diff --git a/poc-agent/src/context.rs b/poc-agent/src/context.rs
new file mode 100644
index 0000000..3c7e1c9
--- /dev/null
+++ b/poc-agent/src/context.rs
@@ -0,0 +1,232 @@
+// context.rs — Context window building and management
+//
+// Pure functions for building the agent's context window from journal
+// entries and conversation messages. No mutable state.
+
+use crate::journal;
+use crate::types::{ContextPlan, ContextState, Message};
+use chrono::{DateTime, Utc};
+use tiktoken_rs::CoreBPE;
+
+/// Build a context window from conversation messages + journal entries.
+pub fn build_context_window(
+    context: &ContextState,
+    conversation: &[Message],
+    model: &str,
+    tokenizer: &CoreBPE,
+) -> (Vec<Message>, String) {
+    let journal_path = journal::default_journal_path();
+    let all_entries = journal::parse_journal(&journal_path);
+    crate::dbglog!("[ctx] {} journal entries from {}", all_entries.len(), journal_path.display());
+    let count = |s: &str| tokenizer.encode_with_special_tokens(s).len();
+
+    let system_prompt = context.system_prompt.clone();
+    let context_message = context.render_context_message();
+
+    let max_tokens = context_budget_tokens(model);
+    let memory_cap = max_tokens / 2;
+    let memory_tokens = count(&context_message);
+    let context_message = if memory_tokens > memory_cap {
+        crate::dbglog!("[ctx] memory too large: {} tokens > {} cap, truncating", memory_tokens, memory_cap);
+        truncate_at_section(&context_message, memory_cap, &count)
+    } else {
+        context_message
+    };
+
+    let recent_start = find_journal_cutoff(conversation, all_entries.last());
+    let recent = &conversation[recent_start..];
+
+    let plan = plan_context(&system_prompt, &context_message, recent, &all_entries, model, &count);
+    let journal_text = render_journal_text(&all_entries, &plan);
+    
+    let messages = assemble_context(system_prompt, context_message, &journal_text, recent, &plan);
+    (messages, journal_text)
+}
+
+pub fn model_context_window(model: &str) -> usize {
+    let m = model.to_lowercase();
+    if m.contains("opus") || m.contains("sonnet") { 200_000 }
+    else if m.contains("qwen") { 131_072 }
+    else { 128_000 }
+}
+
+fn context_budget_tokens(model: &str) -> usize {
+    model_context_window(model) * 60 / 100
+}
+
+fn plan_context(
+    system_prompt: &str,
+    context_message: &str,
+    recent: &[Message],
+    entries: &[journal::JournalEntry],
+    model: &str,
+    count: &dyn Fn(&str) -> usize,
+) -> ContextPlan {
+    let max_tokens = context_budget_tokens(model);
+    let identity_cost = count(system_prompt);
+    let memory_cost = count(context_message);
+    let reserve = max_tokens / 4;
+    let available = max_tokens.saturating_sub(identity_cost).saturating_sub(memory_cost).saturating_sub(reserve);
+
+    let conv_costs: Vec<usize> = recent.iter().map(|m| msg_token_count_fn(m, count)).collect();
+    let total_conv: usize = conv_costs.iter().sum();
+
+    let journal_min = available * 15 / 100;
+    let journal_budget = available.saturating_sub(total_conv).max(journal_min);
+    let full_budget = journal_budget * 70 / 100;
+    let header_budget = journal_budget.saturating_sub(full_budget);
+
+    let mut full_used = 0;
+    let mut n_full = 0;
+    for entry in entries.iter().rev() {
+        let cost = count(&entry.content) + 10;
+        if full_used + cost > full_budget { break; }
+        full_used += cost;
+        n_full += 1;
+    }
+    let full_start = entries.len().saturating_sub(n_full);
+
+    let mut header_used = 0;
+    let mut n_headers = 0;
+    for entry in entries[..full_start].iter().rev() {
+        let first_line = entry.content.lines().find(|l| !l.trim().is_empty()).unwrap_or("(empty)");
+        let cost = count(first_line) + 10;
+        if header_used + cost > header_budget { break; }
+        header_used += cost;
+        n_headers += 1;
+    }
+    let header_start = full_start.saturating_sub(n_headers);
+
+    let journal_used = full_used + header_used;
+    let mut conv_trim = 0;
+    let mut trimmed_conv = total_conv;
+    while trimmed_conv + journal_used > available && conv_trim < recent.len() {
+        trimmed_conv -= conv_costs[conv_trim];
+        conv_trim += 1;
+    }
+    while conv_trim < recent.len() && recent[conv_trim].role != crate::types::Role::User {
+        conv_trim += 1;
+    }
+
+    ContextPlan {
+        header_start, full_start, entry_count: entries.len(), conv_trim,
+        _conv_count: recent.len(), _full_tokens: full_used, _header_tokens: header_used,
+        _conv_tokens: trimmed_conv, _available: available,
+    }
+}
+
+fn render_journal_text(entries: &[journal::JournalEntry], plan: &ContextPlan) -> String {
