aimodel

A Go SDK for AI model APIs with multi-protocol support (OpenAI, Anthropic). Zero external dependencies.

Design Scope

This SDK is a thin API wrapper — it translates requests, manages connections, and normalizes responses across protocols. It intentionally does not include retry, rate limiting, request validation, caching / persistence, logging / metrics. This keeps the SDK minimal and composable. Control mechanisms belong in the layer above, where you have full context over your application's requirements.

Official API References

aimodel is a thin wrapper over the following official protocols. Each wrapper maps one-to-one to its official documentation:

Protocol	Official docs	Wrapper code
OpenAI (OpenAI-compatible)	https://platform.openai.com/docs/api-reference/chat	openai_chat.go / openai_stream.go
Anthropic Messages API	https://platform.claude.com/docs/en/api/messages	anthropic.go / anthropic_chat.go / anthropic_stream.go

The sync status against the official APIs (target version, change summary) is recorded in CHANGES.md.

Maintenance convention: when an official API changes, update all three in sync — the wrapper code, this document, and CHANGES.md — keeping them consistent and continuously up to date.

Usage

import "github.com/vogo/aimodel"

Set env vars AI_API_KEY and AI_BASE_URL (or OPENAI_API_KEY / OPENAI_BASE_URL).

Chat Completion

client, _ := aimodel.NewClient()

resp, _ := client.ChatCompletion(context.Background(), &aimodel.ChatRequest{
    Model: aimodel.ModelOpenaiGPT4o,
    Messages: []aimodel.Message{
        {Role: aimodel.RoleUser, Content: aimodel.NewTextContent("Hello!")},
    },
})

fmt.Println(resp.Choices[0].Message.Content.Text())

Token limits

• MaxCompletionTokens → max_completion_tokens: the current OpenAI cap. Its limit covers both visible output tokens and internal reasoning tokens, and it is the only token-cap field accepted by reasoning models (o-series, GPT-5.x, …). Prefer it.
• MaxTokens → max_tokens: deprecated by OpenAI and rejected by reasoning models. Keep it only for older / non-reasoning models that still accept it.

Both fields are *int with omitempty. For the Anthropic protocol (which always uses max_tokens), the translator prefers MaxCompletionTokens over MaxTokens, defaulting to 4096 when neither is set.

Reasoning effort & verbosity

• ReasoningEffort → OpenAI reasoning_effort / Anthropic top-level effort: how many reasoning tokens the model spends. Use the ReasoningEffort* constants — none / minimal / low / medium / high / xhigh (GPT-5.1 defaults to none). On the Anthropic protocol this effort field (GA 2026-02-05) supersedes thinking.budget_tokens.
• Verbosity → verbosity: how detailed the output is. Use the Verbosity* constants — low / medium / high.

Both fields stay plain string with omitempty, so you can also pass any value a custom OpenAI-compatible backend accepts.

For Anthropic extended thinking, Thinking carries Type (enabled / disabled / adaptive), the now-deprecated BudgetTokens (prefer ReasoningEffort or Type:"adaptive"), and Display — set Display: "omitted" to suppress thinking content and speed up streaming.

resp, _ := client.ChatCompletion(context.Background(), &aimodel.ChatRequest{
    Model:           aimodel.ModelOpenaiGPT4o,
    ReasoningEffort: aimodel.ReasoningEffortHigh,
    Verbosity:       aimodel.VerbosityLow,
    Messages: []aimodel.Message{
        {Role: aimodel.RoleUser, Content: aimodel.NewTextContent("Hello!")},
    },
})

Common request fields

All optional and omitempty, mapping one-to-one to OpenAI's Chat Completions parameters:

• TopK *int → top_k: top-k truncation sampling (restrict sampling to the K most-likely tokens). Native to Anthropic, where toAnthropicRequest maps it straight through. OpenAI's Chat Completions has no top_k, so it is simply omitted when unset and passed through verbatim when set.
• Logprobs *bool → logprobs, TopLogprobs *int → top_logprobs: per-token log probabilities (and the N most-likely alternatives per position) for observability.
• LogitBias map[string]int → logit_bias: per-token-ID bias in [-100, 100].
• ParallelToolCalls *bool → parallel_tool_calls: whether the model may emit multiple tool calls in one turn. On Anthropic an explicit false maps to tool_choice.disable_parallel_tool_use:true (defaulting the choice to {type:"auto"} when none is named and tools are present; never attached to {type:"none"}).
• ServiceTier string → service_tier: latency/throughput tier (e.g. auto / default / flex / priority); plain string for pass-through.
• Store *bool → store, Metadata map[string]string → metadata: persist the completion and attach up to 16 key/value pairs.
• PromptCacheKey string → prompt_cache_key: route requests sharing a prefix to the same cache to improve hit rates.
• AutoCache bool + AutoCacheTTL string (Anthropic only; both struct-local, never on the OpenAI wire): enable Anthropic's automatic caching — a single request-root cache_control ({type:"ephemeral"}, or with ttl "1h"; empty TTL keeps the default 5-minute cache). The server caches the last cacheable block and advances the breakpoint forward as the conversation grows, no per-block markers needed. It coexists with the explicit per-block Message.CacheBreakpoint / Tool.CacheBreakpoint flags. OpenAI-compatible backends ignore it.

