Introduction
You've set up Claude Code and sent your first prompt. Now the question is: how does it actually understand what you wrote?
This guide covers what happens under the hood — how Claude reads code, what tokens and context mean in practice, and why it sometimes drops details from earlier in a conversation. Understanding these mechanics will change how you prompt.
How Claude Reads Code
Claude doesn't execute your code. It processes text. Three stages build its understanding:
1. Parsing and context formation. Claude breaks your prompt into tokens and maps relationships between them — which function calls which, where a variable appears, how control flow branches. The output is a structural model of your code.
2. Reasoning and pattern matching. Claude compares that model against patterns from training. A sorting function that resembles merge sort gets treated like merge sort. That match drives what Claude predicts you need next.
3. Generation and justification. Claude writes its response token by token, checking for coherence and safety at each step. The comments you see — "This variable is unused," "This may fail on empty lists" — come from this stage, not from running the code.
Claude is probabilistic. Two similar prompts may produce different answers. That's reasoning, not a bug.
Tokens and Context Windows
A token is a small chunk of text. Claude averages 4 characters per token, or about 1.5 tokens per word in English. For code that number rises — brackets, operators, and keywords don't map cleanly to natural language tokens.
Practical scale:
-
print("Hello")is 3–4 tokens - A typical function is 50–200 tokens
- 1,000 tokens is roughly 750 words of English prose
The context window is Claude's working memory for a session. It holds your prompts, Claude's responses, uploaded files, and tool outputs — all from the same token budget. Current models as of May 2026:
| Model | Context | Best For |
|---|---|---|
| Claude Haiku 4.5 | 200K | Quick fixes, low-latency tasks |
| Claude Sonnet 4.6 | 1M | General coding, refactoring, multi-file work |
| Claude Opus 4.6 / 4.7 | 1M | Deep analysis, full-stack architecture, large codebases |
The 1M window on Sonnet 4.6 and Opus 4.6 went live at standard pricing in March 2026. The older Claude 3 series is deprecated. Claude 3 Haiku is fully retired — requests to it now return errors.
When a session fills up, Claude drops the oldest tokens first. Recent messages stay. That's why early details can go missing in long conversations.
One more thing worth knowing: research has documented a "lost in the middle" effect — attention quality drops for content buried in the center of a very large context. Put your most important constraints, framework choices, and goals at the top of a session.
The Request-Response Lifecycle
Four stages happen every time you send a prompt.
Stage 1 — Tokenization. Claude converts your input to token IDs. It processes your message together with conversation history, loaded files, system prompts, and any command outputs. Longer sessions accumulate tokens faster because the input compounds.
Stage 2 — Context assimilation. Claude merges the new prompt with history and runs self-attention, weighting which parts matter most. Constitutional AI constraints apply here — unsafe requests get flagged before generation starts.
Stage 3 — Generation. Claude produces output token by token. It checks coherence and safety continuously. You'll often see Claude frame its approach before writing code — that framing is the alignment step.
Stage 4 — Memory update. Claude adds the response to its in-session context. That's why "now optimize the async performance" works — Claude knows which code you mean. Close the session and that context is gone.
Memory and Conversation State
Within a Session
Claude holds everything in the active context window. As it fills, the oldest tokens drop. Re-anchoring every few turns keeps Claude oriented:
"We're still on the FastAPI e-commerce API with JWT auth and PostgreSQL. Now add order history."
Across Sessions
By default, Claude Code starts each session from scratch. Stateless architecture is simpler to reason about and easier to scale — this is intentional, not an oversight.
Anthropic rolled out auto-memory for Claude Code in early 2026. Claude now builds and maintains a MEMORY.md file during sessions, storing key decisions and context that load automatically next time. The feature is on a gradual rollout; your account may need it enabled.
The manual alternative: keep a .claude/ directory with a context.md or decisions/ folder. Claude reads these at session start.
A session_notes.txt with key decisions and code snippets also works well. Paste it in at the top of each new session if auto-memory isn't available yet.
API vs. IDE
Claude Web and IDE integrations maintain context within the open session automatically. The Claude API is stateless by default — you send conversation history with each request to maintain continuity. More work, but also more control.
Which Model to Use
| Model | Context | Use When |
|---|---|---|
| Haiku 4.5 | 200K | Repetitive tasks, test generation, quick completions |
| Sonnet 4.6 | 1M | Everyday coding — best cost-to-capability ratio |
| Opus 4.6 / 4.7 | 1M | Legacy refactors, complex multi-file reasoning, long-context analysis |
Use Haiku for "add a docstring to every function." Use Sonnet for most development work. Bring in Opus when you need to untangle something genuinely complex — a legacy monolith, an architecture decision with many constraints, a long document to reason over. Using Opus for a typo fix costs you money and time.
Best Practices
For Beginners
- Start small. Token awareness develops naturally as you work.
- If Claude loses track of earlier context, restate your goal explicitly.
- The web interface handles context management for you.
For Power Users
- Front-load key facts. Your framework, language, and hard constraints belong at the top of the session, not buried in message 15.
- Re-anchor every few turns. "Still on the FastAPI project, PostgreSQL backend" keeps Claude from drifting as context accumulates.
- Segment sessions by topic. Start a new session when you switch from backend to frontend work.
-
Build explicit context for the API. Send prior messages in each request, or use the
MEMORY.md/.claude/pattern to persist decisions.
Common Mistakes
- Assuming Claude remembers yesterday. It doesn't by default. Save summaries or enable auto-memory.
- Pasting 50,000-line files wholesale. Large undifferentiated inputs dilute attention. Start with the relevant files and expand from there.
- Reaching for Opus by default. Sonnet 4.6 handles most development tasks well and costs significantly less.
- Referencing retired models. Claude 3 Haiku returns errors now. Claude 3 Sonnet and Opus are deprecated. Use the 4.x series.
Conclusion
Claude Code is a reasoning engine with a finite but large working memory. You now understand how it reads code, how tokens accumulate across a session, the four stages it runs on every request, and how to manage context across sessions and model choices.
Try this: Open a fresh session and build a small API across several turns. After a few exchanges, change the subject entirely, then bring Claude back to the original task. Watching how it re-anchors — or loses the thread — makes the mechanics concrete.
Acknowledgment: Based on official Anthropic documentation and community research. Technical details reflect the state of Claude Code as of May 2026.
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