ds-implement

name	ds-implement
description	Phase 3 of the /ds workflow — analysis task execution. Invoked by the ds-plan chain; not user-invocable.
user-invocable	false
disable-model-invocation	true
allowed-tools	Read, Grep, Glob, Bash, Skill, TodoWrite, Agent
hooks	{"PostToolUse":[{"matcher":"Agent","hooks":[{"type":"command","command":"uv run python3 ${CLAUDE_PLUGIN_ROOT}/hooks/ds-post-subagent-guard.py"}]}],"PreToolUse":[{"matcher":"Agent","hooks":[{"type":"command","command":"uv run python3 ${CLAUDE_PLUGIN_ROOT}/hooks/ds-pre-subagent-clear.py"}]},{"matcher":"Read","hooks":[{"type":"command","command":"uv run python3 ${CLAUDE_PLUGIN_ROOT}/hooks/ds-read-after-subagent-guard.py"}]},{"matcher":"Grep","hooks":[{"type":"command","command":"uv run python3 ${CLAUDE_PLUGIN_ROOT}/hooks/ds-read-after-subagent-guard.py"}]},{"matcher":"Glob","hooks":[{"type":"command","command":"uv run python3 ${CLAUDE_PLUGIN_ROOT}/hooks/ds-read-after-subagent-guard.py"}]},{"matcher":"Write","hooks":[{"type":"command","command":"uv run python3 ${CLAUDE_PLUGIN_ROOT}/hooks/ds-no-main-chat-code-guard.py"}]},{"matcher":"Edit","hooks":[{"type":"command","command":"uv run python3 ${CLAUDE_PLUGIN_ROOT}/hooks/ds-no-main-chat-code-guard.py"}]},{"matcher":"Bash","hooks":[{"type":"command","command":"uv run python3 ${CLAUDE_PLUGIN_ROOT}/hooks/ds-no-main-chat-code-guard.py"}]},{"matcher":"Write|Edit|Agent|Workflow","hooks":[{"type":"command","command":"GATE_ARTIFACT=.planning/PLAN_REVIEWED.md GATE_STATUS=APPROVED GATE_DESCRIPTION=\"Plan review\" GATE_REMEDY=\"Return to ds-plan and run ds-plan-reviewer; implementation is gated until PLAN.md is APPROVED.\" GATE_BLOCKED_TOOLS=Write,Edit,Agent,Workflow uv run python3 ${CLAUDE_PLUGIN_ROOT}/hooks/phase-gate-guard.py"}]}]}

Overview

Apply output-first verification at every step of analysis implementation. This is Phase 3 of the /ds workflow.

Contents

• The Iron Law of DS Implementation - EVERY step MUST produce visible output
• Delegation - Main chat orchestrates, subagents analyze
• What Output-First Means
• Output-First Facts
• SAS Language Routing - Load SAS enforcement when PLAN.md specifies SAS
• Implementation Process
• Verification Patterns - See references/verification-patterns.md
• Common Failures
• Gate: Exit Implementation

Implementation (Output-First Verification)

Implement analysis with mandatory visible output at every step. NO TDD - instead, every code step MUST produce and verify output.

## The Iron Law of DS Implementation

EVERY CODE STEP MUST PRODUCE VISIBLE OUTPUT. This is not negotiable.

Before moving to the next step, you MUST:

1. Run the code
2. See the output (print, display, plot)
3. Verify output is correct/reasonable
4. Document in .planning/LEARNINGS.md
5. Only THEN proceed to next step

This applies even when YOU think:

• "I know this works"
• "It's just a simple transformation"
• "I'll check results at the end"
• "The code is straightforward"

If you're about to write code without outputting results, STOP.

Delegation

**YOU MUST NOT WRITE ANALYSIS CODE IN MAIN CHAT. This is not negotiable.**

You orchestrate the ds-implement ultracode workflow, which reads the hardened PLAN.md Task Breakdown table, builds the data-flow DAG, and runs each dependency level's tasks output-first (one ds-analyst/ds-engineer per task, writing directly to the project). You drive the level loop; the workflow's implementers do the analysis/ETL.

0. Set the goal (once): /goal All tasks in PLAN.md are marked [x], each task's Verify
   assertion exits 0, and .planning/VALIDATION.md status is `validated`. Stop after [N] turns.