clone() deep-copies the LogitBias and Metadata maps, so mutating a cloned request never affects the original.

When Logprobs is true, each ChatResponse.Choices[i].LogProbs (a *LogProbs) carries the parsed content / refusal token log probabilities, each token exposing Logprob, Bytes, and TopLogprobs.

logprobs := true
topLogprobs := 5
resp, _ := client.ChatCompletion(context.Background(), &aimodel.ChatRequest{
    Model:          aimodel.ModelOpenaiGPT4o,
    Logprobs:       &logprobs,
    TopLogprobs:    &topLogprobs,
    ServiceTier:    "priority",
    PromptCacheKey: "tenant-42",
    Metadata:       map[string]string{"env": "prod"},
    Messages: []aimodel.Message{
        {Role: aimodel.RoleUser, Content: aimodel.NewTextContent("Hello!")},
    },
})

Response usage

ChatResponse.Usage normalizes per-request token counts:

• PromptTokens / CompletionTokens / TotalTokens → the OpenAI-compatible top-level counts.
• CacheReadTokens → cache-hit prompt tokens, parsed from OpenAI's nested prompt_tokens_details.cached_tokens (an explicit top-level cache_read_tokens takes precedence).
• CacheWriteTokens → prompt-cache write tokens (Anthropic's cache_creation_input_tokens, total). CacheWrite5mTokens / CacheWrite1hTokens break it down by TTL (Anthropic's usage.cache_creation.{ephemeral_5m_input_tokens, ephemeral_1h_input_tokens}, summing to CacheWriteTokens). Cache read/write tokens are subsets of PromptTokens, surfaced separately for observability; OpenAI leaves them at 0.
• ReasoningTokens → tokens spent on a reasoning model's internal thinking, parsed from OpenAI's nested completion_tokens_details.reasoning_tokens (an explicit top-level reasoning_tokens takes precedence).

Usage.Add accumulates all of the above, which is handy when aggregating multi-turn or multi-call usage.

ChatResponse.Choices[i].FinishReason mirrors OpenAI's finish_reason: stop, length, tool_calls, content_filter, and the legacy function_call. Anthropic also emits stop reasons with no OpenAI canonical equivalent; these pass through verbatim and are named for readability: model_context_window_exceeded (input + output exceeded the model's context window — distinct from length), refusal (streaming classifiers intervened on a policy violation), and pause_turn (a long-running/server-tool turn was paused and may be replayed to continue).

When a refusal carries a classification, both ChatResponse.Choices[i].StopDetails (non-streaming) and StreamChunk.Choices[i].StopDetails (the terminal message_delta) expose it as StopDetails{Type, Category, Explanation} (e.g. {type:"refusal", category:"cyber", explanation:"…"}); the field is nil when absent. Explanation is best-effort and not guaranteed stable across model versions.

maxCompletionTokens := 1024
resp, _ := client.ChatCompletion(context.Background(), &aimodel.ChatRequest{
    Model:               aimodel.ModelOpenaiO3, // reasoning model
    MaxCompletionTokens: &maxCompletionTokens,
    Messages: []aimodel.Message{
        {Role: aimodel.RoleUser, Content: aimodel.NewTextContent("Hello!")},
    },
})

Multimodal input & audio output

Content is polymorphic — NewTextContent for plain text, NewPartsContent for a multimodal array. Each ContentPart carries one payload selected by Type:

• text → Text
• image_url → ImageURL{URL, Detail}
• input_audio → InputAudio{Data, Format} — base64 audio plus encoding (wav / mp3)
• file → FilePart{FileID} (reference an uploaded file) or FilePart{Filename, FileData} (inline base64 contents)

To request audio output, set Modalities (e.g. ["text", "audio"]) and Audio (AudioConfig{Voice, Format}); the generated audio comes back on resp.Choices[i].Message.Audio (MessageAudio{ID, Data, Transcript, ExpiresAt}).

