Add draft on stretching context in Foundation Models

src/content/blog/stretching-context-in-foundation-models.md

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+---
+title: "Stretching Context in Apple’s Foundation Models"
+description: "Strategies to estimate usage and gracefully manage Apple’s 4,096-token limit."
+pubDate: 2025-09-15
+draft: true
+---
+
+# Stretching Context in Apple’s Foundation Models
+
+When I published Counting Tokens in Foundation Models, I showed a practical way to approximate usage inside Apple’s 4,096-token hard limit. That post ended with a promise: a follow-up on how to stretch effective context so your app degrades gracefully instead of slamming into the ceiling. This is that follow-up.
+
+## Why Token Estimation Matters
+
+Apple’s Foundation Models don’t expose a tokenizer or usage telemetry. You only find out you’ve crossed the boundary when generation fails. That’s not acceptable for production apps.
+
+The fix is to build a guardrail: track characters across prompts, instructions, and responses, then convert to an estimated token count. In my runs, ~4.2 characters per token was a reliable baseline for English prose. Cross-checking with OpenAI’s tiktoken confirmed the trend: Apple heuristics overstate usage, but the failure mode is still abrupt.
+
+Here’s the pattern I use:
+
+```swift
+let result = await TokenUsageEstimator.buildSummary(
+    for: session.transcript,
+    maxContextTokens: 4096
+)
+self.stableEstimatedTokens = result.stableEstimatedTokens
+self.tokenEstimatesSummary = result.summary
+```
+
+That single number — `stableEstimatedTokens` — becomes the trigger for every context management decision downstream.
+
+## Strategy 1: Sliding Windows
+
+The simplest way to avoid blowing the budget is to keep only the last few turns. Drop the oldest once you cross a threshold (say, 3000 tokens).
+
+```swift
+if estimator.stableEstimatedTokens > 3000 {
+    transcript = transcript.keepRecentTurns(count: 6)
+}
+```
+
+This works well for short, focused exchanges. But if your app supports longer conversations, you’ll need more than blunt trimming.
+
+## Strategy 2: Opportunistic Summarization
+
+Instead of throwing away history, compress it. When the budget tightens, collapse older turns into compact summaries: “User asked about X, model replied Y.” This preserves semantic continuity while making room for new input.
+
+```swift
+if estimator.stableEstimatedTokens > 3200 {
+    transcript = transcript.summarizeOlderEntries()
+}
+```
+
+The key is to do this proactively, before you hit the wall. Summarization itself consumes tokens, so keep the summaries short and role-aware.
+
+## Strategy 3: Hierarchical Condensation
+
+Flat summaries only take you so far. A better pattern is layered condensation:
+
+- First pass: shorten individual messages. For example:
+  - Original: “Sure, I can walk you through the setup of the provisioning API step by step with full code examples and background theory.”
+  - Condensed: “Explained provisioning API setup with examples.”
+- Second pass: roll multiple compressed messages into topic summaries. For example:
+  - Original thread of 10 turns about debugging SQLite.
+  - Topic summary: “Resolved SQLite concurrency issues, recommended using read-only connections and GRDB notifications.”
+
+This gives you a semantic spine of the conversation that survives multiple rounds of compression, while still retaining the key technical outcomes.
+
+## Strategy 4: Selective Retention
+
+Not all context is equally important. Tool outputs and logs can eat your budget quickly. The trick is to retain essential state while discarding noise.
+
+- **Keep:** parameters, IDs, results that the model must reuse. For example: “Calendar event created: ID 87213, date 2025-09-15.”
+- **Drop:** verbose logs like JSON payloads, intermediate HTTP traces, or multi-line stack traces, unless they’re required for reasoning.
+
+Code-wise, this can look like:
+
+```swift
+transcript = transcript.retainImportantTools { toolCall in
+    return toolCall.isCritical || toolCall.containsIdentifiers
+}
+```
+
+By treating structured data as state rather than dialogue, you prevent irrelevant clutter from consuming tokens.
+
+## A Graceful Degradation Policy
+
+Put these strategies together into a pipeline:
+
+1. Monitor tokens continuously.
+2. Trim the earliest turns above 75% of budget.
+3. Summarize mid-aged turns above 85%.
+4. Apply hierarchical condensation if history is long and repetitive.
+5. Strip tool chatter above 90%.
+6. Fail-safe: block input or force summarization if >95%.
+
+The goal isn’t to never hit 4096 — it’s to never hit it suddenly.
+
+## Closing Thoughts
+
+In this post we covered the why and how of token estimation, and walked through strategies to stretch context without tripping over Apple’s hard 4096 limit. Estimation provides the guardrail; context management provides the steering wheel. Together they turn an abrupt ceiling into a manageable budget.
+
+Next time I’ll focus on Strategy 2 — opportunistic summarization — as a happy medium between complexity and efficiency, and show how to implement it cleanly in production.
+