LOOP (one turn per level, under the active /goal):
  1. Workflow(name="ds-implement", args={
       "projectDir": "<absolute project root (cwd)>",
       "pluginRoot": "<resolve ${CLAUDE_SKILL_DIR}/../../workflows>"
     })
     → runs the lowest level's pending tasks output-first, returns { overallPass, level,
       tasksRemaining, tasks, findings, tasksThatFailed, reviews }. Outputs are already on disk.
  2. GROUND-TRUTH: run ds-validate-coverage (or the full pipeline) on the level's outputs —
     per-task Verify ran in isolation; this confirms requirement coverage / no regression.
  3. If result.overallPass AND coverage clean: mark this level's PLAN rows [x], log to
     LEARNINGS.md, END THE TURN (the /goal re-fires for the next level, or closes if
     tasksRemaining=0). No pause.
  4. If result.overallPass is false: read result.findings, fix the cause, re-invoke with
     onlyChecks=result.tasksThatFailed + priorReviews=result.reviews. An R4 (schema change,
     new data source, methodology pivot) is critical — STOP and escalate to the user.

The legacy per-task ds-delegate template is now embedded in the workflow's implementer prompt; ds-delegate remains for ad-hoc single-task dispatch outside this phase. If you're about to write analysis code directly, STOP — the workflow's implementers do that, and ds-no-main-chat-code-guard forbids you (you may only touch .planning/).

Delete & Restart Protocol

Scenario	Action
You wrote > 3 lines of analysis code in main chat	DELETE immediately. Restart via Task agent.
You ran a cell, realized it should have been in Task agent	DELETE the cell output and cell. Re-do via Task agent.
You started a transformation in main chat	STOP. DELETE what you've done. Spawn Task agent instead.
"Just finish this quick analysis here"	STOP — if it's quick enough to finish, it's quick enough for a Task agent. Delete and restart.

Helpfulness Check: If you kept main-chat code "because it worked," you bypassed the orchestration protocol. Working code written in the wrong place skips verification and review — it is anti-helpful to the user. Delete it.

What Output-First Means

DO	DON'T
Print shape after each transform	Chain operations silently
Display sample rows	Trust transformations work
Show summary stats	Wait until end to check
Verify row counts	Assume merges worked
Check for unexpected nulls	Skip intermediate checks
Plot distributions	Move on without looking

The Mantra: If not visible, it cannot be trusted.

Output-First Facts

• "The merge worked fine" without printed numbers is an unverified claim — show the counts, compared against .planning/PLAN.md expected output.
• Combined operations hide which step failed: when the error surfaces at the end, the root cause is buried under every later transform and cannot be isolated. One operation per verification cycle.
• A "COMPLETE" logged in .planning/LEARNINGS.md without verified output is a false claim that review inherits — the task may have silently failed, and the user acts on results that don't exist. Logging a verified completion takes 30 seconds; an unlogged step is invisible to review.
• An agent's summary can gloss over errors its full output reports — deferring the read means running blind. Read agent output immediately and verify claims against the actual data.

Implementation Strategy Choice

After prerequisites pass and PLAN.md verified, check for parallelization potential:

Skip this choice when:

• PLAN.md has fewer than 4 tasks
• All tasks are dependent (every task is after N with no independent groups)
• Tasks form a pipeline (clean → merge → aggregate → model)
• CLAUDE_CODE_EXPERIMENTAL_AGENT_TEAMS is not available

Otherwise, ask the user:

AskUserQuestion(questions=[{
  "question": "How should we implement the analysis tasks in PLAN.md?",
  "header": "Strategy",
  "options": [
    {"label": "Sequential (Default)", "description": "One task at a time with output-first verification. Safest, most DS work is sequential."},
    {"label": "Agent team (parallel)", "description": "Spawn analyst per independent task group. Only for truly independent analysis branches (descriptive stats by subgroup, model comparisons). Requires reconciliation."}
  ],
  "multiSelect": false
}])

If Sequential: Proceed to Implementation Process below (current behavior).

If Agent team: Skip to Agent Team Implementation (Parallel).

SAS Language Routing

If PLAN.md specifies Implementation Language: SAS or Mixed, load SAS enforcement BEFORE dispatching any SAS tasks. Paste the enforcement block into every SAS subagent prompt.

