# Research: Project Schedule Tools

**Date**: 2026-01-06
**Branch**: `001-schedule-tools`

## XER File Format

### Decision: Parse tab-delimited format with %T table headers

**Rationale**: XER is Primavera P6's native export format. It uses a simple text-based structure that's straightforward to parse without external libraries.

**Format Structure**:
```
ERMHDR	...header info...
%T	TABLE_NAME
%F	field1	field2	field3	...
%R	value1	value2	value3	...
%R	value1	value2	value3	...
%T	NEXT_TABLE
...
%E
```

**Key Tables for Schedule Tools**:
| Table | Purpose | Key Fields |
|-------|---------|------------|
| PROJECT | Project metadata | proj_id, proj_short_name, plan_start_date, plan_end_date |
| TASK | Activities | task_id, task_code, task_name, task_type, target_start_date, target_end_date, act_start_date, act_end_date, driving_path_flag |
| TASKPRED | Relationships | task_pred_id, task_id, pred_task_id, pred_type, lag_hr_cnt |
| PROJWBS | WBS structure | wbs_id, wbs_short_name, wbs_name, parent_wbs_id, proj_id |
| CALENDAR | Work calendars | clndr_id, clndr_name, day_hr_cnt |

**Alternatives Considered**:
- XML export: More complex to parse, larger file sizes
- Database direct access: Requires P6 installation, not portable

## MCP Python SDK

### Decision: Use `mcp` package with stdio transport

**Rationale**: The official MCP Python SDK provides a clean async interface for building MCP servers. Stdio transport is simplest for local tools.

**Implementation Pattern**:
```python
from mcp.server import Server
from mcp.server.stdio import stdio_server

server = Server("xer-mcp")

@server.tool()
async def load_xer(file_path: str, project_id: str | None = None) -> dict:
    """Load an XER file and optionally select a project."""
    ...

async def main():
    async with stdio_server() as (read_stream, write_stream):
        await server.run(read_stream, write_stream, server.create_initialization_options())
```

**Key Considerations**:
- All tools are async functions decorated with `@server.tool()`
- Tool parameters use Python type hints for JSON schema generation
- Return values are automatically serialized to JSON
- Errors should raise `McpError` with appropriate error codes

**Alternatives Considered**:
- SSE transport: Adds complexity, not needed for local use
- Custom protocol: Would break MCP compatibility

## SQLite Schema Design

### Decision: In-memory SQLite with normalized tables

**Rationale**: SQLite in-memory mode provides fast queries without file I/O overhead. Normalized tables map directly to XER structure while enabling efficient JOINs for relationship queries.

**Schema Design**:
```sql
-- Project table
CREATE TABLE projects (
    proj_id TEXT PRIMARY KEY,
    proj_short_name TEXT NOT NULL,
    plan_start_date TEXT,  -- ISO8601
    plan_end_date TEXT,
    loaded_at TEXT NOT NULL
);

-- Activities table
CREATE TABLE activities (
    task_id TEXT PRIMARY KEY,
    proj_id TEXT NOT NULL REFERENCES projects(proj_id),
    wbs_id TEXT,
    task_code TEXT NOT NULL,
    task_name TEXT NOT NULL,
    task_type TEXT,  -- TT_Task, TT_Mile, TT_LOE, etc.
    target_start_date TEXT,
    target_end_date TEXT,
    act_start_date TEXT,
    act_end_date TEXT,
    total_float_hr_cnt REAL,
    driving_path_flag TEXT,  -- 'Y' or 'N'
    status_code TEXT
);

-- Relationships table
CREATE TABLE relationships (
    task_pred_id TEXT PRIMARY KEY,
    task_id TEXT NOT NULL REFERENCES activities(task_id),
    pred_task_id TEXT NOT NULL REFERENCES activities(task_id),
    pred_type TEXT NOT NULL,  -- PR_FS, PR_SS, PR_FF, PR_SF
    lag_hr_cnt REAL DEFAULT 0
);

-- WBS table
CREATE TABLE wbs (
    wbs_id TEXT PRIMARY KEY,
    proj_id TEXT NOT NULL REFERENCES projects(proj_id),
    parent_wbs_id TEXT REFERENCES wbs(wbs_id),
    wbs_short_name TEXT NOT NULL,
    wbs_name TEXT
);

-- Indexes for common queries
CREATE INDEX idx_activities_proj ON activities(proj_id);
CREATE INDEX idx_activities_wbs ON activities(wbs_id);
CREATE INDEX idx_activities_type ON activities(task_type);
CREATE INDEX idx_activities_dates ON activities(target_start_date, target_end_date);
CREATE INDEX idx_relationships_task ON relationships(task_id);
CREATE INDEX idx_relationships_pred ON relationships(pred_task_id);
CREATE INDEX idx_wbs_parent ON wbs(parent_wbs_id);
```

**Query Patterns**:
- Pagination: `LIMIT ? OFFSET ?` with `COUNT(*)` for total
- Date filtering: `WHERE target_start_date >= ? AND target_end_date <= ?`
- Critical path: `WHERE driving_path_flag = 'Y'`
- Predecessors: `SELECT * FROM relationships WHERE task_id = ?`
- Successors: `SELECT * FROM relationships WHERE pred_task_id = ?`

**Alternatives Considered**:
- File-based SQLite: Adds complexity for file management, not needed for single-session use
- In-memory dictionaries: Would require custom indexing for efficient queries
- DuckDB: Overkill for this use case, larger dependency

## XER Parsing Strategy

### Decision: Streaming line-by-line parser with table handler registry

**Rationale**: XER files can be large (50K+ activities). Streaming avoids loading entire file into memory. Table handler registry enables extensibility per constitution.

