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Secure BEAM sandbox runtime for LLM code mode and MCP aggregation. Run concurrent LLM/tool clients safely while agents orchestrate approved tools, call upstream MCP/OpenAPI servers, and transform data.

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ptc_runner docs plans float-significant-digits.md
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docs/plans/float-significant-digits.md 

# Change float_precision to significant digits



## Context



Float values in planner prompts show excessive precision (e.g., `0.0076017543859649124`) wasting LLM tokens. The current `float_precision` option uses fixed decimal places (`Float.round(value, N)`), which breaks for small numbers: `Float.round(0.0076, 2)` → `0.01`, losing meaning. Changing to **significant digits** preserves meaning at any magnitude.



## Changes



### 1. Change `round_floats` to use significant digits

**File:** `lib/ptc_runner/lisp.ex` (`round_floats/2`)



Replace `Float.round(value, precision)` with a `round_significant/2` helper:

- `0.0` → `0.0` (special case)

- Compute `d = ceil(log10(abs(value)))`, then `decimal_places = sig_figs - trunc(d)`

- If `decimal_places >= 0`: use `Float.round(value, decimal_places)`

- If `decimal_places < 0`: use `Float.round(value / 10^(-decimal_places), 0) * 10^(-decimal_places)` (handles large numbers)



Examples with 4 significant digits:

- `0.0076017543859649124` → `0.007602`

- `3.3333333` → `3.333`

- `12345.678` → `12350.0`



### 2. Update SubAgent default from 2 to 4

**File:** `lib/ptc_runner/sub_agent/definition.ex`



Change `float_precision: 2` → `float_precision: 4`



### 3. Update Lisp.run docs

**File:** `lib/ptc_runner/lisp.ex`



Update the `@doc` to say "significant digits" instead of "decimal places". Update examples.



### 4. Update SubAgent docs

**File:** `lib/ptc_runner/sub_agent.ex`



Change doc from "Decimal places for floats" to "Significant digits for floats (default: 4)"



### 5. Update guide docs

- `docs/guides/subagent-concepts.md` — update `float_precision` table row



### 6. Update tests

**File:** `test/ptc_runner/lisp/lisp_options_test.exs`

- Update assertions to reflect significant digits behavior

- Add tests for small numbers (the motivating case)



**File:** `test/ptc_runner/sub_agent/run_test.exs`

- Update default assertion from 2 to 4

- Update expected values to match significant digits



**File:** `test/ptc_runner/sub_agent/loop/lisp_opts_test.exs`

- Update expectations for the default propagated through `PtcRunner.SubAgent.Loop.LispOpts`



### 7. Confirm all SubAgent execution paths use the new default

**Files:**

- `lib/ptc_runner/sub_agent.ex` (single-shot mode)

- `lib/ptc_runner/sub_agent/loop/lisp_opts.ex` (loop transports)

- `lib/ptc_runner/sub_agent/loop.ex`

- `lib/ptc_runner/sub_agent/loop/ptc_tool_call.ex`

- `lib/ptc_runner/sub_agent/loop/text_mode.ex`



The old `lib/ptc_runner/plan_runner.ex` direct-task path no longer exists. Current loop transports delegate through `PtcRunner.SubAgent.Loop.LispOpts`, which already passes `agent.float_precision` to `PtcRunner.Lisp.run/2`. Single-shot mode also passes `agent.float_precision` directly. After changing the default, verify these paths pick up `4` without adding duplicated option builders.



## Verification



```bash

mix test test/ptc_runner/lisp/lisp_options_test.exs

mix test test/ptc_runner/sub_agent/run_test.exs

mix precommit

```