Full SAS enforcement rules: See references/sas-enforcement.md

Implementation Process Flowchart

┌─────────────────────────┐
│  Read PLAN.md + Load    │
│  ds-delegate + ETL refs │
└───────────┬─────────────┘
            ▼
┌─────────────────────────┐
│  For each task in PLAN  │◄──────────────────────┐
│  (in dependency order)  │                       │
└───────────┬─────────────┘                       │
            ▼                                     │
┌─────────────────────────┐                       │
│  Dispatch Task agent    │                       │
│  (per ds-delegate)      │                       │
└───────────┬─────────────┘                       │
            ▼                                     │
┌─────────────────────────┐     ┌──────────────┐  │
│  Read agent output      │────→│ Output wrong │  │
│  Verify output present  │     │ or missing?  │  │
│  + reasonable            │     └──────┬───────┘  │
└───────────┬─────────────┘            │           │
            │ OK                       ▼           │
            │                 ┌──────────────────┐ │
            │                 │ STOP. Investigate │ │
            │                 │ Log issue. Fix.   │ │
            │                 │ Re-verify.        │ │
            │                 └──────────────────┘ │
            ▼                                     │
┌─────────────────────────┐                       │
│  Log to LEARNINGS.md    │                       │
│  (Task N: COMPLETE)     │                       │
└───────────┬─────────────┘                       │
            ▼                                     │
        More tasks? ──── YES ─────────────────────┘
            │
            NO
            ▼
┌─────────────────────────┐
│  Exit Gate: Compare     │
│  PLAN.md vs LEARNINGS   │
│  (all tasks accounted?) │
└───────────┬─────────────┘
            ▼
┌─────────────────────────┐
│  Invoke ds-validate     │
└─────────────────────────┘

This flowchart IS the specification. If the narrative below and this flowchart disagree, the flowchart wins.

Topic Change Protocol

If user sends an off-topic message during implementation, follow C6 from ds-common-constraints.md:

1. Announce: "Pausing ds-implement to address your request."
2. Handle: Process the request (normal tools allowed outside the loop).
3. Announce: "Resuming ds-implement. Reading state files for current progress."
4. Reload: Read LEARNINGS.md and PLAN.md to restore context.
5. Resume: Continue from where you left off.

Do NOT silently switch context. Silent switches kill the implementation loop.

Implementation Process

Step 1: Read Plan, Load Shared Enforcement, and Delegation Skill

Auto-load all constraints matching applies-to: ds-implement:

!uv run python3 ${CLAUDE_SKILL_DIR}/../../scripts/load-constraints.py ds-implement

You MUST have these constraints loaded before proceeding. No claiming you "remember" them.

Read(".planning/PLAN.md")

Read ${CLAUDE_SKILL_DIR}/../../skills/ds-delegate/SKILL.md and follow its instructions.

Follow the task order defined in the plan. Use ds-delegate's templates for every task.

ETL Strategy Enforcement — load domain-specific references based on PLAN.md:

If PLAN.md contains an ## ETL Strategy section, the user made decisions during planning that MUST be enforced during implementation. Check each subsection and load the corresponding enforcement:

PLAN.md Section	Enforcement Reference	Inject Into
row_pk / event_key declared (any data with a grain)	ETL enforcement (skills/ds-implement/references/etl-enforcement.md) § Key & Grain Carry-Through	Every data load/transform subagent prompt
Filters & Parameters table present	parameter-transparency: reference the named config location by name, NO inline literals (see Parameter Centralization)	Every subagent prompt that filters/caps/winsorizes/windows data
Implementation Language: SAS or Mixed	SAS ETL enforcement (skills/wrds/references/sas-etl.md)	Every SAS subagent prompt
Filter Strategy table present	ETL enforcement (skills/ds-implement/references/etl-enforcement.md) § Filter Push-Down	Subagent prompts for data loading tasks
Parallelism Plan table present	ETL enforcement (skills/ds-implement/references/etl-enforcement.md) § Parallelism	Implementation strategy choice
Data Flow with intermediates	ETL enforcement (skills/ds-implement/references/etl-enforcement.md) § Caching	Subagent prompts for tasks producing/consuming intermediates
Scale-Up Testing Plan table present	ETL enforcement (skills/ds-implement/references/etl-enforcement.md) § Scale-Up + domain reference (e.g., gemini-batch/references/scale-up-testing.md)	Before any batch submission task

To load these references, discover the plugin cache path first:

• Read ${CLAUDE_SKILL_DIR}/../../skills/wrds/references/sas-etl.md and follow its instructions.
• Read ${CLAUDE_SKILL_DIR}/../../skills/ds-implement/references/etl-enforcement.md and follow its instructions.

