diagnose

diagnose — Root-Cause Isolation

Operating Invariants

• Investigation Boundary: Read production source and existing tests; write temporary harnesses, fixtures, trace replays, and report files only inside .agents/scratch/. End at a root-cause report; leave production fixes, permanent tests, and fix changesets to subsequent work.
• Working Tree Preservation: Preserve pre-existing changes and scratch artifacts. Run probes against isolated copies or existing read-only seams; keep instrumentation out of production source. Tag temporary probes [DEBUG-<hex>] and remove only session-owned instrumentation after collecting evidence.
• Sensitive Evidence & Environment: Redact secrets before logging; never inspect secret files (.env*, *.key, *.pem). Ground runners in .agents/dev-environment.md; confine execution strictly to local scratch boundaries without touching production or host-level containers.

Domain Rubric

Intake and Causal Evidence

• Accept a defect description, error, log, stack trace, or reproduction. Identify expected behavior, observed behavior, relevant environment, and the execution seam; distinguish missing evidence from contradicted evidence.
• Begin investigation with available evidence even when an executable RED is unavailable. Treat a failing assertion as symptom evidence; establish cause through a discriminating probe, a conclusive execution trace, or a verified causal chain.
• Separate trigger and context from the mechanism that produces the failure. For each defensible hypothesis, state a prediction: If <cause>, then <controlled probe> changes <observation>.
• Test competing explanations when evidence permits; vary one causal factor at a time and record results that support or falsify each candidate. Prefer direct traces for obvious syntax errors or deterministic failures over unnecessary harnesses.

Reproduction and Feedback Loops

Choose the tightest loop faithful to the symptom and available runtime:

Seam	Investigation method
Existing test	Run the focused assertion at the failing call site
Running process	Invoke the CLI or HTTP path and compare expected/actual output
Isolated callable	Build a minimal scratch harness; disclose substituted I/O
Captured event	Replay sanitized payloads, logs, or HAR data in scratch
Historical regression	Compare revisions in an isolated scratch checkout
Interactive-only trigger	Adapt the local loop template in scratch; capture redacted responses

• Minimize inputs, configuration, and steps while preserving the exact symptom; retain every factor needed for fidelity, including relevant integration behavior.
• For intermittent failures, control seeds, clocks, event ordering, or concurrency where feasible. Report attempts, failures, and intervention results; use bounded repetitions rather than treating one lucky pass as proof.
• Preserve real architectural seams. If mocks omit the suspected mechanism or no faithful seam is available, document that proof gap and continue evidence-based investigation within scope.

Conclusion States

• CONFIRMED: Evidence verifies the causal mechanism, including the link from that mechanism to the reported symptom.
• INFERRED: Evidence supports a specific causal hypothesis, but a stated gap prevents confirmation.
• REJECTED: Discriminating evidence falsifies the investigated causal claim in the stated scope; identify the claim rather than implying the defect is disproven.
• UNRESOLVED: Available evidence supports no defensible cause, or an input, environment, or access blocker prevents a meaningful conclusion.

Canonical Output Contract

Produce one compact report. Include commands, relevant observations, and source locations sufficient to reproduce or inspect the reasoning; keep raw logs in sanitized scratch artifacts when needed.

# Root-Cause Report: <defect>

State: <CONFIRMED | INFERRED | REJECTED | UNRESOLVED>
Conclusion: <causal mechanism, supported hypothesis, rejected claim, or unresolved question>
Scope: <symptom, runtime/environment, and inspected seam>
Evidence: <commands or trace references; observed results; discriminating causal link>
Conditions and proof limits: <assumptions, substitutions, missing evidence, and applicability limits>

## Competing explanations *(omit if none were investigated)*
<claim -> probe/prediction -> observation -> supported or falsified>

## Reproduction *(omit if no executable or interactive loop was established)*
<scratch path or existing command; required setup; assertion and observed symptom>

Routing

• An existing candidate and direction need solution proof → (via /spike).
• Root-cause findings need a solution direction → (via /architect-decision).

Execution Protocol

1. Ground the symptom in supplied evidence and inspect the relevant call chain. Record the narrow causal question and available verification seam.
2. Establish and minimize a faithful RED loop when possible. When execution is unavailable, continue through traces and source evidence while recording the missing proof.
3. Probe ranked hypotheses until a cause is confirmed, supported with an explicit gap, falsified, or unresolved. Stop an investigation branch when further probes require unavailable input/access or merely repeat the same signal.
4. Present the root-cause report with its state and proof limits. Preserve useful sanitized reproduction artifacts, remove session-owned instrumentation, and verify production files remain unchanged.