resp, _ := client.ChatCompletion(context.Background(), &aimodel.ChatRequest{
    Model:      aimodel.ModelOpenaiGPT4o,
    Modalities: []string{"text", "audio"},
    Audio:      &aimodel.AudioConfig{Voice: "alloy", Format: "wav"},
    Messages: []aimodel.Message{
        {Role: aimodel.RoleUser, Content: aimodel.NewPartsContent(
            aimodel.ContentPart{Type: "text", Text: "What is said in this clip?"},
            aimodel.ContentPart{Type: "input_audio", InputAudio: &aimodel.InputAudio{
                Data: base64Audio, Format: "wav",
            }},
        )},
    },
})

if a := resp.Choices[0].Message.Audio; a != nil {
    fmt.Println(a.Transcript) // generated audio's text transcript
}

Streaming

stream, _ := client.ChatCompletionStream(context.Background(), &aimodel.ChatRequest{
    Model: aimodel.ModelOpenaiGPT4o,
    Messages: []aimodel.Message{
        {Role: aimodel.RoleUser, Content: aimodel.NewTextContent("Hello!")},
    },
})
defer stream.Close()

for {
    chunk, err := stream.Recv()
    if errors.Is(err, io.EOF) {
        break
    }
    fmt.Print(chunk.Choices[0].Delta.Content.Text())
}

Anthropic Protocol

Use WithProtocol to select the Anthropic Messages API:

client, _ := aimodel.NewClient(
    aimodel.WithAPIKey("sk-ant-xxx"),
    aimodel.WithProtocol(aimodel.ProtocolAnthropic),
)

resp, _ := client.ChatCompletion(context.Background(), &aimodel.ChatRequest{
    Model: aimodel.ModelAnthropicClaude4Sonnet,
    Messages: []aimodel.Message{
        {Role: aimodel.RoleUser, Content: aimodel.NewTextContent("Hello!")},
    },
})

The same ChatCompletion / ChatCompletionStream methods work for all protocols — routing is handled internally.

System message translation: only the leading run of RoleSystem messages (those before the first user/assistant turn) is hoisted into Anthropic's top-level system field. A RoleSystem message that appears mid-conversation is kept inline as a role:"system" message in its original position (supported since Opus 4.8), so you can switch instructions mid-session without losing prompt-cache hits.

Tool choice translation: "auto" → {type:"auto"}, "required" → {type:"any"}, "none" → {type:"none"} (explicitly forbid any call — distinct from omitting the field, which lets the model choose), and a specific function → {type:"tool", name:...}. ParallelToolCalls: false adds disable_parallel_tool_use:true to the resulting choice (see the request-field note above).

Client Options

client, _ := aimodel.NewClient(
    aimodel.WithAPIKey("your-key"),
    aimodel.WithBaseURL("https://api.example.com/v1"),
    aimodel.WithProtocol(aimodel.ProtocolAnthropic),
    aimodel.WithTimeout(30 * time.Second),
)

Anthropic header options (no effect under the OpenAI protocol):

client, _ := aimodel.NewClient(
    aimodel.WithAPIKey("sk-ant-xxx"),
    aimodel.WithProtocol(aimodel.ProtocolAnthropic),
    // Opt into beta capabilities via the "anthropic-beta" header.
    // Values append across calls and are comma-joined on the wire.
    aimodel.WithAnthropicBeta("context-1m-2025-08-07"),
    // Override the "anthropic-version" header (default "2023-06-01").
    aimodel.WithAnthropicVersion("2023-06-01"),
)

WithAnthropicBeta ignores empty strings and, with no value configured, the anthropic-beta header is omitted entirely (default behavior). WithAnthropicVersion("") keeps the default version. These are the infrastructure for enabling beta features (compaction, context-editing, structured-outputs, fast-mode, advisor, …).

Multi-Model Compose

The composes package dispatches requests across multiple backends with failover, random, or weighted strategies:

import "github.com/vogo/aimodel/composes"

openai, _ := aimodel.NewClient(aimodel.WithAPIKey("sk-openai"), aimodel.WithBaseURL("https://api.openai.com/v1"))
anthropic, _ := aimodel.NewClient(
    aimodel.WithAPIKey("sk-ant"),
    aimodel.WithProtocol(aimodel.ProtocolAnthropic),
)

cc, _ := composes.NewComposeClient(composes.StrategyFailover, []composes.ModelEntry{
    {Name: "gpt-4o", Client: openai},
    {Name: "claude-sonnet", Client: anthropic},
})

resp, _ := cc.ChatCompletion(ctx, req)