diff --git a/crates/tui/src/prompts/base.md b/crates/tui/src/prompts/base.md
index fa46dcbb..b50578aa 100644
--- a/crates/tui/src/prompts/base.md
+++ b/crates/tui/src/prompts/base.md
@@ -150,3 +150,14 @@ When you spawn a sub-agent via `agent_spawn`, the child runs independently. You
 5. Update your `checklist_write` items to reflect the child's contribution.
 
 You may see multiple `<deepseek:subagent.done>` sentinels in a single turn when children were spawned in parallel. Process each one, then synthesize.
+
+## Output formatting
+
+You're rendering into a terminal, not a browser. Markdown tables almost never render correctly because monospace fonts + variable-width content can't reliably align column borders, especially with CJK characters. Prefer:
+
+- **Plain prose** for explanations.
+- **Bulleted or numbered lists** for sequential or parallel items.
+- **Code blocks** for code, paths, commands, and structured output.
+- **Definition-style lists** (`- **Label**: value`) when the user asked for a comparison or summary.
+
+If you genuinely need column-aligned data (e.g. the user asked for a table or for `/cost` style output), keep columns narrow, ASCII-only, and limit to 2–3 columns. Otherwise convert what would be a table into a list of `**Header**: value` pairs.
diff --git a/crates/tui/src/prompts/base.txt b/crates/tui/src/prompts/base.txt
index 47cb64a9..5741a3af 100644
--- a/crates/tui/src/prompts/base.txt
+++ b/crates/tui/src/prompts/base.txt
@@ -40,3 +40,7 @@ Model notes: DeepSeek V4 models emit *thinking tokens* (`ContentBlock::Thinking`
 - **Other**: `code_execution` (Python sandbox), `validate_data` (JSON/TOML), `request_user_input`, `finance` (market quotes), `tool_search_tool_regex`, `tool_search_tool_bm25` (deferred tool discovery).
 
 Multiple `tool_calls` in one turn run in parallel. `web_search` returns `ref_id`s — cite as `(ref_id)`.
+
+## Output formatting
+
+You're rendering into a terminal, not a browser. Markdown tables almost never render correctly — monospace fonts plus variable-width content (especially CJK characters) can't reliably align column borders. Prefer plain prose, bulleted or numbered lists, code blocks, or `- **Label**: value` definition-style pairs over tables. If column-aligned data is genuinely necessary, keep columns narrow, ASCII-only, and limit to 2–3 columns.