feat: user agent

[meowgorithm]

This revision sets a default user agent (e.g. Charm Fantasy/0.11.0) and allows end users to configure the user agent on both the agent and provider level.

// Set it on the provider level
provider, _ := open router.New(
    openrouter.WithAPIKey(key),
    openrouter.WithUserAgent("MyHotAgent/0.1.0"),
)

// Set it on the agent level
agent := open router.New(
    model,
    fantasy.WithUserAgent("MyHotterAgent/0.1.0"),
)

The precedence is:

1. Agent-level WithUserAgent()
2. Provider-level WithUserAgent()
3. Provider-level WithHeaders(map[string]string{"User-Agent": "…"})
4. Default (Charm Fantasy/<version>)

…where higher numbers take prescience.

Related: charmbracelet/crush#2323

[Copilot]

Pull request overview

Adds consistent User-Agent handling across the SDK by introducing a default UA (Charm Fantasy/<version>) and allowing overrides at provider, call, and agent levels, with shared header-resolution helpers and tests to validate precedence.

Changes:

• Embed SDK version from version.txt and expose it as fantasy.Version for default UA generation.
• Add call-level (Call.UserAgent, ObjectCall.UserAgent) and agent-level (fantasy.WithUserAgent) UA overrides and wire them through providers.
• Introduce shared providers/internal/httpheaders utilities for default UA + precedence resolution; add provider-specific WithUserAgent options and tests across providers.

Reviewed changes

Copilot reviewed 27 out of 27 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
version.txt	Adds the SDK version source used for default UA strings.
version.go	Embeds version.txt and exposes fantasy.Version.
providers/vercel/vercel.go	Adds provider-level WithUserAgent passthrough to OpenAI provider options.
providers/openrouter/useragent_test.go	Tests OpenRouter UA default + override precedence.
providers/openrouter/openrouter.go	Adds provider-level WithUserAgent passthrough to OpenAI provider options.
providers/openaicompat/openaicompat.go	Adds provider-level WithUserAgent passthrough to OpenAI provider options.
providers/openai/responses_language_model.go	Applies per-call UA request options to Responses API calls/streams.
providers/openai/openai_test.go	Adds tests covering default/provider/call/agent UA precedence for OpenAI provider.
providers/openai/openai.go	Adds provider-level WithUserAgent and resolves headers with default UA.
providers/openai/language_model.go	Applies per-call UA request options for chat completions and object flows.
providers/openai/call_useragent.go	Adds helpers to translate call UA override into OpenAI per-request options.
providers/internal/httpheaders/httpheaders_test.go	Adds unit tests for default UA and precedence resolution utilities.
providers/internal/httpheaders/httpheaders.go	Introduces shared default UA and header precedence resolution utilities.
providers/google/useragent_test.go	Tests Google provider UA default + override precedence.
providers/google/google.go	Adds provider-level WithUserAgent, default UA resolution, and wires per-call UA via wrapped HTTP client.
providers/google/call_useragent.go	Implements per-call UA override via context + custom RoundTripper.
providers/bedrock/useragent_test.go	Tests Bedrock provider UA default + override precedence.
providers/bedrock/bedrock.go	Adds provider-level WithUserAgent passthrough to Anthropic provider options.
providers/azure/useragent_test.go	Tests Azure provider UA default + override precedence.
providers/azure/azure.go	Adds provider-level WithUserAgent passthrough to OpenAI provider options.
providers/anthropic/useragent_test.go	Tests Anthropic provider UA default + override precedence.
providers/anthropic/call_useragent.go	Adds helper to translate call UA override into Anthropic per-request options.
providers/anthropic/anthropic.go	Adds provider-level WithUserAgent, default UA resolution, and applies per-call UA request options.
object.go	Adds ObjectCall.UserAgent to support per-object-call UA override.
model.go	Adds Call.UserAgent to support per-call UA override.
agent_useragent_test.go	Tests that agent-level UA is propagated into underlying fantasy.Call.
agent.go	Adds agent-level WithUserAgent and propagates it into per-step calls.

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[andreynering]

This looks good.

As a sanity check, I'd hold merge until we have the Crush PR opened so we can test both together and ensure everything keep working.

The User-Agent change per se is unlikely to break anything, but I mean mostly because of the added HTTP transports, etc.

[andreynering]

There are two failures on CI.

• lint: just need to format with gofumpt
• govulncheck: just need a rebase or merge from main. A dependency was updated.

[meowgorithm]

Alrighty, good catches. Updated task release, gofumpt-ed, and rebased on main.