**Implementation Approach**:
1. Read file line by line
2. Track current table context (%T lines)
3. Parse %F lines as field headers
4. Parse %R lines as records using current field map
5. Dispatch to registered table handler
6. Handler converts to model and inserts into SQLite

**Encoding Handling**:
- XER files use Windows-1252 encoding by default
- Attempt UTF-8 first, fallback to Windows-1252
- Log encoding detection result

## Pagination Implementation

### Decision: Offset-based pagination with metadata

**Rationale**: Simple to implement with SQLite's LIMIT/OFFSET. Metadata enables clients to navigate results.

**Response Format**:
```python
@dataclass
class PaginatedResponse:
    items: list[dict]
    pagination: PaginationMetadata

@dataclass
class PaginationMetadata:
    total_count: int
    offset: int
    limit: int
    has_more: bool
```

**Default Limit**: 100 items (per spec clarification)

## Error Handling

### Decision: Structured MCP errors with codes

**Rationale**: MCP protocol defines error format. Consistent error codes help clients handle failures.

**Error Codes**:
| Code | Name | When Used |
|------|------|-----------|
| -32001 | FILE_NOT_FOUND | XER file path doesn't exist |
| -32002 | PARSE_ERROR | XER file is malformed |
| -32003 | NO_FILE_LOADED | Query attempted before load |
| -32004 | PROJECT_SELECTION_REQUIRED | Multi-project file without selection |
| -32005 | ACTIVITY_NOT_FOUND | Requested activity ID doesn't exist |
| -32006 | INVALID_PARAMETER | Bad filter/pagination parameters |

## Driving Relationship Flag

**Research Date**: 2026-01-06

### Question: What field in the XER TASKPRED table contains the driving relationship flag?

**Finding**: The TASKPRED table in P6 XER files does NOT contain a direct `driving_flag` field.

**Evidence**: Analysis of sample XER file (S48019R - Proposal Schedule):
```
%F  task_pred_id  task_id  pred_task_id  proj_id  pred_proj_id  pred_type  lag_hr_cnt  comments  float_path  aref  arls
```

Fields available:
- `task_pred_id` - Unique relationship identifier
- `task_id` - Successor activity ID
- `pred_task_id` - Predecessor activity ID
- `proj_id` / `pred_proj_id` - Project identifiers
- `pred_type` - Relationship type (PR_FS, PR_SS, PR_FF, PR_SF)
- `lag_hr_cnt` - Lag duration in hours
- `comments` - User comments
- `float_path` - Float path indicator (contains dates, not boolean)
- `aref` / `arls` - Activity reference dates

### Question: Where is driving/critical path information stored in P6 XER files?

**Finding**: The `driving_path_flag` is stored at the ACTIVITY level on the TASK table, not on individual relationships.

**Evidence**:
```
TASK table includes: driving_path_flag (Y/N)
```

This flag indicates whether an activity is on the driving/critical path, but does not indicate which specific predecessor relationship is driving that activity's dates.

### Question: Can driving relationships be derived from available data?

**Finding**: Yes, driving relationships can be computed using schedule date comparison logic.

A relationship is "driving" when the successor activity's early start is constrained by the predecessor's completion. For a Finish-to-Start (FS) relationship:

```
driving = (predecessor.early_end_date + lag_hours ≈ successor.early_start_date)
```

### Decision: Compute driving flag at query time using early dates

**Rationale**:
1. P6 does not export a pre-computed driving flag per relationship
2. The driving relationship determination can be computed from activity dates
3. This matches how P6 itself determines driving relationships in the UI

**Implementation Approach**:
1. Early dates (`early_start_date`, `early_end_date`) are already parsed from TASK table
2. When querying relationships, compute `driving` by comparing dates
3. For FS: Compare `pred.early_end_date + lag` to `succ.early_start_date`
4. Use 1-hour tolerance for floating point date arithmetic

**Alternatives Considered**:
1. **Static flag from XER**: Not available in standard exports
2. **Always false**: Would not provide value to users
3. **Require user to specify**: Adds complexity, not aligned with P6 behavior

### Schema Impact

No schema changes needed for relationships table. Required activity date columns are already present:
- `activities.early_start_date` - Already in schema ✓
- `activities.early_end_date` - Already in schema ✓

The driving flag will be computed at query time via JOIN on activity dates.

### Validation Plan

- [ ] Verify early_start_date and early_end_date are parsed correctly from TASK table
- [ ] Test driving computation against known P6 schedules
- [ ] Confirm results match P6 "show driving" feature where possible