 47 when ripgrep returns 8MB of matches, and rebuild the truncation layer. That is the point of this capstone.

Concept

The harness has four surfaces. Plan maintains a TodoWrite-style state object that the model rewrites each turn. Act dispatches tool calls (read, edit, run, search, git). Observe captures stdout / stderr / exit codes, truncates, and feeds the summary back. Recover handles tool errors without blowing the context window or looping forever. The 2026 shape adds one more thing: hooks. PreToolUse, PostToolUse, SessionStart, SessionEnd, UserPromptSubmit, Notification, Stop, and PreCompact — configurable extension points where the operator injects policy, telemetry, and guardrails.

The sandbox is E2B or Daytona. Each task runs in a fresh devcontainer with a git worktree mounted read-write. The harness never touches the host filesystem. The worktree gets torn down on success or failure. Cost control is enforced at three layers: a per-turn token ceiling, a per-session dollar budget, and a hard turn limit (typically 50). The observability layer is OpenTelemetry spans with GenAI semantic conventions, shipped to a self-hosted Langfuse.

Architecture

  user CLI  ->  harness (Bun + Ink TUI)
                  |
                  v
           plan / act / observe loop  <--->  Claude Sonnet 4.7 / GPT-5.4-Codex / Gemini 3 Pro
                  |                          (via OpenRouter, model-agnostic)
                  v
           tool dispatcher (MCP StreamableHTTP client)
                  |
     +------------+------------+----------+
     v            v            v          v
  read/edit    ripgrep     tree-sitter   git/run
     |            |            |          |
     +------------+------------+----------+
                  |
                  v
           E2B / Daytona sandbox  (worktree isolated)
                  |
                  v
           hooks: Pre/Post, Session, Prompt, Compact
                  |
                  v
           OpenTelemetry -> Langfuse (spans, tokens, $)
                  |
                  v
           PR via GitHub app

Stack

• Harness runtime: Bun 1.2 + Ink 5 (React-in-terminal)
• Model access: OpenRouter unified API with Claude Sonnet 4.7, GPT-5.4-Codex, Gemini 3 Pro, Opus 4.5 (for hardest tasks)
• Tool transport: Model Context Protocol StreamableHTTP (MCP 2026 revision)
• Sandbox: E2B sandboxes (JS SDK) or Daytona devcontainers
• Code search: ripgrep subprocess, tree-sitter parsers for 17 languages (pre-compiled)
• Isolation: git worktree add per task, cleanup on success / failure
• Eval harness: SWE-bench Pro (verified subset) + Terminal-Bench 2.0 + your own 30-task holdout
• Observability: OpenTelemetry SDK with gen_ai.* semconv → self-hosted Langfuse
• PR posting: GitHub App with fine-grained token, scope limited to the target repo

ce-agent-loop

Build It

1. TUI and command loop. Scaffold a Bun project with Ink. Accept agent run <repo> "<task>". Print a split view: plan pane (top), tool-call stream (middle), token budget (bottom). Add cancel on Ctrl-C that fires SessionEnd hook before exit.

2. Plan state. Define a typed TodoWrite schema (pending / in_progress / done items with notes). Model rewrites the full state each turn as a tool call — do not let it mutate incrementally. Persist plan to .agent/state.json so crashes can resume.

3. Tool surface. Define six tools: read_file, edit_file (with diff preview), ripgrep, tree_sitter_symbols, run_shell (with timeout), git (status / diff / commit / push). Expose over MCP StreamableHTTP so the harness is transport-agnostic. Every tool returns truncated output (cap at 4k tokens per call).

4. Sandbox wrapping. Each task spawns an E2B sandbox. git worktree add -b agent/$TASK_ID a fresh branch. All tool calls execute inside the sandbox. Host filesystem is unreachable.

5. Hooks. Implement all eight 2026 hook types. Wire at least four user-authored hooks: (a) PreToolUse destructive-command guard that blocks rm -rf outside the worktree, (b) PostToolUse token accounting, (c) SessionStart budget initialization, (d) Stop writes a final trace bundle.

6. Eval loop. Clone a 30-issue subset of SWE-bench Pro Python. Run your harness against each. Compare to mini-swe-agent (the minimal baseline) on pass@1, turns-per-task, and $-per-task. Write the results to eval/results.jsonl.

7. Cost control. Hard cutoffs: 50 turns, 200k context, $5 per task. PreCompact hook summarizes older turns into a prior-state block at the 150k mark, freeing room for new observations without losing the plan.

8. PR posting. On success, the final step is git push + a GitHub API call that opens a PR with the plan and the diff summary in the body.

Use It

$ agent run ./my-repo "Fix the race condition in worker.rs"
[plan]  1 locate worker.rs and enumerate mutex uses
        2 identify shared state under contention
        3 propose fix, verify tests
[tool]  ripgrep mutex.*lock -t rust           (44 matches, truncated)
[tool]  read_file src/worker.rs 120..180
[tool]  edit_file src/worker.rs (+8 -3)
[tool]  run_shell cargo test worker::          (passed)
[plan]  1 done · 2 done · 3 done
[done]  PR opened: #482   turns=9   tokens=38k   cost=$0.41

Ship It

The deliverable skill lives in outputs/skill-terminal-coding-agent.md. Given a repo path and a task description, it runs the full plan-act-observe loop in a sandbox and returns a PR URL plus a trace bundle. The rubric for this capstone:

Exercises

1. Swap the backing model from Claude Sonnet 4.7 to Qwen3-Coder-30B served on vLLM. Compare pass@1 and $-per-task. Report where the open model underperforms.

2. Add a reviewer sub-agent that reads the diff before PR posting and can request a revision loop. Measure whether false-positive reviews drop SWE-bench pass rate below the single-agent baseline (hint: usually yes).

3. Stress-test the sandbox: write a task that tries to curl an external URL and a task that writes outside the worktree. Confirm both are blocked by the PreToolUse hook. Log the attempts.

4. Implement PreCompact summarization with a smaller model (Haiku 4.5). Measure how much plan fidelity is lost at 3x compaction.

5. Swap MCP StreamableHTTP transport for stdio. Benchmark cold-start and per-call latency. Pick a winner for local-only use.

Key Terms

Further Reading

• Claude Code documentation — reference harness from Anthropic
• Cursor 3 changelog — Agent Tabs and Composer 2 product notes
• mini-swe-agent — minimal baseline for SWE-bench harness comparison
• Live-SWE-agent — 79.2% SWE-bench Verified with Opus 4.5
• OpenCode — open harness, 112k stars
• SWE-bench Pro leaderboard — the evaluation this capstone targets
• Model Context Protocol 2026 roadmap — StreamableHTTP, capability metadata
• OpenTelemetry GenAI semantic conventions — span schema for tool calls and token usage