fix(providers): keep a third-party agent's own User-Agent on OpenCode Go

[yourspraveen]

⚠️ base-red inherited: #15306

Closes #15311

Problem

OpencodeExecutor.buildHeaders feeds the CLI defaults into forwardOpencodeClientHeaders. Its applyCliDefaults replaces every User-Agent that isn't a versioned OpenCode CLI. That rewrite exists for the Zen free tier (#5997, Cloudflare refusing generic UAs from datacenter IPs), but it also ran on the authenticated OpenCode Go path. As a result, a third-party agent sending my-coding-agent/1.0 reached Go as opencode/<version> unless CLI synthesis was switched off globally.

OpenCode Go's client requirements (opencode.ai/docs/go, "Where can I use it?") ask the opposite: a client should

Identify itself with its own user agent, such as my-coding-agent/1.0, rather than a generic SDK or HTTP-library name.

and send a stable x-opencode-session.

Fix

• forwardOpencodeClientHeaders gains a keepAgentUserAgent option. When it is set, a client User-Agent that names the agent itself is kept.
• The new isGenericClientUserAgent matches anchored, bounded SDK / HTTP-library defaults: curl, python-requests/httpx, OpenAI/Anthropic SDKs, axios, node-fetch, undici, Go-http-client, okhttp, Bun and others. Those UAs are still replaced, and a missing UA is still filled.
• buildHeaders enables the option only on the Go surface (identified by registry baseUrl, so aliases don't matter) and outside the free-tier gate. The Zen free-tier policy is unchanged.

Tests

tests/unit/opencode-go-client-user-agent-15311.test.ts covers:

• R1: Go keeps the agent's UA with synthesis unset and with synthesis on
• R2: the session is unchanged across turns
• R3: generic SDK / library UAs are replaced, and a missing UA is filled
• R4: a genuine opencode/1.18.31 UA is kept
• R5: the Zen free tier still rewrites a non-CLI UA
• R6: with synthesis off, the UA and session are forwarded as before

R1 and R2 failed before the fix. All OpenCode tests pass after it (850/850), and so do the existing #5997 synthesis tests.

Related

• fix(opencode): send CLI identity + x-opencode-session on background calls #13005 (@alvinveroy) gives OpenCode background calls (model discovery, quota) a CLI identity and x-opencode-session. That is a different path (buildOpencodeBackgroundHeaders) and doesn't conflict with this generation-path change.
• fix(sse): reproduce OpenCode CLI requests and isolate model cooldowns (#14977) #15143 (free-tier pause) and fix(opencode): background calls (model discovery, quota) send UA "Bun fetch" without x-opencode-session — hard errors announced from 09/06 #12657 / fix(providers): [403]: Error from provider (Console): OpenCode's free tier can only be used from within OpenCode #15220, which the issue mentions, concern the Zen free tier. Its policy is deliberately unchanged here, and R5 pins that.

Local quality gates

I ran the quality.yml fast-gate set locally against release/v3.8.52:

• Passing (34), including file-size and complexity-ratchets. The User-Agent decision is in its own keepsClientUserAgent, so applyCliDefaults gains no complexity and opencode.ts stays under its frozen size.
• Failing (3), none caused by this change:
  • cycles and agent-skills-sync fail identically on the untouched base.
  • workflows needs actionlint and zizmor, which aren't installed locally.

[yourspraveen]

CI note for reviewers:

The red checks on this PR are inherited from release/v3.8.52 (#15306) or from CI infrastructure. None of them involves the files this PR changes:

Check	Cause	Why it is not this PR
Fast Quality Gates	only check:cycles fails (14 SCCs)	fails identically on the untouched base; this PR adds no import edges in those cycles
Docs Sync (Strict)	[i18n-check] drift detected	i18n mirrors drifted on the base; this PR touches no translated file
Merge integrity	check:agent-skills-sync regenerates omni-providers	same dry-run diff on the base; no skill or provider-catalog change here
Lint	stops at npm run audit:deps: 44 advisories (incl. next, node-forge)	new advisories against the base lockfile; this PR changes no dependency
Build	The runner has received a shutdown signal	runner infrastructure, cancelled mid-job
Unit Tests fast-path (2/4)	11582 pass, 1 fail: transientBackendRetry: default sleep respects AbortSignal without custom sleep	a timing test in open-sse/services/transientBackendRetry, which this PR doesn't touch; it passes 3/3 locally on this branch and on the base, so it looks like a flake on a loaded runner

The checks that exercise this change pass: API Route Typecheck, the other unit-test shards and Vitest. Before pushing I ran the quality.yml fast-gate set locally (34 passing, including file-size and complexity-ratchets) and all OpenCode tests (850/850).