If PLAN.md has NO ETL Strategy section: Skip this — proceed directly to Step 2.

Context Monitoring

Before starting each task, check context availability:

Level	Remaining Context	Action
Normal	>35%	Proceed with task
Warning	25-35%	Complete current task, then invoke ds-handoff
Critical	≤25%	Invoke ds-handoff immediately — no new tasks

At Warning level: After current task completes, invoke: Read ${CLAUDE_SKILL_DIR}/../../skills/ds-handoff/SKILL.md and follow its instructions.

Why: A multi-task analysis pipeline with 20% context remaining produces degraded output. Better to handoff cleanly and resume fresh.

Step 2: Execute Each Task via Delegation

For each task in PLAN.md:

1. Dispatch analyst subagent (per ds-delegate pattern)
2. Verify outputs are present and reasonable
3. Dispatch methodology reviewer (for statistical tasks)
4. Log findings to LEARNINGS.md

Step 3: Log to LEARNINGS.md

Document every significant step:

## Task N: [Description] - COMPLETE

**Input:** [Describe input state]

**Operation:** [What was done]

**Output:**
- Shape: [final shape]
- Key findings: [observations]

**Verification:** [How you confirmed it worked]

**Next:** [What comes next]

Task Summary (MANDATORY after each task)

After a task passes review, append a structured summary to LEARNINGS.md:

## Task N: [task description]

---
task: N
status: completed
implements: [DATA-01, STAT-03]
affects: [notebooks/analysis.py, data/processed/]
key-files:
  created: [list of new files]
  modified: [list of changed files]
deviations: {r1: 0, r2: 1, r3: 0, r4: 0}
---

One-liner: [SUBSTANTIVE summary — not "Task complete" but "Merged CRSP-Compustat panel with winsorized returns at 1%/99%"]

Changes: [what was added/modified and why]
Output: [output files produced and their contents]

One-liner rule: Must be SUBSTANTIVE. Good: "Panel regression with firm and year FE, clustered SEs, 3 robustness checks". Bad: "Completed task 3".

Dataset Construction Diagram (Maintain As You Build)

If PLAN.md has a ## Dataset Construction Diagram (the master-datasets mermaid flowchart: raw → merges → filters → master datasets → exhibits), it is a required doc deliverable the pipeline must keep current. PLAN.md's diagram is the intended construction; the docs carry the construction that actually ran.

• After a master-build task lands, update the diagram in the analysis docs (e.g. docs/INVESTIGATION or the analysis README/notebook header) with the real merge keys and the actual row-drops each filter produced (the profiled numbers, not the planned estimates).
• If the built pipeline diverges from the plan (an extra filter, a different join key, a master split in two), update the diagram AND note the divergence in LEARNINGS.md. A stale diagram that contradicts the code is worse than none — the reader trusts the picture over the script.
• Keep master nodes as [(rounded)] and label every edge with its key or filter+row-drop. An edgeless box diagram hides the sample funnel, which is the one thing the diagram exists to show.

This is cheap to edit per-task and expensive to reconstruct from memory at write-up time. The final diagram is what ds-handoff records and ds-review checks against the code.

Parameter Centralization (No Inline Literals)

If PLAN.md has a ## Filters & Parameters table, it names a single config location (default: a plain src/config.py of named constants with rationale in inline comments). Every task subagent MUST read parameters from that location by name and write NO inline numeric literals for any analysis decision — filters, bands, caps, winsorization levels, date windows, min-obs counts.

