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Debugging Policies

This guide explains how to use OPA trace output to debug policy evaluations. When you need to understand why a policy granted or denied access—not just what the outcome was—trace output shows you the step-by-step evaluation path.

Overview: Two Debugging Tools

PolicyEngine provides two complementary debugging tools:

Tool	Shows	Best For
AccessRecord	What happened: which policies voted GRANT/DENY, final decision	Understanding the overall decision flow
OPA Trace	How each rule evaluated: variable bindings, condition checks, failures	Debugging why a specific rule matched or failed

Use AccessRecords first to identify which policy caused an unexpected outcome, then use trace output to understand why that policy behaved as it did.

Enabling Trace Output

Enable OPA trace logging with the --trace flag:

mpe --trace test decision -b my-domain.yml -i input.json

Combine with --pretty-log for easier reading of the AccessRecord:

mpe --trace --pretty-log test decision -b my-domain.yml -i input.json

Filtering Trace Output

When evaluating multiple policies, trace output can be overwhelming. Use --trace-filter to show traces only from specific policies:

# Trace only the unix-permissions policy
mpe --trace --trace-filter "mrn:iam:policy:unix-permissions" test decision -b my-domain.yml -i input.json

# Trace multiple policies (can specify --trace-filter multiple times)
mpe --trace \
  --trace-filter "mrn:iam:policy:require-auth" \
  --trace-filter "mrn:iam:policy:unix-permissions" \
  test decision -b my-domain.yml -i input.json

# Use regex patterns to match policy families
mpe --trace --trace-filter "mrn:iam:policy:.*permissions.*" test decision -b my-domain.yml -i input.json

Each --trace-filter value is a regex pattern matched against the policy MRN. If any filter matches, that policy's evaluation will produce trace output.

:::tip Start by identifying the problematic policy from the AccessRecord (check which policy in references has an unexpected decision), then use --trace-filter to focus on just that policy's evaluation. :::

Output Streams

• Trace output goes to stderr
• AccessRecord goes to stdout

This means you can redirect them separately:

# Save trace to file, view AccessRecord on terminal
mpe --trace --pretty-log test decision -b my-domain.yml -i input.json 2> trace.log

# Save AccessRecord to file, view trace on terminal
mpe --trace test decision -b my-domain.yml -i input.json > access-record.json

Understanding OPA Trace Output

The trace shows every step of OPA's evaluation. Each line has this format:

<location>    <depth>    <operation> <expression>

Column	Description
Location	Policy MRN and line number (e.g., mrn:iam:policy:unix-permissions:14)
Depth	Indentation showing call depth (pipes \| indicate nesting)
Operation	What OPA is doing (Enter, Exit, Eval, Unify, Fail, Redo)
Expression	The Rego expression being evaluated

Trace Operations

Operation	Meaning
Enter	Starting evaluation of a rule or function
Exit	Successfully completed evaluation (produced a result)
Eval	Evaluating an expression
Unify	Binding a variable to a value
Fail	Expression evaluated to false—this path won't succeed
Redo	Backtracking to try alternative matches (common with iteration)

Key Patterns to Look For

Successful evaluation path:

policy:9     | Enter data.authz.allow
policy:10    | | Eval some_condition
policy:9     | | Exit data.authz.allow early

The Exit confirms the rule succeeded.

Failed evaluation:

policy:9     | Enter data.authz.allow
policy:10    | | Eval principal.sub = resource.owner
policy:10    | | Fail principal.sub = resource.owner

The Fail shows exactly which condition didn't match.

Iteration with backtracking:

library:28   | | | Eval suffix = __local20__[__local10__]
library:28   | | | Unify ":get" = suffix
library:29   | | | Eval endswith(operation, suffix)
library:29   | | | Fail endswith(operation, suffix)
library:28   | | | Redo suffix = __local20__[__local10__]
library:28   | | | Unify ":list" = suffix

Redo shows OPA trying each element in a set/array until one matches or all fail.

Practical Examples

These examples use the Unix Filesystem example. Download the PolicyDomain and input files from that page to follow along, or clone the repository:

git clone https://github.com/manetu/policyengine.git
cd policyengine/docs/static/examples/unix-filesystem

Example 1: Group Read Access (GRANT)

Scenario: Bob (in the developers group) reads a file shared with that group.

mpe --trace --pretty-log test decision \
  -b policydomain.yml \
  -i input-3-group-read.json

Key trace excerpt (Resource phase evaluation):

mrn:iam:policy:unix-permissions:14     | | Eval data.unix_perms.permission_class(...)
mrn:iam:library:unix-perms:7           | | Enter data.unix_perms.permission_class
mrn:iam:library:unix-perms:8           | | | Eval principal.sub = resource.owner
mrn:iam:library:unix-perms:8           | | | Unify "bob" = resource.owner
mrn:iam:library:unix-perms:8           | | | Unify "alice" = "bob"
mrn:iam:library:unix-perms:8           | | | Fail principal.sub = resource.owner     ← Bob isn't the owner
mrn:iam:library:unix-perms:11          | | Enter data.unix_perms.permission_class    ← Try "group" class
mrn:iam:library:unix-perms:12          | | | Eval neq(__local24__, __local25__)      ← bob != alice ✓
mrn:iam:library:unix-perms:13          | | | Eval internal.member_2(__local26__, __local27__)
mrn:iam:library:unix-perms:11          | | | Exit data.unix_perms.permission_class   ← Returns "group"

