Why token counting isn't a solved problem when building across providers

Building AI products that span multiple LLM providers involves a challenge most developers don't anticipate until they hit it: context windows are not interoperable.

On the surface, managing context in a multi-LLM system seems straightforward. You track how long conversations get, trim when needed, and move on. In practice, it's considerably more complex — and if you're routing requests across providers like OpenAI, Anthropic, Google, Cohere, or xAI, there's a fundamental mismatch that can break your product in subtle ways.

The Tokenization Problem

Every major LLM provider uses its own tokenizer. These tokenizers don't agree. The same block of text produces different token counts depending on which model processes it. The difference is often 10–20%, sometimes more.

What this means in practice: a conversation that fits comfortably in one model's context window may silently overflow another's. A prompt routed to OpenAI might count as 1,200 tokens; the same prompt routed to Claude might count as 1,450. That gap matters.

Where It Breaks

The failure modes tend to show up at the boundaries. When you switch providers mid-conversation, the new model has to ingest the full prior context. If your context management layer was calibrated to the previous model's tokenizer, the new model may see a context that's already at or over the limit — before it's even responded to anything new.

This produces three common failure patterns:

• Unexpected context-window overflow: the conversation that worked before now breaches the limit
• Inconsistent truncation: different models truncate at different points, changing what prior context the model actually sees
• Routing failures that are unpredictable because the numbers your system used don't match the numbers the model actually used

Why Simple Estimates Fail

The instinct is to maintain a single "token estimate" with a generous safety margin. The problem is that the margin you'd need varies by provider, model version, and content type (code tokenizes differently than prose). A margin calibrated for one use case will either be too tight for another, causing failures, or too generous, causing unnecessary truncation that degrades conversation quality.

The Solution: Provider-Aware Token Counting

A robust multi-LLM context management layer makes token counting provider-specific. Rather than maintaining a single estimate, it measures each prompt the way the actual target model will measure it. The routing layer uses these per-provider measurements to make decisions before requests are sent.

This lets the system stay ahead of context limits: it knows when a conversation is approaching an edge, trims or compresses history calibrated to the specific model receiving the request, and avoids the pricing and failure surprises that come from miscounted tokens.

The end result is what users should see: a smooth conversation experience, regardless of which model is serving it. The complexity of "every model speaks a slightly different token language" stays inside the infrastructure layer, invisible to the people using the product.

This is the approach we've taken in our adaptive context window management component, and it's become a foundational part of how we think about multi-LLM routing more broadly.

Rob Imbeault
Apr 17, 2026