agent

A small REPL agent that talks to an Anthropic-compatible Messages API and exposes three tools to the model: read_file, list_files, edit_file, and bash. Two implementations live in this repo:

• main.go — the original implementation (Go, two third-party deps: anthropic-sdk-go + invopop/jsonschema).
• main.zig — a std-lib-only rewrite in Zig 0.16.0.

Running

To run the Go implementation with an OpenCode Go Anthropic-compatible model:

ANTHROPIC_BASE_URL='https://opencode.ai/zen/go' ANTHROPIC_API_KEY='<redacted>' go run main.go

To run the Zig implementation with an OpenCode Go Anthropic-compatible model:

ANTHROPIC_BASE_URL='https://opencode.ai/zen/go' API_KEY='<redacted>' zig run main.zig

OpenCode Go endpoints are documented here.

Configuration

The default endpoint is the OpenCode Zen Go Anthropic-compatible endpoint, and the default model is minimax-m3. Both are overridable via env vars.

Env var	Required	Default
API_KEY	yes (exits 1 if missing/empty, Zig only)	—
ANTHROPIC_BASE_URL	no	https://opencode.ai/zen/go
ANTHROPIC_API_KEY	yes (Go only — API_KEY is ignored)	—
MODEL	no (Zig only; the Go version hard-codes minimax-m3)	minimax-m3
SSL_CERT_FILE	no (Zig only)	unset — ca_bundle.rescan() picks the platform default (only used if the file is set)

Zig override example

ANTHROPIC_BASE_URL=https://other-host   \
MODEL=other-model                       \
API_KEY=sk-...                          \
zig run main.zig

Behavior differences between the two implementations

These are intentional, documented in main.zig at the call sites:

	Go (main.go)	Zig (main.zig)
list_files depth	Recursive (filepath.Walk)	One-level only (also described in the tool description the model sees)
Empty user prompt	Sends an empty user message to the model	Re-prompts, sends nothing
edit_file replace	All occurrences (strings.Replace(..., -1))	All occurrences (single std.mem.replaceOwned — already replaces all internally)
read_file / edit_file file size	Unbounded	Capped at 1 MiB
HTTP response size	Unbounded	Capped at 1 MiB
Exit code on API_KEY missing	0 (Go recovers)	1 (explicit std.process.exit(1))
Dependencies	Two third-party Go modules	None (Zig std-lib only)
TLS / CA bundle	crypto/tls defaults	SSL_CERT_FILE (if set) or client.ca_bundle.rescan() (Zig 0.16 doesn't auto-load one); rescan is platform-aware (Linux/macOS/Windows/BSD)
HTTP compression	n/a	accept-encoding: identity forced — std.http.Client advertises gzip but doesn't decompress, which would make responses opaque to the JSON parser
Tool input parsing strictness	Go's json.Unmarshal ignores unknown fields	parseFromValueLeaky(..., .{ .ignore_unknown_fields = true }) — same as Go
Bash signal exit code	-1 (Go's cmd.ProcessState.ExitCode())	-1 (matching Go for parity)
bash duration_ms	time.Since(start).Milliseconds()	Io.Clock.awake start/end Timestamp.durationTo / ns_per_ms (matching Go)

Zig 0.16.0 notes

A few std-lib quirks that the rewrite has to work around (call sites in main.zig are commented with these rationales):

• All FS ops need an io: std.Io parameter. openFile, close, reader, writer, iterate, createFile, createDirPath, etc. Initialize once in main via Io.Threaded.init and pass it through to every call.
• std.http.Client.FetchOptions has no size-limiting field in 0.16 (no max_append_size, max_chunk_size, or max_size). To cap the response body you have to drop to the lower-level client.request() / req.sendBodyComplete() / req.receiveHead() + response.reader() API and cap on the reader side. sendMessage does exactly this.
• std.mem.replaceOwned already replaces all occurrences internally (it delegates to replace which loops). The Zig port calls it once per edit_file invocation, matching the Go version's strings.Replace(..., -1).
• std.json.Stringify.value(allocator, value, .{}, writer) is the replacement for the removed top-level std.json.stringify helper. The allocator is the first argument. Stringify.valueAlloc is the convenience that allocates a fresh []u8 and returns it.
• std.process.Environ.getPosix walks the env block captured at startup. std.c.getenv (libc) does NOT work under Zig 0.16's standalone runtime because it doesn't populate libc's environ global. Use getEnvOr in main.zig for any env access.
• Privileged HTTP headers (e.g. accept-encoding) are set via the structured RequestOptions.headers field, not extra_headers. The accept-encoding: identity override is required because std.http.Client advertises gzip but does not decompress; without the override, a gzipped response would be opaque to the JSON parser.

Out of scope (neither implementation)

• No streaming.
• No conversation history truncation / token counting.
• No retry / backoff.
• No multi-provider support (OpenAI etc.).
• No tests.
• No build.zig — running is zig run main.zig directly.
• No recursive directory walk for list_files in the Zig port.
• No slash commands (/exit, etc.).
• No proxy / custom TLS / custom CA bundle configuration beyond SSL_CERT_FILE.