What happened:

1. The permission_class function first tried to match Bob as the owner → failed (Bob ≠ Alice)
2. Then tried the "group" class → succeeded (Bob is in developers, the file's group)
3. The permission check then verified group.read = true

Example 2: Group Write Denied (DENY)

Scenario: Bob tries to write to the same file (group only has read permission).

mpe --trace --pretty-log test decision \
  -b policydomain.yml \
  -i input-4-group-write-denied.json

Key trace excerpt (the critical failure):

mrn:iam:policy:unix-permissions:17     | | Eval required = __local23__
mrn:iam:policy:unix-permissions:17     | | Unify required = "write"           ← Operation requires "write"
mrn:iam:policy:unix-permissions:20     | | Eval data.unix_perms.has_permission(mode, class, required)
mrn:iam:library:unix-perms:22          | | Enter data.unix_perms.has_permission
mrn:iam:library:unix-perms:22          | | | Unify "group" = class             ← Bob is in "group" class
mrn:iam:library:unix-perms:22          | | | Unify "write" = permission        ← Checking write permission
mrn:iam:library:unix-perms:23          | | | Eval mode[class][permission] = true
mrn:iam:library:unix-perms:23          | | | Unify false = true                ← group.write is false!
mrn:iam:library:unix-perms:23          | | | Fail mode[class][permission] = true
mrn:iam:policy:unix-permissions:20     | | Fail data.unix_perms.has_permission(...)

What happened:

1. Bob was classified as "group" (same as Example 1)
2. The operation file:document:write requires "write" permission
3. The has_permission check found mode.group.write = false
4. The main allow rule failed, falling through to default allow = false

Example 3: Superuser Bypass (GRANT)

Scenario: Root writes to a locked file (no permissions for anyone).

mpe --trace --pretty-log test decision \
  -b policydomain.yml \
  -i input-7-superuser.json

Key trace excerpt (superuser rule short-circuits):

mrn:iam:policy:unix-permissions:24     | Enter data.authz.allow               ← Superuser rule
mrn:iam:policy:unix-permissions:25     | | Eval __local35__ = input.principal.mroles
mrn:iam:policy:unix-permissions:25     | | Unify ["mrn:iam:role:superuser"] = __local35__
mrn:iam:policy:unix-permissions:25     | | Eval internal.member_2("mrn:iam:role:superuser", __local35__)
mrn:iam:policy:unix-permissions:24     | | Exit data.authz.allow early        ← Success! No permission check needed

What happened:

1. The policy has two allow rules: main permission check and superuser bypass
2. OPA tried the superuser rule first (line 24)
3. Root has mrn:iam:role:superuser in their roles
4. The rule matched and exited early—permission bits were never checked

Adding Custom Trace Messages

For complex debugging, add trace() calls to your policies:

allow if {
    trace(sprintf("Checking user %s against resource owner %s", [input.principal.sub, input.resource.owner]))
    input.principal.sub == input.resource.owner
}

This produces a Note entry in the trace:

policy:10    | | | Note "Checking user bob against resource owner alice"

:::warning Remove in Production The trace() builtin adds overhead. Remove or comment out trace calls before deploying to production. :::

Debugging Workflow

When a policy isn't behaving as expected:

1. Reproduce with trace enabled:

   mpe --trace --pretty-log test decision -b bundle.yml -i input.json 2> trace.log
   

2. Check the AccessRecord first:

   • Which phase failed? (SYSTEM, IDENTITY, RESOURCE, SCOPE)
   • Which specific policy voted DENY?

3. Find that policy's trace section: Search for the policy MRN in the trace output (e.g., mrn:iam:policy:unix-permissions)

4. Locate the failure point: Search for Fail lines within that policy's trace

5. Work backwards from the failure:

   • What value was being compared?
   • What did OPA expect vs. what it found?
   • Check the Unify lines to see actual values

6. Add trace() calls if needed: If the built-in trace isn't clear enough, add custom trace messages

7. Verify your fix: Re-run with trace to confirm the expected path now succeeds

Combining Trace with AccessRecords

Use both tools together for efficient debugging:

Start With	Then Use	When
AccessRecord	Trace	You know which policy failed but not why
Trace	AccessRecord	You see unexpected evaluation but need the big picture

Example workflow:

# Step 1: Get the high-level picture
mpe --pretty-log test decision -b bundle.yml -i input.json | jq '.references[] | select(.decision == "DENY")'

# Output shows: mrn:iam:policy:unix-permissions in RESOURCE phase

# Step 2: Drill into why that policy denied
mpe --trace test decision -b bundle.yml -i input.json 2>&1 | grep -A 20 "mrn:iam:policy:unix-permissions"

Related Resources

• Testing Policies — How to run policy tests
• Reading Access Records — Interpreting AccessRecord output
• Policies Concept — How policies are structured
• CLI Reference: mpe test — Complete command reference