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"Non-Deterministic workflow detected" error from deterministic code

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Assessment

Difficulty
4/5
Estimated time
3-5 days
Newbie friendliness
45/100
Issue type
Bug
Clarity
Mostly clear
Activity status
Quiet
Tech stack
azure, python
Domain
backend, distributed-systems

Research direction

Start by running the minimal reproduction from repro_nondeterminism.py with the function_app.py setup, using the supplied exception string and the shorter "x" comparison. Trace how the activity exception is reported through orchestration replay and determinism checks; done means the workflow reaches Failed status and exposes the activity exception without a non-deterministic error.

Written by the indexing model from the issue text.

Description

blocked bug fixed-in-v2 known-regression

🐛 Describe the bug
I have a minimal repro which makes the Function fail with "Non-Deterministic workflow detected" error, even if the code is completely deterministic. If I replace the RuntimeError argument string with "x", then I get the expected behavior. There is something in the exception string which triggers the undesired behavior.

🤔 Expected behavior
The Function should end with Failed status with the exception from the activity in the output.

☕ Steps to reproduce
Here is the code for a minimal repro: HTTP trigger + trivial orchestration and activity. The activity throws an exception every time. (the SAS signature is fake)

import logging
import azure.functions as func
import azure.durable_functions as df

bp = df.Blueprint()
logger = logging.getLogger(__name__)

# ── Trigger ────────────────────────
@bp.route(route="repro")
@bp.durable_client_input(client_name="orchestration_client")
async def http_start_repro(
    req: func.HttpRequest, orchestration_client: df.DurableOrchestrationClient
) -> func.HttpResponse:
    instance_id = await orchestration_client.start_new("repro_orchestrator")
    logger.info(f"Started repro orchestrator with ID '{instance_id}'")
    return orchestration_client.create_check_status_response(req, instance_id)

# ── Orchestrator ──────────────────
@bp.orchestration_trigger(context_name="context")
def repro_orchestrator(context: df.DurableOrchestrationContext):
    try:
        yield context.call_activity("repro_activity")
    except Exception as e:
        logger.error(f"Repro orchestrator caught: {e}")
        raise

# ── Activity ──────────────────────
@bp.activity_trigger(input_name="input_data")
def repro_activity(input_data):
    # The sig= value contains /, +, and = which trigger the bug.
    raise RuntimeError(
        "Download failed: https://example.blob.core.windows.net/container/test.file"
        "?sv=2025-07-05&spr=https&st=2026-04-01T10:58:55Z"
        "&se=2026-04-02T10:58:55Z&sr=b&sp=r"
        "&sig=aB3dE/fG5hI+jK7lM9nO="
    )

# ── function_app.py ──────────────
# Save the following as function_app.py in the same directory:

import azure.functions as func
import azure.durable_functions as df
from repro_nondeterminism import bp

app = df.DFApp(http_auth_level=func.AuthLevel.ANONYMOUS)
app.register_blueprint(bp)

⚡If deployed to Azure
I see the same behavior if deployed to Azure. Please refer to the code.

Dominant language
Python
Stars
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Forks
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