+    if plan.header_start >= plan.entry_count { return String::new(); }
+    
+    let mut text = String::from("[Earlier in this conversation — from your journal]\n\n");
+    
+    for entry in &entries[plan.header_start..plan.full_start] {
+        let first_line = entry.content.lines().find(|l| !l.trim().is_empty()).unwrap_or("(empty)");
+        text.push_str(&format!("## {} — {}\n", entry.timestamp.format("%Y-%m-%dT%H:%M"), first_line));
+    }
+
+    let n_headers = plan.full_start - plan.header_start;
+    let n_full = plan.entry_count - plan.full_start;
+    if n_headers > 0 && n_full > 0 { text.push_str("\n---\n\n"); }
+
+    for entry in &entries[plan.full_start..] {
+        text.push_str(&format!("## {}\n\n{}\n\n", entry.timestamp.format("%Y-%m-%dT%H:%M"), entry.content));
+    }
+    text
+}
+
+fn assemble_context(
+    system_prompt: String,
+    context_message: String,
+    journal_text: &str,
+    recent: &[Message],
+    plan: &ContextPlan,
+) -> Vec<Message> {
+    let mut messages = vec![Message::system(system_prompt)];
+    if !context_message.is_empty() { messages.push(Message::user(context_message)); }
+    
+    let final_recent = &recent[plan.conv_trim..];
+    
+    if !journal_text.is_empty() {
+        messages.push(Message::user(journal_text.to_string()));
+    } else if !final_recent.is_empty() {
+        messages.push(Message::user(
+            "Your context was just rebuilt. Memory files have been \
+             reloaded. Your recent conversation continues below. \
+             Earlier context is in your journal and memory files."
+                .to_string(),
+        ));
+    }
+    messages.extend(final_recent.iter().cloned());
+    messages
+}
+
+fn truncate_at_section(text: &str, max_tokens: usize, count: &dyn Fn(&str) -> usize) -> String {
+    let mut boundaries = vec![0usize];
+    for (i, line) in text.lines().enumerate() {
+        if line.trim() == "---" || line.starts_with("## ") {
+            let offset = text.lines().take(i).map(|l| l.len() + 1).sum::<usize>();
+            boundaries.push(offset);
+        }
+    }
+    boundaries.push(text.len());
+
+    let mut best = 0;
+    for &end in &boundaries[1..] {
+        let slice = &text[..end];
+        if count(slice) <= max_tokens { best = end; }
+        else { break; }
+    }
+    if best == 0 { best = text.len().min(max_tokens * 3); }
+
+    let truncated = &text[..best];
+    crate::dbglog!("[ctx] truncated memory from {} to {} chars ({} tokens)", text.len(), truncated.len(), count(truncated));
+    truncated.to_string()
+}
+
+fn find_journal_cutoff(conversation: &[Message], newest_entry: Option<&journal::JournalEntry>) -> usize {
+    let cutoff = match newest_entry { Some(entry) => entry.timestamp, None => return 0 };
+    
+    let mut split = conversation.len();
+    for (i, msg) in conversation.iter().enumerate() {
+        if let Some(ts) = parse_msg_timestamp(msg) {
+            if ts > cutoff { split = i; break; }
+        }
+    }
+    while split > 0 && split < conversation.len() && conversation[split].role != crate::types::Role::User {
+        split -= 1;
+    }
+    split
+}
+
+fn msg_token_count_fn(msg: &Message, count: &dyn Fn(&str) -> usize) -> usize {
+    let content = msg.content.as_ref().map_or(0, |c| match c {
+        crate::types::MessageContent::Text(s) => count(s),
+        crate::types::MessageContent::Parts(parts) => parts.iter().map(|p| match p {
+            crate::types::ContentPart::Text { text } => count(text),
+            crate::types::ContentPart::ImageUrl { .. } => 85,
+        }).sum(),
+    });
+    let tools = msg.tool_calls.as_ref().map_or(0, |calls| calls.iter().map(|c| count(&c.function.arguments) + count(&c.function.name)).sum());
+    content + tools
+}
+
+fn parse_msg_timestamp(msg: &Message) -> Option<DateTime<Utc>> {
+    msg.timestamp.as_ref().and_then(|ts| DateTime::parse_from_rfc3339(ts).ok()).map(|dt| dt.with_timezone(&Utc))
+}
+
+pub fn is_context_overflow(err: &anyhow::Error) -> bool {
+    let msg = err.to_string().to_lowercase();
+    msg.contains("context length") || msg.contains("token limit") || msg.contains("too many tokens")
+        || msg.contains("maximum context") || msg.contains("prompt is too long") || msg.contains("request too large")
+        || msg.contains("input validation error") || msg.contains("content length limit")
+        || (msg.contains("400") && msg.contains("tokens"))
+}
+
+pub fn is_stream_error(err: &anyhow::Error) -> bool {
+    err.to_string().contains("model stream error")
+}
+
+pub fn msg_token_count(tokenizer: &CoreBPE, msg: &Message) -> usize {
+    msg_token_count_fn(msg, &|s| tokenizer.encode_with_special_tokens(s).len())
+}