feat(llm): add prompt caching configuration

[Linxiushen]

Summary

• add a global llm.prompt_caching setting while preserving the current enabled-by-default behavior
• apply the setting to Anthropic system blocks and tool definitions across provider, legacy, and interactive configuration flows
• add CLI, resolver, request serialization, persistence, and TUI coverage, plus configuration documentation

Testing

• go test ./... -count=1 -skip 'TestResolveBackgroundFilePath|TestSaveConfig|TestResolveRuleEntries_SymlinkSafety|TestFinalizeSurfacesWriterCreationErrorWithoutStdout|TestHandleRepos_PermissionDenied|TestDiscoverRepos_SkipsUnreadableSubdir|TestListSessions_SkipsUnreadableFiles'
• go vet ./...
• go build -o .tmp/ocr.exe ./cmd/opencodereview

Closes #700

[CLAassistant]

All committers have signed the CLA.

[github-actions]

🔍 OpenCodeReview found 3 issue(s) in this PR.

• ✅ Successfully posted inline: 2 comment(s)
• ❌ Failed to post inline: 1 comment(s)

---

[bug · medium]

📄 cmd/opencodereview/provider_tui.go (L1100-L1107)

⚠️ GitHub could not post this as an inline comment: Lines 1100-1107 could not be resolved (outside PR diff hunks)

The custom provider form is missing the prompt caching UI step for Anthropic protocol providers. While the official provider flow transitions to stepPromptCaching and the manual provider flow includes manualStepPromptCaching, the custom provider create/edit form skips this entirely. After completing the auth header step, handleCustomFormEnter calls finishCustomProviderForm() directly without checking if the protocol is Anthropic. The promptCaching value is initialized from config and included in the result, but users have no way to modify it through the UI for custom Anthropic providers. This creates an inconsistent experience where two of three provider paths offer the toggle but custom providers do not.

💡 Suggested Change

Before:

	case cpStepAuthHeader:
		raw := m.cpAuthInput.Value()
		if _, err := llm.NormalizeAuthHeader(raw); err != nil {
			m.formError = authHeaderFormError(raw)
			return m, nil
		}
		m.cpAuthInput.Blur()
		return m.finishCustomProviderForm()

After:

	case cpStepAuthHeader:
		raw := m.cpAuthInput.Value()
		if _, err := llm.NormalizeAuthHeader(raw); err != nil {
			m.formError = authHeaderFormError(raw)
			return m, nil
		}
		m.cpAuthInput.Blur()
		if cpProtocols[m.cpProtocolIdx] == llm.ProtocolAnthropic {
			m.cpStep = cpStepPromptCaching
			return m, nil
		}
		return m.finishCustomProviderForm()

[github-actions]

[maintainability · low]
Potential issue: When switching from Anthropic to a non-Anthropic protocol, the old PromptCaching value remains in the config file. While this doesn't cause functional problems (the setting is only used for Anthropic), it leaves stale configuration data. Consider clearing the field when the protocol is not Anthropic:

if result.promptCaching != nil {
    cfg.Llm.PromptCaching = result.promptCaching
} else if llm.NormalizeProtocol(result.protocol) != llm.ProtocolAnthropic {
    cfg.Llm.PromptCaching = nil
}

This same pattern appears in all three apply functions (applyManualConfig, applyCustomProviderConfig, applyOfficialProviderConfig) and should be updated consistently.

[Linxiushen]

Addressed in c07b2a7. The manual, custom, and official apply paths now clear stale prompt-caching state when the effective protocol is not Anthropic. Regression coverage includes all three switch paths and preserves the active Anthropic setting when an inactive OpenAI provider is edited.

[github-actions]

[maintainability · low]
Inconsistent sourcing: PromptCaching is read from cfg.Llm.PromptCaching (the top-level llm block), while other analogous per-provider fields like ExtraBody, ExtraHeaders, and TimeoutSec are sourced from the provider entry. Furthermore, providerEntryConfig has no PromptCaching field, so users cannot configure this setting per-provider.

If PromptCaching is intentionally a global-only setting, this is acceptable, but a brief comment explaining the design decision would prevent future confusion. Otherwise, add PromptCaching *bool to providerEntryConfig and read it from entry (falling back to cfg.Llm.PromptCaching if unset) for consistency.

[Linxiushen]

Confirmed as global by design: the toggle follows whichever Anthropic provider is active rather than being duplicated across provider entries. c07b2a7 documents that contract in both the persisted config and resolver, while keeping provider-specific transport fields on each entry.

[cometkim]

I think this may duplicate #332, which adds a generalized option for prompt caching, for multiple providers.