diff --git a/context.lua b/context.lua
index 82f2ca3..b00e955 100644
--- a/context.lua
+++ b/context.lua
@@ -1,28 +1,77 @@
 -- context.lua — in-memory conversation history + token budget.
--- Phase 0: ordered turn list, sliding window eviction.
--- Tokenization is char/4 heuristic in Phase 0; accurate count is Phase 2.
--- See docs/PHASE0.md §8.
+-- Phase 0: ordered turn list, sliding-window eviction by max_turns.
+-- Tokenization is char/4 heuristic in Phase 0; accurate count is Phase 3 (Q1).
+-- See docs/PHASE0.md §6, §8.
 
 local M = {}
 
--- Construct a Context table from config.context.
+-- The §6 default system prompt. The `CMD: ` (exact prefix, single space)
+-- contract is locked per §3 invariants — do not edit without amending PHASE0.
+local DEFAULT_SYSTEM_PROMPT = [[
+You are aish, an AI-augmented shell assistant. You help the user execute shell
+commands, write and debug code, and re-engineer software. When suggesting shell
+commands, output them on a line beginning with exactly "CMD: " so aish can
+identify and optionally execute them. Be concise. Prefer concrete actions over
+explanations unless asked.]]
+
+local Context = {}
+Context.__index = Context
+
 function M.new(opts)
-    error("context.new: not implemented (Phase 0 pending)")
+    opts = opts or {}
+    return setmetatable({
+        system_prompt = opts.system_prompt or DEFAULT_SYSTEM_PROMPT,
+        turns         = {},
+        max_turns     = opts.max_turns    or 40,
+        token_budget  = opts.token_budget or 4096,
+    }, Context)
 end
 
--- Append a turn { role = ..., content = ... }.
-function M:append(turn)
-    error("context:append: not implemented (Phase 0 pending)")
+function Context:append(turn)
+    assert(type(turn) == "table" and turn.role and turn.content,
+           "context:append requires { role = ..., content = ... }")
+    self.turns[#self.turns + 1] = { role = turn.role, content = turn.content }
 end
 
--- Render messages array suitable for broker.chat (system prompt prepended).
-function M:to_messages()
-    error("context:to_messages: not implemented (Phase 0 pending)")
+-- Render the messages array for broker.chat (system prompt prepended; turns
+-- in order). The system prompt is NOT stored in self.turns per §6.
+function Context:to_messages()
+    local msgs = { { role = "system", content = self.system_prompt } }
+    for _, t in ipairs(self.turns) do
+        msgs[#msgs + 1] = { role = t.role, content = t.content }
+    end
+    return msgs
 end
 
--- Apply max_turns eviction policy. Returns number of turns evicted.
-function M:enforce_budget()
-    error("context:enforce_budget: not implemented (Phase 0 pending)")
+-- Evict the oldest pair (user + assistant) while we exceed max_turns. Returns
+-- total turns evicted. Caller is responsible for rendering the §8 status line.
+function Context:enforce_budget()
+    local evicted = 0
+    while #self.turns > self.max_turns do
+        table.remove(self.turns, 1)
+        evicted = evicted + 1
+        if #self.turns > self.max_turns or evicted % 2 == 1 then
+            if #self.turns > 0 then
+                table.remove(self.turns, 1)
+                evicted = evicted + 1
+            end
+        end
+    end
+    return evicted
+end
+
+-- Coarse char/4 token estimate per §8. Phase 0 visibility only; accurate
+-- tokenization is Q1 (target Phase 3).
+function Context:estimate_tokens()
+    local n = #self.system_prompt
+    for _, t in ipairs(self.turns) do
+        n = n + #t.content
+    end
+    return math.floor(n / 4)
+end
+
+function Context:reset()
+    self.turns = {}
 end
 
 return M