Summary

Four prompt-side improvements bundled because they all live in the FM prompt channel and were all measurable against the eval suite from the parent stack:

• Trailing context. The suffix (text after the caret) was captured but never sent to FM. Mid-line insertions now get a Text after the caret: section so the model can bridge into existing text rather than overwrite it.
• Length hint. The user's word-count preset (Return only the next 7 to 12 words.) is reintroduced on the prompt channel — not instructions, so it does not bust the cached session prefix from PR 2. Apple's model lands cleaner stopping points with a natural-language cue than with maximumResponseTokens alone.
• Per-app tone hint. A small bundle-ID dictionary maps focused-app categories (code editor, email, chat, browser) to one-line tone cues injected into the prompt. Unknown bundles fall through to no hint.
• Bigger FM-side prefix budget. SuggestionConfiguration gains FM-only prefix caps (2500 chars / 150 words) so Apple's larger context can take more surrounding sentences without crowding instructions. Llama keeps its existing tighter window.

Stacked on refactor/fm-positive-frame. Use the eval suite from the head of the stack to compare mid-word truncation, mid-line bridging, and category drift.

Validation

xcodebuild -project Cotabby.xcodeproj -scheme Cotabby -destination 'platform=macOS' build CODE_SIGNING_ALLOWED=NO
→ ** BUILD SUCCEEDED **

xcodebuild test ... -only-testing:CotabbyTests/FoundationModelPromptRendererTests -only-testing:CotabbyTests/SuggestionRequestFactoryTests -only-testing:CotabbyTests/SuggestionEngineRouterTests
→ Executed 33 tests, with 0 failures (0 unexpected)

swiftlint lint --quiet → no violations.

Manual: focus a real text field in Mail / Slack / Xcode / Safari and verify the diagnostics panel shows the matching per-app tone hint and (where applicable) the trailing-text section.

Linked issues

Refs the FM-quality investigation.

Risk / rollout notes

• SuggestionConfiguration gained two new memberwise fields (maxPrefixWordsFoundationModel, maxPrefixCharactersFoundationModel). Production uses .standard, which is updated; the two explicit-constructor test sites were updated alongside.
• SuggestionRequestFactory.truncatedPromptPrefix now takes an optional engine: parameter (defaulting to .llamaOpenSource) so external callers stay source-compatible. The internal call from buildRequest and the coordinator's clipboard-filter call both pass the active engine explicitly.
• The per-app tone-hint dictionary is intentionally small. Adding bundle IDs is a one-line change in FoundationModelPromptRenderer; the absence of a hint is the safe default.
• Diagnostic preview now surfaces the length cue (it lives on the prompt channel), so the test_promptPreview assertion was flipped — and a new assertion ensures the cue still does NOT appear in instructions (so the session-cache key remains stable across word-count preset changes... wait, those should invalidate; correction: word-count preset only changes the prompt, not instructions, so cache stays warm).

Greptile Summary

This PR enriches the Foundation Models prompt channel with four coordinated improvements: trailing-context injection (bounded by maxSuffixCharacters), a natural-language length hint on the per-request prompt, per-app tone cues keyed by bundle-ID prefix, and a separate, larger prefix budget (2500 chars / 150 words) for the FM path.

• Trailing text is now capped at maxSuffixCharacters before being sent, closing the previously identified gap where a caret at the top of a long document would flood the 4096-token context window.
• SuggestionConfiguration gains two new FM-specific prefix cap fields; all existing explicit constructors in tests were updated alongside, and truncatedPromptPrefix selects the right budget via an engine switch with a backward-compatible default.
• The bundle-ID tone-hint dictionary deliberately excludes terminal emulators and Cursor (whose ToDesktop hash is unstable), with tests covering both known-match and exclusion cases; one entry (com.apple.mobilesms) references an iOS-only bundle ID that will not match macOS Messages.

Confidence Score: 5/5

Safe to merge — all prompt-channel changes are additive and the most significant correctness fix (trailing-text overflow) is properly addressed.

The trailing-text cap is correctly applied before injection, the engine-aware prefix budget switch is clean with a backward-compatible default, and the tone-hint dictionary's safe-default-on-no-match design means unknown or mismatched bundle IDs degrade gracefully. The one dead entry (com.apple.mobilesms) silently misses macOS Messages but causes no harm. Tests cover the new code paths thoroughly.

FoundationModelPromptRenderer.swift — the chatBundlePrefixes list contains one iOS-only entry that will not fire on macOS.

Important Files Changed

Sequence Diagram

sequenceDiagram
    participant Coord as SuggestionCoordinator
    participant Factory as SuggestionRequestFactory
    participant FM as FoundationModelPromptRenderer
    participant Apple as Apple FM API

    Coord->>Factory: truncatedPromptPrefix(engine: .appleIntelligence)
    Note over Factory: Selects maxPrefixCharactersFoundationModel=2500<br/>maxPrefixWordsFoundationModel=150
    Factory-->>Coord: bounded prefixText

    Coord->>Factory: buildRequest(context, settings, configuration)
    Factory->>Factory: truncatedPromptPrefix(engine: appleIntelligence)
    Factory-->>Coord: SuggestionRequestBuildResult

    Coord->>FM: promptPreview(for: request)
    FM->>FM: sessionInstructions(for: request)
    Note over FM: Role + output contract + examples<br/>(no length hint, no tone hint)
    FM->>FM: prompt(for: request)
    Note over FM: appToneHint(bundleIdentifier)<br/>+ Text before caret (prefixText)<br/>+ Text after caret (trailingText, capped at maxSuffixCharacters)<br/>+ Write only the next continuation fragment.<br/>+ completionLengthInstruction
    FM-->>Coord: instructions + prompt

    Coord->>Apple: generate(instructions, prompt)
    Apple-->>Coord: completion text

_{Reviews (4): Last reviewed commit: "Address Greptile review on #344" | Re-trigger Greptile}