• Inject the config-location instruction into every implementation subagent prompt: "All filter/threshold/cap/window values come from <config location> referenced by name. Do NOT hard-code numeric literals for analysis decisions — if a value you need is missing from the config, ADD it there (with a Filters & Parameters row), don't inline it."
• df[df.price > 100] is a magic number; df[df.price > MIN_PRICE] is correct. Loop indices, unit conversions (* 100 for percent), and array offsets are not parameters — leave them inline.
• When a task introduces a new parameter not in the inventory, the subagent ADDS it to the config location AND appends a row to the ## Filters & Parameters table (constant · value · applied in · rationale/source · principled? · disposition). A new convenience (⚠) parameter needs a disposition (robustness panel / verified-redundant / display-only) — log it for ds-review; principled (✓) requires a cited source or a validation result.
• A scattered literal is a [Rule 2 - Missing Critical] deviation: centralize it (move to config, reference by name), verify the output is unchanged, and track it.

A magic number that reaches the final pipeline is a replication landmine the reviewer will flag — centralizing as you write costs one import; retrofitting after literals scatter costs an audit pass (the exact rework Edwin's muni magic-numbers audit is paying down).

Verification Patterns

See references/verification-patterns.md for detailed code patterns for:

• Data loading, filtering, merging
• Aggregation and model training
• Batch pipeline scale-up testing (submission, validation, cost extrapolation)
• Quick reference table by operation type

See references/etl-enforcement.md for ETL strategy enforcement:

• Filter push-down (database vs application vs hybrid)
• Parallelism (Task agents vs SGE vs sequential)
• Intermediate caching (parquet vs CSV vs SQLite)
• Scale-up testing domain routing

Scale-Up Testing Protocol (Batch/ETL Operations)

Triggers when PLAN.md includes a Scale-Up Testing Plan table. NO FULL BATCH WITHOUT A SUCCESSFUL TEST BATCH. This is not negotiable.

Three stages: Test (~10 items, always required) -> Intermediate (~100, if total >500) -> Large (~1,000, if total >5,000). Each stage has quality gates that must pass before scaling up.

Full protocol, scale-up facts, and red flags: See references/scale-up-testing.md

Common Failures to Avoid

Failure	Why It Happens	Prevention
Silent data loss	Merge drops rows	Print row counts before/after
Hidden nulls	Join introduces nulls	Check null counts after joins
Wrong aggregation	Groupby logic error	Display sample groups
Type coercion	Pandas silent conversion	Verify dtypes after load
Off-by-one	Date filtering edge cases	Print min/max dates

If Output Looks Wrong

1. STOP - do not proceed
2. Investigate - print more details
3. Document - log the issue in LEARNINGS.md
4. Ask - if unclear, ask user for guidance
5. Fix - only proceed after output verified

Never hide failures. Bad output documented is better than silent failure.

No Pause Between Tasks

**After completing task N, IMMEDIATELY start task N+1. You MUST NOT pause.**

The user sees results at the END and is waiting for completion, not interim check-ins — a courtesy pause costs a full turn round-trip and delivers nothing. Pause only when ALL tasks are complete or you are blocked.

Your pausing between tasks is procrastination disguised as courtesy.

Autonomous Execution: Re-Read & Blocker Handling

Dynamic plan re-read. After each task completes, RE-READ .planning/PLAN.md before starting the next task. A prior task (or a Rule-2/Rule-3 deviation) may have inserted, reordered, or removed tasks. Trusting a stale in-memory task list silently skips dynamically-added work. The on-disk PLAN.md is the source of truth.

Blocker handling. When a task cannot proceed (subagent reports failure it cannot auto-fix under R1-R3, missing dependency, environment error), do NOT silently stop. Present the blocker with three options and act on the choice:

Option	When	Action
Retry	Transient / fixable cause	Re-dispatch the task subagent with the blocker context added
Skip	Task is non-blocking for downstream work	Mark the task blocked in PLAN.md, log to LEARNINGS.md, continue to the next independent task
Stop	Blocker invalidates the plan (R4-class)	Invoke ds-handoff, escalate to the user

In autonomous/auto-advance mode, default to Retry once, then Skip if still blocked and the task is non-critical, then Stop. Record the chosen path in LEARNINGS.md.

Deviation Rules

When subagents encounter unplanned issues during implementation, follow this 4-rule system:

Rule	Trigger	Action	Permission
R1: Bug	Data integrity bugs, wrong joins, type errors, off-by-one in date ranges, NaN propagation, index alignment errors	Fix → verify output with output-first protocol → track [Rule 1 - Bug]	Auto
R2: Missing Critical	Missing null handling, no dedup check after merge, missing row count verification, no dtype validation, missing outlier handling	Add → verify → track [Rule 2 - Missing Critical]	Auto
R3: Blocking	Missing dependency/package, wrong file path, data file unavailable, API rate limit, memory error on large data	Fix blocker → verify proceeds → track [Rule 3 - Blocking]	Auto
R4a: Data Assumption	Data doesn't match expected shape/schema/distribution — expected panel but got cross-section, unexpected nulls in key column, different date range than specified, unexpected categories	STOP → present finding with evidence → track [Rule 4a - Data Assumption]	Ask user
R4b: Methodology Change	Analysis approach needs changing — different model needed, different sample definition, different variable construction, need to add/remove control variables	STOP → present decision with alternatives → track [Rule 4b - Methodology]	Ask user

Priority: R4a/R4b (STOP) > R1-R3 (auto) > unsure → escalate as R4.

Edge cases:

• Unexpected nulls in non-key column → R2 (add handling)
• Unexpected nulls in key/ID column → R4a (data assumption violated)
• Package version mismatch → R3 (blocking)
• Need different statistical test → R4b (methodology change)
• Wrong merge type (left vs inner) → R1 (bug)
• Data has different granularity than expected → R4a (assumption)

Tracking format per task: Each task summary in .planning/LEARNINGS.md should end with: Deviations: N auto-fixed (R1: X, R2: Y, R3: Z). R4 escalations: [list or "none"].

Agent Team Implementation (Parallel)

Full protocol: See references/agent-team-protocol.md for prerequisites, spawn prompt template, lead monitoring, reconciliation (3 passes), and usage guidelines.

Key points:

1. Run foundation tasks sequentially FIRST, then spawn parallel teammates
2. Each teammate gets exclusive data scope and output files
3. After all teammates complete, lead performs 3 reconciliation passes (Collect, Verify, Methodology)
4. Default is sequential. Only use agent teams for 4+ tasks with true independence (different datasets/subsets, no shared output files)

Gate: Exit Implementation

Checkpoint type: human-verify (all tasks pass — machine-verifiable)

**You MUST NOT proceed to review without verifying ALL tasks are complete. This is not negotiable.**

Before proceeding to validation, execute this gate:

1. IDENTIFY: Read .planning/PLAN.md — list every task by number and name
2. RUN: Read .planning/LEARNINGS.md — find entries for each task
3. READ: For each task, confirm LEARNINGS.md contains:
   • A "Task N: [Name] - COMPLETE" entry
   • Verified output (shape, stats, or sample)
   • No unresolved issues flagged
4. VERIFY: Count tasks in PLAN.md vs completed entries in LEARNINGS.md. They MUST match.

Staleness Check: LEARNINGS.md must be updated in THIS session, not reused from prior work.

• Does each task entry reference current outputs (file paths, cell numbers)?
• If LEARNINGS.md is stale from a prior session, UPDATE it with fresh entries before claiming completion.

Stale LEARNINGS.md = false gate pass = unverified work = the user gets results no one actually checked.

5. CLAIM: Only if all tasks accounted for, write the completion sentinel, then proceed to validation:

Write(".planning/IMPLEMENT_COMPLETE.md", """---
status: COMPLETE
tasks_total: [N]
date: [ISO 8601]
---
All PLAN.md tasks complete and verified in LEARNINGS.md. ds-validate may proceed.
""")

If ANY task is missing from LEARNINGS.md, implement it before proceeding. Do NOT write the sentinel until the task counts match.

Claiming all tasks are done without checking LEARNINGS.md against PLAN.md is NOT HELPFUL — missing tasks mean incomplete analysis the user relies on.

Phase Complete

After passing the exit gate (sentinel written), IMMEDIATELY discover and read the validation phase: Read ${CLAUDE_SKILL_DIR}/../../skills/ds-validate/SKILL.md and follow its instructions. Follow its instructions to validate outputs before review.

This gate is hook-enforced: ds-validate declares a PreToolUse phase-gate-guard.py hook that blocks its validator dispatch until .planning/IMPLEMENT_COMPLETE.md exists with status: COMPLETE.